Return 429 response on HTTP max connection limit. Instead of silently closing
the connection, return a `429 Too Many Requests` HTTP response with a helpful
error message to aid debugging when the connection limit is unintentionally
reached.
Set a 10-millisecond write timeout and rate limiter for connection-limit 429
response to prevent writing the HTTP response from consuming too many server
resources.
Add `nomad.agent.http.exceeded metric` counting the number of HTTP connections
exceeding concurrency limit.
* docs: tighten up parameterized job metrics docs
* docs: improve alloc status descriptions
Remove `nomad.client.allocations.start` as it doesn't exist.
Related to #13740
- blocked_evals.total_blocked is the number of evals blocked for *any*
reason
- blocked_evals.total_quota_limit is the number of evals blocked by
quota limits, but critically: their resources are *not* counted in the
cpu/memory
Plan rejections occur when the scheduler work and the leader plan
applier disagree on the feasibility of a plan. This may happen for valid
reasons: since Nomad does parallel scheduling, it is expected that
different workers will have a different state when computing placements.
As the final plan reaches the leader plan applier, it may no longer be
valid due to a concurrent scheduling taking up intended resources. In
these situations the plan applier will notify the worker that the plan
was rejected and that they should refresh their state before trying
again.
In some rare and unexpected circumstances it has been observed that
workers will repeatedly submit the same plan, even if they are always
rejected.
While the root cause is still unknown this mitigation has been put in
place. The plan applier will now track the history of plan rejections
per client and include in the plan result a list of node IDs that should
be set as ineligible if the number of rejections in a given time window
crosses a certain threshold. The window size and threshold value can be
adjusted in the server configuration.
To avoid marking several nodes as ineligible at one, the operation is rate
limited to 5 nodes every 30min, with an initial burst of 10 operations.
- Moved federation docs to the bottom since *everyone* is potentially
affected by the other sections on the page, but only users of
federation are affected by it.
- Added section on the plan for node rejected bug since it is fairly
easy to diagnose and removing affected nodes is a fairly reliable
workaround.
- Mention 5s cliff for wait_for_index.
- Remove the lie that we do not have job status metrics! How old was
that?!
- Reinforce the importance of monitoring basic system resources
* Fixed name of `nomad.scheduler.allocs.reschedule` metric
* Added new metrics to metrics reference documentation
* Expanded definitions of "waiting" metrics
* Changelog entry for #10236 and #10237
This changeset adds more specific recommendations as to what metrics
to monitor, and what resources should be examined during incident
response.
It also renames the "Telemetry" section to "Monitoring Nomad" to
surface the material better and distinguish it from the "Metric
Reference".
Co-authored-by: Charlie Voiselle <464492+angrycub@users.noreply.github.com>
Old description of `{plan,worker}.wait_for_index` described the metric
in terms of waiting for a snapshot which has two problems:
1. "Snapshot" is an overloaded term in Nomad and operators can't be
expected to know which use we're referring to here.
2. The most important thing about the metric is what we're waiting *on*
before taking a snapshot: the raft index of the object to be
processed (plan or eval).
The new description tries to cram all of that context into the tiny
space provided.
See #5791 for details about the `wait_for_index` mechanism in general.
