# 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 to our application by implementing the Asynchronous Server Gateway Interface or 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 use to RabbitMQ for the same purpose.

Inside Tower we use the emit_channel_notification which places messages on to 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 but 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...]
            'control': ['limit_reached'],
    }

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

## Deployment

This section will specifically discuss deployment in the context of websockets and the path your request takes 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 I will point out, 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 have 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 connect 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, that queue is 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
 - uwsgi, daphne, and runworker are started with honcho instead of supervisord
