Templates

Instead of formatting output with the format specification as described above, mqttwarn has provision for rendering the output message from Jinja2 templates, probably particularly interesting for the smtp or nntp and file targets.

Consider the following example topic configuration, where we illustrate using a template instead of format (which is commented out).

[nn/+]
targets = nntp:jpaa
; format = {name}: {number} => {_dthhmm}
template = demo.j2

mqttwarn loads Jinja2 templates from the templates/ directory relative to the configured directory. Assuming we have the following content in the file templates/demo.j2

{#
    this is a comment
    in Jinja2
    See http://jinja.pocoo.org/docs/templates/ for information
    on Jinja2 templates.
#}
{% set upname = name | upper %}
{% set width = 60 %}
{% for n in range(0, width) %}-{% endfor %}

Name.................: {{ upname }}
Number...............: {{ number }}
Timestamp............: {{ _dthhmm }}
Original payload.....: {{ payload }}

could produce the following message, on any target which uses this configuration.

------------------------------------------------------------
Name.................: JANE JOLIE
Number...............: 47
Timestamp............: 19:15
Original payload.....: {"name":"Jane Jolie","number":47, "id":91}

One of the template variables you may be interested in is called {{ payload }}; this carries the original MQTT message in it. Also, if the payload was JSON, those are available also (as shown in the above example), together with all the other transformation data.

If the template cannot be rendered, say, it contains a Jinja2 error or the template file cannot be found, etc., the original raw message is used in lieu on output.

As mentioned already, we think this is useful for targets which expect a certain amount of text (file, smtp, and nntp come to mind).

Use of this feature requires Jinja2, but you don't have to install it if you don't need templating.