Some useful hints regarding how the zmq package works.
A small introduction to ROUTER sockets, for more details see zmq guide chapter 3.
The ROUTER socket is mostly used in a server role as it can maintain connections to many peers. In order to distinguish peers, it assigns a random identity to each connected peer. Application code does not know which peer gets which identity, however, the identity of a peer stays the same for the lifetime of the connection.
If, for example, two Components CA, CB connect to a ROUTER socket, the socket assigns identities, which will be called IA, IB here (they can be any byte sequence).
Whenever a message is sent to a ROUTER socket from a peer, the socket prepends that identity in front of the message frames, before handing the message to the application code.
For example, if CA sends the message Request A, the ROUTER socket will read IA|Request A.
That way, an answer to that message can be returned to this same peer, and not another one (a ROUTER socket may have many connected peers).
Consequently, in order to send such an answer, the identity has to be prepended to the frames to send: Calling the ROUTER's send command with IA|Reply A, the socket will send Reply A to the peer, whose identity is IA, in this case that is CA.
The following diagram shows this example communication with two Components: :::{mermaid} sequenceDiagram participant Code as Message handling participant ROUTER as ROUTER socket Note over Code, ROUTER: Coordinator CA ->> ROUTER: "Request A" ROUTER ->> Code: "IA|Request A" Code ->> ROUTER: "IA|Reply A" ROUTER ->> CA: "Reply A" CB ->> ROUTER: "Request B" ROUTER ->> Code: "IB|Request B" Code ->> ROUTER: "IB|Reply B" ROUTER ->> CB: "Reply B" :::