Prerequisites

  • Knowledge of Ignition and Module installation process: Cirrus Link Module Installation
  • Have Ignition 8.0.16 or greater installed
  • Have MQTT Engine 4.0.10 or later installed

Abstract

MQTT Engine supports 'String Replacement' for both MQTT Topics and Payloads for character sequences that will become part of an Ignition tag path. In many cases with Custom Namespaces this can be required if one does not have full control of the topics and payloads that are sent.

This can also be true with certain Sparkplug based payloads where metric names may have unusual characters in them. While MQTT and Sparkplug both support characters such as . & % =, Ignition does not support these as valid characters in a tag path or tag name. As a result, sometimes it may be necessary to tell MQTT Engine to replace certain characters or strings of characters with something else so the tag path and tag names can be properly created in Ignition.

String Replacement Usage

The String Conversion feature can be used in any Custom Namespace or in the Sparkplug B Namespace. The String Replacement feature supports the following.

  • Replacements are executed using the 'Order Index' order from lowest to highest.
  • Single character to single character replacement
  • String to string replacement
  • Character or string to nothing (i.e. removes a string or character)
  • Regular Expression (regex) to string, character, or nothing replacement (i.e. uses a regex for pattern matching rather than a literal string)
  • Character replacements will only occur for characters that will be part of the tag path. Character replacements are not applied to Ignition tag values
  • Null or empty replacement string

Examples:

  • Remove all '@' symbols
  • Replace all '@' symbols with '_'
  • Replace all 'abc' character sequences with 'def'
  • Replace all characters between the literal strings 'start' and 'end' with a '_' character using a regex
     

How do I determine what string replacement I need?

When an Ignition tag is created, there are some rules that need to be followed:

  • The first character of the Tag name must be one of the following:
    • Letter - specifically, a letter as recognized by Unicode's Letter (L) category.
    • Number
    • Underscore
  • The second character, and every character after that can then be one of the following:
    • Letter
    • Number
    • Underscore
    • Space
    • Any of the following special characters: ', -, :, (, )
All other special characters are not allowed in a Tag name.


To determine the string replacement needed for the namespace:

  • Look at the published MQTT topic and a sample payload, if parsing as JSON, and identify any characters that may be problematic.
  • Review these characters against the rules above or open Ignition designer and test if a tag can be created with the specific characters.
  • If a tag cannot be created, created a string replacement rule.


Consider an incoming MQTT message with the characteristics:

  • Topic = a/b
  • Payload = { "tag 1": 123, "tag@2": "some.value" }


A custom namespace with a subscription a/# will be able to receive the message however this will fail to be handled by the Custom Namespace and an error will be logged that looks like:

INFO   | jvm 1    | 2022/01/06 11:33:29 | E [c.c.m.e.g.j.JsonPayloadHandler] [19:33:29]: Failed to handle tagPath - not adding tag: a/b/tag@2 - Remove it from future payloads


Using 'String Conversions' can allow this tag to be created. Consider the following String Conversion:

This tells MQTT Engine that for this custom namespace, convert any '@' characters that would normally be part of the tag path to '_' characters. So now when we publish the same MQTT message, we don't get an error message and the tag gets properly created as shown below with the '@' character replaced by a '_' character.


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