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ZeroMQ

Sends or receives messages over ZeroMQ sockets (PUSH/PULL, PUB/SUB, REQ/REP). The socket_type selects the pattern and bind decides whether the endpoint binds or connects.

URL format

zeromq://<transport>?socket_type=<type>[&bind=true]

zeromq:// and zmq:// are both accepted. The URL is a ZeroMQ transport address such as tcp://127.0.0.1:5555. Choose bind=true on exactly one side of a socket pair; the other connects.

Examples

Pull from a PUSH producer and write to a file, continuous:

mqb copy \
  --from 'zeromq://tcp://127.0.0.1:5555?socket_type=pull&bind=true' \
  --to file:///data/events.jsonl?format=json

Publish a Kafka topic to a PUB socket, continuous:

mqb copy \
  --from kafka://kafka.local:9092?topic=events \
  --to 'zeromq://tcp://0.0.0.0:5556?socket_type=pub&bind=true'

Subscribe to a topic on a remote PUB socket, continuous:

mqb copy \
  --from 'zeromq://tcp://feed.local:5556?socket_type=sub&topic=orders' \
  --to 'postgres://user:pass@localhost/app?table=orders&auto_create_table=true'

Key options

OptionPurpose
socket_typepush, pull, pub, sub, req, or rep.
bindBind to the address instead of connecting (default false).
topicConsumer-only: topic filter for sub sockets.
formatWire format: raw_framed (default — payload bytes with a JSON metadata frame in front), raw (payload bytes only, no metadata), json (wraps the whole message).
backendtry_omq (default — use omq when the zeromq-omq feature is compiled in, else zmq), or zmq / omq to require that backend.

Wire format changed in 0.4.0. format used to default to json; it is now raw_framed, which is binary-safe and still carries headers. A 0.4 peer and a 0.3 peer no longer understand each other on the same socket unless one of them uses format=json in the URL query. REQ/REP replies are the exception to format entirely: a REP peer always answers with a JSON array of canonical messages, and a REQ publisher always decodes one.

Both backends cover the whole socket set, REQ/REP included. backend: omq is the faster path when the zeromq-omq feature is compiled in; naming omq or zmq explicitly makes that backend a hard requirement rather than a preference. On omq, backpressure is applied by the socket’s high-water mark and internal_buffer_size is ignored.

Full field list: reference/zeromq.md.