Asterisk stasis message bus

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Asterisk stasis message bus 내용 정리

Message Bus

The overall stasis-core API can be best described as a publish/subscribe message bus.

Data that needs to be published on the message bus is encapsulated in a stasis_message, which associates the message data(which is simply an AO2 managed void *) along with a stasis_message_type. (The message also has the timeval when it was created, which is generally useful, since messages are received asynchronously).

Messages are published to a stasis_topic. To receive the messages published to a topic, use stasis_subscribe() to create a subscription.

When topics are subscribed to/unsubscribed to, stasis_subscription_change are sent to all of the topic's subscribers. On unsubscribe, the stasis_subscription_change will be the laste message received by the unsubscriber, which can be used to kick off any cleanup that might be necessary. The convenience function stasis_subscription_final_message() can be used to check if a message is the final unsubscribe message.

To forward all of the messages from one topic to another, use stasis_forward_all(). This is useful for creating aggregation topics, like ast_channel_topic_all(), which collect all of the messages published to a set of other topics.

Routing and caching

In addition to these fundamental concepts, there are a few extra objects in the Stasis Bus arsenal.

For subscriptions that deal with many different types of messages, the stasis_message_router can be used to route messages based on type. This gets rid of a lot of ugly looking if/else chains and replaces them with callback dispatch.

Another common use case within Asterisk is to have a subscription that monitors state changes for snapshots that are published to a topic. To aid in this, we have stasis_cachig_topic. This is a topic filter, which presents its own topic with filtered and modified messages from an original topic. In the case of the caching topic, it watches for snapshot messages, comapres them with their prior values. For these messages, it publishes a stasis_cache_update_message with both the old and new values of the snapshot.

Design philosophy

By convention (since there's no effective way to enforce this in C), messages are immutable. Not only the stasis_message type itself, but also the data it contains. This allows messages to be shared freely throughout the system without locking. If you find yourself in the situation where you want to modify the contents of a stasis_message, what you actually want to do is copy the message, and change the copy. While on the surface this seems wasteful and inefficient, the gains in reducing lock contention win the day.

Messages are opaque to the message bus, so you don't have to modify stasis.c or stasis.h to add a new topic or message type.

The stasis_topic object is thread safe. Subscribes, unsubscribes and publishes many happen from any thread. There are some ordering guarantees about message delivery, but not many.

  • A stasis_publish() that begins after a stasis_publish() to the same topic completes will be delivered in order.
    • In general, this means that publications from different threads to the same topic are unordered, but publications from the same thread are.
  • Publications to different topics are not ordered.
  • The final message delivered to a subscription will be the stasis_subscription_final_message().

The callback invocations for a stasis_topic are serialized, although invocations can happen out of any thread from the Stasis thread pool. This is a comfortable middle ground between allocating a thread for every subscription(which would result in too many threads in the system) and invoking callbacks in parallel (which can cause insanity). If your particular handling can benefit from being parallelized, then you can dispatch it to a thread pool in your callback.

See also