[meta] Tail-based sampling (TBS) improvements

[carsonip]

This is a meta-issue on tail-based sampling.

Tail-based sampling comes up frequently in bug reports, as there is minimal documentation and guidance on TBS configuration. It is not clear to users how TBS works, which leads to misconfigured TBS storage size, and consequently apm-server and ES issues.

When TBS local storage (badger) is filled, it results in error in writing traces (where apm-server logs received error writing sampled trace: configured storage limit reached (current: 127210377485, limit: 126000000000)) and bypassing TBS as sampling rate jumps to 100%, causing a performance cliff and downstream effects: surprising significant increase on writes to ES, and either slowing ES and causing backpressure to apm-server, or unexpected high storage usage in ES.

The task list contains tasks to either document it properly, investigate/fix bugs, and to provide escape hatches for compromises.

Impact: TBS is a popular feature among heavy apm-server users who rely on TBS to reduce ES storage requirements while retaining the value of the sampled traces. We need to ensure and show that TBS is good for high load, like the rest of apm-server.

Tasks

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1. Benchmark and document tail-based sampling performance #11346
   docs performance +2
2. Configurable option to handle events failed to be processed by TBS #11127
   enhancement +1
3. Expose TBS TTL config via integration policy #13525
   enhancement +1
4. TBS: Investigate cases where apm-server keeps exceeding storage limit and lsm size >> vlog size #14923
   bug +1
   
5. Update badger to latest version #11546
   enhancement +1
   
6. Revisit default TBS storage size limit sampling.tail.storage_size #14933
   9.0-candidate enhancement +2
7. TBS: Document monitoring of disk space used by Tail Based Sampling in public docs. #14996
   enhancement +1

[lucabelluccini]

• [ESS/ECE only] Ability to see the disk size on Integration servers on the fly (even better, the available live disk usage) in Admin Console https://github.com/elastic/cloud/issues/128879
  • Mitigation until then: guide users to know what is the disk size via documentation pointers
• [ESS priority] Ability to automatically set the TBS max disk usage in the Integration policy as percentage of the whole disk OR set it automatically to a sane max value and freeze it (so the customer cannot exceed the maximum)
• [ESS/ECE and on-premise] Ability to monitor TBS disk-related metrics on self-hosted APM Servers, Integration Servers and Integration Servers in ESS via at least a Dashboard (likely not possible to add new graphs to Stack Monitoring). The dashboards could be shipped with the apm input package or with the Elastic Agent
  • [ALL], make sure the necessary metrics are shipped monitoring: apm-server not shiping all of its monitoring metrics #14247 and available for search & aggregations
  • [ESS/ECE only], the prerequisite is to enable Metrics shipping via L&M on the deployment. This has to be documented.
  • [On-premise], the prerequisite is to put in place a dedicated Metricbeat to monitor the Integration Server, which is odd
    • It would be great to have this integrated with the monitoring of all the other components via the EA Monitoring collection instead of relying on an external Metricbeat. I do not get why we are able to collect metrics from Filebeat, Metricbeat and other components, but not APM Server.
      • An alternative, might be to develop a beats integration able to collect monitoring data reusing the Metricbeat module https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-module-beat.html
    • [DOCS] The instructions we give here are no more necessary. Since 8.15 it is possible to customize the Elastic Agent policy to set agent.monitoring.http.enabled to true (but not for the Elastic Cloud agent policy) via the dedicated Monitoring settings. See screenshot. Opened issue at [DOCS] Better / improved instructions for APM Integration monitoring for on-premise #14707
      
• [ALL] Once the metrics are shipped, it would be nice to provide an out-of-the-box alert if disk is getting full due to TBS or hit the soft limit in order to be aware when APM Server will let through all the transactions.