What happens when APM Server or Elasticsearch is down?

If Elasticsearch is down

APM Server does not have an internal queue to buffer requests, but instead leverages an HTTP request timeout to act as back-pressure. If Elasticsearch goes down, the APM Server will eventually deny incoming requests. Both the APM Server and APM agent(s) will issue logs accordingly.

If APM Server is down

Some agents have internal queues or buffers that will temporarily store data if the APM Server goes down. As a general rule of thumb, queues fill up quickly. Assume data will be lost if APM Server goes down. Adjusting these queues/buffers can increase the agent’s overhead, so use caution when updating default values.