opentelemetry-erlang-contrib/instrumentation/opentelemetry_elli/README.md

# opentelemetry_elli

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Elli middleware for OpenTelemetry instrumentation.

## Setup and Configuration

``` erlang
{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:

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



OpenTelemetry's [HTTP Semantic Conventions](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/data-http.md#http-server) 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:

``` erlang
{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](https://w3c.github.io/trace-context/) and [B3](https://github.com/openzipkin/b3-propagation) 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](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/data-http.md#name) 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:

``` erlang
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](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/data-http.md).

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`:

``` erlang

{opentelemetry_elli, [{excluded_urls, ["/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:

``` erlang
$ rebar3 ct
```