Channels and contexts
Contexts can subscribe to particular channels to receive events delivered to those channels from adapters and from other contexts. See
Channels and input events and
Subscribing to channels. Contexts that are public, that is, they were created with a
true flag in the context constructor, have a permanent subscription to the default channel. The name of the default channel is the empty string.
Contexts can send events to channels without knowledge of whether the event is required by contexts, clients, adapters, or some combination. When an event is sent from a context to a channel the event is received by all contexts subscribed to that channel and by all external receivers that are listening on that channel. See
Generating events with the send command.
An Apama query automatically runs in a context that has a permanent subscription to the default channel and to the com.apama.queries channel.
Channels are useful for:
Identifying service monitors — If many monitors need to send events to a service monitor you can use a well known name (which can appear in EPL as a string literal or string constant) as a channel name. The service monitor (and only the service monitor) should subscribe to the channel and other monitors send events to that channel. When a request-response event protocol is required the sender can specify a channel to which it is subscribed, or a context to send the response to.
Applications that have different contexts that consume different streams of data can use channels to send the data to the intended contexts, even if many contexts require the same data stream or one context requires multiple data streams. For example, statistical arbitrage trading strategies could run in many contexts, each subscribed to a channel for the pair of stock symbols it is trading against each other. If the adapter where the events are coming from is able to use a separate connection per channel, then the application will scale very well as more trading strategies on different symbols are added.
Different components of an application can be de-coupled by using an event protocol that sends events to channels for each interaction point between components. This allows adapters to be replaced with monitors that simulate those adapters for testing, and makes it easy to scale an application across several hosts by running different parts on different correlators and then connecting them.
