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opentelemetry_elli

Elli middleware for OpenTelemetry instrumentation.

Setup and Configuration

{deps, [opentelemetry_elli]}.

While using the elli_middleware callback, place oc_elli_middelware as the first module to be called in the list of handlers:

[{callback, elli_middleware},
 {callback_args, [{mods, [{otel_elli_middleware, []},
                          {<YOUR HANDLER>, []}]}]}]

OpenTelemetry's HTTP Semantic Conventions for a server details the attributes added to a Span automatically by using this middleware. One such attribute must be set in your sys.config under the opentelemetry_elli application like:

{opentelemetry_elli, [{server_name, <<"my-http-server">>}]}.

It is strongly recommended to set this environment variable so the attribute can be included:

http.server_name has shown great value in practice, as bogus HTTP Host headers occur often in the wild. It is strongly recommended to set http.server_name to allow associating requests with some logical server entity.

Use

Including the Middleware and setting Span names

The middleware takes care of extracting the parent Span from the requests headers, both the W3C and B3 formats are supported.

Because Elli has no router, there is no way to get a very descriptive Span name automatically. See the OpenTelemetry docs Semantic conventions for HTTP spans for why you don't want to set the Span name to the raw path of the request. Thus, the Span has the name "HTTP {METHOD_NAME}".

The macro update_name from opentelemetry_api/include/otel_tracer.hrl allows you to update the name of the Span after it has started:

handle(Req, Args) ->
    handle(Req#req.path, Req, Args).

handle([<<"hello">>, Who], Req, _Args) ->
    ?update_name(<<"/hello/{who}">>),
    {ok, [], <<"Hello ", Who/binary>>}.

Attributes set by the middleware can be found in the OpenTelemetry docs Semantic conventions for HTTP spans.

Updating the Span name is not required. Another option is to create a child Span with a more descriptive name, whose parent will be the Span named "HTTP {METHOD_NAME}".

Excluding Paths

Not all paths are created equal. It is likely you don't want to Trace a health check endpoint or /metrics if using Prometheus to scrape metrics. This library offers two ways to exclude requests from potentially creating Spans by filtering on the raw path in the URL.

An application environment variable is read in on Elli start, so the following can be added to sys.config to exclude the URLs /health and /metrics:


{opentelemetry_elli, [{excluded_paths, ["/health", "/metrics"]}]}

An OS environment variable, OTEL_ELLI_EXCLUDED_URLS, is also read and is a comma separated list of paths. Note that the URLs in the variable are split with a simple string:split call, so commas are not support in any URL. No escaping or quoting are recognized, if a comma in a URL's path is required it should be percent encoded as %2C, this is how the URL it is compared against will appear in Elli anyway.

The lists from both are merged.

Testing

A Common Test suite which starts an Elli server and tests the exported Spans created by a test handle is found under test/. Run with:

$ rebar3 ct

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