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= cowboy_tracer_h(3)

== Name

cowboy_tracer_h - Tracer stream handler

== Description

The module `cowboy_tracer_h` can be used to conditionally
trace streams based on information found in the request.
Trace messages are given to the configured callback.

== Options

[source,erlang]
----
opts() :: #{
    tracer_callback    => Callback,
    tracer_flags       => [atom()],
    tracer_match_specs => [MatchSpec]
}

Callback :: fun((init | terminate | tuple(), State) -> State)

MatchSpec :: MatchPredicate
           | {method, binary()}
           | {host, binary()}
           | {path, binary()}
           | {path_start, binary()}
           | {header, binary()}
           | {header, binary(), binary()}
           | {peer_ip, inet:ip_address()}

MatchPredicate :: fun((cowboy_stream:streamid(),
                       cowboy_req:req(),
                       cowboy:opts()) -> boolean())
}
----

Configuration for the tracer stream handler.

This module will not set trace patterns. Those must be
set by the user directly, either from the callback's
`init` or, preferably, in advance.

tracer_callback::

The function that will be called for each trace
events. It will also be called before any trace
event with an argument `init`, and when the
stream is terminated with an argument `terminate`.
+
This option is required for tracing to be enabled.
The tracer stream handler does nothing otherwise.

tracer_flags::

Trace flags to enable. See the documentation
of `erlang:trace/3` for details. Note that all
trace flags are allowed except for the `tracer`
flag.

tracer_match_specs::

A list of match conditions that must all be
fulfilled for the stream to be traced. Cowboy
will compare these with the information found
in the request and only enable tracing if all
matches succeed.
+
This option is required for tracing to be enabled.
The tracer stream handler does nothing otherwise.

== Events

The tracer stream handler does not produce any event.

== Changelog

* *2.7*: Module introduced.

== See also

link:man:cowboy(7)[cowboy(7)],
link:man:cowboy_stream(3)[cowboy_stream(3)],
link:man:cowboy_compress_h(3)[cowboy_compress_h(3)],
link:man:cowboy_metrics_h(3)[cowboy_metrics_h(3)],
link:man:cowboy_stream_h(3)[cowboy_stream_h(3)]