SpandexDatadog

A datadog adapter for the spandex library.

Installation

The package can be installed by adding spandex_datadog to your list of dependencies in mix.exs:

def deps do
  [
    {:spandex_datadog, "~> 0.5.0"}
  ]
end

To start the datadog adapter, add a worker to your application's supervisor

# Example configuration
opts =
  [
    host: System.get_env("DATADOG_HOST") || "localhost",
    port: System.get_env("DATADOG_PORT") || 8126,
    batch_size: System.get_env("SPANDEX_BATCH_SIZE") || 10,
    sync_threshold: System.get_env("SPANDEX_SYNC_THRESHOLD") || 100,
    http: HTTPoison
  ]

# in your supervision tree

worker(SpandexDatadog.ApiServer, [opts])

Distributed Tracing

Distributed tracing is supported via headers x-datadog-trace-id, x-datadog-parent-id, and x-datadog-sampling-priority. If they are set, the Spandex.Plug.StartTrace plug will act accordingly, continuing that trace and span instead of starting a new one. Both x-datadog-trace-id and x-datadog-parent-id must be set for distributed tracing to work. You can learn more about the behavior of x-datadog-sampling-priority in the Datadog priority sampling documentation.

API Sender Performance

Originally, the library had an API server and spans were sent via GenServer.cast, but we've seen the need to introduce backpressure, and limit the overall amount of requests made. As such, the Datadog API sender accepts batch_size and sync_threshold options.

Batch size refers to traces, not spans, so if you send a large amount of spans per trace, then you probably want to keep that number low. If you send only a few spans, then you could set it significantly higher.

Sync threshold is how many simultaneous HTTP pushes will be going to Datadog before it blocks/throttles your application by making the tracing call synchronous instead of async.

Ideally, the sync threshold would be set to a point that you wouldn't reasonably reach often, but that is low enough to not cause systemic performance issues if you don't apply backpressure.

A simple way to think about it is that if you are seeing 1000 request per second and batch_size is 10, then you'll be making 100 requests per second to Datadog (probably a bad config). With sync_threshold set to 10, only 10 of those requests can be processed concurrently before trace calls become synchronous.

This concept of backpressure is very important, and strategies for switching to synchronous operation are often surprisingly far more performant than purely asynchronous strategies (and much more predictable).