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Scalable Realtime Architectures in Python

Summary

This talk will focus on you can readily implement highly scalable and fault tolerant realtime architectures, such as dashboards, using Python and tools like Storm, Kafka, and ZooKeeper. We will focus on two related aspects: composing reliable systems using at-least-once and idempotence semantics and how to partition for locality.

Description

Increasingly we are interested in implementing highly scalable and fault tolerant realtime architectures such as the following:

  • Realtime aggregation. This is the realtime analogue of working with batched map-reduce in systems like Hadoop.
  • Realtime dashboards. Continuously updated views on all your customers, systems, and the like, without breaking a sweat.
  • Realtime decision making. Given a set of input streams, policy on what you like to do, and models learned by machine learning, optimize a business process. One example includes autoscaling a set of servers.

(We use realtime in the soft sense: systems that are continuously computing on input streams of data and make a best effort to keep up; it certainly does not imply hard realtime systems that strictly bound their computation times.)

Obvious tooling for such implementations include Storm (for event processing), Kafka (for queueing), and ZooKeeper (for tracking and configuration). Such components, written respectively in Clojure (Storm), Scala (Kafka), and Java (ZooKeeper), provide the desired scalability and reliability. But what may not be so obvious at first glance is that we can work with other languages, including Python, for the application level of such architectures. (If so inclined, you can also try reimplementing such components in Python, but why not use something that's been proven to be robust?)

In fact Python is likely a better language for the app level, given that it is concise, high level, dynamically typed, and has great libraries. Not to mention fun to write code in! This is especially true when we consider the types of tasks we need to write: they are very much like the data transformations and analyses we would have written of say a standard Unix pipeline. And no one is going to argue that writing such a filter in say Java is fun, concise, or even considerably faster in running time.

So let's look at how you might solve such larger problems. Given that it was straightforward to solve a small problem, we might approach as follows. Simply divide up larger problems in small one. For example, perhaps work with one customer at a time. And if failure is an ever present reality, then simply ensure your code retries, just like you might have re-run your pipeline against some input files.

Unfortunately both require distributed coordination at scale. And distributed coordination is challenging, especially for real systems, that will break at scale. Just putting a box in your architecture labeled "ZooKeeper" doesn't magically solve things, even if ZooKeeper can be a very helpful part of an actual solution.

Enter the Storm framework. While Storm certainly doesn't solve all problems in this space, it can support many different types of realtime architectures and works well with Python. In particular, Storm solves two key problems for you.

Partitioning. Storm lets you partition streams, so you can break down the size of your problem. But if the a node running your code fails, Storm will restart it. Storm also ensures such topology invariants as the number of nodes (spouts and bolts in Storm's lingo) that are running, making it very easy to recover from such failures.

This is where the cleverness really begins. What can you do if you can ensure that all the data you need for a given continuously updated computation - what is the state of this customer's account? - can be put in exactly one place, then flow the supporting data through it over time? We will look at how you can readily use such locality in your own Python code.

Retries. Storm tracks success and failure of events being processed efficiently through a batching scheme and other cleverness. Your code can then choose to retry as necessary. Although Storm also supports exactly-once event processing semantics, we will focus on the simpler model of at-least-once semantics. This means your code must tolerate retry, or in a word, is idempotent. But this is straightforward. We have often written code like the following:

seen = set()
for record in stream:
    k = uniquifier(record)
    if k not in seen:
       seen.add(k)
       process(record)

Except of course that any such real usage has to ensure it doesn't attempt to store all observations (first, download the Internet! ;), but removes them by implementing some sort of window or uses data structures like HyperLogLog, as we will discuss.

One more aspect of reliability we will discuss is how to compose reliable systems out of reliable components; we will show how this can be readily done with a real example of consuming Kafka and tracking consumption progress in ZooKeeper.

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