Stream Framework Design¶
The first approach
A first feed solution usually looks something like this:
SELECT * FROM tweets JOIN follow ON (follow.target_id = tweet.user_id) WHERE follow.user_id = 13
This works in the beginning, and with a well tuned database will keep on working nicely for quite some time. However at some point the load becomes too much and this approach falls apart. Unfortunately it’s very hard to split up the tweets in a meaningfull way. You could split it up by date or user, but every query will still hit many of your shards. Eventually this system collapses, read more about this in Facebook’s presentation.
Push or Push/Pull
In general there are two similar solutions to this problem.
In the push approach you publish your activity (ie a tweet on twitter) to all of your followers. So basically you create a small list per user to which you insert the activities created by the people they follow. This involves a huge number of writes, but reads are really fast they can easily be sharded.
For the push/pull approach you implement the push based systems for a subset of your users. At Fashiolista for instance we used to have a push based approach for active users. For inactive users we only kept a small feed and eventually used a fallback to the database when we ran out of results.
Stream Framework allows you to easily use Cassndra/Redis and Celery (an awesome task broker) to build infinitely scalable feeds. The high level functionality is located in 4 classes.
- Feed managers
Activities are the blocks of content which are stored in a feed. It follows the nomenclatura from the [activity stream spec] [astream] [astream]: http://activitystrea.ms/specs/atom/1.0/#activity.summary Every activity therefor stores at least:
- Time (the time of the activity)
- Verb (the action, ie loved, liked, followed)
- Actor (the user id doing the action)
- Object (the object the action is related to)
- Extra context (Used for whatever else you need to store at the activity level)
Optionally you can also add a target (which is best explained in the activity docs)
Feeds are sorted containers of activities. You can easily add and remove activities from them.
Stream Framework classes (feed managers) handle the logic used in addressing the feed objects. They handle the complex bits of fanning out to all your followers when you create a new object (such as a tweet).
In addition there are several utility classes which you will encounter
- Serializers (classes handling serialization of Activity objects)
- Aggregators (utility classes for creating smart/computed feeds based on algorithms)
- Timeline Storage (cassandra or redis specific storage functions for sorted storage)
- Activity Storage (cassandra or redis specific storage for hash/dict based storage)