Date: 2021-10-18
Accepted (lazy consensus).
Implemented.
Notification for newly received emails is a common feature of modern mail applications. Furthermore, real time updates across devices are a common feature. We wish to provide support for this on top of Apache James JMAP implementation for a large variety of devices.
Dealing with mobile devices yield a unique set of challenges at the protocol level, including low bandwidth connection, long latencies, and the need to save battery. As such, mobile devices operating systems tend to limit background connections for applications. Thus, in order to receive notifications when the application is running on the background, one cannot rely on ADR 0047 (JMAP websocket push) - as it would imply the application maintaining a persistent connection to the server, which is battery/network consuming.
As such, RFC-8620 section 7.2 JMAP specification for web push introduces the use of RFC-8030 Generic Event Delivery Using HTTP Push using an intermediate push server: a Push Gateway is used by the device to multiplex all push of all applications on a single connection. The device then register the URL of its push gateway on the application server (here JMAP server) for it to forward StateChange events to the push gateway.
Implement RFC-8620 section 7.2 JMAP specification for web push.
We will store StateChanges, provide a Cassandra implementation of this storage for the Distributed application.
We will implement a PushClient based on reactor-netty to send events to the push gateway.
We will rely on a Group listener plugged on the JMAP event bus to handle the web push - See ADR 37: Event bus.
We will implement the PushSubscription/get and PushSubscription/set JMAP methods.
We will also implement the verification code mechanism (a first round-trip to verify the push gateway works well).
We also decided to allow a single push subscription per devideId (which includes device and APP identifier...)
The notification traffic transits by some third party infrastructure which can both be a security and privacy risk. RFC-8291 Message Encryption for Web Push introduces encryption for web push messages and is integrated to JMAP web push mechanisms. This SHOULD be implemented. google/tink library provides utilities to easily do this.
Our testing strategy will rely on a mock HTTP server mimicking the behavior of a push gateway.
The push gateway can support rate limiting and might reject some of the messages pushed to it (HTTP 429 too many requests). Please note JMAP StateChanges is resilient to message loss (as only the latest state of the underlying object is transmitted, missing intermediate states does not imply missing changes). We do not plan on implementing retries nor adaptive push rate.
Generic Event Delivery Using HTTP PushMessage Encryption for Web Push