The difference between the two types of remote queue definition is that with synchronous a message put to a remote queue definition is sent over the network in real-time and put to the queue on the remote queue manager, whereas with asynchronous the message is put to a temporary store and transmitted when a network connection becomes available.
See more at Message delivery.
This allows applications to operate on the queue when the device is offline. Consequently, asynchronous queues require a message store in order that messages can be temporarily stored at the sending queue manager whilst awaiting transmission.
This diagram shows an example of a remote queue set up for synchronous operation and a remote queue setup for asynchronous operation.
In both the synchronous and asynchronous examples queue manager qm2 has a local queue invQ.
In the synchronous example, queue manager qm1 has a remote queue definition of queue invQ. invQ resides on queue manager qm2. The mode of operation is set to synchronous.
An application using queue manager qm1 and putting messages to queue qm2.invQ establishes a network connection to queue manager qm2 (if it does not already exist) and the message is immediately put on the real queue. If the network connection cannot be established then the application receives an exception that it must handle.
In the asynchronous example, queue manager qm1 has a remote queue definition of queue invQ. invQ resides on queue manager qm2. The mode of operation is set to asynchronous.
An application using queue manager qm1 and putting messages to queue qm2.invQ stores messages temporarily on the remote queue on qm1. When the transmission rules allow, the message is moved to the real queue on queue manager qm2. The message remains on the remote queue until the transmission is successful.