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Note
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.

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usage
usage
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.

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  • 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
     

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Replacement
Replacement
How do I determine what string replacement I need?

When an Ignition tag is created, there are some rules than 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: ', -, :, (, )

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To determine the string replacement needed for the namespace:

  • each incoming topic:

Consider an incoming MQTT message with the characteristics:

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

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  • 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 Now consider an incoming MQTT message with the characteristics:

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


This 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:

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