Test an MQTT broker

The mqtt driver does one job per iteration: connect, do the job, close. The job is set with -opt mode=.... The body is the message payload.

# Publish a message and wait for the broker's ack (the default mode).
vegaload run -target mqtt://127.0.0.1:1883 -protocol mqtt -body 'hello' \
  -opt topic=load/test -opt qos=1 -vus 20 -duration 30s

# Publish, and wait until the same message comes back. This measures
# delivery through the broker. {id} is a unique client id for each iteration.
vegaload run -target mqtt://127.0.0.1:1883 -protocol mqtt -body 'ping {id}' \
  -opt mode=roundtrip -opt topic='load/{id}' -opt qos=1

# Subscribe and wait for 5 messages that contain "ok".
vegaload run -target mqtt://127.0.0.1:1883 -protocol mqtt \
  -opt mode=subscribe -opt topic='sensors/#' -opt count=5 -opt expect=ok

The options are mode (publish, subscribe, roundtrip), topic, qos, retain, username, password_env, client_id, count, expect and keepalive. Put the password in an environment variable and pass its name with -opt password_env=NAME (it needs username), so it is not on the command line. In roundtrip mode the body must contain {id}, so each user can tell its own message from the messages of other users. Use mqtts:// for TLS, with -insecure if the certificate is not trusted. mqtts:// takes the four options under "TLS with your own CA". Every iteration uses a new client id, because a broker closes an older connection that has the same id. The same host allowlist and caps apply as for HTTP targets.