= Request Tracing

Gerrit supports on-demand tracing of single requests that results in
additional logs with debug information that are written to the
`error_log`. The logs that correspond to a traced request are
associated with a unique trace ID. This trace ID is returned with the
response and can be used by an administrator to find the matching log
entries.

How tracing is enabled and how the trace ID is returned depends on the
request type:

* REST API: For REST calls tracing can be enabled by setting the
  `trace` request parameter, the trace ID is returned as
  `X-Gerrit-Trace` header. More information about this can be found in
  the link:rest-api.html#tracing[Request Tracing] section of the
  link:rest-api.html[REST API documentation].
* SSH API: For SSH calls tracing can be enabled by setting the
  `--trace` option. More information about this can be found in
  the link:cmd-index.html#trace[Trace] section of the
  link:cmd-index.html[SSH command documentation].
* Git: For Git pushes tracing can be enabled by setting the
  `trace` push option, the trace ID is returned in the command output.
  More information about this can be found in
  the link:user-upload.html#trace[Trace] section of the
  link:user-upload.html[upload documentation]. Tracing for Git requests
  other than Git push is not supported.

Since tracing consumes additional server resources tracing should only
be enabled for single requests if there is a concrete need for
debugging. In particular bots should never enable tracing for all their
requests by default.

== Find log entries for a trace ID

If tracing is enabled all log messages that correspond to the traced
request have a `TRACE_ID` tag set, e.g.:

----
[2018-08-13 15:28:08,913] [HTTP-76] TRACE com.google.gerrit.httpd.restapi.RestApiServlet : Received REST request: GET /a/accounts/self (parameters: [trace]) [CONTEXT forced=true TRACE_ID="1534166888910-3985dfba" ]
[2018-08-13 15:28:08,914] [HTTP-76] TRACE com.google.gerrit.httpd.restapi.RestApiServlet : Calling user: admin [CONTEXT forced=true TRACE_ID="1534166888910-3985dfba" ]
[2018-08-13 15:28:08,942] [HTTP-76] TRACE com.google.gerrit.httpd.restapi.RestApiServlet : REST call succeeded: 200 [CONTEXT forced=true TRACE_ID="1534166888910-3985dfba" ]
----

By doing a grep with the trace ID over the error log the log entries
that correspond to the request can be found.