turbo-replay

turbo-replay assigns a sequence number to broadcasted messages and caches them. When a client disconnects because of flaky network, we're able to resend (or replay, hence the name) missed messages in the same order they were originally sent.

Installation

Important: Make sure you have installed hotwired/turbo-rails before using this gem!

Add this line to your application's Gemfile:

gem "turbo-replay"

Execute the following commands to install the gem and generate an initializer:

$ bin/bundle install
$ bin/rails turbo-replay:install

Replace the import for @hotwired/turbo-rails in your application.js

- import "@hotwired/turbo-rails"
+ import "turbo-replay"

Now reload your server - and that's it!

Javascript events

// The user connected for the first time to a channel.
window.addEventListener('turbo-replay:connected', (ev) => {
  console.log('connected', ev.detail.channel)
})

// The user disconnected from a channel and we're retrying to reconnect.
// It's good to show some 'reconnecting...' indication here.
window.addEventListener('turbo-replay:disconnected', (ev) => {
  console.log('disconnected', ev.detail.channel)
})

// The user reconnected after being offline a little bit.
// Hide the 'reconnecting...' indicationk here.
window.addEventListener('turbo-replay:reconnected', (ev) => {
  console.log('reconnected', ev.detail.channel)
})

// The user reconnected, but the latest received message was older
// than the oldest message in the cache. There's nothing we can do
// here to recover the state. You can reload the whole application
// or show some indication asking the user to reload.
window.addEventListener('turbo-replay:unrecoverable', (ev) => {
  console.log('unrecoverable', ev.detail.channel)
})

How does it work?

turbo-replay stores broadcasted messages in a cache. Each message is assigned a sequence number.

Because the sequence number is sequential, clients know what the next value is expected to be. If an arrived sequence_number doesn't match the expected value, it means the client missed a message.

The sequence number ensures clients can detect missed messages even without a disconnect event.

When the client notices they missed an event, they ask the server to resend messages after the last known sequence number.

I like to broadcast lots of stuff, isn't the cache too much overhead?

Maybe, it depends on your use case. Take a look at config/initializers/turbo_replay.rb in your application to tweak the cache's retention policy.

What if the client stays offline too long?

There's nothing we can do if the client's latest received message is older than the oldest message in the cache.

Suggestion: handle the turbo-replay:unrecoverable event and display a message asking the user to reload the app. For example:

window.addEventListener('turbo-replay:unrecoverable', () => {
  window.alert("You're offline for too long. Please reload the page.")
})

License

The gem is available as open source under the terms of the MIT License.