47. JMAP PUSH over WebSockets

Date: 2021-01-26

Status

Accepted (lazy consensus).

Relates to 45. Support JMAP Push with Mailbox/changes implementation

Requires 46. Generalize EventBus

Context

JMAP Push notifications allow clients to efficiently update (almost) instantly to stay in sync with data changes on the server.

In order to support this, we need to have the server emit newest state changes to the client over a PUSH channel.

Decision

We will implement RFC-8887 - A JSON Meta Application Protocol (JMAP) Subprotocol for WebSocket as a transport mechanism for PUSH notifications.

We will generalize EventBus in order to achieve an out-of-the box Publish-Subscribe system for JMAP related events, that does not fit in the mailbox API.

We will implement listeners registered on the JMAP event bus for WebSocket clients in order to carry over state changes to the client.

Consequences

We expect clients using the PUSH to lead to a drastic performance enhancement, as less data needs to be transmitted upon resynchronisation.

As mentioned in RFC-8887 the usage of webSockets allows other performance optimizations:

  • Requests can easily be compressed, which is not doable over HTTP for most available implementation (HTTP request compression is not ubiquitous).
  • WebSockets being connected, authentication can be performed once, when establishing the connection. This can allow to reduce the load, if needed, on authentication systems. It might ease the use for instance of custom OpenId connect providers.

People deploying JMAP need to be aware that load-balancing webSockets requires session stickiness.

Sequence

  1. Bob authenticates against the ws:// endpoints. Upgrade to websockets is granted.
  2. Bob registers Email and Mailbox updates. A listener listens for state changes related to Bob account.
  3. Bob receives a mail. The MailboxManager adds it to Bob's mailbox. An Added event is fired on the mailbox event bus.
  4. The MailboxChangeListener processes the Added event, handles delegation, records the state change, and fires related events for each account on the JMAP event bus, for both Email (as there is an addition) and Mailbox (as the counts were updated).
  5. Bob's webSocket listener receives a message from RabbitMQ and pushes it to Bob.
  6. Bob's MUA is aware it needs to re-synchronize. It will perform resynch requests combining Email/changes, Email/get, Mailbox/changes and Mailbox/get.

Event bus listener are created for each socket upon client requests, and removed upon disconnection or when PUSH is explicitly canceled by the client.

In a multi-node setup, the event bus registration key mechanism described in this ADR and its distributed implementation ensure events are routed to the James servers holding client PUSH registration. We will use the AccountId as a base for a registration key.

Alternatives

The JMAP RFC defines event source (Server Sent Events) as a supported transport medium for PUSH notification. Yet:

  • Active JMAP contributors lack production experience on Event source while they do already deploy webSockets
  • Performance enhancements (authentication and request compression) unlocked by webSockets are not achievable via event source.

Note that nothing refrains about future implementation of event source mechanism, that, if need be can be ignored or disabled via a reverse proxy.

References