What Are the .z Message Handlers?

Question

A q process exposes a small set of built-in callback functions in the .z namespace that fire at specific points during an IPC connection's life. What are the main ones involved in opening a connection and handling incoming requests, and what does each one do?

Solution

Handler	Fires when...
.z.pw	a username/password needs validating, before a connection is accepted
.z.po	a connection has just been opened (after validation succeeds)
.z.pg	a synchronous ("get") request arrives over an open connection
.z.ps	an asynchronous ("set") request arrives over an open connection
.z.pc	a connection closes

Explanation

These five cover a connection's full lifecycle: validate (.z.pw) → open (.z.po) → handle requests (.z.pg sync / .z.ps async) → close (.z.pc).

q ships a working default for each, so a process that never touches .z still behaves correctly. Overriding one — .z.pg, say — lets a process log, reject, or check incoming requests before evaluating them, with no change needed on the calling side.

The naming follows "get vs set": .z.pg handles requests expecting something back, .z.ps handles fire-and-forget ones — mirroring the sync/async split exactly.