# 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`:

```elixir
def deps do
  [
    {:spandex_datadog, "~> 0.1.0"}
  ]
end
```

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

```elixir
# 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

Individual adapters can support distributed tracing. See their documentation for more information.

Distributed tracing is supported via headers `x-datadog-trace-id` and `x-datadog-parent-id`. If they are set, the `StartTrace` plug will act accordingly, continuing that trace and span instead of starting a new one. *Both* must be set for distributed tracing to work.

## 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 refers to the *number of processes concurrently sending spans*. *NOT* the number of traces queued up waiting to be sent. It is used to apply backpressure while still taking advantage of parallelism. 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 your batch size is 10, then you'll be making 100 requests per second to datadog(probably a bad config). But if your sync_threshold is set to 10, you'll almost certainly exceed that because 100 requests in 1 second will likely overlap in that way. So when that is exceeded, the work is done synchronously, (not waiting for the asynchronous ones to complete even). 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).