Prevent indefinite unsent metrics loop in outpus.azure_monitor

[Hr0bar]

Use Case

Hi, we are using outputs.azure_monitor for more than 3 years now and only now we ran into this issue which triggered this feature request. Basically a global option to drop unsent metrics by a time limit instead of buffer size limit could be a solution, or output plugin specific solution could be added?

The issue we experienced:

For more than 30 minutes the Azure endpoint was Timeouting:

Since around Sep 13 07:00:00:
Sep 13 07:47:19 HOSTNAME.com telegraf[1305]: 2024-09-13T07:47:19Z E! [agent] Error writing to outputs.azure_monitor: Post "https://northeurope.monitoring.azure.com/subscriptions/CENSORED/metrics": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
Sep 13 07:48:19 HOSTNAME.com telegraf[1305]: 2024-09-13T07:48:19Z E! [agent] Error writing to outputs.azure_monitor: Post "https://northeurope.monitoring.azure.com/subscriptions/CENSORED/metrics": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
Sep 13 07:49:19 HOSTNAME.com telegraf[1305]: 2024-09-13T07:49:19Z E! [agent] Error writing to outputs.azure_monitor: Post "https://northeurope.monitoring.azure.com/subscriptions/CENSORED/metrics": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
Sep 13 07:50:19 HOSTNAME.com telegraf[1305]: 2024-09-13T07:50:19Z E! [agent] Error writing to outputs.azure_monitor: Post "https://northeurope.monitoring.azure.com/subscriptions/CENSORED/metrics": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
Sep 13 07:51:09 HOSTNAME.com telegraf[1305]: 2024-09-13T07:51:09Z E! [agent] Error writing to outputs.azure_monitor: Post "https://northeurope.monitoring.azure.com/subscriptions/CENSORED/metrics": read tcp 172.CENSORED.CENSORED.CENSORED:43962->20.50.65.82:443: read: connection timed out

Then Azure endpoint became responsive again, but since it was down for more than 30 minutes, all outputs Telegraf has buffered are now rejected by Azure, resulting in indefinite loop of never sent metrics:

Sep 13 07:52:10 HOSTNAME.com telegraf[1305]: 2024-09-13T07:52:10Z E! [agent] Error writing to outputs.azure_monitor: failed to write batch: [400] 400 Bad Request: {"error":{"code":"BadRequest","message":"'time' should not be older than 30 minutes and not more than 4 minutes in the future\r\n"}}
Sep 13 07:53:09 HOSTNAME.com telegraf[1305]: 2024-09-13T07:53:09Z E! [agent] Error writing to outputs.azure_monitor: failed to write batch: [400] 400 Bad Request: {"error":{"code":"BadRequest","message":"'time' should not be older than 30 minutes and not more than 4 minutes in the future\r\n"}}
Sep 13 07:54:09 HOSTNAME.com telegraf[1305]: 2024-09-13T07:54:09Z E! [agent] Error writing to outputs.azure_monitor: failed to write batch: [400] 400 Bad Request: {"error":{"code":"BadRequest","message":"'time' should not be older than 30 minutes and not more than 4 minutes in the future\r\n"}}
Sep 13 07:55:09 HOSTNAME.com telegraf[1305]: 2024-09-13T07:55:09Z E! [agent] Error writing to outputs.azure_monitor: failed to write batch: [400] 400 Bad Request: {"error":{"code":"BadRequest","message":"'time' should not be older than 30 minutes and not more than 4 minutes in the future\r\n"}}

...Forever until Telegraf restarts

For now we will drastically reduce the buffer size limit to avoid this. But wee add/remove metrics to monitoring all the time, its not feasible to recalculate proper buffer limits (metric_batch_size and metric_buffer_limit) every time to achieve desired unsent metrics time limit before dropping them, so this solution doesnt seem very "clean" ?

One extra note on documentation, we had to take a look at the Telegraf source code comments to find out an important info that oldest metrics are overwritten, which is important for us to know that reducing the buffer size will help us in most cases:

	// MetricBufferLimit is the max number of metrics that each output plugin
	// will cache. The buffer is cleared when a successful write occurs. When
	// full, the oldest metrics will be overwritten. This number should be a
	// multiple of MetricBatchSize. Due to current implementation, this could
	// not be less than 2 times MetricBatchSize.

The official docs say just:

metric_buffer_limit: Maximum number of unwritten metrics per output. Increasing this value allows for longer periods of output downtime without dropping metrics at the cost of higher maximum memory usage.

Which is not enough to make an educated configuration decision, perhaps it could be made more verbose ?

Expected behavior

Reliably configure to drop unsent metrics by time instead of buffer sizes. We add/remove metrics to monitoring all the time, its not feasible to recalculate proper buffer limits (metric_batch_size and metric_buffer_limit) every time to achieve desired unsent metrics time limit before dropping them.

Alternatively, a force drop of old metrics could occur when
E! [agent] Error writing to outputs.azure_monitor: failed to write batch: [400] 400 Bad Request: {"error":{"code":"BadRequest","message":"'time' should not be older than 30 minutes and not more than 4 minutes in the future\r\n"}}

is detected in Azure output plugin, as a one plugin only solution.

Alternatively, perhaps a documentation update in Azure output plugin could be added and be enough, warning all users that aggressive buffer sizes should be used to avoid this issue. The rationale is that a global option to drop by time limit could be an overkill / not warranted enough, but that is for the Telegraf project to decide :)

Actual behavior

Azure metrics / possibly other output destinations will reject metrics older than certain time limit. During network/other issues resulting into outputs not being sent, Telegraf will buffer metrics older than that time limit indefinitely in a loop where metrics older than the time limit are scheduled to be sent (at the top of the stack) indefinitely.

Additional info

Similar to #13273

[Hr0bar]

Actually just found #14928 where it says:

Currently, when a Telegraf output metric queue fills, either due to incoming
metrics being too fast or various issues with writing to the output, new
metrics are dropped and never written to the output.

While in code comments we see this:

	// MetricBufferLimit is the max number of metrics that each output plugin
	// will cache. The buffer is cleared when a successful write occurs. When
	// full, the oldest metrics will be overwritten. This number should be a
	// multiple of MetricBatchSize. Due to current implementation, this could
	// not be less than 2 times MetricBatchSize.

Im not so sure anymore that the workaround with aggressive buffer limit will help now, since its not very clear whether old are dropped, or old kept and new dropped, in which case aggressive buffer limit wont help.

[srebhan]

The code comment is correct and the spec needs correction. Are you willing to put up a PR to fix the spec wording? I can confirm that actually the oldest metrics in the buffer are dropped (overwritten by new metrics) first.

[srebhan]

Regarding your issue, I'm preparing a new way of selectively dropping metrics on outputs and I want to hold this up until this is ready. Is that OK for you?

[Hr0bar]

Thanks, yes we are fine waiting for some more comprehensive solution. Ill have a look at PR for wording in the spec (and possibly also expanding the wording in "metric_buffer_limit" description in docs, perhaps even azure output plugin docs could use some mention of the Azure limit, some Azure users may not be aware).

[srebhan]

@Hr0bar would love to get your feedback on PR #16034!

[Hr0bar]

Well thats one way to solve it - by dropping all rejected metrics only, that would certainly work for this use case as well (rather than relying on time based drop).

But... I would assume this could be difficult to generalize for many output plugins, as I imagine some external systems (in this case Azure) reject whole batch, some provide info what specifically they rejected and why, some dont etc.

No idea how Azure monitor output behaves for example... is it sent one by one metric? or full batch? What happens when only one metric in full batch is rejected ? Do we drop whole batch (dont think so)? How do we know which one caused the reject by service error if its only one out of many?

[srebhan]

Did you check the spec? It extends the current batch handling to be able to do exactly this, drop specific metrics of a batch or drop the whole batch or do whatever the output requires. Currently, you can either accept a batch (removing it from the buffer) or keep the batch (requeueing the whole thing on next write)...