# Tracing

This document describes how the event tracing subsystem works in tor so
developers can add events to the code base but also hook them to an event
tracing framework.

## Basics

Event tracing is separated in two concepts, trace events and a tracer. The
tracing subsystem can be found in `src/trace`. The `events.h` header file is
the main file that maps the different tracers to trace events.

### Events

A trace event is basically a function from which we can pass any data that
we want to collect. In addition, we specify a context for the event such as
a subsystem and an event name.

A trace event in tor has the following standard format:

	tor_trace(subsystem, event\_name, args...)

The `subsystem` parameter is the name of the subsytem the trace event is in.
For example that could be "scheduler" or "vote" or "hs". The idea is to add
some context to the event so when we collect them we know where it's coming
from. The `event_name` is the name of the event which helps a lot with
adding some semantic to the event. Finally, `args` is any number of
arguments we want to collect.

Here is an example of a possible tracepoint in main():

	tor_trace(main, init_phase, argc)

The above is a tracepoint in the `main` subsystem with `init_phase` as the
event name and the `int argc` is passed to the event as well.

How `argc` is collected or used has nothing to do with the instrumentation
(adding trace events to the code). It is the work of the tracer so this is why
the trace events and collection framework (tracer) are decoupled. You _can_
have trace events without a tracer.

### Tracer

In `src/trace/events.h`, we map the `tor_trace()` function to the right
tracer. A tracer support is only enabled at compile time. For instance, the
file `src/trace/debug.h` contains the mapping of the generic tracing function
`tor_trace()` to the `log_debug()` function. More specialized function can be
mapped depending on the tracepoint.

## Build System

This section describes how it is integrated into the build system of tor.

By default, every tracing events are disabled in tor that is `tor_trace()`
is a NOP.

To enable a tracer, there is a configure option on the form of:

	--enable-tracing-<tracer>

We have an option that will send every trace events to a `log_debug()` (as
mentionned above) which will print you the subsystem and name of the event but
not the arguments for technical reasons. This is useful if you want to quickly
see if your trace event is being hit or well written. To do so, use this
configure option:

	--enable-tracing-debug

## Instrument Tor

This is pretty easy. Let's say you want to add a trace event in
`src/feature/rend/rendcache.c`, you only have to add this include statement:

	#include "trace/events.h"

Once done, you can add as many as you want `tor_trace()` that you need.
Please use the right subsystem (here it would be `hs`) and a unique name that
tells what the event is for. For example:

	tor_trace(hs, store_desc_as_client, desc, desc_id);

If you look in `src/trace/events.h`, you'll see that if tracing is enabled it
will be mapped to a function called:

	tor_trace_hs_store_desc_as_client(desc, desc_id)

And the point of all this is for that function to be defined in a new file
that you might want to add named `src/trace/hs.{c|h}` which would defined how
to collect the data for the `tor_trace_hs_store_desc_as_client()` function
like for instance sending it to a `log_debug()` or do more complex operations
or use a userspace tracer like LTTng (https://lttng.org).
