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StoryBoard: Subscriptions and Events

https://storyboard.openstack.org/#!/story/96

StoryBoard needs to notify a user when changes occur to a resource which they have decided to be notified about.

A key feature needed by all ticket tracking systems is the ability to notify a user when topics which they care about have changed. A common way to describe this is “Subscription”, where a single user will ask to be notified about certain types of changes for certain resources. More complicated implementations include filtering, notification alert levels, summaries of events that all impact the same resource, or automatic notification based on inferred or calculated relevance.

Problem description

In its simplest form, we would like each user to be able to indicate which resources they are interested in, and be able to retrieve a date-sorted list of which of those resources have changed recently. This will then be shown in the UI as a list of events that occurred that are relevant to the user. More advanced features would be the ability to filter these events, or to receive notification of new events in “real time”.

Requirements are as follows: * A user should be able to manage their subscriptions * Subscriptions should be as up-to-date as possible with as little data loss as possible. * A user should be able to subscribe to tasks, stories, projects, and project groups. * Subscriptions should have a minimal impact on API performance. * When a subscription is added, all changes from that point forward should be reported. Historical changes do not need to be generated. * When a subscription is removed, a user’s subscription list does not have to be recalculated to extract no-longer-relevant events. * StoryBoard should be able to run on a single server with no crazy additional infrastructure required. * Oslo libraries should be taken into account.

Subscriptions are the classic, personalized, pub-sub problem, where a user’s list of subscriptions can be large, and the matrix of events that could cause a subscription to be notified can be complex and processor intensive. Classically, there are four ways to handle this problem: Push, Pull, Async, or JIT.

Push

The push approach assumes that you will notify all subscribers when the event occurs. In the case of our API, this means that during a PUT/POST/DELETE, all subscribers to that resource are checked to see whether they need to be notified. In most small-scale system this works fine, however as subscriptions increase this approach is generally not stable, as the time required to process the request (and the number of errors that could possibly be raised) will start to impact the client making the API request. Things that would raise concerns are timeout, as well as the number of points of failure, and as a result of that this is not an appropriate approach.

Pull

The pull approach believes in restricting the processing load described above to only those users who care enough to ask for subscription events. In this case, a list is generated fresh every time GET /subscriptions (or similar) is called. This approach is appropriate when usage is expected to be low, similar to the generation of a report. Given that a subscription list is a fairly frequently polled resource, this is not an appropriate approach.

Async

An asynchronous approach makes use of deferred, distributed processing to “eventually” update a user’s list of subscription events. Worker management systems such as Gearman or Rabbit are notified whenever a resource changes (likely via Pecan API hooks), and they ‘eventually’ ask a worker process to go figure out which subscriptions need to be updated. Advantages of this approach is that we can avoid the Python Global Interpreter Lock by having separate worker processes, and any errors encountered during subscription processing will be isolated and thus not impact the actual API request. The challenge with this is that most queueing/worker management systems are resource intensive (kafka), do not guarantee delivery (gearman), or have known issues with split-brain clustering (rabbit). Any approach will have to accommodate the chosen system.

Streaming

A streaming approach begins by emitting an event whenever a resource changes, and to notify all subscribers that are currently connected via a socket. Persistence of events may be handled by creating individual processes that listen to the stream and persist the received data much like a subscribed client might update its UI. This approach solves the real-time problem with a hot sexy technology, however coordinating listeners and ensuring that persistence is handled properly raises the same problems as the Async or Push/Pull problem. As a result this approach is unnecessarily more complex than it needs to be.

Proposed change

Summary

StoryBoard will emit events whenever a resource changes. Since most resources map directly to database changes, the majority of these changes can be handled via Pecan post-request hooks.

Events, when emitted, will be written to a deferred processing queue. If the queue is unavailable or misconfigured, a warning should be written to the log, however the system should complete the original request normally and discard the change event altogether.

The queuing system to be used is RabbitMQ, because it guarantees delivery and recovers after a crash. This decision is based on the assumption that the number of events dispatched by the database will not be sufficient to require a full RabbitMQ cluster, which means we don’t have to worry about split-brain problems.

The StoryBoard server will spin up a series of processes that listen to the event queue and perform actions based on the type of event received. In the case of subscriptions, a process would read the event, load the impacted resource and its change, search for any subscriptions to the impacted resource, update each subscription’s owners’ subscription feed, and then notify rabbit that the message has been received and processed. If updating one subscription fails, the process should still attempt to complete the other ones.

The number of events that are retained per user should be configurable by age, with a default of 1 month.

A user’s event feed should be retained in the database in its own table.

API: Subscriptions

StoryBoard will expose a new endpoint at /v1/users/ID/subscriptions. This endpoint will support basic CRUD operations by which a user can manage their subscriptions.

API: Feed

StoryBoard will expose a new endpoint at /v1/users/ID/feed. This endpoint will provide a list of events that have been emitted by a user’s subscriptions, sorted by date in descending order.

Alternatives

Alternative approaches have been listed in the problem description. Alternative queueing systems include ZeroMQ & gearman, which was disqualified because it does not guarantee delivery, Kafka which was disqualified because (anecdotally) it requires a cluster to perform properly.

Implementation

Assignee(s)

Primary Assignee:
TBD

Work Items

  • Create an API to add subscriptions for projects, project groups, stories, and tasks.
  • Teach the storyboard-webclient to allow subscription on projects, project groups, stories, and tasks.
  • Install RabbitMQ on StoryBoard Server.
  • Use Oslo.messaging to create an SQLAlchemy hook that broadcasts change events for project groups, projects, stories, and tasks.
  • Add configuration to StoryBoard for the AMQP connection string and optionally an enabling flag for the whole feature.
  • Create a storyboard-worker process that connects to AMQP and receives messages for processing.
  • Create a way for the storyboard-worker process to process lots of different kinds of events (event hooks of some sort? processor factory?)
  • Build a subscription event handler which is run by storyboard-worker and updates a subscriber’s feed.
  • Create an API endpoint that exposes the feed.
  • Teach the storyboard-webclient to display the feed.

Repositories

No new repositories.

Servers

No new servers. storyboard.openstack.org will need to have a running RabbitMQ instance.

DNS Entries

No new DNS entries.

Dependencies

See above. Puppet module for storyboard will need to be updated. Additional dependencies are on oslo.messaging, rabbitmq-server, upstart, etc.