This reporter acts as subscription handler such that other services (e.g. http server) can fetch metrics from it.

Add exometer_fetch to your list of dependencies in rebar.config:

{deps, [
    {exometer_fetch, ".*", {git, "https://github.com/travelping/exometer_fetch.git", "master"}}
]}.

Ensure exometer_fetch is started before your application:

{applications, [exometer_fetch]}.

Configure it:

{exometer,
    {reporters, [
        {exometer_report_fetch, [
            {autosubscribe, true},
            {subscriptions_module, exometer_fetch_subscribe_mod}
        ]}
    ]}
}.

It is possible to create a subscription automatically for each newly created metric entry. By default this is disabled. You can enable it in the reporter options. You must also provide a callback module which handles the entries. Apart from that there are no further options.

The callback module may look like:

-module(exometer_fetch_subscribe_mod).
-export([subscribe/2]).

subscribe([test, metric] = Metric, histogram) ->
    {Metric, [max, min], [{key, <<"some_key">>}]};
subscribe(_, _) -> [].

subscribe/2 calls for each new entry and it should return a (possibly empty) list or just one subscription. Here a single subscription has the following layout:

{exometer_report:metric(), exometer_report:datapoints(), exometer_report:extra()}

exometer_report:subscribe(exometer_report_fetch, [erlang, memory], total, manual, [{key, <<"some_key">>}]).

Check if everything is working:

exometer_fetch:fetch(<<"some_key">>).

Further it is possible to return only the value of a specific datapoint:

exometer_fetch:fetch(<<"some_key">>, total).

Keys always need to be given as binary. If a key is not found then all metrics which have this key as prefix will be returned (or an error if nothing is found).

The report interval in subscriptions should be set to manual such that the metric is actually never reported using time interval triggers. The metric value will be retrieved from exometer directly when one sends a request to the corresponding key.