# Appendix

## ZMQ

Some useful hints regarding how the zmq package works.

### ROUTER sockets

A small introduction to ROUTER sockets, for more details see [zmq guide chapter 3](https://zguide.zeromq.org/docs/chapter3/#Exploring-ROUTER-Sockets).

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"
:::