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# Description

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## Overview

The two major components of the `Event Tracer (ET)` tool is a graphical sequence chart viewer (`et_viewer`) and its backing storage (`et_collector`). One `Collector` may be used as backing storage for several simultaneous `Viewers` where each one may display a different view of the same trace data.

The interface between the `Collector` and its `Viewers` is public in order to enable other types of `Viewers`. However in the following text we will focus on usage of the `et_viewer`.

The main start function is `et_viewer:start/1`. By default it will start both an `et_collector` and an `et_viewer`:

    % erl -pa et/examples
    Erlang R13B03 (erts-5.7.4) [64-bit] [smp:4:4] [rq:4] [async-threads:0] [kernel-poll:false]

    Eshell V5.7.4  (abort with ^G)
    1> {ok, Viewer} = et_viewer:start([]).
    {ok,<0.40.0>}

A `Viewer` gets trace `Events` from its `Collector` by polling it regularly for more `Events` to display. `Events` are for example reported to the `Collector` with `et_collector:report_event/6`:

    2> Collector = et_viewer:get_collector_pid(Viewer).
    <0.39.0>
    3> et_collector:report_event(Collector, 60, my_shell, mnesia_tm, start_outer,
    3>                           "Start outer transaction"),
    3> et_collector:report_event(Collector, 40, mnesia_tm, my_shell, new_tid,
    3>                           "New transaction id is 4711"),
    3> et_collector:report_event(Collector, 20, my_shell, mnesia_locker, try_write_lock,
    3>                           "Acquire write lock for {my_tab, key}"),
    3> et_collector:report_event(Collector, 10, mnesia_locker, my_shell, granted,
    3>                           "You got the write lock for {my_tab, key}"),
    3> et_collector:report_event(Collector, 60, my_shell, do_commit,
    3>                           "Perform  transaction commit"),
    3> et_collector:report_event(Collector, 40, my_shell, mnesia_locker, release_tid,
    3>                           "Release all locks for transaction 4711"),
    3> et_collector:report_event(Collector, 60, my_shell, mnesia_tm, delete_transaction,
    3>                           "End of outer transaction"),
    3> et_collector:report_event(Collector, 20, my_shell, end_outer,
    3>                           "Transaction returned {atomic, ok}").
    {ok,{table_handle,<0.39.0>,16402,trace_ts,
         #Fun<et_collector.0.62831470>}}

This actually is a simulation of the process `Events` caused by a `Mnesia` transaction that writes a record in a local table:

    mnesia:transaction(fun() -> mnesia:write({my_tab, key, val}) end).

At this stage when we have a couple of `Events`, it is time to show how it looks like in the graphical interface of `et_viewer`:

In the sequence chart, the actors (which symbolically has performed the `Event`) are shown as named vertical bars. The order of the actors may be altered by dragging (hold mouse button 1 pressed during the operation) the name tag of an actor and drop it elsewhere:

An `Event` may be an action performed by one single actor (blue text label) or it may involve two actors and is then depicted as an arrow directed from one actor to another (red text label). Details of an `Event` can be shown by clicking (press and release the mouse button 1) on the event label text or on the arrow. When doing that a `Contents Viewer` window pops up. It may look like this:

## Filters and dictionary

The `Event Tracer (ET)` uses named filters in various contexts. An Event Trace filter is an `Erlang fun` that takes some trace data as input and returns a possibly modified version of it:

    filter(TraceData) -> false | true | {true, NewEvent}

    TraceData = Event | erlang_trace_data()
    Event = #event{}
    NewEvent = #event{}

The interface of the filter function is the same as the the filter functions for the good old `lists:filtermap/2`. If the filter returns `false` it means that the trace data should silently be dropped. `true` means that the trace data data already is an `Event Record` and that it should be kept as it is. `true` means that the `TraceData` already is an `Event Record` and that it should be kept as it is. `{true, NewEvent}` means that the original trace data should be replaced with `Event`. This provides means to get rid of unwanted `Events` as well as enabling alternate views of an `Event`.

The first filter that the trace data is exposed for is the `Collector Filter`. When a trace `Event` is reported with `et_collector:report/2` (or `et_collector:report_event/5,6`) the first thing that happens, is that a message is sent to the `Collector` process to fetch a handle that contains some useful stuff, such as the `Collector Filter Fun` and an Ets table identifier. Then the `Collector Filter Fun` is applied and if it returns `true` (or `{true, NewEvent}`), the `Event` will be stored in an Ets table. As an optimization, subsequent calls to `et_collector:report`-functions can use the handle directly instead of the `Collector Pid`.

All filters (registered in a `Collector` or in a `Viewer`) must be able to handle an `Event record` as input. The `Collector Filter` (that is the filter named `all`) is a little bit special, as its input also may be raw `Erlang Trace Data`

The `Collector` manages a key/value based dictionary, where the filters are stored. Updates of the dictionary is propagated to all subscribing processes. When a `Viewer` is started it is registered as a subscriber of dictionary updates.

In each `Viewer` there is only one filter that is active and all trace `Events` that the `Viewer` gets from the `Collector` will pass thru that filter. By writing clever filters it is possible to customize how the `Events` looks like in the viewer. The following filter in `et/examples/et_demo.erl` replaces the actor names `mnesia_tm` and `mnesia_locker` and leaves everything else in the record as it was:

    mgr_actors(E) when is_record(E, event) ->
        Actor = fun(A) ->
                   case A of
                       mnesia_tm     -> trans_mgr;
                       mnesia_locker -> lock_mgr;
                       _             -> A
                   end
                end,
        {true, E#event{from = Actor(E#event.from),
                       to = Actor(E#event.to),
                       contents = [{orig_from, E#event.from},
                                   {orig_to,   E#event.to},
                                   {orig_contents, E#event.contents}]}}.

If we now add the filter to the running `Collector`:

    4> Fun = fun(E) -> et_demo:mgr_actors(E) end.
    #Fun<erl_eval.6.13229925>
    5> et_collector:dict_insert(Collector, {filter, mgr_actors}, Fun).
    ok

you will see that the `Filter` menu in all viewers have got a new entry called `mgr_actors`. Select it, and a new `Viewer` window will pop up:

In order to see the nitty gritty details of an `Event` you may click on the `Event` in order to start a `Contents Viewer` for that `Event`. In the `Contents Viewer` there also is a filter menu that enables inspection of the `Event` from other views than the one selected in the viewer. A click on the `new_tid` `Event` will cause a `Contents Viewer` window to pop up, showing the `Event` in the `mgr_actors` view:

Select the `all` entry in the `Filters` menu and a new `Contents Viewer window` will pop up showing the same trace `Event` in the collectors view:

## Trace clients

As you have seen, it is possible to use the `et_collector:report_event/5,6` functions explicitly. By using those functions you can write your own trace client that reads trace data from any source stored in any format and just feed the `Collector` with it. You may replace the default `Collector Filter` with a filter that converts new exciting trace data formats to `Event Records` or you may convert it to an `Event Record` before you invoke `et_collector:report/2` and then rely on the default `Collector Filter` to handle the new format.

There are also existing functions in the API that reads from various sources and calls `et_collector:report/2`:

- The trace `Events` that are hosted by the `Collector` may be stored to file and later be loaded by selecting `save` and `load` entries in the `Viewers` `File` menu or via the `et_collector` API.

- It is also possible to perform live tracing of a running system by making use of the built-in trace support in the Erlang emulator. These Erlang traces can be directed to files or to ports. See the reference manual for `erlang:trace/3`, `erlang:trace_pattern/3`, `dbg` and `ttb` for more info.

  There are also corresponding trace client types that can read the Erlang trace data format from such files or ports. The `et_collector:start_trace_client/3` function makes use of these Erlang trace clients and redirects the trace data to the `Collector`.

  The default `Collector Filter` converts the raw Erlang trace data format into `Event Records`. If you want to perform this differently you can of course write your own `Collector Filter` from scratch. But it may probably save you some efforts if you first apply the default filter in `et_selector:parse_event/2` before you apply your own conversions of its output.

## Global tracing

Setting up an Erlang tracer on a set of nodes and connecting trace clients to the ports of these tracers is not intuitive. In order to make this it easier the `Event Tracer` has a notion of global tracing. When used, the `et_collector` process will monitor Erlang nodes and when one connects, an Erlang tracer will automatically be started on the newly connected node. A corresponding trace client will also be started on the `Collector` node in order to automatically forward the trace `Events` to the `Collector`. Set the boolean parameter `trace_global` to `true` for either the `et_collector` or `et_viewer` in order to activate the global tracing. There is no restriction on how many concurrent (anonymous) collectors you can have, but you can only have one global `Collector` as its name is registered in `global`.

In order to further simplify the tracing, you can make use of the `et:trace_me/4,5` functions. These functions are intended to be invoked from other applications when there are interesting `Events`, in your application that needs to be highlighted. The functions are extremely light weight as they do nothing besides returning an atom. These functions are specifically designed to be traced for. As the caller explicitly provides the values for the `Event Record` fields, the default `Collector Filter` is able to automatically provide a customized `Event Record` without any user defined filter functions.

In normal operation, the `et:trace_me/4,5` calls are almost for free. When tracing is needed, you can either activate tracing on these functions explicitly. Or you can combine the usage of `trace_global` with the usage of `trace_pattern`. When set, the `trace_pattern` will automatically be activated on all connected nodes.

One nice thing with the `trace_pattern` is that it provides a very simple way of minimizing the amount of generated trace data by allowing you to explicitly control the detail level of the tracing. As you may have seen the `et_viewer` have a slider called `"Detail Level"` that allows you to control the detail level of the trace `Events` displayed in the `Viewer`. On the other hand if you set a low detail level in the `trace_pattern`, lots of the trace data will never be generated and thus not sent over the socket to the trace client and stored in the `Collector`.

## Viewer window

Almost all functionality available in the `et_viewer` is also available via shortcuts. Which key that has the same effect as selecting a menu entry is shown enclosed in parentheses. For example pressing the key `r` is equivalent to selecting the menu entry `Viewer->Refresh`.

File menu:

- `Clear all events in the Collector` - Deletes all `Events` stored in the `Collector` and notifies all connected `Viewers` about this.
- `Load events to the Collector from file` - Loads the `Collector` with `Events` from a file and notifies all connected `Viewers` about this.
- `Save all events in the Collector to file` - Saves all `Events` stored in the `Collector` to file.
- `Print setup` - Enables editing of printer setting, such as paper and layout.
- `Print current page` - Prints the events on the current page. The page size is dependent of the selected paper type.
- `Print all pages` - Prints all events. The page size is dependent of the selected paper type.
- `Close this Viewer` - Closes this `Viewer` window, but keeps all other `Viewers` windows and the `Collector` process.
- `Close other Viewers, but this` - Keeps this `Viewer` window and its `Collector` process, but closes all other `Viewers` windowsconnected to the same `Collector`.
- `Close all Viewers and the Collector` - Closes the `Collector` and all `Viewers` connected to it.

Viewer menu:

- `First` - Scrolls `this` viewer to the first `Event` in the `Collector`.
- `Last` - Scrolls `this` viewer to the last `Event` in the `Collector`.
- `Prev` - Scrolls `this` viewer one page backwards.
- `Next` - Scrolls `this` viewer one page forward.
- `Refresh` - Clears `this` viewer and re-read its `Events` from the `Collector`.
- `Up` - Scrolls a few `Events` backwards.
- `Down` - Scrolls a few `Events` forward.
- `Display all actors.` - Reset the settings for hidden and/or highlighted actors.

Collector menu:

- `First` - Scrolls`all` viewers to the first `Event` in the `Collector`.
- `Last` - Scrolls `all` viewers to the last `Event` in the `Collector`.
- `Prev` - Scrolls `all` viewers one page backwards.
- `Next` - Scrolls `all` viewers one page forward.
- `Refresh` - Clears `all` viewers and re-read their `Events` from the `Collector`.

Filters and scaling menu:

- `ActiveFilter (=)` - Starts a new `Viewer` window with the same active filter and scale as the current one.
- `ActiveFilter (+)` - Starts a new `Viewer` window with the same active filter but a larger scale than the current one.
- `ActiveFilter (-)` - Starts a new `Viewer`window with the same active filter but a smaller scale than the current one.
- `all (0)` - Starts a new `Viewer` with the `Collector Filter` as active filter. It will cause all events in the collector to be viewed.
- `AnotherFilter (2)` - If more filters are inserted into the dictionary, these will turn up here as entries in the `Filters` menu. The second filter will get the shortcut number 2, the next one number 3 etc. The names are sorted.

Slider and radio buttons:

- `Hide From=To` - When true, this means that the `Viewer` will hide all `Events` where the from-actor equals to its to-actor. These events are sometimes called actions.
- `Hide (excluded actors)` - When true, this means that the `Viewer` will hide all `Events` whose actors are marked as excluded. Excluded actors are normally enclosed in round brackets when they are displayed in the `Viewer`.
- `Detail level` - This slider controls the resolution of the `Viewer`. Only `Events` with a detail level `smaller` than the selected one (default=100=max) are displayed.

Other features:

- `Vertical scroll` - Use mouse wheel and up/down arrows to scroll little. Use page up/down and home/end buttons to scroll more.
- `Display details of an event` - Left mouse click on the event label or the arrowand a new `Contents Viewer` window will pop up, displaying the contents of an `Event`.
- `Highlight actor (toggle)` - Left mouse click on the actor name tag. The actor name will be enclosed in square brackets `[]`. When one or more actors are highlighted, only events related to those actors are displayed. All others are hidden.
- `Exclude actor (toggle)` - Right mouse click on the actor name tag. The actor name will be enclosed in round brackets `()`. When an actor is excluded, all events related to this actor is hidden. If the checkbox `Hide (excluded actors)` is checked, even the name tags and corresponding vertical line of excluded actors will be hidden.
- `Move actor` - Left mouse button drag and drop on actor name tag. Move the actor by first clicking on the actor name, keeping the button pressed while moving the cursor to a new location and release the button where the actor should be moved to.
- `Display all actors` - Press the 'a' button. Reset the settings for hidden and/or highlighted actors.

## Configuration

The `Event Records` in the Ets table are ordered by their timestamp. Which timestamp that should be used is controlled via the `event_order` parameter. Default is `trace_ts` which means the time when the trace data was generated. `event_ts` means the time when the trace data was parsed (transformed into an `Event Record`).

## Contents viewer window

File menu:

- `Close` - Close this window.
- `Save` - Save the contents of this window to file.

Filters menu:

- `ActiveFilter` - Start a new `Contents Viewer window` with the same active filter.
- `AnotherFilter (2)` - If more filters are inserted into the dictionary, these will turn up here as entries in the `Filters` menu. The second filter will be number 2, the next one number 3 etc. The names are sorted.

Hide menu:

- `Hide actor in viewer` - Known actors are shown as a named vertical bars in the `Viewer` window. By hiding the actor, its vertical bar will be removed and the `Viewer` will be refreshed.

  `Hiding the actor` is only useful if the `max_actors` threshold has been reached, as it then will imply that the "hidden" actor will be displayed as if it were `"UNKNOWN"`. If the `max_actors` threshold not have been reached, the actor will re-appear as a vertical bar in the `Viewer`.

- `Show actor in viewer` - This implies that the actor will be added as a known actor in the `Viewer` with its own vertical bar.

Search menu:

- `Forward from this event` - Set this event to be the first event in the viewer and change its display mode to be enter forward search mode. The actor of this event (from, to or both) will be added to the list of selected actors.
- `Reverse from this event` - Set this event to be the first `Event` in the `Viewer` and change its display mode to be enter reverse search mode. The actor of this `Event` (from, to or both) will be added to the list of selected actors. Observe, that the `Events` will be shown in reverse order.
- `Abort search. Display all` - Switch the display mode of the `Viewer` to show all `Events` regardless of any ongoing searches. Abort the searches.

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