Erlang stats collection and distributed tracing framework

Using with Rebar3 project

Add as dependency to rebar.config:

{deps, [opencensus]}.

Or to use the latest from git master branch:

{deps, [{opencensus, {git, "https://github.com/census-instrumentation/opencensus-erlang.git", {branch, "master"}}}]}.

Creating Spans

Parse Transform

The parse transform provides an attribute to decorate functions with that will start a span, wrap the contents in a try and finish the span in an after clause. Add the parse transform to the compile opts in rebar.config:

{erl_opts, [{parse_transform, oc_transform}]}.

And use:

-trace([]).
function_to_trace() ->
  ... body ...

Process Dictionary

some_fun() ->
  ocp:start_span(<<"some_fun/0">>),
  try
    ... body ...
  after
    ocp:finish_span()
  end.

Manual

Span’s can be tracked manually in a variable:

some_fun() ->
  Span0 = opencensus:start_span(<<"some_fun/0">>, opencensus:generate_trace_id(), undefined),
  try
    ... body ...
  after
    opencensus:finish_span(Span0)
  end.

Samplers

• oc_sampler_never: Never enable a new trace, but keeps a trace enabled if its propagated context is enabled.
• oc_sampler_always: Enable every new trace for sampling.
• oc_sampler_probability: Takes a probability, default 0.5, that any new trace will be sampled.

Reporters

• Google Cloud Trace: Support for v1 in master, v2 and grpc coming soon;
• Prometheus: Exports spans as Prometheus metrics.

Working with Spans

Attributes

A span has a map of attributes providing details about the span. The key is a binary string and the value of the attribute can be a binary string, integer, or boolean.

Span1 = opencensus:put_attribute(<<"/instance_id">>, <<"my-instance">>, Span),

Time Events

A time event is a timestamped annotation with user-supplied key-value pairs or a message event to represent a message (not specificly an Erlang message) sent to or received from another span.

The message_event consists of a type, identifier and size of the message. Id is an identifier for the event’s message that can be used to match SENT and RECEIVEDmessage_events. For example, this field could represent a sequence ID for a streaming RPC. It is recommended to be unique within a Span. If CompressedSize is 0 it is assumed to be the same as UncompressedSize.

Event = opencensus:message_event(?MESSAGE_EVENT_TYPE_SENT, Id, UncompressedSize, CompressedSize)
Span1 = opencensus:add_time_event(Event, Span),

Links

Links are useful in cases like a job queue. A job is created with a span context and when run wants to report a new span. The job isn’t a direct child of the span that inserted it into the queue, but it is related. The job creates a link to the span that created it.

Span = opencensus:start_span(<<"running job">>, TraceId),
Link = link(?LINK_TYPE_PARENT_LINKED_SPAN, TraceId, ParentSpanId, #{}),
Span1 = opencensus:add_link(Link, Span),
... run job ...
opencensus:finish_span(Span1).

Stats

Development

$ rebar3 compile

Running tests:

$ rebar3 ct

Updating OpenCensus standard protobuf encoder and decoder

Language independent interface types for Census are found in the opencensus-proto repo. The opencensus Erlang app provides functionality for converting from the apps internal representation to the standard protobuf interface. Below are the steps to update the Erlang module and header for encoding and decoding the protobufs:

$ git clone https://github.com/census-instrumentation/opencensus-proto priv/opencensus-proto
$ rebar3 protobuf compile