If State Is Hell, SOA Is Satan

More and more companies are describing their success stories regarding the switch to a service-oriented architecture. As with any technological upswing, there’s a clear and palpable hype factor involved (Big Data™ or The Cloud™ anyone?), but obviously it’s not just puff.

While microservices and SOA have seen a staggering rate of adoption in recent years, the mindset of developers often seems to be stuck in the past. I think this is, at least in part, because we seek a mental model we can reason about. It’s why we build abstractions in the first place. In a sense, I would argue there’s a comparison to be made between the explosion of OOP in the early 90’s and today’s SOA trend. After all, SOA is as much about people scale as it is about workload scale, so it makes sense from an organizational perspective.

The Perils of Good Abstractions

While systems are becoming more and more distributed, abstractions are attempting to make them less and less complex. Mesosphere is a perfect example of this, attempting to provide the “datacenter operating system.” Apache Mesos allows you to “program against your datacenter like it’s a single pool of resources.” It’s an appealing proposition to say the least. PaaS like Google App Engine and Heroku offer similar abstractions—write your code without thinking about scale. The problem is you absolutely have to think about scale or you’re bound to run into problems down the road. And while these abstractions are nice, they can be dangerous just the same. Welcome to the perils of good abstractions.

I like to talk about App Engine because I have firsthand experience with it. It’s an easy sell for startups. It handles spinning up instances when you need them, turning them down when you don’t. It’s your app server, database, caching, job scheduler, task queue all in one, and it does it at scale. There’s vendor lock-in, sure, yet it means no ops, no sysadmins, no overhead. Push to deploy. But it’s a leaky abstraction. It has to be. App Engine scales because it’s distributed, but it allows—no, encourages—you to write your system as a monolith. The datastore, memcache, and task queue accesses are masked as RPCs. This is great for our developer mental model, but it will bite you if you’re not careful. App Engine imposes certain limitations to encourage good design; for instance, front-end requests and datastore calls are limited to 60 seconds (it used to be much less), but the leakiness goes beyond that.

RPC is consistently at odds with distributed systems. I would go so far as to say it’s an anti-pattern in many cases. RPC encourages writing synchronous code, but distributed systems are inherently asynchronous. The network is not reliable. The network is not fast. The network is not your friend. Developers who either don’t understand this or don’t realize what’s happening when they make an RPC will write code as if they were calling a function. It will sure as hell look like just calling a function. When we think synchronously, we end up with systems that are slow, fault intolerant, and generally not scalable. To be quite honest, however, this is perfectly acceptable for 90% of startups as they are getting off the ground because they don’t have workloads at meaningful scale.

There’s certainly some irony here. One of the selling points of App Engine is its ability to scale to large amounts of traffic, yet the vast majority of startups would be perfectly suited to scaling up rather than out, perhaps with some failover in place for good measure. Stack Overflow is the poster child of scale-up architecture. In truth, your architecture should be a function of your access patterns, not the other way around (and App Engine is very much tailored to a specific set of access patterns). Nonetheless, it shows that vertical scaling can work. I would bet a lot of startups could sufficiently run on a large, adequately specced machine or maybe a small handful of them.

The cruel irony is that once you hit a certain scale with App Engine, both in terms of your development organization and user base, you’ve reached a point where you have to migrate off it. And if your data model isn’t properly thought out, you will without a doubt hit scale problems. It’s to the point where you need someone with deep knowledge of how App Engine works in order to build quality systems on it. Good luck hiring a team of engineers who understand it. GAE is great at accelerating you to 100 mph, but you better have some nice airbags for the brick wall it launches you into. In fairness, this is a problem every org hits—Conway’s law is very much a reality and every startup has growing pains. To be clear, this isn’t a jab at GAE, which is actually very effective at accelerating a product using little capital and can sustain long-term success given the right use case. Instead, I use it to illustrate a point.

Peering Through the Abstraction

Eventually SOA makes sense, but our abstractions can cause problems if we don’t understand what’s going on behind the curtain (hence the leakiness). Partial failure is all but guaranteed, and latency, partitioning, and other network pressure happens all the time.

Ken Arnold is famed with once saying “state is hell” in reference to designing distributed systems. In the past, I’ve written how scaling shared data is hard, but with SOA it’s practically a requirement. Ken is right though—state is hell, and SOA is fundamentally competing with consistency. The FLP Impossibility result and the CAP theorem can prove it formally, but really this should be intuitively obvious if we accept the laws of physics.

On the other hand, if you store information that I can’t reconstruct, then a whole host of questions suddenly surface. One question is, “Are you now a single point of failure?” I have to talk to you now. I can’t talk to anyone else. So what happens if you go down?

To deal with that, you could be replicated. But now you have to worry about replication strategies. What if I talk to one replicant and modify some data, then I talk to another? Is that modification guaranteed to have already arrived there? What is the replication strategy? What kind of consistency do you need—tight or loose? What happens if the network gets partitioned and the replicants can’t talk to each other? Can anybody proceed?

Essentially, the more stateful your system is, the harder it’s going to be to scale it because distributing that state introduces a rich tapestry of problems. In practice, we often can’t eliminate state wholesale, but basically everything that can be stateless should be stateless.

Making servers disposable allows you a great deal of flexibility. Former Netflix Cloud Architect Adrian Cockcroft articulates this idea well:

You want to think of servers like cattle, not pets. If you have a machine in production that performs a specialized function, and you know it by name, and everyone gets sad when it goes down, it’s a pet. Instead you should think of your servers like a herd of cows. What you care about is how many gallons of milk you get. If one day you notice you’re getting less milk than usual, you find out which cows aren’t producing well and replace them.

This is effectively how App Engine achieves its scalability. With lightweight, stateless, and disposable instances, it can spin them up and down on the fly without worrying about being in an invalid state.

App Engine also relies on eventual consistency as the default model for datastore interactions. This makes queries fast and highly available, while snapshot isolation can be achieved using entity-group transactions if necessary. The latter, of course, can result in a lot of contention and latency. Yet, people seem to have a hard time grappling with the reality of eventual consistency in distributed systems. State is hell, but calling SOA “satan” is clearly a hyperbole. It is a tough problem nevertheless.

A State of Mind

In the situations where we need state, we have to reconcile with the realities of distributed systems. This means understanding the limitations and accepting the complexities, not papering over them. It doesn’t mean throwing away abstractions. Fortunately, distributed computing is the focus of a lot of great research, so there are primitives with which we can build: immutability, causal ordering, eventual consistency, CRDTs, and other ideas.

As long as we recognize the trade-offs, we can design around them. The crux is knowing they exist in the first place. We can’t have ACID semantics while remaining highly available, but we can use Highly Available Transactions to provide strong-enough guarantees. At the same time, not all operations require coordination or concurrency control. The sooner we view eventual consistency as a solution and not a consequence, the sooner we can let go of this existential crisis. Other interesting research includes BOOM, which seeks to provide a high-level, declarative approach to distributed programming.

State might be hell, but it’s a hell we have to live. I don’t advocate an all-out microservice architecture for a company just getting its start. The complications far outweigh any benefits to be gained, but it becomes a necessity at a certain point. The key is having an exit strategy. PaaS providers make this difficult due to vendor lock-in and architectural constraints. Weigh their advantages carefully.

Once you do transition to a SOA, make as many of those services, or the pieces backing them, as stateless as possible. For those which aren’t stateless, know that the problem typically isn’t novel. These problems have been solved or are continuing to be solved in new and interesting ways. Academic research is naturally at the bleeding edge with industry often lagging behind. OOP concepts date back to as early as the 60’s but didn’t gain widespread adoption until several decades later. Distributed computing is no different. SOA is just a state of mind.

Stream Processing and Probabilistic Methods: Data at Scale

Stream processing and related abstractions have become all the rage following the rise of systems like Apache Kafka, Samza, and the Lambda architecture. Applying the idea of immutable, append-only event sourcing means we’re storing more data than ever before. However, as the cost of storage continues to decline, it’s becoming more feasible to store more data for longer periods of time. With immutability, how the data lives isn’t interesting anymore. It’s all about how it moves.

The shifting landscape of data architecture parallels the way we’re designing systems today. Specifically, the evolution of monolithic to service-oriented architecture necessitates a change in the way we do data integration. The traditional normalization approach doesn’t cut it. Our systems are composed of databases, caches, search indexes, data warehouses, and a multitude of other components. Moreover, there’s an increasing demand for online, real-time processing of this data that’s tantamount to the growing popularity of large-scale, offline processing along the lines of Hadoop. This presents an interesting set of new challenges, namely, how do we drink from the firehose without getting drenched?

The answer most likely lies in frameworks like Samza, Storm, and Spark Streaming. Similarly, tools like StatsD solve the problem of collecting real-time analytics. However, this discussion is meant to explore some of the primitives used in stream processing. The ideas extend far beyond event sourcing, generalizing to any type of data stream, unbounded or not.

Batch vs. Streaming

With offline or batch processing, we often have some heuristics which provide insight into our data set, and we can typically afford multiple passes of the data. Without a strict time constraint, data structures are less important. We can store the entire data set on disk (or perhaps across a distributed file system) and process it in batches.

With streaming data, however, there’s a need for near real-time processing—collecting analytics, monitoring and alerting, updating search indexes and caches, etc. With web crawlers, we process a stream of URLs and documents and produce indexed content. With websites, we process a stream of page views and update various counters and gauges. With email, we process a stream of text and produce a filtered, spam-free inbox. These cases involve massive, often limitless data sets. Processing that data online can’t be done with the conventional ETL or MapReduce-style methods, and unless you’re doing windowed processing, it’s entirely impractical to store that data in memory.

Framing the Problem

As a concrete example of stream processing, imagine we want to count the number of distinct document views across a large corpus, say, Wikipedia. A naive solution would be to use a hash table which maps a document to a count. Wikipedia has roughly 35 million pages. Let’s assume each document is identified by a 16-byte GUID, and the counters are stored as 8-byte integers. This means we need in the ball park of a gigabyte of memory. Given today’s hardware, this might not sound completely unreasonable, but now let’s say we want to track views per unique IP address. We see that this approach quickly becomes intractable.

To illustrate further, consider how to count the cardinality of IP addresses which access our website. Instead of a hash table, we can use a bitmap or sparse bit array to count addresses. There are over four billion possible distinct IPv4 addresses. Sure, we could allocate half a gigabyte of RAM, but if you’re developing performance-critical systems, large heap sizes are going to kill you, and this overhead doesn’t help.1

A Probabilistic Approach

Instead, we turn to probabilistic ways of solving these problems. Probabilistic algorithms and data structures seem to be oft-overlooked, or maybe purposely ignored, by system designers despite the theory behind them having been around for a long time in many cases. The goal should be to move these from the world of academia to industry because they are immensely useful and widely neglected.

Probabilistic solutions trade space and performance for accuracy. While a loss in precision may be a cause for concern to some, the fact is with large data streams, we have to trade something off to get real-time insight. Much like the CAP theorem, we must choose between consistency (accuracy) and availability (online). With CAP, we can typically adjust the level in which that trade-off occurs. This space/performance-accuracy exchange behaves very much the same way.

The literature can get pretty dense, so let’s look at what some of these approaches are and the problems they solve in a way that’s (hopefully) understandable.

Bloom Filters

The Bloom filter is probably the most well-known and, conceptually, simplest probabilistic data structure. It also serves as a good foundation because there are a lot of twists you can put on it. The theory provides a kernel from which many other probabilistic solutions are derived, as we will see.

Bloom filters answer a simple question: is this element a member of a set? A normal set requires linear space because it stores every element. A Bloom filter doesn’t store the actual elements, it merely stores the “membership” of them. It uses sub-linear space opening the possibility for false positives, meaning there’s a non-zero probability it reports an item is in the set when it’s actually not. This has a wide range of applications. For example, a Bloom filter can be placed in front of a database. When a query for a piece of data comes in and the filter doesn’t contain it, we completely bypass the database.

The Bloom filter consists of a bit array of length and hash functions. Both of these parameters are configurable, but we can optimize them based on a desired rate of false positives. Each bit in the array is initially unset. When an element is “added” to the filter, it’s hashed by each of the functions, h1…hk, and modded by m, resulting in indices into the bit array. Each bit at the respective index is set.

To query the membership of an element, we hash it and check if each bit is set. If any of them are zero, we know the item isn’t in the set. What happens when two different elements hash to the same index? This is where false positives are introduced. If the bits aren’t set, we know the element wasn’t added, but if they are, it’s possible that some other element or group of elements hashed to the same indices. Bloom filters have false positives, but false negatives are impossible—a member will never be reported incorrectly as a non-member. Unfortunately, this also means items can’t be removed from the filter.

It’s clear that the likelihood of false positives increases with each element added to the filter. We can target a specific probability of false positives by selecting an optimal value for m and k for up to n insertions.2 While this implementation works well in practice, it has a drawback in that some elements are potentially more sensitive to false positives than others. We can solve this problem by partitioning the m bits among the k hash functions such that each one produces an index over its respective partition. As a result, each element is always described by exactly bits. This prevents any one element from being especially sensitive to false positives. Since calculating k hashes for every element is computationally expensive, we can actually perform a single hash and derive k hashes from it for a significant speedup.3

A Bloom filter eventually reaches a point where all bits are set, which means every query will indicate membership, effectively making the probability of false positives one. The problem with this is it requires a priori knowledge of the data set in order to select optimal parameters and avoid “overfilling.” Consequently, Bloom filters are ideal for offline processing and not so great for dealing with streams. Next, we’ll look at some variations of the Bloom filter which attempt to deal with this issue.

Scalable Bloom Filter

As we saw earlier, traditional Bloom filters are a great way to deal with set-membership problems in a space-efficient way, but they require knowing the size of the data set ahead of time in order to be effective. The Scalable Bloom Filter (SBF) was introduced by Almeida et al. as a way to cope with the capacity dilemma.

The Scalable Bloom Filter dynamically adapts to the size of the data set while enforcing a tight upper bound on the rate of false positives. Like the classic Bloom filter, false negatives are impossible. The SBF is essentially an array of Bloom filters with geometrically decreasing false-positive rates. New elements are added to the last filter. When this filter becomes “full”—more specifically, when it reaches a target fill ratio—a new filter is added with a tightened error probability. A tightening ratio, r, controls the growth of new filters.

Testing membership of an element consists of checking each of the filters. The geometrically decreasing false-positive rate of each filter is an interesting property. Since the fill ratio determines when a new filter is added, it turns out this progression can be modeled as a Taylor series which allows us to provide a tight upper bound on false positives using an optimal fill ratio of 0.5 (once the filter is 50% full, a new one is added). The compounded probability over the whole series converges to a target value, even accounting for an infinite series.

We can limit the error rate, but obviously the trade-off is that we end up allocating memory proportional to the size of the data set since we’re continuously adding filters. We also pay some computational cost on adds. Amortized, the cost of filter insertions is negligible, but for every add we must compute the current fill ratio of the filter to determine if a new filter needs to be added. Fortunately, we can optimize this by computing an estimated fill ratio. If we keep a running count of the items added to the filter, n, the approximate current fill ratio, p, can be obtained from the Taylor series expansion with p \approx 1-e^{-n/m} where m is the number of bits in the filter. Calculating this estimate turns out to be quite a bit faster than computing the actual fill ratio—fewer memory reads.

To provide some context around performance, adds in my Go implementation of a classic Bloom filter take about 166 ns on my MacBook Pro, and membership tests take 118 ns. With the SBF, adds take 422 ns and tests 113 ns. This isn’t a completely fair comparison since, as the SBF grows over time, tests require scanning through each filter, but certainly the add numbers are intuitive.

Scalable Bloom Filters are useful for cases where the size of the data set isn’t known a priori and memory constraints aren’t of particular concern. It’s an effective variation of a regular Bloom filter which generally requires space allocation orders-of-magnitude larger than the data set to allow enough headroom.

Stable Bloom Filter

Classic Bloom filters aren’t great for streaming data, and Scalable Bloom Filters, while a better option, still present a memory problem. Another derivative, the Stable Bloom Filter (SBF), was proposed by Deng and Rafiei as a technique for detecting duplicates in unbounded data streams with limited space. Most solutions work by dividing the stream into fixed-size windows and solving the problem within that discrete space. This allows the use of traditional methods like the Bloom filter.

The SBF attempts to approximate duplicates without the use of windows. Clearly, we can’t store the entire history of an infinite stream in limited memory. Instead, the thinking is more recent data has more value than stale data in many scenarios. As an example, web crawlers may not care about redundantly fetching a web page that was crawled a long time ago compared to fetching a page that was crawled more recently. With this impetus, the SBF works by representing more recent data while discarding old information.

The Stable Bloom Filter tweaks the classic Bloom filter by replacing the bit array with an array of m cells, each allocated d bits. The cells are simply counters, initially set to zero. The maximum value, Max, of a cell is 2^d-1. When an element is added, we first have to ensure space for it. This is done by selecting P random cells and decrementing their counters by one, clamping to zero. Next, we hash the element to k cells and set their counters to Max. Testing membership consists of probing the k cells. If any of them are zero, it’s not a member. The Bloom filter is actually a special case of an SBF where d is one and P is zero.

Like traditional Bloom filters, an SBF has a non-zero probability of false positives, which is controlled by several parameters. Unlike the Bloom filter, an SBF has a tight upper bound on the rate of false positives while introducing a non-zero rate of false negatives. The false-positive rate of a classic Bloom filter eventually reaches one, after which all queries result in a false positive. The stable-point property of an SBF means the false-positive rate asymptotically approaches a configurable fixed constant.

Stable Bloom Filters lend themselves to situations where the size of the data set isn’t known ahead of time and memory is bounded. For example, an SBF can be used to deduplicate events from an unbounded event stream with a specified upper bound on false positives and minimal false negatives. In particular, if the stream is not uniformly distributed, meaning duplicates are likely to be grouped closer together, the rate of false positives becomes immaterial.

Counting Bloom Filter

Conventional Bloom filters don’t allow items to be removed. The Stable Bloom Filter, on the other hand, works by evicting old data, but its API doesn’t expose a way to remove specific elements. The Counting Bloom Filter (CBF) is yet another twist on this structure. Introduced by Fan et al. in Summary Cache: A Scalable Wide-Area Web Cache Sharing Protocol, the CBF provides a way of deleting data from a filter.

It’s actually a very straightforward approach. The filter comprises an array of n-bit buckets similar to the Stable Bloom Filter. When an item is added, the corresponding counters are incremented, and when it’s removed, the counters are decremented.

Obviously, a CBF takes n-times more space than a regular Bloom filter, but it also has a scalability limit. Unless items are removed frequently enough, a counting filter’s false-positive probability will approach one. We need to dimension it accordingly, and this normally requires inferring from our data set. Likewise, since the CBF allows removals, it exposes an opportunity for false negatives. The multi-bit counters diminish the rate, but it’s important to be cognizant of this property so the CBF can be applied appropriately.

Inverse Bloom Filter

One of the distinguishing features of Bloom filters is the guarantee of no false negatives. This can be a valuable invariant, but what if we want the opposite of that? Is there an efficient data structure that can offer us no false positives with the possibility of false negatives? The answer is deceptively simple.

Jeff Hodges describes a rather trivial—yet strikingly pragmatic—approach which he calls the “opposite of a Bloom filter.” Since that name is a bit of a mouthful, I’m referring to this as the Inverse Bloom Filter (IBF).

The Inverse Bloom Filter may report a false negative but can never report a false positive. That is, it may indicate that an item has not been seen when it actually has, but it will never report an item as seen which it hasn’t come across. The IBF behaves in a similar manner to a fixed-size hash map of m buckets which doesn’t handle conflicts, but it provides lock-free concurrency using an underlying CAS.

The test-and-add operation hashes an element to an index in the filter. We take the value at that index while atomically storing the new value, then the old value is compared to the new one. If they differ, the new value wasn’t a member. If the values are equivalent, it was in the set.

The Inverse Bloom Filter is a nice option for dealing with unbounded streams or large data sets due to its limited memory usage. If duplicates are close together, the rate of false negatives becomes vanishingly small with an adequately sized filter.

HyperLogLog

Let’s revisit the problem of counting distinct IP addresses visiting a website. One solution might be to allocate a sparse bit array proportional to the number of IPv4 addresses. Alternatively, we could combine a Bloom filter with a counter. When an address comes in and it’s not in the filter, increment the count. While these both work, they’re not likely to scale very well. Rather, we can apply a probabilistic data structure known as the HyperLogLog (HLL). First presented by Flajolet et al. in 2007, HyperLogLog is an algorithm which approximately counts the number of distinct elements, or cardinality, of a multiset (a set which allows multiple occurrences of its elements).

Imagine our stream as a series of coin tosses. Someone tells you the most heads they flipped in a row was three. You can guess that they didn’t flip the coin very many times. However, if they flipped 20 heads in a row, it probably took them quite a while. This seems like a really unconvincing way of estimating the number of tosses, but from this kernel of thought grows a fruitful idea.

HLL replaces heads and tails with zeros and ones. Instead of counting the longest run of heads, it counts the longest run of leading zeros in a binary hash. Using a good hash function means that the values are, more or less, statistically independent and uniformly distributed. If the maximum run of leading zeros is n, the number of distinct elements in the set is approximately 2^n. But wait, what’s the math behind this? If we think about it, half of all binary numbers start with 1. Each additional bit divides the probability of a run further in half; that is, 25% start with 01, 12.5% start with 001, 6.25% start with 0001, ad infinitum. Thus, the probability of a run of length n is 2^{-(n+1)}.

Like flipping a coin, there’s potential for an enormous amount of variance with this technique. For instance, it’s entirely possible—though unlikely—that you flip 20 heads in a row on your first try. One experiment doesn’t provide enough data, so instead you flip 10 coins. HyperLogLog uses a similar strategy to reduce variance by splitting the stream up across a set of buckets, or registers, and applying the counting algorithm on the values in each one. It tracks the longest run of zeros in every register, and the total cardinality is computed by taking the harmonic mean across all registers. The harmonic mean is used to discount outliers since the distribution of values tends to skew towards the right. We can increase accuracy by adding more registers, which of course comes at the expense of performance and memory.

HyperLogLog is a remarkable algorithm. It almost feels like magic, and it’s exceptionally useful for working with data streams. HLL has a fraction of the memory footprint of other solutions. In fact, it’s usually several orders of magnitude while remaining surprisingly accurate.

Count-Min Sketch

HyperLogLog is a great option to efficiently count the number of distinct elements in a stream using a minimal amount of space, but it only gives us cardinality. What about counting the frequencies of specific elements? Cormode and Muthukrishnan developed the Count-Min sketch (CM sketch) to approximate the occurrences of different event types from a stream of events.

In contrast to a hash table, the Count-Min sketch uses sub-linear space to count frequencies. It consists of a matrix with w columns and d rows. These parameters determine the trade-off between space/time constraints and accuracy. Each row has an associated hash function. When an element arrives, it’s hashed for each row. The corresponding index in the rows are incremented by one. In this regard, the CM sketch shares some similarities with the Bloom filter.

count_min_sketch

The frequency of an element is estimated by taking the minimum of all the element’s respective counter values. The thought process here is that there is possibility for collisions between elements, which affects the counters for multiple items.  Taking the minimum count results in a closer approximation.

The Count-Min sketch is an excellent tool for counting problems for the same reasons as HyperLogLog. It has a lot of similarities to Bloom filters, but where Bloom filters effectively represent sets, the CM sketch considers multisets.

MinHash

The last probabilistic technique we’ll briefly look at is MinHash. This algorithm, invented by Andrei Broder, is used to quickly estimate the similarity between two sets. This has a variety of uses, such as detecting duplicate bodies of text and clustering or comparing documents.

MinHash works, in part, by using the Jaccard coefficient, a statistic which represents the size of the intersection of two sets divided by the size of the union:

J(A,B) = {{|A \cap B|}\over{|A \cup B|}}

This provides a measure of how similar the two sets are, but computing the intersection and union is expensive. Instead, MinHash essentially works by comparing randomly selected subsets through element hashing. It resembles locality-sensitive hashing (LSH), which means that similar items have a greater tendency to map to the same hash values. This varies from most hashing schemes, which attempt to avoid collisions. Instead, LSH is designed to maximize collisions of similar elements.

MinHash is one of the more difficult algorithms to fully grasp, and I can’t even begin to provide a proper explanation. If you’re interested in how it works, read the literature but know that there are a number of variations of it. The important thing is to know it exists and what problems it can be applied to.

In Practice

We’ve gone over several fundamental primitives for processing large data sets and streams while solving a few different types of problems. Bloom filters can be used to reduce disk reads and other expensive operations. They can also be purposed to detect duplicate events in a stream or prune a large decision tree. HyperLogLog excels at counting the number of distinct items in a stream while using minimal space, and the Count-Min sketch tracks the frequency of particular elements. Lastly, the MinHash method provides an efficient mechanism for comparing the similarity between documents. This has a number of applications, namely around identifying duplicates and characterizing content.

While there are no doubt countless implementations for most of these data structures and algorithms, I’ve been putting together a Go library geared towards providing efficient techniques for stream processing called Boom Filters. It includes implementations for all of the probabilistic data structures outlined above. As streaming data and real-time consumption grow more and more prominent, it will become important that we have the tools and understanding to deal with them. If nothing else, it’s valuable to be aware of probabilistic methods because they often provide accurate results at a fraction of the cost. These things aren’t just academic research, they form the basis for building online, high-performance systems at scale.

  1. Granted, if you’re sensitive to garbage-collection pauses, you may be better off using something like C or Rust, but that’s not always the most practical option. []
  2. The math behind determining optimal m and k is left as an exercise for the reader. []
  3. Less Hashing, Same Performance: Building a Better Bloom Filter discusses the use of two hash functions to simulate additional hashes. We can use a 64-bit hash, such as FNV, and use the upper and lower 32-bits as two different hashes. []

Fast, Scalable Networking in Go with Mangos

In the past, I’ve looked at nanomsg and why it’s a formidable alternative to the well-regarded ZeroMQ. Like ZeroMQ, nanomsg is a native library which markets itself as a way to build fast and scalable networking layers. I won’t go into detail on how nanomsg accomplishes this since my analysis of it already covers that fairly extensively, but instead I want to talk about a Go implementation of the protocol called Mangos.1 If you’re not familiar with nanomsg or Scalability Protocols, I recommend reading my overview of those first.

nanomsg is a shared library written in C. This, combined with its zero-copy API, makes it an extremely low-latency transport layer. While there are a lot of client bindings which allow you to use nanomsg from other languages, dealing with shared libraries can often be a pain—not to mention it complicates deployment.

More and more companies are starting to use Go for backend development because of its speed and concurrency primitives. It’s really good at building server components that scale. Go obviously provides the APIs needed for socket networking, but building a scalable distributed system that’s reliable using these primitives can be somewhat onerous. Solutions like nanomsg’s Scalability Protocols and ZeroMQ attempt to make this much easier by providing useful communication patterns and by taking care of other messaging concerns like queueing.

Naturally, there are Go bindings for nanomsg and ZeroMQ, but like I said, dealing with shared libraries can be fraught with peril. In Go (and often other languages), we tend to avoid loading native libraries if we can. It’s much easier to reason about, debug, and deploy a single binary than multiple. Fortunately, there’s a really nice implementation of nanomsg’s Scalability Protocols in pure Go called Mangos by Garrett D’Amore of illumos fame.

Mangos offers an idiomatic Go implementation and interface which affords us the same messaging patterns that nanomsg provides while maintaining compatibility. Pub/Sub, Pair, Req/Rep, Pipeline, Bus, and Survey are all there. It also supports the same pluggable transport model, allowing additional transports to be added (and extended2) on top of the base TCP, IPC, and inproc ones.3 Mangos has been tested for interoperability with nanomsg using the nanocat command-line interface.

One of the advantages of using a language like C is that it’s not garbage collected. However, if you’re using Go with nanomsg, you’re already paying the cost of GC. Mangos makes use of object pools in order to reduce pressure on the garbage collector. We can’t turn Go’s GC off, but we can make an effort to minimize pauses. This is critical for high-throughput systems, and Mangos tends to perform quite comparably to nanomsg.

Mangos (and nanomsg) has a very familiar, socket-like API. To show what this looks like, the code below illustrates a simple example of how the Pub/Sub protocol is used to build a fan-out messaging system.

My message queue test framework, Flotilla, uses the Req/Rep protocol to allow clients to send requests to distributed daemon processes, which handle them and respond. While this is a very simple use case where you could just as easily get away with raw TCP sockets, there are more advanced cases where Scalability Protocols make sense. We also get the added advantage of transport abstraction, so we’re not strictly tied to TCP sockets.

I’ve been building a distributed messaging system using Mangos as a means of federated communication. Pub/Sub enables a fan-out, interest-based broadcast and Bus facilitates many-to-many messaging. Both of these are exceptionally useful for connecting disparate systems. Mangos also supports an experimental new protocol called Star. This pattern is like Bus, but when a message is received by an immediate peer, it’s propagated to all other members of the topology.

My favorite Scalability Protocol is Survey. As I discussed in my nanomsg overview, there are a lot of really interesting applications of this. Survey allows a process to query the state of multiple peers in one shot. It’s similar to Pub/Sub in that the surveyor publishes a single message which is received by all the respondents (although there’s no topic subscriptions). The respondents then send a message back, and the surveyor collects these responses. We can also enforce a deadline on the respondent replies, which makes Survey particularly useful for service discovery.

With my messaging system, I’ve used Survey to implement a heartbeat protocol. When a broker spins up, it begins broadcasting a heartbeat using a Survey socket. New brokers can connect to existing ones, and they reply to the heartbeat which allows brokers to “discover” each other. If a heartbeat isn’t received before the deadline, the peer is removed. Mangos also handles reconnects, so if a broker goes offline and comes back up, peers will automatically reconnect.

To summarize, if you’re building distributed systems in Go, consider taking a look at Mangos. You can certainly roll your own messaging layer with raw sockets, but you’re going to end up writing a lot of logic for a robust system. Mangos, and nanomsg in general, gives you the right abstraction to quickly build systems that scale and are fast.

  1. Full disclosure: I am a contributor on the Mangos project, but only because I was a user first! []
  2. Mangos supports TLS with the TCP transport as an experimental extension. []
  3. A nanomsg WebSocket transport is currently in the works. []

From Mainframe to Microservice: An Introduction to Distributed Systems

I gave a talk at Iowa Code Camp this weekend on distributed systems. It was primarily an introduction to them, so it explored some core concepts at a high level.  We looked at why distributed systems are difficult to build (right), the CAP theorem, consensus, scaling shared data and CRDTs.

There was some interest in making the slides available online. I’m not sure how useful they are without narration, but here they are anyway for posterity.

Dissecting Message Queues

Continuing my series on message queues, I spent this weekend dissecting various libraries for performing distributed messaging. In this analysis, I look at a few different aspects, including API characteristics, ease of deployment and maintenance, and performance qualities. The message queues have been categorized into two groups: brokerless and brokered. Brokerless message queues are peer-to-peer such that there is no middleman involved in the transmission of messages, while brokered queues have some sort of server in between endpoints.

The systems I’ll be analyzing are:

Brokerless
nanomsg
ZeroMQ

Brokered
ActiveMQ
NATS
Kafka
Kestrel
NSQ
RabbitMQ
Redis
ruby-nats

To start, let’s look at the performance metrics since this is arguably what people care the most about. I’ve measured two key metrics: throughput and latency. All tests were run on a MacBook Pro 2.6 GHz i7, 16GB RAM. These tests are evaluating a publish-subscribe topology with a single producer and single consumer. This provides a good baseline. It would be interesting to benchmark a scaled-up topology but requires more instrumentation.

The code used for benchmarking, written in Go, is available on GitHub. The results below shouldn’t be taken as gospel as there are likely optimizations that can be made to squeeze out performance gains. Pull requests are welcome.

Throughput Benchmarks

Throughput is the number of messages per second the system is able to process, but what’s important to note here is that there is no single “throughput” that a queue might have. We’re sending messages between two different endpoints, so what we observe is a “sender” throughput and a “receiver” throughput—that is, the number of messages that can be sent per second and the number of messages that can be received per second.

This test was performed by sending 1,000,000 1KB messages and measuring the time to send and receive on each side. Many performance tests tend to use smaller messages in the range of 100 to 500 bytes. I chose 1KB because it’s more representative of what you might see in a production environment, although this varies case by case. For message-oriented middleware systems, only one broker was used. In most cases, a clustered environment would yield much better results.Unsurprisingly, there’s higher throughput on the sending side. What’s interesting, however, is the disparity in the sender-to-receiver ratios. ZeroMQ is capable of sending over 5,000,000 messages per second but is only able to receive about 600,000/second. In contrast, nanomsg sends shy of 3,000,000/second but can receive almost 2,000,000.

Now let’s take a look at the brokered message queues. Intuitively, we observe that brokered message queues have dramatically less throughput than their brokerless counterparts by a couple orders of magnitude for the most part. Half the brokered queues have a throughput below 25,000 messages/second. The numbers for Redis might be a bit misleading though. Despite providing pub/sub functionality, it’s not really designed to operate as a robust messaging queue. In a similar fashion to ZeroMQ, Redis disconnects slow clients, and it’s important to point out that it was not able to reliably handle this volume of messaging. As such, we consider it an outlier. Kafka and ruby-nats have similar performance characteristics to Redis but were able to reliably handle the message volume without intermittent failures. The Go implementation of NATS, gnatsd, has exceptional throughput for a brokered message queue.

Outliers aside, we see that the brokered queues have fairly uniform throughputs. Unlike the brokerless libraries, there is little-to-no disparity in the sender-to-receiver ratios, which themselves are all very close to one.

Latency Benchmarks

The second key performance metric is message latency. This measures how long it takes for a message to be transmitted between endpoints. Intuition might tell us that this is simply the inverse of throughput, i.e. if throughput is messages/second, latency is seconds/message. However, by looking closely at this image borrowed from a ZeroMQ white paper, we can see that this isn’t quite the case. latency The reality is that the latency per message sent over the wire is not uniform. It can vary wildly for each one. In truth, the relationship between latency and throughput is a bit more involved. Unlike throughput, however, latency is not measured at the sender or the receiver but rather as a whole. But since each message has its own latency, we will look at the averages of all of them. Going further, we will see how the average message latency fluctuates in relation to the number of messages sent. Again, intuition tells us that more messages means more queueing, which means higher latency.

As we did before, we’ll start by looking at the brokerless systems.
In general, our hypothesis proves correct in that, as more messages are sent through the system, the latency of each message increases. What’s interesting is the tapering at the 500,000-point in which latency appears to increase at a slower rate as we approach 1,000,000 messages. Another interesting observation is the initial spike in latency between 1,000 and 5,000 messages, which is more pronounced with nanomsg. It’s difficult to pinpoint causation, but these changes might be indicative of how message batching and other network-stack traversal optimizations are implemented in each library. More data points may provide better visibility.

We see some similar patterns with brokered queues and also some interesting new ones.

Redis behaves in a similar manner as before, with an initial latency spike and then a quick tapering off. It differs in that the tapering becomes essentially constant right after 5,000 messages. NSQ doesn’t exhibit the same spike in latency and behaves, more or less, linearly. Kestrel fits our hypothesis.

Notice that ruby-nats and NATS hardly even register on the chart. They exhibited surprisingly low latencies and unexpected relationships with the number of messages.Remarkably, the message latencies for ruby-nats and NATS appear to be constant. This is counterintuitive to our hypothesis.

You may have noticed that Kafka, ActiveMQ, and RabbitMQ were absent from the above charts. This was because their latencies tended to be orders-of-magnitude higher than the other brokered message queues, so ActiveMQ and RabbitMQ were grouped into their own AMQP category. I’ve also included Kafka since it’s in the same ballpark.

Here we see that RabbitMQ’s latency is constant, while ActiveMQ and Kafka are linear. What’s unclear is the apparent disconnect between their throughput and mean latencies.

Qualitative Analysis

Now that we’ve seen some empirical data on how these different libraries perform, I’ll take a look at how they work from a pragmatic point of view. Message throughput and speed is important, but it isn’t very practical if the library is difficult to use, deploy, or maintain.

ZeroMQ and Nanomsg

Technically speaking, nanomsg isn’t a message queue but rather a socket-style library for performing distributed messaging through a variety of convenient patterns. As a result, there’s nothing to deploy aside from embedding the library itself within your application. This makes deployment a non-issue.

Nanomsg is written by one of the ZeroMQ authors, and as I discussed before, works in a very similar way to that library. From a development standpoint, nanomsg provides an overall cleaner API. Unlike ZeroMQ, there is no notion of a context in which sockets are bound to. Furthermore, nanomsg provides pluggable transport and messaging protocols, which make it more open to extension. Its additional built-in scalability protocols also make it quite appealing.

Like ZeroMQ, it guarantees that messages will be delivered atomically intact and ordered but does not guarantee the delivery of them. Partial messages will not be delivered, and it’s possible that some messages won’t be delivered at all. The library’s author, Martin Sustrik, makes this abundantly clear:

Guaranteed delivery is a myth. Nothing is 100% guaranteed. That’s the nature of the world we live in. What we should do instead is to build an internet-like system that is resilient in face of failures and routes around damage.

The philosophy is to use a combination of topologies to build resilient systems that add in these guarantees in a best-effort sort of way.

On the other hand, nanomsg is still in beta and may not be considered production-ready. Consequently, there aren’t a lot of resources available and not much of a development community around it.

ZeroMQ is a battle-tested messaging library that’s been around since 2007. Some may perceive it as a predecessor to nanomsg, but what nano lacks is where ZeroMQ thrives—a flourishing developer community and a deluge of resources and supporting material. For many, it’s the de facto tool for building fast, asynchronous distributed messaging systems that scale.

Like nanomsg, ZeroMQ is not a message-oriented middleware and simply operates as a socket abstraction. In terms of usability, it’s very much the same as nanomsg, although its API is marginally more involved.

ActiveMQ and RabbitMQ

ActiveMQ and RabbitMQ are implementations of AMQP. They act as brokers which ensure messages are delivered. ActiveMQ and RabbitMQ support both persistent and non-persistent delivery. By default, messages are written to disk such that they survive a broker restart. They also support synchronous and asynchronous sending of messages with the former having substantial impact on latency. To guarantee delivery, these brokers use message acknowledgements which also incurs a massive latency penalty.

As far as availability and fault tolerance goes, these brokers support clustering through shared storage or shared nothing. Queues can be replicated across clustered nodes so there is no single point of failure or message loss.

AMQP is a non-trivial protocol which its creators claim to be over-engineered. These additional guarantees are made at the expense of major complexity and performance trade-offs. Fundamentally, clients are more difficult to implement and use.

Since they’re message brokers, ActiveMQ and RabbitMQ are additional moving parts that need to be managed in your distributed system, which brings deployment and maintenance costs. The same is true for the remaining message queues being discussed.

NATS and Ruby-NATS

NATS (gnatsd) is a pure Go implementation of the ruby-nats messaging system. NATS is distributed messaging rethought to be less enterprisey and more lightweight (this is in direct contrast to systems like ActiveMQ, RabbitMQ, and others). Apcera’s Derek Collison, the library’s author and former TIBCO architect, describes NATS as “more like a nervous system” than an enterprise message queue. It doesn’t do persistence or message transactions, but it’s fast and easy to use. Clustering is supported so it can be built on top of with high availability and failover in mind, and clients can be sharded. Unfortunately, TLS and SSL are not yet supported in NATS (they are in the ruby-nats) but on the roadmap.

As we observed earlier, NATS performs far better than the original Ruby implementation. Clients can be used interchangeably with NATS and ruby-nats.

Kafka

Originally developed by LinkedIn, Kafka implements publish-subscribe messaging through a distributed commit log. It’s designed to operate as a cluster that can be consumed by large amounts of clients. Horizontal scaling is done effortlessly using ZooKeeper so that additional consumers and brokers can be introduced seamlessly. It also transparently takes care of cluster rebalancing.

Kafka uses a persistent commit log to store messages on the broker. Unlike other durable queues which usually remove persisted messages on consumption, Kafka retains them for a configured period of time. This means that messages can be “replayed” in the event that a consumer fails.

ZooKeeper makes managing Kafka clusters relatively easy, but it does introduce yet another element that needs to be maintained. That said, Kafka exposes a great API and Shopify has an excellent Go client called Sarama that makes interfacing with Kafka very accessible.

Kestrel

Kestrel is a distributed message queue open sourced by Twitter. It’s intended to be fast and lightweight. Because of this, it has no concept of clustering or failover. While Kafka is built from the ground up to be clustered through ZooKeeper, the onus of message partitioning is put upon the clients of Kestrel. There is no cross-communication between nodes. It makes this trade-off in the name of simplicity. It features durable queues, item expiration, transactional reads, and fanout queues while operating over Thrift or memcache protocols.

Kestrel is designed to be small, but this means that more work must be done by the developer to build out a robust messaging system on top of it. Kafka seems to be a more “all-in-one” solution.

NSQ

NSQ is a messaging platform built by Bitly. I use the word platform because there’s a lot of tooling built around NSQ to make it useful for real-time distributed messaging. The daemon that receives, queues, and delivers messages to clients is called nsqd. The daemon can run standalone, but NSQ is designed to run in as a distributed, decentralized topology. To achieve this, it leverages another daemon called nsqlookupd. Nsqlookupd acts as a service-discovery mechanism for nsqd instances. NSQ also provides nsqadmin, which is a web UI that displays real-time cluster statistics and acts as a way to perform various administrative tasks like clearing queues and managing topics.

By default, messages in NSQ are not durable. It’s primarily designed to be an in-memory message queue, but queue sizes can be configured such that after a certain point, messages will be written to disk. Despite this, there is no built-in replication. NSQ uses acknowledgements to guarantee message delivery, but the order of delivery is not guaranteed. Messages can also be delivered more than once, so it’s the developer’s responsibility to introduce idempotence.

Similar to Kafka, additional nodes can be added to an NSQ cluster seamlessly. It also exposes both an HTTP and TCP API, which means you don’t actually need a client library to push messages into the system. Despite all the moving parts, it’s actually quite easy to deploy. Its API is also easy to use and there are a number of client libraries available.

Redis

Last up is Redis. While Redis is great for lightweight messaging and transient storage, I can’t advocate its use as the backbone of a distributed messaging system. Its pub/sub is fast but its capabilities are limited. It would require a lot of work to build a robust system. There are solutions better suited to the problem, such as those described above, and there are also some scaling concerns with it.

These matters aside, Redis is easy to use, it’s easy to deploy and manage, and it has a relatively small footprint. Depending on the use case, it can be a great choice for real-time messaging as I’ve explored before.

Conclusion

The purpose of this analysis is not to present some sort of “winner” but instead showcase a few different options for distributed messaging. There is no “one-size-fits-all” option because it depends entirely on your needs. Some use cases require fast, fire-and-forget messages, others require delivery guarantees. In fact, many systems will call for a combination of these. My hope is that this dissection will offer some insight into which solutions work best for a given problem so that you can make an intelligent decision.

A Look at Nanomsg and Scalability Protocols (Why ZeroMQ Shouldn’t Be Your First Choice)

Earlier this month, I explored ZeroMQ and how it proves to be a promising solution for building fast, high-throughput, and scalable distributed systems. Despite lending itself quite well to these types of problems, ZeroMQ is not without its flaws. Its creators have attempted to rectify many of these shortcomings through spiritual successors Crossroads I/O and nanomsg.

The now-defunct Crossroads I/O is a proper fork of ZeroMQ with the true intention being to build a viable commercial ecosystem around it. Nanomsg, however, is a reimagining of ZeroMQ—a complete rewrite in C1. It builds upon ZeroMQ’s rock-solid performance characteristics while providing several vital improvements, both internal and external. It also attempts to address many of the strange behaviors that ZeroMQ can often exhibit. Today, I’ll take a look at what differentiates nanomsg from its predecessor and implement a use case for it in the form of service discovery.

Nanomsg vs. ZeroMQ

A common gripe people have with ZeroMQ is that it doesn’t provide an API for new transport protocols, which essentially limits you to TCP, PGM, IPC, and ITC. Nanomsg addresses this problem by providing a pluggable interface for transports and messaging protocols. This means support for new transports (e.g. WebSockets) and new messaging patterns beyond the standard set of PUB/SUB, REQ/REP, etc.

Nanomsg is also fully POSIX-compliant, giving it a cleaner API and better compatibility. No longer are sockets represented as void pointers and tied to a context—simply initialize a new socket and begin using it in one step. With ZeroMQ, the context internally acts as a storage mechanism for global state and, to the user, as a pool of I/O threads. This concept has been completely removed from nanomsg.

In addition to POSIX compliance, nanomsg is hoping to be interoperable at the API and protocol levels, which would allow it to be a drop-in replacement for, or otherwise interoperate with, ZeroMQ and other libraries which implement ZMTP/1.0 and ZMTP/2.0. It has yet to reach full parity, however.

ZeroMQ has a fundamental flaw in its architecture. Its sockets are not thread-safe. In and of itself, this is not problematic and, in fact, is beneficial in some cases. By isolating each object in its own thread, the need for semaphores and mutexes is removed. Threads don’t touch each other and, instead, concurrency is achieved with message passing. This pattern works well for objects managed by worker threads but breaks down when objects are managed in user threads. If the thread is executing another task, the object is blocked. Nanomsg does away with the one-to-one relationship between objects and threads. Rather than relying on message passing, interactions are modeled as sets of state machines. Consequently, nanomsg sockets are thread-safe.

Nanomsg has a number of other internal optimizations aimed at improving memory and CPU efficiency. ZeroMQ uses a simple trie structure to store and match PUB/SUB subscriptions, which performs nicely for sub-10,000 subscriptions but quickly becomes unreasonable for anything beyond that number. Nanomsg uses a space-optimized trie called a radix tree to store subscriptions. Unlike its predecessor, the library also offers a true zero-copy API which greatly improves performance by allowing memory to be copied from machine to machine while completely bypassing the CPU.

ZeroMQ implements load balancing using a round-robin algorithm. While it provides equal distribution of work, it has its limitations. Suppose you have two datacenters, one in New York and one in London, and each site hosts instances of “foo” services. Ideally, a request made for foo from New York shouldn’t get routed to the London datacenter and vice versa. With ZeroMQ’s round-robin balancing, this is entirely possible unfortunately. One of the new user-facing features that nanomsg offers is priority routing for outbound traffic. We avoid this latency problem by assigning priority one to foo services hosted in New York for applications also hosted there. Priority two is then assigned to foo services hosted in London, giving us a failover in the event that foos in New York are unavailable.

Additionally, nanomsg offers a command-line tool for interfacing with the system called nanocat. This tool lets you send and receive data via nanomsg sockets, which is useful for debugging and health checks.

Scalability Protocols

Perhaps most interesting is nanomsg’s philosophical departure from ZeroMQ. Instead of acting as a generic networking library, nanomsg intends to provide the “Lego bricks” for building scalable and performant distributed systems by implementing what it refers to as “scalability protocols.” These scalability protocols are communication patterns which are an abstraction on top of the network stack’s transport layer. The protocols are fully separated from each other such that each can embody a well-defined distributed algorithm. The intention, as stated by nanomsg’s author Martin Sustrik, is to have the protocol specifications standardized through the IETF.

Nanomsg currently defines six different scalability protocols: PAIR, REQREP, PIPELINE, BUS, PUBSUB, and SURVEY.

PAIR (Bidirectional Communication)

PAIR implements simple one-to-one, bidirectional communication between two endpoints. Two nodes can send messages back and forth to each other.

REQREP (Client Requests, Server Replies)

The REQREP protocol defines a pattern for building stateless services to process user requests. A client sends a request, the server receives the request, does some processing, and returns a response.

PIPELINE (One-Way Dataflow)

PIPELINE provides unidirectional dataflow which is useful for creating load-balanced processing pipelines. A producer node submits work that is distributed among consumer nodes.

BUS (Many-to-Many Communication)

BUS allows messages sent from each peer to be delivered to every other peer in the group.

PUBSUB (Topic Broadcasting)

PUBSUB allows publishers to multicast messages to zero or more subscribers. Subscribers, which can connect to multiple publishers, can subscribe to specific topics, allowing them to receive only messages that are relevant to them.

SURVEY (Ask Group a Question)

The last scalability protocol, and the one in which I will further examine by implementing a use case with, is SURVEY. The SURVEY pattern is similar to PUBSUB in that a message from one node is broadcasted to the entire group, but where it differs is that each node in the group responds to the message. This opens up a wide variety of applications because it allows you to quickly and easily query the state of a large number of systems in one go. The survey respondents must respond within a time window configured by the surveyor.

Implementing Service Discovery

As I pointed out, the SURVEY protocol has a lot of interesting applications. For example:

  • What data do you have for this record?
  • What price will you offer for this item?
  • Who can handle this request?

To continue exploring it, I will implement a basic service-discovery pattern. Service discovery is a pretty simple question that’s well-suited for SURVEY: what services are out there? Our solution will work by periodically submitting the question. As services spin up, they will connect with our service discovery system so they can identify themselves. We can tweak parameters like how often we survey the group to ensure we have an accurate list of services and how long services have to respond.

This is great because 1) the discovery system doesn’t need to be aware of what services there are—it just blindly submits the survey—and 2) when a service spins up, it will be discovered and if it dies, it will be “undiscovered.”

Here is the ServiceDiscovery class:

The discover method submits the survey and then collects the responses. Notice we construct a SURVEYOR socket and set the SURVEYOR_DEADLINE option on it. This deadline is the number of milliseconds from when a survey is submitted to when a response must be received—adjust it accordingly based on your network topology. Once the survey deadline has been reached, a NanoMsgAPIError is raised and we break the loop. The resolve method will take the name of a service and randomly select an available provider from our discovered services.

We can then wrap ServiceDiscovery with a daemon that will periodically run discover.

The discovery parameters are configured through environment variables which I inject into a Docker container.

Services must connect to the discovery system when they start up. When they receive a survey, they should respond by identifying what service they provide and where the service is located. One such service might look like the following:

Once again, we configure parameters through environment variables set on a container. Note that we connect to the discovery system with a RESPONDENT socket which then responds to service queries with the service name and address. The service itself uses a REP socket that simply responds to any requests with “The answer is 42,” but it could take any number of forms such as HTTP, raw socket, etc.

The full code for this example, including Dockerfiles, can be found on GitHub.

Nanomsg or ZeroMQ?

Based on all the improvements that nanomsg makes on top of ZeroMQ, you might be wondering why you would use the latter at all. Nanomsg is still relatively young. Although it has numerous language bindings, it hasn’t reached the maturity of ZeroMQ which has a thriving development community. ZeroMQ has extensive documentation and other resources to help developers make use of the library, while nanomsg has very little. Doing a quick Google search will give you an idea of the difference (about 500,000 results for ZeroMQ to nanomsg’s 13,500).

That said, nanomsg’s improvements and, in particular, its scalability protocols make it very appealing. A lot of the strange behaviors that ZeroMQ exposes have been resolved completely or at least mitigated. It’s actively being developed and is quickly gaining more and more traction. Technically, nanomsg has been in beta since March, but it’s starting to look production-ready if it’s not there already.

  1. The author explains why he should have originally written ZeroMQ in C instead of C++. []

Distributed Messaging with ZeroMQ

“A distributed system is one in which the failure of a computer you didn’t even know existed can render your own computer unusable.” -Leslie Lamport

With the increased prevalence and accessibility of cloud computing, distributed systems architecture has largely supplanted more monolithic constructs. The implication of using a service-oriented architecture, of course, is that you now have to deal with a myriad of difficulties that previously never existed, such as fault tolerance, availability, and horizontal scaling. Another interesting layer of complexity is providing consistency across nodes, which itself is a problem surrounded with endless research. Algorithms like Paxos and Raft attempt to provide solutions for managing replicated data, while other solutions offer eventual consistency.

Building scalable, distributed systems is not a trivial feat, but it pales in comparison to building real-time systems of a similar nature. Distributed architecture is a well-understood problem and the fact is, most applications have a high tolerance for latency. Few systems have a demonstrable need for real-time communication, but the few that do present an interesting challenge for developers. In this article, I explore the use of ZeroMQ to approach the problem of distributed, real-time messaging in a scalable manner while also considering the notion of eventual consistency.

The Intelligent Transport Layer

ZeroMQ is a high-performance asynchronous messaging library written in C++. It’s not a dedicated message broker but rather an embeddable concurrency framework with support for direct and fan-out endpoint connections over a variety of transports. ZeroMQ implements a number of different communication patterns like request-reply, pub-sub, and push-pull through TCP, PGM (multicast), in-process, and inter-process channels. The glaring lack of UDP support is, more or less, by design because ZeroMQ was conceived to provide guaranteed-ish delivery of atomic messages. The library makes no actual guarantee of delivery, but it does make a best effort. What ZeroMQ does guarantee, however, is that you will never receive a partial message, and messages will be received in order. This is important because UDP’s performance gains really only manifest themselves in lossy or congested environments.

The comprehensive list of messaging patterns and transports alone make ZeroMQ an appealing choice for building distributed applications, but it particularly excels due to its reliability, scalability and high throughput. ZeroMQ and related technologies are popular within high-frequency trading, where packet loss of financial data is often unacceptable1. In 2011, CERN actually performed a study comparing CORBA, Ice, Thrift, ZeroMQ, and several other protocols for use in its particle accelerators and ranked ZeroMQ the highest.

cern

ZeroMQ uses some tricks that allow it to actually outperform TCP sockets in terms of throughput such as intelligent message batching, minimizing network-stack traversals, and disabling Nagle’s algorithm. By default (and when possible), messages are queued on the subscriber, which attempts to avoid the problem of slow subscribers. However, when this isn’t sufficient, ZeroMQ employs a pattern called the “Suicidal Snail.” When a subscriber is running slow and is unable to keep up with incoming messages, ZeroMQ convinces the subscriber to kill itself. “Slow” is determined by a configurable high-water mark. The idea here is that it’s better to fail fast and allow the issue to be resolved quickly than to potentially allow stale data to flow downstream. Again, think about the high-frequency trading use case.

A Distributed, Scalable, and Fast Messaging Architecture

ZeroMQ makes a convincing case for use as a transport layer. Let’s explore a little deeper to see how it could be used to build a messaging framework for use in a real-time system. ZeroMQ is fairly intuitive to use and offers a plethora of bindings for various languages, so we’ll focus more on the architecture and messaging paradigms than the actual code.

About a year ago, while I first started investigating ZeroMQ, I built a framework to perform real-time messaging and document syncing called Zinc. A “document,” in this sense, is any well-structured and mutable piece of data—think text document, spreadsheet, canvas, etc. While purely academic, the goal was to provide developers with a framework for building rich, collaborative experiences in a distributed manner.

The framework actually had two implementations, one backed by the native ZeroMQ, and one backed by the pure Java implementation, JeroMQ2. It was really designed to allow any transport layer to be used though.

Zinc is structured around just a few core concepts: Endpoints, ChannelListeners, MessageHandlers, and Messages. An Endpoint represents a single node in an application cluster and provides functionality for sending and receiving messages to and from other Endpoints. It has outbound and inbound channels for transmitting messages to peers and receiving them, respectively.

endpoint

ChannelListeners essentially act as daemons listening for incoming messages when the inbound channel is open on an Endpoint. When a message is received, it’s passed to a thread pool to be processed by a MessageHandler. Therefore, Messages are processed asynchronously in the order they are received, and as mentioned earlier, ZeroMQ guarantees in-order message delivery. As an aside, this is before I began learning Go, which would make for an ideal replacement for Java here as it’s quite well-suited to the problem :)

Messages are simply the data being exchanged between Endpoints, from which we can build upon with Documents and DocumentFragments. A Document is the structured data defined by an application, while DocumentFragment represents a partial Document, or delta, which can be as fine- or coarse- grained as needed.

Zinc is built around the publish-subscribe and push-pull messaging patterns. One Endpoint will act as the host of a cluster, while the others act as clients. With this architecture, the host acts as a publisher and the clients as subscribers. Thus, when a host fires off a Message, it’s delivered to every subscribing client in a multicast-like fashion. Conversely, clients also act as “push” Endpoints with the host being a “pull” Endpoint. Clients can then push Messages into the host’s Message queue from which the host is pulling from in a first-in-first-out manner.

This architecture allows Messages to be propagated across the entire cluster—a client makes a change which is sent to the host, who propagates this delta to all clients. This means that the client who initiated the change will receive an “echo” delta, but it will be discarded by checking the Message origin, a UUID which uniquely identifies an Endpoint. Clients are then responsible for preserving data consistency if necessary, perhaps through operational transformation or by maintaining a single source of truth from which clients can reconcile.

cluster

One of the advantages of this architecture is that it scales reasonably well due to its composability. Specifically, we can construct our cluster as a tree of clients with arbitrary breadth and depth. Obviously, the more we scale horizontally or vertically, the more latency we introduce between edge nodes. Coupled with eventual consistency, this can cause problems for some applications but might be acceptable to others.

scalability

The downside is this inherently introduces a single point of failure characterized by the client-server model. One solution might be to promote another node when the host fails and balance the tree.

Once again, this framework was mostly academic and acted as a way for me to test-drive ZeroMQ, although there are some other interesting applications of it. Since the framework supports multicast message delivery via push-pull or publish-subscribe mechanisms, one such use case is autonomous load balancing.

Paired with something like ZooKeeper, etcd, or some other service-discovery protocol, clients would be capable of discovering hosts, who act as load balancers. Once a client has discovered a host, it can request to become a part of that host’s cluster. If the host accepts the request, the client can begin to send messages to the host (and, as a result, to the rest of the cluster) and, likewise, receive messages from the host (and the rest of the cluster). This enables clients and hosts to submit work to the cluster such that it’s processed in an evenly distributed way, and workers can determine whether to pass work on further down the tree or process it themselves. Clients can choose to participate in load-balancing clusters at their own will and when they become available, making them mostly autonomous. Clients could then be quickly spun-up and spun-down using, for example, Docker containers.

ZeroMQ is great for achieving reliable, fast, and scalable distributed messaging, but it’s equally useful for performing parallel computation on a single machine or several locally networked ones by facilitating in- and inter- process communication using the same patterns. It also scales in the sense that it can effortlessly leverage multiple cores on each machine. ZeroMQ is not a replacement for a message broker, but it can work in unison with traditional message-oriented middleware. Combined with Protocol Buffers and other serialization methods, ZeroMQ makes it easy to build extremely high-throughput messaging frameworks.

  1. ZeroMQ’s founder, iMatix, was responsible for moving JPMorgan Chase and the Dow Jones Industrial Average trading platforms to OpenAMQ []
  2. In systems where near real-time is sufficient, JeroMQ is adequate and benefits by not requiring any native linking. []