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Multithreading

The pn_proactor allows you to create multi-threaded client and server applications. You can call pn_proactor_run in multiple threads to create a thread pool for proton to handle events and IO.

The Proactor functions are safe to call from any thread. Most other functions are not safe to call concurrently. The following rules applies: objects and events associated with different connections can safely be used concurrently. Objects and events associated with the same connection cannot.

The proactor helps you to handle connections safely: pn_proactor_get() and pn_proactor_wait() will never return events associated with the same connection to different threads concurrently. As long as you process only the objects available from the event returned by the proactor, you are in no danger.

When you need to process objects belonging to a different connection (for example if you receive a message on one connection and want to send it on another) you must use locks and thread safe data structures and the pn_connection_wake() function.

## The wake function

pn_connection_wake() can be called from any thread as long as the pn_connection_t has not been freed. It allows any thread to "wake up" a connection by generating a PN_CONNECTION_WAKE event for that connection.

For example, supposed a message arrives on connection A and must be sent on connection B. The thread that receives events for A from the proactor can safely call pn_link_recv() on A's link to decode the Message. However it cannot call pn_link_send() on any link belonging to B.

The decoded Message is independent of any connection, so the thread handling A can store the message in a thread-safe data structure, and call pn_connection_wake() to notify B. Shortly after, some thread will get a PN_CONNECTION_WAKE event for B, and can retrieve the message and send it on B's link.

Note
  Be careful to ensure (using a mutex for example) that connection B cannot be deleted before connection A calls pn_connection_wake. Connections are automatically deleted by the proactor after the PN_TRANSPORT_CLOSED event git