"Error in plugin: key "...." not in Fields" triggered by Starlark processor plugin

Telegraf
,
1

The problem

  • Telegraf gives errors
    **2020-12-02T18:58:07Z E! [processors.starlark] Error in plugin: key “Allocated Virtual Memory” not in Fields.

=====
telegraf.conf has:

[[inputs.snmp]]
agents = [“udp://127.0.0.1:161”]
name = “Local VM”
:
[[inputs.snmp]]
agents = [“udp://192.168.9.1:161,udp://192.168.9.1:162”]
name = “remote VM”
:
[[processors.starlark]]
script = “/home/hn1961/starlark/mine.star”

===

The starlark script ‘mine.star’ has the following:

def apply(metric):
allocatedMem = metric.fields[‘Allocated Virtual Memory’]
usedMem = metric.fields[‘Used Virtual Memory’]
metric.fields[‘freeMem’]= allocatedMem - usedMem
return metric

===

The goal
To have a new metric ‘freeMem’ with the desirable result.

==

Observations

  • Chronograf shows:
  • Only 127.0.0.1 (with measurement=“Local VM”) have metric names and data.
  • Other hosts (192.168.x.x) associated with name = “remote VM” have metric names but no data for any metric field.
  • It appears that Telegraf stopped collecting metrics for hosts that do not have metrics mentioned in the starlark script.

=======

The environment:

  • Both ‘Allocated Virtual Memory’ and ‘Used Virtual Memory’ are custom MIBs
  • Only agent with measurement=“Local VM” has metrics ‘Allocated Virtual Memory’ and ‘Used Virtual Memory’
  • Agents with Measurement = “remote VM” do not have the custom MIBs, hence, the metrics are not applicable to them.

======

I tried adding IF-ELSE statement in the starlark script so that the calculation are done for the ‘Local VM’ measurement.
However, I am unable to determine the Condition parameter, for instance,
def apply(metric):
if MEASUREMENT[‘Name’] == “Local VM”:
allocatedMem = metric.fields[‘Allocated Virtual Memory’]
usedMem = metric.fields[‘Used Virtual Memory’]
metric.fields[‘freeMem’]= allocatedMem - usedMem
return metric
else:
return metric

I get errors:
2020-12-02T19:08:56Z E! [telegraf] Error running agent: could not initialize processor starlark: /home/hn1961/starlark/sam.star:2:6: undefined: MEASUREMENT

Next Steps
Any pointer is greatly appreciated.

2

Hello there, it seems like your issue is that you want the starlark script to only process on the Local VM measurement. I believe a simple way to do this is with a ‘namepass’ in the processor’s config. You could do something like namepass = [“Local VM”] underneath [[processors.starlark]] and I think that should act to filter to only the Local VM measurements. Just be sure to remove the if MEASUREMENT[‘Name’] == “Local VM” line for that solution.

3

Note that the measurement is called metric.name, not MEASUREMENT.

also if you are not sure if the field exists, you could use metric.fields.get() instead of metric.fields[] to get the field value. Note that you may want to limit the metrics that the processor is looking at with namepass.

4

Using namepass as a filter works well. See script below.

[[processors.starlark]]
namepass = [‘Local VM’]
source = ‘’’
def apply(metric):
allocatedMem = metric.fields[‘Allocated Virtual Memory’]
usedMem = metric.fields[‘Used Virtual Memory’]
metric.fields[‘freeMem’]= allocatedMem - usedMem
return metric
‘’’

This way, the processor only acts on collection targets that have the measurement in the script.

As a heads up, a Processor script can stop reporting metrics unintentionally if not planned carefully (as I reported above). Hope it will be more robust than it is now. In the meantime, I will try metric.fields.get() as a safe guard.

Thank you so much for the help.

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