---
title: APM Server performance diagnostic
description: When Elasticsearch is under excessive load or indexing pressure, APM Server could experience the downstream backpressure when indexing new documents into...
url: https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/4491/troubleshoot/observability/apm/apm-server-performance-diagnostic
products:
  - Elastic Observability
---

# APM Server performance diagnostic
## Diagnosing backpressure from Elasticsearch

When Elasticsearch is under excessive load or indexing pressure, APM Server could experience the downstream backpressure when indexing new documents into Elasticsearch. Most commonly, backpressure from Elasticsearch will manifest itself in the form of higher indexing latency and/or rejected requests, which in return could lead APM Server to deny incoming requests. As a result, APM agents connected to the affected APM Server will suffer from throttling and/or request timeout when shipping APM events.
To quickly identify possible issues try looking for similar error logs lines in APM Server logs:
```json
...
{"log.level":"error","@timestamp":"2024-07-27T23:46:28.529Z","log.origin":{"function":"github.com/elastic/go-docappender/v2.(*Appender).flush","file.name":"v2@v2.2.0/appender.go","file.line":370},"message":"bulk indexing request failed","service.name":"apm-server","error":{"message":"flush failed (429): [429 Too Many Requests]"},"ecs.version":"1.6.0"}
{"log.level":"error","@timestamp":"2024-07-27T23:55:38.612Z","log.origin":{"function":"github.com/elastic/go-docappender/v2.(*Appender).flush","file.name":"v2@v2.2.0/appender.go","file.line":370},"message":"bulk indexing request failed","service.name":"apm-server","error":{"message":"flush failed (503): [503 Service Unavailable]"},"ecs.version":"1.6.0"}
...
```

To gain better insight into APM Server health and performance, consider enabling the monitoring feature by following the steps in [Monitor APM Server](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/4491/solutions/observability/apm/apm-server/monitor). When enabled, APM Server will additionally report a set of vital metrics to help you identify any performance degradation.
Pay careful attention to the next metric fields:
- `beats_stats.metrics.libbeat.output.events.active` that represents the number of buffered pending documents waiting to be ingested; (*if this value is increasing rapidly it may indicate Elasticsearch backpressure*)
- `beats_stats.metrics.libbeat.output.events.acked` that represents the total number of documents that have been ingested successfully;
- `beats_stats.metrics.libbeat.output.events.failed` that represents the total number of documents that failed to ingest; (*if this value is increasing rapidly it may indicate Elasticsearch backpressure*)
- `beats_stats.metrics.libbeat.output.events.toomany` that represents the number of documents that failed to ingest due to Elasticsearch responding with 429 Too many Requests; (*if this value is increasing rapidly it may indicate Elasticsearch backpressure*)
- `beats_stats.output.elasticsearch.bulk_requests.available` that represents the number of bulk indexers available for making bulk index requests; (*if this value is equal to 0 it may indicate Elasticsearch backpressure*)
- `beats_stats.output.elasticsearch.bulk_requests.completed` that represents the number of already completed bulk requests;
- `beats_stats.metrics.output.elasticsearch.indexers.active` that represents the number of active bulk indexers that are concurrently processing batches;

See [Metricbeat documentation](https://docs-v3-preview.elastic.dev/elastic/beats/tree/main/reference/metricbeat/exported-fields-beat) for the full list of exported metric fields.
One likely cause of excessive indexing pressure or rejected requests is undersized Elasticsearch. To mitigate this, follow the guidance in [Rejected requests](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/4491/troubleshoot/elasticsearch/rejected-requests).
(Not recommended) If scaling Elasticsearch resources up is not an option, you can adjust the `flush_bytes`, `flush_interval`, `max_retries` and `timeout` settings described in [Configure the Elasticsearch output for APM Server](https://docs-v3-preview.elastic.dev/elastic/docs-content/pull/4491/solutions/observability/apm/apm-server/configure-elasticsearch-output) to reduce APM Server indexing pressure. However, consider that increasing number of buffered documents and/or reducing retries may lead to a higher rate of dropped APM events. Down below a custom configuration example is listed where the number of default buffered documents is roughly doubled while Elasticsearch indexing retries are decreased simultaneously. This configuration provides a generic example and might not be applicable to your situation. Try adjusting the settings further to see what works for you.
```yaml
output.elasticsearch:
  flush_bytes: "2MB"
  flush_interval: "2s"
  max_retries: 1
  timeout: 60
```