influxdata/telegraf

Feature request for mqtt_consumer: ignore retained messages

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#10,824 opened on Mar 16, 2022

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Description

Feature Request

When using the MQTT consumer input plugin, you can subscribe to topics that have a retained value. At the moment, retained messages are treated as any other message and their value it stored.

Proposal:

Add a configuration setting to allow the mqtt_consumer input plugin to ignore retained messages.

Current behavior:

  1. Telegraf starts
  2. mqtt_consumer subscribes to configured topics
  3. Broker sends the retained messages for the configured topics
  4. Telegraf stores the values in configures output.

Desired behavior:

  1. Telegraf starts
  2. mqtt_consumer subscribes to configured topics
  3. Broker sends the retained messages for the configured topics
  4. Telegraf checks the config for the plugin, and if the option to ignore retained messages is set, it will NOT store the retained values.

Use case:

I have sensors that publish their data to MQTT with the retain flag set. Currently, every time I restart telegraf, it stores the retained value again when I only want it to update when new messages arrive.

Contributor guide

Tech stack
go
Domain
databackend
Issue type
feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Basic Go knowledgeUnderstanding of MQTTFamiliarity with Telegraf plugin system
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45
Research direction
Examine the mqtt consumer plugin located in plugins/inputs/mqtt consumer/. Look at how other plugins handle configuration options, e.g., in plugins/inputs/ and their respective READMEs. The main work involves adding a config field like 'ignore retained' and checking the retained flag on incoming messages in the OnMessage callback. Reference the MQTT library's retained property (e.g., paho.mqtt.golang). After implementation, add tests and update the plugin's sample config.