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Streaming analytics with Python and Kafka

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We discuss the challenges of building realtime streaming systems, key concepts such as push vs pull, agents, and backpressure. We present a simple yet robust approach to doing that with Python-only code using asyncio and Kafka.

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Streaming analytics with Python and Kafka

  1. 1. © MOSAIC SMART DATA 1 Egor Kraev, Head of AI, Mosaic Smart Data PyData, April 4, 2017 Streaming analytics with asynchronous Python and Kafka
  2. 2. © MOSAIC SMART DATA 2 Overview ▪ This talk will show what streaming graphs are, why you really want to use them, and what the pitfalls are ▪ It then presents a simple, lightweight, yet reasonably robust way of structuring your Python code as a streaming graph, with the help of asyncio and Kafka
  3. 3. © MOSAIC SMART DATA 3 A simple streaming system ▪ The processing nodes are often stateful, need to process the messages in the correct sequence, and update their internal state after each message (an exponential average calculator is a basic example) ▪ The graphs often contain cycles, so for example A ->B -> C -> A ▪ The graphs nearly always have some nodes containing and emitting multiple streams
  4. 4. © MOSAIC SMART DATA 4 Why structure your system as a streaming graph? ▪ Makes the code clearer ▪ Makes the code more granular and testable ▪ Allows for organic scaling ▪ Start out with the whole graph in one file, can gradually split it up to each node being a microservice with multiple workers ▪ As the system grows, nodes can run in different languages/frameworks ▪ Makes it easier to run the same code on historic and live data ▪ Treating your historical run as replay also solves some realtime problems such as aligning different streams correctly
  5. 5. © MOSAIC SMART DATA 5 Two key features of a streaming graph framework 1. Language for graph definition ▪ Ideally, the same person who writes the business logic in the processing nodes should define the graph structure as well ▪ This means the graph definition language must be simple and natural 2. Once the graph is defined, scheduling is an entirely separate, hard problem ▪ If we have multiple nodes in a complex graph, with branchings, cycles, etc, what order do we call them in? ▪ Different consumers of the same message stream, consuming at different rates - what to do? ▪ If one node has multiple inputs, what order does it receive and process them in? ▪ What if an upstream node produces more data than a downstream node can process?
  6. 6. © MOSAIC SMART DATA 6 Popular kinds of scheduling logic 1. Agents ▪ Each node autonomously decides what messages to send ▪ Each node accepts messages sends to it ▪ Logic for buffering and message scheduling needs to be defined in each node ▪ For example, pykka 2. 'Push' approach ▪ First attempt at event-driven systems tends to be ‘push’ ▪ For example 'reactive' systems, eg Microsoft’s RXPy ▪ When an external event appears, it’s fed to the entry point node. ▪ Each node processes what it receives, once done, triggers its downstream nodes ▪ Benefit: simpler logic in the nodes; each node must only have a list of its downsteam nodes to send messages to
  7. 7. © MOSAIC SMART DATA 7 Problems with the Push approach 1. What if the downstream can't cope? ▪ Solution: 'backpressure': downstream nodes are allowed to signal upstream when they're not coping ▪ That limits the amount of buffering we need to do internally, but can bring its own problems. ▪ Backpressure needs to be implemented well at framework level, else we end up with a callback nightmare: each node must have callbacks to both upstream and downstream, and manage these as well as an internal message buffer (RXPy as example) ▪ Backpressure combined with multiple dowstreams can lead to processing locking up accidentally, 2. Push does really badly at aligning merging streams ▪ Even if individual streams are ordered, different streams are often out of sync ▪ What if the graph branches and then re-converges, how do we make sure the 'right' messages from both branches are processed together?
  8. 8. © MOSAIC SMART DATA 8 The Pull approach ▪ Let's turn the problem on its head! ▪ Let's say each node doesn't need to know its downstream, only its parents. ▪ The execution is controlled by the downmost node. When it's ready, it requests more messages from its parents ▪ No buffering needed ▪ When streams merge, the merging node is in control, decides which stream to consume from first Limitations: ▪ The sources must be able to wait until queried ▪ Has problems with two downstream nodes wanting to consume the same message stream
  9. 9. © MOSAIC SMART DATA 9 The challenge I set out to find or create an architecture with the following properties: ▪ Allows realtime processing ▪ All user-defined logic is in Python with fairly simple syntax ▪ Both processing nodes and graph structure ▪ Lightweight approach, thin layer on top of core Python ▪ Can run on a laptop ▪ Scheduling happens transparently to the user ▪ No need to buffer data inside the Python process (unless you want to) ▪ Must scale gracefully to larger data volumes
  10. 10. © MOSAIC SMART DATA 10 What is out there? ▪ In the JVM world, there's no shortage of great streaming systems ▪ Akka Streams: a mature library ▪ Kafka Streams: allows you to treat Kafka logs as database tables, do joins etc ▪ Flink: Stream processing framework that is good at stateful nodes ▪ On the Python side, a couple of frameworks are almost what I want ▪ Google Dataflow only supports streaming Python when running in Google Cloud, local runner only supports finite datasets ▪ Spark has awesome Python support, but it's basic approach is map-reduce on steroids, doesn't fit that well with stateful nodes and cyclical graphs
  11. 11. © MOSAIC SMART DATA 11 Collaborative multitasking, event loop, and asyncio ▪ The event loop pattern, collaborative multitasking ▪ An ‘event loop’ keeps track of multiple functions that want to be executed ▪ Each function can signal to it whether it’s ready to execute or waiting for input ▪ The event loop runs the next function until it has nothing to process, it then surrenders control back to event loop ▪ A great way of running multiple bits of logic ‘simultaneously’ without worrying about threading – runs well on a single thread ▪ Asyncio is Python’s official event loop implementation
  12. 12. © MOSAIC SMART DATA 12 Kafka ▪ A simple yet powerful messaging system ▪ A producer client can create a topic in Kafka and write messages to it ▪ Multiple consumer clients can then read these messages in sequence, each at their own pace ▪ Partitioning of topics - if multiple consumers in the same group, each sees a distinct subset of partitions of the topic ▪ It's a proper Big Data application with many other nice properties, the only one that concerns us is that it's designed to deal with lots of data and lots of clients, fast! ▪ Can spawn an instance locally in seconds, using Docker, eg using the image at https://hub.docker.com/r/flozano/kafka/
  13. 13. © MOSAIC SMART DATA 13 Now let’s put it all together! ▪ Structure your graph as a collection of pull-only subgraphs, that consume from and publish to multiple Kafka topics ▪ Inside each subgraph, can merge streams; can also choose to send each message of a stream to one of many sources, ▪ Inside each subgraph, each message goes to at most one downstream! ▪ If two consumers want to consume the same stream, push that stream to Kafka and let them each read from Kafka at their own pace ▪ If you have a 'hot' source that won't wait: just run a separate process that just pushes the output of that source into a Kafka topic, then consume at leisure
  14. 14. © MOSAIC SMART DATA 14 Our example streaming graph sliced up according to the pattern ▪ The ‘active’ nodes are green – exactly one per subgraph ▪ All buffering happens in Kafka, it was built to handle it!
  15. 15. © MOSAIC SMART DATA 15 Scaling ▪ Thanks to asyncio, can run multiple subgraphs in the same Python process and thread, so can in principle have a whole graph in one file (two if you want one dedicated to user input) ▪ Scale using Kafka partitioning to begin with: for slow subgraphs, spawn multiple nodes each looking at its own partitions of a topic ▪ If that doesn't help, replace the problematic subgraphs by applications in other languages/frameworks ▪ So stateful Python nodes and Spark subgraphs can coexist happily, communicating via Kafka
  16. 16. © MOSAIC SMART DATA 16 Example application ▪ To give a nice syntax to users, we implement a thin façade over the AsyncIterator interface, adding overloading of operators | and > ▪ So a data source is just an async iterator with some operator overloading on top: ▪ The | operator applies an operator (such as ‘map’) to a source, returning a new source ▪ The a > b operator creates a coroutine that, when run, will iterate over a and feed the results to b, a can be an iterable or async iterable ▪ The ‘run’ command asks the event loop to run all its arguments ▪ The kafka interface classes are a bit of syntactic sugar on top of aiokafka
  17. 17. © MOSAIC SMART DATA 17 Summary ▪ Pull-driven subgraphs ▪ Asyncio and async iterators to run many subgraphs at once ▪ Kafka to glue it all together (and to the world) Questions? Comments? ▪ Please feel free to contact me at egor@dagon.ai

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