# servant-tracing
[](https://travis-ci.org/ChrisCoffey/haskell-opentracing-light)
This repository is the minimum required for publishing trace data to Zipkin or Jaeger. It adheres to the [Open Tracing Standard] (https://github.com/opentracing/specification/blob/master/specification.md) but is not complete. See the documentation on Hackage for module-level details.
### Using the library
The OpenTracing standard revolves around a single function, `recordSpan`. `recordSpan` is responsible for creating new spans (see the standard for the definition of a span) and ensuring child spans use the new id. In order to properly build this tree of calls library users must provide the necessary environment via a `MonadTracer` instance (see haddocks). Library users are responsible for defining their own publish loop. There is a default `Zipkin` publisher in `Tracing.Zipkin` which works with Jaeger & Zipkin, but the loop to drain the `spanBuffer` must be provided by the user.
```
foo :: (MonadIO m, MonadTracer m) =>
Int
-> m String
foo str = recordSpan
Nothing
[Tag "Ultimate Answer to Life, The Universe and Everything", Tag 42]
"Compute Ultimate Question"
$ pure "Oops"
```
The code above logs a new span to the `spanBuffer`, where it will sit until published. If it turns out that `foo` is called from an active span, then it will be recorded as a child of said higher span.
### Testing Locally with the Demo App
You can start up a compatible server for (Zipkin)[] or (Jaeger)[] via a standalone docker container. From there its a matter of seting the following environment variables:
- *TRACING_ENDPOINT*: a `String` with the fully url to a running tracing server
- *TRACING_SERVICE*: a `String` name for your service
#### Pending Features
- Concurrent tracing support.
- Thrift support
- Additional clients
- Pluggable samplers