Since version 0.7, Faye includes an API for monitoring activity going on within the engine. This means you can attach event listeners to monitor the creation and destruction of client sessions, find out when clients subscribe and unsubscribe from channels, and watch published messages.
You attach an event listener to your server like so:
bayeux = Faye::RackAdapter.new(:mount => '/faye', :timeout => 25) bayeux.on(:handshake) do |client_id| # event listener logic end
The available events are:
:handshake [client_id]– Triggered when a new client connects and is issued with an ID.
:subscribe [client_id, channel]– Triggered when a client subscribes to a channel. This does not fire if a
/meta/subscribemessage is received for a subscription that already exists.
:unsubscribe [client_id, channel]– Triggered when a client unsubscribes from a channel. This can fire either because the client explicitly sent a
/meta/unsubscribemessage, or because its session was timed out by the server.
:publish [client_id, channel, data]– Triggered when a non-
/meta/**message is published. Includes the client ID of the publisher (which may be
nil), the channel the message was sent to and the data payload.
:disconnect [client_id]– Triggered when a client session ends, either because it explicitly sent a
/meta/disconnectmessage or because its session was timed out by the server.
Events versus extensions
On the surface, this API seems similar to the facilities provided by the extensions system. Historically, a common use case for extensions has been to monitor what’s going on in your Faye service. The difference is that extensions operate within the protocol layer, and monitoring events are attached at a lower level: they come from the engine and are independent of the protocol. See the architecture overview for more information.
This means that they can notify you about events that are not caused by
incoming messages. For example, although clients are supposed to explicitly
end their session by sending a
/meta/disconnect message, they often fail
to do this and their session is ended when the server notices that no messages
have been received from that client for a certain amount of time. In this
:disconnect event is emitted, along with
:unsubscribe events for
all the client’s subscriptions, even though no message was received.
The flip side of this is that these events cannot use or modify any data being transmitted via the protocol. If you need to implement authentication, or use message extension data, then you should write an extension. If not, these events provide a lightweight way to monitor messaging activity.
Monitoring in multi-process servers
If you’re using the Redis engine to run multiple front-end server processes,
you should keep in mind that monitoring events only fire once, in the process
that handled the relevant event. For example if a client subscribes to a
:subscribe event will only fire in the process that handled
/meta/subscribe message, not in any other process.
This has implications if you’re storing state derived from this activity. For example, if you’re storing subscriber counts for channels in memory but running multiple Faye servers, each server will have its own local and incorrect value of the count for a channel.
A word of warning
While this monitoring API is certainly useful for some applications, you
are advised to think carefully before using it to implement decisions within
your application. For example, many people building a chat room initially
think that listening for
:disconnect is a good way to monitor presence. In
reality, a user in your app will create many Faye clients as they navigate
from page to page, so a Faye session may not have a one-to-one relationship
with an entity in your app.
You should consider whether it would be better to implement your own heartbeat/timeout system on top of Faye for your use case before diving in and using monitoring events for this. Binding your application logic to Faye client IDs may create unwelcome couplings between your application and your network communication stack.