Experiences in Asynchronous Application Development with Crossbar.io by Adam Jorgensen

Recently at OpenDNA we've been rebuilding one of our core internal applications from the ground-up. Early on we decided that we wanted to make the new system completely asynchronous, leveraging the ability of modern browsers to communicate with the server in a bi-directional fashion unconstrained by the usual flow of a RESTful application.

There are a number of ways to implement this kind of application in Python but only one of them (in our humble opinion) is really worth investing time in: Crossbar.io

In this talk I will cover the process of developing a purely asynchronous application using Crossbar from top to bottom, detailing everything from high-level design considerations to low-level code decisions to potential pitfalls and how to avoid them.

The aim: To convince you that your next application should be built using Crossbar!

This talk should be suitable for any audience but will be of particular interest to developers looking for a way to leverage WebSockets and/or WAMP to develop highly or even purely asynchronous applications.

Transcript

1. Experiences in Asynchronous Application Development with Crossbar.io By Adam Jorgensen, https://github.com/oopman/
2. Who am I and what is ? • I’m Adam (you probably guessed that already though…) • Senior Backend Developer at OpenDNA • OpenDNA is a company leveraging Machine Learning and Python to • Perform User Behaviour and Smart Content Analyses • Provide Real-time Insights and Psychographic Profiles • We work primarily in Python 3 and leverage various AWS services • We’re hiring! Checkout out http://opendna.ai for more details… • As per ISO 476105, there is a good coffee machine!
3. WAMP, Crossbar.io, Autobahn, Asyncio, fizz, buzz, etc! asyncio • Part of the Python 3.4+ standard library • Provides tools for writing single- threaded co-operative concurrent code…and more!
4. HTTP Protocol vs. WAMP Protocol Feature HTTP Protocol WAMP Protocol State Stateless* Stateful Directionality Uni-directional Bi-directional Duplex Half-duplex Full-duplex Connection Transient**, optionally encrypted Persistent, optionally encrypted Message methods/verbs GET, HEAD, POST, PUT, DELETE, TRACE, OPTIONS, CONNECT, PATCH Call, Register, Publish, Subscribe Message format Plain text JSON/MessagePack/CBOR Authentication Static/dynamic Basic access, Digest access, Custom challenge-response implemented in application Static/dynamic versions of Anonymous, Ticket, CRA, Cryptosign, Cookie, TLS Authorization Implemented in application Static/dynamic URI-based * State is usually implemented at the application level using various methods ** HTTP/1.1 provides a keep-alive mechanism which sees common usage
5. Getting started with Crossbar and Autobahn • Crossbar/Autobahn Installation • Option 1: Docker • See https://crossbar.io/docs/Getting-Started/ • Option 2: Virtualenv • See https://crossbar.io/docs/Installation/ • Crossbar Project management • CLI tool • Node config file • WAMP Components • Easy to write • Running can be a bit more tricky… • asyncio components vs. twisted components: autobahn-python-runners to the rescue!
6. Coming from a synchronous world… • asyncio library ecosystem is small but growing… • aio-libs organization on GitHub has clients for: HTTP, Redis, MySQL, PostgreSQL, ODBC, ZeroMQ, Kafka and more! • aioes is out and official elasticsearch-py-async is in  • ORM situation is tricky 1. sqlalchemy_aio exists but doesn’t allow usage of the ORM component 2. peewee-async provides Django-like Manager layer over peewee Option 2 is superior • No asyncio options? Don’t worry, there’s still hope! • Easy: Expensive sync call results can be cached for async access via aioredis • Less easy: Leverage asyncio.run_in_executor
7. Framework not included… …but don’t worry, you won’t really miss it  • Compared to HTTP web frameworks, Crossbar and AutoBahn is quite minimal… • Autobahn-Python models each component as a class • Routed RPC takes the place of HTTP methods • PubSub has no direct comparison in HTTP web frameworks • No concept of request or response objects/thread-locals • Data supplied to end-points available via arguments and keyword arguments • Coercion of input/output data to and from Python types automatic • WSGI-style middleware replaced by decorators and/or inheritance • Everything else is up to you!
8. Component-based Application Design • WAMP components • Are inherently modular • Have locational transparency • Can behave as both “clients” and “servers” to other WAMP components • In Python, implemented using classes • Do’s and Don’ts for Python components • Don’t store state on component class instances • Don’t interact with other component classes outside of RPC and PubSub • Don’t make blocking calls outside of __init__* • Don’t perform long-running work in end-points** • Do handle RPC call failures gracefully • Do use decorators to encapsulate common functionality * If you must, consider using the asyncio thread/process executor system
9. Authentication and Authorization • Support baked in at Router level • Configured at Node level • Dynamic authenticators/authorizers implemented using components • Authentication end-point • Receives Realm and Authid • Responds with Role • Authorization end-point • Receives Session, URI, and Action • Responds with bool or dict • Overall a nice system • Highly flexible… • But with some limits to keep things clean 
10. Debugging and Testing • Debugging • Initially seems difficult due to Crossbar being a process manager… • ….just run individual Components with PyDevD or normal breakpoints • Breakpoints on await…you can’t just step over them! • Interactive Testing • Initially tricky before our web front-end was up and running… • …so we developed a REPL Component! • OSS version: https://github.com/opn-oss/autobahn-python-repl • Automated Testing • pytest-asyncio • aiounittest
11. Danger, Will Robinson! • Deadlocks possible in single-threaded code! Example: • Function A requests connection from aiomysql pool • Function A calls Function B • Function B requests connection from aiomysql pool • Fire off many calls to Function A, watch deadlock fireworks! • File I/O is blocking • Use aiofiles to get around this • peewee-async can accidentally make synchronous calls… • Calling a co-routine doesn’t automatically start it!
12. Conclusion • Overall • Crossbar and Autobahn have been great to work with • Relative simplicity compared to HTTP frameworks is nice • Flexibility is also extremely impressive • asyncio ecosystem needs more love • Is Crossbar for you? • Bad fit for multi-page web applications • Good fit for single-page web applications and mobile app backends • Great fit for soft real-time applications and IoT • Bonus: Plays nicely in multi-language projects!