Tutorial — et v1.7.3

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# Tutorial

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## Visualizing Message Sequence Charts

The easiest way of using `ET`, is to just use it as a graphical tool for displaying message sequence charts. In order to do that you need to first start a `Viewer` (which by default starts a `Collector`):

          {ok, ViewerPid} = et_viewer:start([{title,"Coffee Order"}]),
          CollectorPid = et_viewer:get_collector_pid(ViewerPid).

Then you send events to the `Collector` with the function `et_collector:report_event/6` like this:

          et_collector:report_event(CollectorPid,85,from,to,message,extra_stuff).

The `Viewer` will automatically pull events from the `Collector` and display them on the screen.

The number (in this case 85) is an integer from 1 to 100 that specifies the "detail level" of the message. The higher the number, the more important it is. This provides a crude form of priority filtering.

The `from`, `to`, and `message` parameters are exactly what they sound like. `from` and `to` are visualized in the `Viewer` as "lifelines", with the message passing from one to the other. If `from` and `to` are the same value, then it is displayed next to the lifeline as an "action". The `extra_stuff`value is simply data that you can attach that will be displayed when someone actually clicks on the action or message in the `Viewer` window.

The module `et/examples/et_display_demo.erl` illustrates how it can be used:

    -module(et_display_demo).

    -export([test/0]).

    test() ->
        {ok, Viewer} = et_viewer:start([{title,"Coffee Order"}, {max_actors,10}]),
        Drink = {drink,iced_chai_latte},
        Size = {size,grande},
        Milk = {milk,whole},
        Flavor = {flavor,vanilla},
        C = et_viewer:get_collector_pid(Viewer),
        et_collector:report_event(C,99,customer,barrista1,place_order,[Drink,Size,Milk,Flavor]),
        et_collector:report_event(C,80,barrista1,register,enter_order,[Drink,Size,Flavor]),
        et_collector:report_event(C,80,register,barrista1,give_total,"$5"),
        et_collector:report_event(C,80,barrista1,barrista1,get_cup,[Drink,Size]),
        et_collector:report_event(C,80,barrista1,barrista2,give_cup,[]),
        et_collector:report_event(C,90,barrista1,customer,request_money,"$5"),
        et_collector:report_event(C,90,customer,barrista1,pay_money,"$5"),
        et_collector:report_event(C,80,barrista2,barrista2,get_chai_mix,[]),
        et_collector:report_event(C,80,barrista2,barrista2,add_flavor,[Flavor]),
        et_collector:report_event(C,80,barrista2,barrista2,add_milk,[Milk]),
        et_collector:report_event(C,80,barrista2,barrista2,add_ice,[]),
        et_collector:report_event(C,80,barrista2,barrista2,swirl,[]),
        et_collector:report_event(C,80,barrista2,customer,give_tasty_beverage,[Drink,Size]),
        ok.

When you run the `et_display_demo:test().` function in the example above, the `Viewer` window will look like this:

## Four Modules

The event tracer framework is made up of four modules:

- `et`
- `et_collector`
- `et_viewer`
- `et_selector`

In addition, you'll probably want to familiarize yourself with the `dbg` module and possibly `seq_trace` module as well.

## The Event Tracer Interface

The `et` module is not like other modules. It contains a function called `et:trace_me/5`. Which is a function that does not do any useful stuff at all. Its sole purpose is to be a function that is easy to trace. A call to it may be something like:

          et:trace_me(85,from,to,message,extra_stuff).

The parameters to `et:trace_me/5` are the same as to `et_collector:report_event/6` in the previous chapter. The big difference between the two is in the semantics of the two functions. The second actually reports an `Event` to the `Collector` while the first does nothing, it just returns the atom `hopefully_traced`. In order to make the parameters to `et:trace_me/5` turn up in the `Collector`, tracing of that function must be activated and the `Collector` must be registered as a `Tracer` of the `Raw Trace Data`.

Erlang tracing is a seething pile of pain that involves reasonably complex knowledge of clever ports, tracing return formats, and specialized tracing `MatchSpecs` (which are really their own special kind of hell). The tracing mechanism is very powerful indeed, but it can be hard to grasp.

Luckily there is a simplified way to start tracing of `et:trace_me/5` function calls. The idea is that you should instrument your code with calls to `et:trace_me/5` in strategic places where you have interesting information available in your program. Then you just start the `Collector` with global tracing enabled:

          et_viewer:start([{trace_global, true}, {trace_pattern, {et,max}}]).

This will start a `Collector`, a `Viewer` and also start the tracing of `et:trace_me/5` function calls. The `Raw Trace Data` is collected by the `Collector` and a view of it is displayed on the screen by the `Viewer`. You can define your own "views" of the data by implementing your own `Filter` functions and register them in the `Viewer`.

## The Collector and Viewer

These two pieces work in concert. Basically, the `Collector` receives `Raw Trace Data` and processes it into `Events` in a `et` specific format (defined in `et/include/et.hrl`). The `Viewer` interrogates the `Collector` and displays an interactive representation of the data.

You might wonder why these aren't just one module. The `Collector` is a generic full-fledged framework that allows processes to "subscribe" to the `Events` that it collects. One `Collector` can serve several `Viewers`. The typical case is that you have one `Viewer` that visualizes `Events` in one flavor and another `Viewer` that visualizes them in another flavor. If you for example are tracing a text based protocol like `HTML` (or `Megaco/H.248`) it would be useful to be able to display the `Events` as plain text as well as the internal representation of the message. The architecture does also allow you to implement your own `Viewer` program as long as it complies to the protocol between the `Collector/Viewer` protocol. Currently two kinds of `Viewers` exists. That is the old `GS` based one and the new based on `wxWidgets`. But if you feel for it you may implement your own `Viewer`, which for example could display the `Events` as ASCII art or whatever you feel useful.

The `Viewer` will by default create a `Collector` for you. With a few options and some configuration settings you can start collecting `Events`.

The `Collector` API does also allow you to save the collected `Events` to file and later load them in a later session.

## The Selector

This is perhaps the most central module in the entirety of the `et` suite. The `Collector` needs "filters" to convert the `Raw Trace Data` into "events" that it can display. The `et_selector` module provides the default `Filter` and some API calls to manage the `Trace Pattern`. The `Selector` provides various functions that achieve the following:

- Convert `Raw Trace Data` into an appropriate `Event`
- Magically notice traces of the `et:trace_me/5` function and make appropriate `Events`
- Carefully prevent translating the `Raw Trace Data` twice
- Manage a `Trace Pattern`

The `Trace Pattern` is basically a tuple of a `module` and a `detail level` (either an integer or the atom max for full detail). In most cases the `Trace Pattern` `{et,max}` does suffice. But if you do not want any runtime dependency of `et` you can implement your own `trace_me/5` function in some module and refer to that module in the `Trace Pattern`.

The specified module flows from your instantiation of the `Viewer`, to the `Collector` that it automatically creates, gets stashed in as the `Trace Pattern`, and eventually goes down into the bowels of the `Selector`.

The module that you specify gets passed down (eventually) into `Selector`'s default `Filter`. The format of the `et:trace_me/5` function call is hardcoded in that `Filter`.

## How To Put It Together

The `Collector` automatically registers itself to listen for trace `Events`, so all you have to do is enable them.

For those people who want to do general tracing, consult the `dbg` module on how to trace whatever you're interested in and let it work its magic. If you just want `et:trace_me/5` to work, do the following:

1. Create a `Collector`
2. Create a `Viewer` (this can do step #1 for you)
3. Turn on and pare down debugging

The module `et/examples/et_trace_demo.erl` achieves this.

    -module(et_trace_demo).

    -export([test/0]).

    test() ->
        et_viewer:start([
            {title,"Coffee Order"},
            {trace_global,true},
            {trace_pattern,{et,max}},
            {max_actors,10}
          ]),
          %% dbg:p(all,call),
          %% dbg:tpl(et, trace_me, 5, []),
          Drink = {drink,iced_chai_latte},
          Size = {size,grande},
          Milk = {milk,whole},
          Flavor = {flavor,vanilla},
          et:trace_me(99,customer,barrista1,place_order,[Drink,Size,Milk,Flavor]),
          et:trace_me(80,barrista1,register,enter_order,[Drink,Size,Flavor]),
          et:trace_me(80,register,barrista1,give_total,"$5"),
          et:trace_me(80,barrista1,barrista1,get_cup,[Drink,Size]),
          et:trace_me(80,barrista1,barrista2,give_cup,[]),
          et:trace_me(90,barrista1,customer,request_money,"$5"),
          et:trace_me(90,customer,barrista1,pay_money,"$5"),
          et:trace_me(80,barrista2,barrista2,get_chai_mix,[]),
          et:trace_me(80,barrista2,barrista2,add_flavor,[Flavor]),
          et:trace_me(80,barrista2,barrista2,add_milk,[Milk]),
          et:trace_me(80,barrista2,barrista2,add_ice,[]),
          et:trace_me(80,barrista2,barrista2,swirl,[]),
          et:trace_me(80,barrista2,customer,give_tasty_beverage,[Drink,Size]),
          ok.

Running through the above, the most important points are:

- Turn on global tracing
- Set a `Trace Pattern`
- Tell `dbg` to trace function Calls
- Tell it specifically to trace the `et:trace_me/5` function

When you run the `et_trace_demo:test()` function above, the `Viewer` window will look like this screenshot:

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