Fritzy.io

Realtime web app architecture with Thoonk: a series of tubes, not tables

andyet postrealtimenode.jsjavascripthowto

Now you're thinking with feeds!

When I look at a single-page webapp, all I see are feeds; I don't even see the UI anymore. I just see lists of items that I care about. Some of which only I have access to and some of which other groups have access to. I can change, delete, re-position, and add to the items on these feeds and they'll propagate to the people and entities that have access to them (even if it is just me on another device or at a later date).

I've seen it this way for years, but I haven't grokked it enough to articulate what I was seeing until now.

What Thoonk Is

Thoonk is a series of higher-level objects built on Redis that sends publish, edit, delete, and position events when they are changed. These objects are feeds for making real-time applications and feed services.

What is a Thoonk feed?

A Thoonk feed is a list of indexed data objects that are limited by topic and by what a single entity might subscribe to. An RSS/ATOM feed qualifies. What makes a Thoonk feed different from a table? A table is limited to a topic, but lacks single entity interest limitations. A Thoonk feed isn't just a message broker, it's a database-store that sends out events when the data changes.

Let's use &bang as an example. Each team-member has a list of tasks. In a relational database we might have a table that looks like this:

team_member_tasks

id | team_id | member_id | description | complete bool | etc.

Whenever a user renders their list, I would query that list, limiting by a specific user and a specific team.

If we converted this table, without changing it, into a Thoonk feed, then we would only be able to subscribe to ALL tasks and not just the tasks of a particular team or member. So, instead, a Thoonk feed might look like:

team:<team_id>:member:<member_id>:tasks

{description: "", completed: false, etc, etc}

Now when the user wants a rendered list of tags, I can do one index look-up rather than three, and I am able to subscribe to changes on the specific team member's tasks, or even to team:353:member:*:tasks to subscribe to all of that team's tasks.

[Note: I suppose you could arrange a relational database this way, but it wouldn't really be able to take advantage of SQL, nor could you subscribe to the table to get changes.]

It's Feeds All the Way Up

If I use Thoonk subscribe-able feeds as my data-storage engine, life gets so much easier. When a user logs in, I can subscribe contextualized callbacks just for them to the feeds of data that they have access to read from. This way, if their data changes for any reason, by any process, by any server, it can bubble all the way up to the user without having to run any queries. I can also subscribe separate processes that can automatically scrub, pre-index, cull, or any number of tasks to any Thoonk feed a particular process cares about. I can use processes in mixed languages to provide monitoring and additional API's to the feeds.

But What About Writes?

Let's not think in terms of writes. Writes are just changes to feed items (publishing, editing, deleting, repositioning) that writes the data to ram/disk and informs any subscribers of the change. Let's instead think in terms of user-actions. A user-action (such as delegating a task to another user in &bang) needs ACL and may affect multiple feeds in a single call. If we defer user-actions to jobs (a special kind of Thoonk feed), we can easily isolate, scale, share, and distribute the business-logic involved in dealing with a user-action.

What Are Thoonk Jobs?

Thoonk Jobs are items that represent business-logic needing to be done reliably, a single time, by any available worker. Jobs are consumed as fast as a worker-pool can consume them. A job feed is a list of job items, each of which may exist in the state of available, in-flight, and stalled. Available jobs are taken and are placed in an in-flight set while they are being processed. When the job is done, the job is removed from the in-flight set, and its item is deleted. If the worker fails to complete the job (either because of an error, distaste, or a monitoring process deciding that the job has timed out), the job may be placed back to the available list or the stalled set.

Why use Thoonk Jobs for User-Actions?

The Last One is a Doozy

Scaling, reliability, monitoring and all of that is nice, but being able to build your application out rather than up is, I believe, the greatest reason for this approach. &bang is written in node.js, but if I have a favorite library for implementing a REST interface or an XMPP interface written in Python or Ruby (or any other language), I can quickly put that together and add it as a process. In fact, I can pretty much add any piece of functionality as a process without having to reload the rest of the application server, and really isolate a feature as its own process. User-actions from this process can be published to Thoonk Job feeds without having to worry about request validation or ACL since that is handled by the worker itself.

Rather than having a very large, complex application, I can have a series of very small processes that automatically cluster and are informed of changes in areas of their specific concerns.

Scaling Beyond Redis

Our testing indicates that Redis will not be a choke point until we have nearly 100,000 active users. The plan to scale beyond that is to shard &bang by teams. A quick look-up will tell us which server a team resides on, and users and processes can subscribe callbacks to connections on those servers. In that way, we can run many Redis servers, and theoretically scale vertically. High-availability is handled by a slave for each shard and a gossip protocol for promoting slaves.

Conflict Resolution and Missed Updates

Henrik's recent post spawned a couple of questions about conflict resolution. First I'll give a deflection, and then I'll give a real answer.

&bang doesn't yet need conflict resolution. None of the writes are actually done on the client as they are all RPC calls which go into a job queue. Then the workers validate the payload, check the ACL, and update some feeds, at which point the data bubbles back up to the client. The feed updates are atomic, and happen quite quickly. Also, two users being able "to edit the same item only comes up with delegated task, in which case the most recent edit wins.

Ok, now the real answer. Thoonk is going to have revision history and incrementing revision numbers for 1.0. Each historical item is the same as the publish/edit/delete/reposition updates that are sent via pubsub. When a user change job is done, the client can send its current revision numbers for the feeds involved, and thus conflicts on an edit can be detected. The historical data should be enough data to facilitate some form of conflict resolution (determined by the application implementer). The revision numbers can also bubble up to the client, so the client can detect missing updates and ask for a replay from a given revision number.

Currently we're punting on missed items. Anytime the &bang user is disconnected, the app is disabled and refreshed when it is able to reconnect. A more elaborate solution using the new Thoonk features I just listed is probably coming and perhaps some real offline-mode support with local "dirty" changes that get resolved when you come back online.

All Combined

Using Thoonk, we were able to make &bang scale to 10s of thousands of active users on a single server, burst user-activity beyond our choke-points, isolate user-action business-logic and ACL, automatically cluster to more servers and processes, choose any Redis client library supported language for individual features and interfaces, bubble data changes all the way up to the user regardless of the source of change, provide an easy way of iterating, and generally create a kick-ass, realtime, single-page webapp.

Can I Use Thoonk Now?

Thoonk.js and Thoonk.py are MIT licensed, and free to use. While we are using Thoonk.js in production and it is stable there, the API is not final. Currently I'm moving the the feed logic to Redis Lua scripts, which will be officially supported in Redis 2.6 with an RC1 promised for this December. I plan to be ready for that. The Lua scripting will give us performance gains, and remove unnecessary extra logic to keep publish/edit/delete/reposition commands atomic, but most importantly it will allow us to share the core code with all implementations of Thoonk, allowing us to easily add and support more languages. As mentioned previously, as I do the Redis Lua scripting, I'll be adding revision history and revision numbers to feeds, which will facilitate conflict detection and replay of missed events.

That said, feel free to comment, contribute, steal, or abuse the project in the meantime. A 1.0 release will indicate API stability, and I will encourage its use in production at that point. I will soon be breaking out the Lua scripts to their own git repo for easy implementation.

If you want to keep an eye on what we're doing, follow me @fritzy and @andyet on twitter. Also be sure to check out &bang for getting stuff done with your team.

If you're building a single page app, keep in mind that &yet offers consulting, training and development services. Shoot Henrik an email (henrik@andyet.net) and tell us what we can do to help.