Channels Overview

Our channels/websocket implementation handles the communication between Tower API and updates in Tower UI.

Architecture

Tower enlists the help of the django-channels library to create our communications layer. django-channels provides us with per-client messaging integration in our application by implementing the Asynchronous Server Gateway Interface (ASGI).

To communicate between our different services we use RabbitMQ to exchange messages. Traditionally, django-channels uses Redis, but Tower uses a custom asgi_amqp library that allows access to RabbitMQ for the same purpose.

Inside Tower we use the emit_channel_notification function which places messages onto the queue. The messages are given an explicit event group and event type which we later use in our wire protocol to control message delivery to the client.

Protocol

You can connect to the Tower channels implementation using any standard websocket library by pointing it to /websocket. You must provide a valid Auth Token in the request URL.

Once you've connected, you are not subscribed to any event groups. You subscribe by sending a json request that looks like the following:

'groups': {
        'jobs': ['status_changed', 'summary'],
        'schedules': ['changed'],
        'ad_hoc_command_events': [ids...],
        'job_events': [ids...],
        'workflow_events': [ids...],
        'project_update_events': [ids...],
        'inventory_update_events': [ids...],
        'system_job_events': [ids...],
        'control': ['limit_reached_<user_id>'],
}

These map to the event group and event type that the user is interested in. Sending in a new groups dictionary will clear all previously-subscribed groups before subscribing to the newly requested ones. This is intentional, and makes the single page navigation much easier since users only need to care about current subscriptions.

Deployment

This section will specifically discuss deployment in the context of websockets and the path those requests take through the system.

Note: The deployment of Tower changes slightly with the introduction of django-channels and websockets. There are some minor differences between production and development deployments that will be pointed out in this document, but the actual services that run the code and handle the requests are identical between the two environments.

Services

Name	Details
`nginx`	listens on ports 80/443, handles HTTPS proxying, serves static assets, routes requests for `daphne` and `uwsgi`
`uwsgi`	listens on port 8050, handles API requests
`daphne`	listens on port 8051, handles websocket requests
`runworker`	no listening port, watches and processes the message queue
`supervisord`	(production-only) handles the process management of all the services except `nginx`

When a request comes in to nginx and has the Upgrade header and is for the path /websocket, then nginx knows that it should be routing that request to our daphne service.

daphne receives the request and generates channel and routing information for the request. The configured event handlers for daphne then unpack and parse the request message using the wire protocol mentioned above. This ensures that the connection has its context limited to only receive messages for events it is interested in. daphne uses internal events to trigger further behavior, which will generate messages and send them to the queue, which is then processed by the runworker.

runworker processes the messages from the queue. This uses the contextual information of the message provided by the daphne server and our asgi_amqp implementation to broadcast messages out to each client.

Development

nginx listens on 8013/8043 instead of 80/443

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