Opencensus.Absinthe

Extends Absinthe to automatically create OpenCensus spans. Designed to work with whatever is producing spans upstream, e.g. Opencensus.Plug.

Installation

• Take the dependency
• Set up the pipeline
• Set up the middleware
• Adjust your schema
• Check it's all working

Dependency

If you're using Absinthe.Plug, add opencensus_absinthe to your deps in mix.exs using a tighter version constraint than:

{:absinthe_plug, ">= 0.0.0"},
{:opencensus_absinthe, ">= 0.0.0"},

Pipeline

Add a :pipeline to your t:Absinthe.Plug.opts/0 to have it call Opencensus.Absinthe.Plug.traced_pipeline/2. If you're using Phoenix.Router.forward/4, for example:

forward(
  path,
  Absinthe.Plug,
  # ... existing config ...
  pipeline: {Opencensus.Absinthe.Plug, :traced_pipeline}
)

If you already have a pipeline, you can define your own and call both to insert their phases. To work with ApolloTracing, for example:

def your_custom_pipeline(config, pipeline_opts \\ []) do
  config
  |> Absinthe.Plug.default_pipeline(pipeline_opts)
  |> ApolloTracing.Pipeline.add_phases()
  |> Opencensus.Absinthe.add_phases()
end

Worst case, you'll need to copy the code from the current pipeline target and add a call to Opencensus.Absinthe.add_phases/1 as above.

Middleware

Your middleware callback needs to run its output through the matching function in Opencensus.Absinthe.Middleware to add the middleware to only the fields that need it:

def middleware(middleware, field, object) do
  Opencensus.Absinthe.middleware(middleware, field, object)
end

If you've already got some middleware, like above, you might need to copy some code around to get the job done:

def middleware(middleware, field, object) do
  ([ApolloTracing.Middleware.Tracing, ApolloTracing.Middleware.Caching] ++ middleware)
  |> Opencensus.Absinthe.middleware(field, object)
end

Schema

Until Absinthe merge and publish their telemetry support (see below) and you upgrade, you'll also need to set :trace in the metadata for any field for which you want tracing to happen:

  query do
    @desc "List all the things"
    field :things, list_of(:thing), meta: [trace: true] do
      resolve(&Resolvers.Account.all_things/2)
    end

Once you're on a telemetry-capable Absinthe, you'll get tracing for every field containing a resolve.

Verification

Check your installation with iex -S mix phx.server, assuming Phoenix, and:

iex> :oc_reporter.register(:oc_reporter_stdout)
:ok

Fire off a few requests and check the {span, <<NAME> lines on standard output.

• If you see names matching your GraphQL route, e.g. <</api>>, you set up opencensus_plug properly.

• If you see <<"Absinthe.Blueprint">>, the pipeline is working.

• If you see <<"YourProject.Schema:thefield">>, the middleware is working and you've either:

  • Added meta: [trace: true] to your field :thefield as above, or

  • Upgraded to a telemetry-capable Absinthe.

Behaviour

Each Absinthe query runs in the process of its caller. If you hook up opencensus_plug, or something else that'll take trace details off the wire, the process dictionary will have an :oc_span_ctx_key key used by opencensus to keep track of spans in flight.

This package adds new phases to your Absinthe Pipeline to start new spans for each resolution and call, using both methods available:

opencensus provides two methods for tracking [trace and span] context, the process dictionary and a variable holding a ctx record.

Specifically, this package:

• Starts a new span registered in the process dictionary for each query, and

• Without any use of the process dictionary, starts a new span for each field, using the query span as the parent.

The latter is necessary because the fields don't necessarily start and stop without overlap. Naïve use of :ocp.with_child_span and :ocp.finish_span will yield incorrect traces.

Development

Dependency management:

• mix deps.get to get your dependencies
• mix deps.compile to compile them
• mix licenses to check their license declarations, recursively

Finding problems:

• mix compile to compile your code
• mix credo to suggest more idiomatic style for it
• mix dialyzer to find problems static typing might spot... slowly
• mix test to run unit tests
• mix test.watch to run the tests again whenever you change something
• mix coveralls to check test coverage

Documentation:

• mix docs to generate documentation for this project
• mix help to find out what else you can do with mix

Next Steps

Obvious next steps include stronger tests and many minor tweaks:

• Rename the outer span according to the schema
• Set some attributes on the outer span
• Trim the path from references so it starts with the closest lib
• Set the span status on completion
• Retire lib/opencensus/absinthe/logger.ex when possible

The biggest looming change would be telemetry integration:

absinthe-graphql/absinthe#663 to add telemetry to Absinthe could give us start and stop calls from within the calling process suitable for calling :ocp.with_child_span and :ocp.finish_span to maintain the main trace. In turn, that'd mean we didn't need the pipeline.

#663 won't help us generate spans for fields, because there's no way to pass state back through :telemetry.execute. That said, it'll automatically set :absinthe_telemetry in the field metadata if query is present.

Rather than push back on the telemetry support to make it better support tracing, we could integrate this capability directly with Absinthe if:

• The community deploy a lot of opencensus
• It proves to be as lightweight and stable as telemetry
• Its impact when not hooked up is minimal or zero

We could then retire this module except for users with older versions.