Sender capacity is the property of a sender object that controls the number of asynchronous sends pending acknowledgement from the server that the sender will permit. These unacknowledged messages are buffered in memory for retransmission in the event of a link failure, so the sender capacity is also known as the sender replay buffer size.

By default, sender capacity is set to UNLIMITED, meaning that the sender will allow an unlimited number of asynchronous calls to be made, and buffer a number of messages that is limited only by the memory limits of the system.

When the sender capacity is set to a number other than UNLIMITED, the sender will allow only that many asynchronous send operations to be outstanding at the same time.

For example: if a sender's capacity is set to 10, then a maximum of 10 asynchronous send operations can be awaiting acknowledgement at the same time for the sender. If 10 asynchronous send operations are invoked, and an 11th operation is attempted before any of those 10 are acknowledged by the broker, then the sender will block until one of the asynchronous send operations is acknowledged by the broker.

Be aware of two things: unbounded sender capacity can have an impact on resources if your sender outpaces the server significantly. Be aware also that upon reaching its capacity a sender will switch from asynchronous to synchronous send behavior, and message sends will block. You should tune your sender capacity with this in mind, and also carefully program your send operations to check the sender's capacity and availability if blocking will be problematic.