# Tracing

Lotus has tracing built into many of its internals. To view the traces, first download [jaeger](https://www.jaegertracing.io/download/) (Choose the 'all-in-one' binary). Then run it somewhere, start up the lotus daemon, and open up localhost:16686 in your browser.

## Open Census

Lotus uses [OpenCensus](https://opencensus.io/) for tracing application flow.
This generates spans
through the execution of annotated code paths.

Currently it is set up to use jaeger, though other tracing backends should be
fairly easy to swap in.

## Running Locally

To easily run and view tracing locally, first, install jaeger. The easiest way
to do this is to [download the binaries](https://www.jaegertracing.io/download/) and then run the `jaeger-all-in-one`
binary. This will start up jaeger, listen for spans on `localhost:6831`, and
expose a web UI for viewing traces on `http://localhost:16686/`.

Now, to start sending traces from lotus to jaeger, set the environment variable
`LOTUS_JAEGER` to `localhost:6831`, and start the `lotus daemon`.

Now, to view any generated traces, open up `http://localhost:16686/` in your
browser.

## Adding Spans

To annotate a new codepath with spans, add the following lines to the top of the function you wish to trace:

```go
ctx, span := trace.StartSpan(ctx, "put function name here")
defer span.End()
```