Commands: feedback / perf_assess / default plugins

Author

Affiliation

Paolo Bosetti

University of Trento

Abstract

mads feedback and mads perf_assess are two small, standalone (non-plugin) agents used mainly for testing a MADS network by hand: a generic subscriber that prints everything, and a generic publisher that emits a steady synthetic stream. This page covers both, together with the default plugins loaded by mads source/filter/sink when no plugin is given — which read/write plain JSON over stdin/stdout, making them a quick way to wire an external script into a MADS network without writing a line of C++.

Related guides: Plugins — writing a real plugin, once stdin/stdout piping isn’t enough.

Synopsis

mads feedback [-b|--dont-block] [-q|--queue-size n] [common options]
mads perf_assess [-p period_ms] [-l payload_bytes] [common options]
mads source|filter|sink   # with no plugin argument: loads the default plugin

mads feedback

A pure subscriber (unlike mads sink, it is a standalone Agent, not a plugin loader): it prints every message it receives as topic: payload, one line per message.

Flag	Effect
-b, --dont-block	Poll without blocking (from ini: dont_block).
-q, --queue-size n	ZMQ socket queue size (default 1000).
(ini) print_width	Truncate the payload to this many characters (default: full pretty-print).
(ini) indent_width	Indent width when pretty-printing (no truncation).

Its sub_topic (like any agent) defaults to whatever the [feedback] ini section says — set it to [""] to see everything on the network.

mads perf_assess

The mirror image: a pure publisher that emits a synthetic payload (id, origin_timestamp, payload — a random fixed-length string, payload_length, period) at a fixed rate, useful for eyeballing a pipeline’s actual timing/throughput with mads feedback, or with the monitor sink plugin (mads sink monitor.plugin), on the receiving end.

Flag	Effect
-p ms	Publish period (default 100 ms).
-l bytes	Length of the random payload string (default 10).
-n, --name / -i, --agent-id	Agent identity options.

flowchart LR
  PA["mads perf_assess
-p 50 -l 256"] -- "steady synthetic
stream" --> Broker[("broker")]
  Broker --> FB["mads feedback"]
  Broker --> MO["mads sink monitor.plugin
(inter-arrival timing)"]

Default plugins

mads source, mads filter, and mads sink each load a default plugin when none is given on the command line — and all three read/write plain JSON on stdin/stdout, which is the fastest way to prototype an agent’s logic in any language, or to inject/inspect traffic by hand.

Loader	Default plugin	Behavior
mads source	publish.plugin	Reads one JSON value per line from stdin, publishes it.
mads filter	bridge.plugin	Prints each received message as {"topic":..., "input":...} to stdout, then blocks reading a replacement JSON line from stdin and publishes that as output.
mads sink	feedback.plugin	Prints each received message as topic: payload to stdout (truncated to print_width, default 60, or pretty-printed with indent_width if print_width is 0).

flowchart LR
  Ext["external process
(any language)"] -- "reads on stdout,
writes on stdin" --> BR["mads filter
(bridge.plugin)"]
  BR <--> Net[("broker")]

This is exactly the pattern used to wrap an external script as a filter — e.g. a Python loop reading process.stdout.readline(), transforming the JSON, and writing it back to process.stdin.

Warning

bridge.plugin’s process() step blocks on getline(cin, ...), and that call runs on the agent’s own main loop thread. If nothing is written to its stdin in response to a printed input, the whole filter stalls — it will not publish, and it will not process the next incoming message either.

ImportantNaming collision

mads feedback (this page, a standalone agent) and feedback.plugin (the default sink plugin, loaded by mads sink) are two different implementations that happen to do a similar thing (print received messages) — one is a full agent binary, the other a tiny plugin loaded by the generic sink loader. Don’t confuse the two when reading logs or scripts.

See also

mads · source / filter / sink · plugin