Trace Tool Builder — observer v2.17

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# Trace Tool Builder

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## Introduction

Trace Tool Builder is a base for building trace tools for single node or distributed Erlang systems. It requires the Runtime_Tools application to be available on the traced node.

The following are the main features of Trace Tool Builder:

- Start tracing to file ports on many nodes with one function call.
- Write more information to a trace information file, which is read during formatting.
- Restore previous configuration by maintaining a history buffer and handling configuration files.
- Provide some simple support for sequential tracing.
- Format binary trace logs and merge logs from multiple nodes.

The intention of Trace Tool Builder is to serve as a base for tailor-made trace tools, but it can also be used directly from the Erlang shell (it can mimic `dbg` behaviour while still providing useful additions, such as match specification shortcuts). Trace Tool Builder only allows the use of file port tracer, so to use other types of trace clients it is better to use `dbg` directly.

## Getting Started

Module `ttb` is the interface to all functions in Trace Tool Builder.

To get started, the least you need to do is to start a tracer with `ttb:tracer/0,1,2`, and set the required trace flags on the processes you want to trace with `ttb:p/2`.

When the tracing is completed, stop the tracer with `ttb:stop/0,1` and format the trace log with `ttb:format/1,2` (if there is anything to format).

Useful functions:

- `ttb:tracer/0,1,2` - Opens a trace port on each node to be traced. By default, trace messages are written to binary files on remote nodes (the binary trace log).

- `ttb:p/2` - Specifies the processes to be traced. Trace flags specified in this call specify what to trace on each process. This function can be called many times if you like different trace flags to be set on different processes.

- `ttb:tp/2,3,4` or `ttb:tpl/2,3,4` - If you want to trace function calls (that is, if you have trace flag `call` set on any process), you must also set trace patterns on the required function(s) with `ttb:tp/2,3,4` or `ttb:tpl/2,3,4`. A function is only traced if it has a trace pattern. The trace pattern specifies how to trace the function by using match specifications. Match specifications are described in the ERTS User's Guide.

- `ttb:stop/0,1` - Stops tracing on all nodes, deletes all trace patterns, and flushes the trace port buffer.

- `ttb:format/1/2` - Translates the binary trace logs into something readable. By default, `ttb` presents each trace message as a line of text, but you can also write your own handler to make more complex interpretations of the trace information. A trace log can also be presented graphically with application Event Tracer (ET).

  If option `format` is specified to `ttb:stop/1`, the formatting is automatically done when stopping `ttb`.

### Tracing Local Node from Erlang Shell

The following small module is used in the subsequent example:

    -module(m).
    -export([f/0]).
    f() ->
       receive
          From when is_pid(From) ->
             Now = erlang:now(),
             From ! {self(),Now}
       end.

The following example shows the basic use of `ttb` from the Erlang shell. Default options are used both for starting the tracer and for formatting (the custom fetch directory is however provided). This gives a trace log named `Node-ttb` in the newly created directory, where `Node` is the node name. The default handler prints the formatted trace messages in the shell:

    (tiger@durin)47> %% First I spawn a process running my test function
    (tiger@durin)47> Pid = spawn(m,f,[]).
    <0.125.0>
    (tiger@durin)48>
    (tiger@durin)48> %% Then I start a tracer...
    (tiger@durin)48> ttb:tracer().
    {ok,[tiger@durin]}
    (tiger@durin)49>
    (tiger@durin)49> %% and activate the new process for tracing
    (tiger@durin)49> %% function calls and sent messages.
    (tiger@durin)49> ttb:p(Pid,[call,send]).
    {ok,[{<0.125.0>,[{matched,tiger@durin,1}]}]}
    (tiger@durin)50>
    (tiger@durin)50> %% Here I set a trace pattern on erlang:now/0
    (tiger@durin)50> %% The trace pattern is a simple match spec
    (tiger@durin)50> %% indicating that the return value should be
    (tiger@durin)50> %% traced. Refer to the reference_manual for
    (tiger@durin)50> %% the full list of match spec shortcuts
    (tiger@durin)50> %% available.
    (tiger@durin)51> ttb:tp(erlang,now,return).
    {ok,[{matched,tiger@durin,1},{saved,1}]}
    (tiger@durin)52>
    (tiger@durin)52> %% I run my test (i.e. send a message to
    (tiger@durin)52> %% my new process)
    (tiger@durin)52> Pid ! self().
    <0.72.0>
    (tiger@durin)53>
    (tiger@durin)53> %% And then I have to stop ttb in order to flush
    (tiger@durin)53> %% the trace port buffer
    (tiger@durin)53> ttb:stop([return, {fetch_dir, "fetch"}]).
    {stopped, "fetch"}
    (tiger@durin)54>
    (tiger@durin)54> %% Finally I format my trace log
    (tiger@durin)54> ttb:format("fetch").
    ({<0.125.0>,{m,f,0},tiger@durin}) call erlang:now()
    ({<0.125.0>,{m,f,0},tiger@durin}) returned from erlang:now/0 ->
    {1031,133451,667611}
    ({<0.125.0>,{m,f,0},tiger@durin}) <0.72.0> !
    {<0.125.0>,{1031,133451,667611}}
    ok

### Build Your Own Tool

The following example shows a simple tool for "debug tracing", that is, tracing of function calls with return values:

    -module(mydebug).
    -export([start/0,trc/1,stop/0,format/1]).
    -export([print/4]).
    %% Include ms_transform.hrl so that I can use dbg:fun2ms/2 to
    %% generate match specifications.
    -include_lib("stdlib/include/ms_transform.hrl").
    %%% -------------Tool API-------------
    %%% ----------------------------------
    %%% Star the "mydebug" tool
    start() ->
        %% The options specify that the binary log shall be named
        %% <Node>-debug_log and that the print/4 function in this
        %% module shall be used as format handler
        ttb:tracer(all,[{file,"debug_log"},{handler,{{?MODULE,print},0}}]),
        %% All processes (existing and new) shall trace function calls
        %% We want trace messages to be sorted upon format, which requires
        %% timestamp flag. The flag is however enabled by default in ttb.
        ttb:p(all,call).

    %%% Set trace pattern on function(s)
    trc(M) when is_atom(M) ->
        trc({M,'_','_'});
    trc({M,F}) when is_atom(M), is_atom(F) ->
        trc({M,F,'_'});
    trc({M,F,_A}=MFA) when is_atom(M), is_atom(F) ->
        %% This match spec shortcut specifies that return values shall
        %% be traced.
        MatchSpec = dbg:fun2ms(fun(_) -> return_trace() end),
        ttb:tpl(MFA,MatchSpec).

    %%% Format a binary trace log
    format(Dir) ->
        ttb:format(Dir).

    %%% Stop the "mydebug" tool
    stop() ->
        ttb:stop(return).

    %%% --------Internal functions--------
    %%% ----------------------------------
    %%% Format handler
    print(_Out,end_of_trace,_TI,N) ->
        N;
    print(Out,Trace,_TI,N) ->
        do_print(Out,Trace,N),
        N+1.

    do_print(Out,{trace_ts,P,call,{M,F,A},Ts},N) ->
        io:format(Out,
                  "~w: ~w, ~w:~n"
                  "Call      : ~w:~w/~w~n"
                  "Arguments :~p~n~n",
                  [N,Ts,P,M,F,length(A),A]);
    do_print(Out,{trace_ts,P,return_from,{M,F,A},R,Ts},N) ->
        io:format(Out,
                  "~w: ~w, ~w:~n"
                  "Return from  : ~w:~w/~w~n"
                  "Return value :~p~n~n",
                  [N,Ts,P,M,F,A,R]).

To distinguish trace logs produced with this tool from other logs, option `file` is used in `tracer/2`. The logs are therefore fetched to a directory named `ttb_upload_debug_log-YYYYMMDD-HHMMSS`

By using option `handler` when starting the tracer, the information about how to format the file is stored in the trace information file (`.ti`). This is not necessary, as it can be specified when formatting instead. However, It can be useful if you, for example, want to format trace logs automatically using option `format` in `ttb:stop/1`. Also, you do not need any knowledge of the content of a binary log to format it the way it is intended. If option `handler` is specified both when starting the tracer and when formatting, the one specified when formatting is used.

Trace flag `call` is set on all processes. This means that any function activated with command `trc/1` is traced on all existing and new processes.

## Running Trace Tool Builder against Remote Node

The Observer application might not always be available on the node to be traced (in the following called the "traced node"). However, Trace Tool Builder can still be run from another node (in the following called the "trace control node") as long as the following is fulfilled:

- The Observer application is available on the trace control node.
- The Runtime_Tools application is available on both the trace control node and the traced node.

If Trace Tool Builder is to be used against a remote node, it is highly recommended to start the trace control node as hidden. This way it can connect to the traced node without being "seen" by it, that is, if the `nodes/0` BIF is called on the traced node, the trace control node does not show. To start a hidden node, add option `-hidden` to the `erl` command, for example:

    % erl -sname trace_control -hidden

### Diskless Node

If the traced node is diskless, `ttb` must be started from a trace control node with disk access, and option `file` must be specified to function `tracer/2` with value `{local, File}`, for example:

    (trace_control@durin)1> ttb:tracer(mynode@diskless,
                                       {file,{local,{wrap,"mytrace"}}}).
    {ok,[mynode@diskless]}

## More Tracing Options

When setting up a trace, the following features can also be activated:

- Time-constrained tracing
- Overload protection
- Autoresume
- `dbg` mode

### Time-Constrained Tracing

It can sometimes be helpful to enable trace for a specified period of time (for example, to monitor a system for 24 hours or half a second). This can be done with option `{timer, TimerSpec}`. If `TimerSpec` has the form of `MSec`, the trace is stopped after `MSec` milliseconds using `ttb:stop/0`. If more options are provided (`TimerSpec = {MSec, Opts}`), `ttb:stop/1` is called instead with `Opts` as argument.

The timer is started with `ttb:p/2`, so any trace patterns must be set up in advance. `ttb:start_trace/4` always sets up all patterns before invoking `ttb:p/2`.

The following example shows how to set up a trace that is automatically stopped and formatted after 5 seconds:

    (tiger@durin)1> ttb:start_trace([node()],
                                    [{erlang, now,[]}],
                                    {all, call},
                                    [{timer, {5000, format}}]).

#### Note

Because of network and processing delays, the period of tracing is approximate.

### Overload Protection

When tracing live systems, always take special care to not overload a node with too heavy tracing. `ttb` provides option `overload` to address this problem.

`{overload, MSec, Module, Function}` instructs the `ttb` back end (a part of the Runtime_Tools application) to perform overload check every `MSec` millisecond. If the check (named `Module:Function(check)`) returns `true`, tracing is disabled on the selected node.

Overload protection activated on one node does not affect other nodes, where the tracing continues as normal. `ttb:stop/0` fetches data from all clients, including everything collected before the activation of overload protection.

#### Note

It is not allowed to change trace details (with `ttb:p/2` and `ttb:tp/tpl...`) once overload protection is activated in one of the traced nodes. This is to avoid trace setup being inconsistent between nodes.

`Module:Function` provided with option `overload` must handle three calls: `init`, `check`, and `stop`. `init` and `stop` allow some setup and teardown required by the check. An overload check module can look as follows:

    -module(overload).
    -export([check/1]).

    check(init) ->
        Pid = sophisticated_module:start(),
        put(pid, Pid);
    check(check) ->
        get(pid) ! is_overloaded,
        receive
            Reply ->
                Reply
        after 5000 ->
                true
        end;
    check(stop) ->
        get(pid) ! stop.

#### Note

`check` is always called by the same process, so `put` and `get` are possible.

### Autoresume

A node can crash (probably a buggy one, hence traced). Use `resume` to resume tracing on the node automatically when it gets back. The failing node then tries to reconnect to trace control node when `Runtime_Tools` is started. This implies that `Runtime_Tools` must be included in the startup chain of other nodes (if not, you can still resume tracing by starting `Runtime_Tools` manually, that is, by an RPC call).

To not lose the data that the failing node stored up to the point of crash, the control node tries to fetch it before restarting trace. This must occur within the allowed time frame, otherwise it is aborted (default is 10 seconds, but it can be changed with `{resume, MSec}`). The data fetched this way is then merged with all other traces.

The autostart feature requires more data to be stored on traced nodes. By default, the data is stored automatically to the file named "ttb_autostart.bin" in the current working directory (cwd) of the traced node. Users can change this behaviour (that is, on diskless nodes) by specifying their own module to handle autostart data storage and retrieval (`ttb_autostart_module` environment variable of `runtime_tools`). For information about the API, see module `ttb`. The following example shows the default handler:

    -module(ttb_autostart).
    -export([read_config/0,
             write_config/1,
             delete_config/0]).

    -define(AUTOSTART_FILENAME, "ttb_autostart.bin").

    delete_config() ->
        file:delete(?AUTOSTART_FILENAME).

    read_config() ->
        case file:read_file(?AUTOSTART_FILENAME) of
            {ok, Data} -> {ok, binary_to_term(Data)};
            Error      -> Error
        end.

    write_config(Data) ->
        file:write_file(?AUTOSTART_FILENAME, term_to_binary(Data)).

#### Note

Remember that file trace ports buffer the data by default. If the node crashes, trace messages are not flushed to the binary log. If the risk of failure is high, it can be a good idea to flush the buffers every now and then automatically. Passing `{flush, MSec}` as an option of `ttb:tracer/2` flushes all buffers every `MSec` millisecond.

### dbg Mode

Option `{shell, ShellType}` allows making `ttb` operation similar to `dbg`. Using `{shell, true}` displays all trace messages in the shell before storing them. `{shell, only}` additionally disables message storage (making the tool to behave exactly like `dbg`). This is allowed only with IP trace ports (`{trace, {local, File}}`).

Command `ttb:tracer(dbg)` is a shortcut for the pure `dbg` mode (`{shell, only}`).

## Trace Information and File .ti

In addition to the trace log file(s), a file with extension `.ti` is created when Trace Tool Builder is started. This is the trace information file. It is a binary file, which contains the process information, trace flags used, the name of the node to which it belongs, and all information written with function `ttb:write_trace_info/2`. `.ti` files are always fetched with other logs when the trace is stopped.

Except for the process information, everything in the trace information file is passed on to the handler function when formatting. Parameter `TI` is a list of `{Key,ValueList}` tuples. The keys `flags`, `handler`, `file`, and `node` are used for information written directly by `ttb`.

Information to the trace information file by can be added by calling `ttb:write_trace_info/2`. Notice that `ValueList` always is a list, and if you call `write_trace_info/2` many times with the same `Key`, the `ValueList` is extended with a new value each time.

Example:

`ttb:write_trace_info(mykey,1)` gives the entry `{mykey,[1]}` in `TI`. Another call, `ttb:write_trace_info(mykey,2)`, changes this entry to `{mykey,[1,2]}`.

## Wrap Logs

If you want to limit the size of the trace logs, you can use wrap logs. This works almost like a circular buffer. You can specify the maximum number of binary logs and the maximum size of each log. `ttb` then creates a new binary log each time a log reaches the maximum size. When the maximum number of logs are reached, the oldest log is deleted before a new one is created.

#### Note

The overall size of data generated by `ttb` can be greater than the wrap specification suggests. If a traced node restarts and autoresume is enabled, the old wrap log is always stored and a new one is created.

Wrap logs can be formatted one by one or all at once. See Formatting.

## Formatting

Formatting can be done automatically when stopping `ttb` (see section Automatically Collect and Format Logs from All Nodes), or explicitly by calling function `ttb:format/1,2`.

Formatting means to read a binary log and present it in a readable format. You can use the default format handler in `ttb` to present each trace message as a line of text, or write your own handler to make more complex interpretations of the trace information. You can also use application ET to present the trace log graphically (see section Presenting Trace Logs with Event Tracer).

The first argument to `ttb:format/1,2` specifies which binary log(s) to format. This is usually the name of a directory that `ttb` created during log fetch. Unless option `disable_sort` is provided, the logs from different files are always sorted according to time-stamp in traces.

The second argument to `ttb:format/2` is a list of options as follows:

- `out` - Specifies the destination to write the formatted text. Default destination is `standard_io`, but a filename can also be specified.

- `handler` - Specifies the format handler to use. If this option is not specified, option `handler` that is specified when starting the tracer is used. If option `handler` is not specified when starting the tracer either, a default handler is used, which prints each trace message as a text line.

- `disable_sort` - Indicates that the logs are not to be merged according to time-stamp, but processed one file after another (this can be a bit faster).

A format handler is a fun taking four arguments. This fun is called for each trace message in the binary log(s). A simple example that only prints each trace message can be as follows:

    fun(Fd, Trace, _TraceInfo, State) ->
       io:format(Fd, "Trace: ~p~n", [Trace]),
       State
    end.

Here, `Fd` is the file descriptor for the destination file, or the atom `standard_io`. `_TraceInfo` contains information from the trace information file (see section Trace Information and File .ti). `State` is a state variable for the format handler fun. The initial value of variable `State` is specified with the handler option, for example:

    ttb:format("tiger@durin-ttb", [{handler, {{Mod,Fun}, initial_state}}])
                                                         ^^^^^^^^^^^^^

Another format handler can be used to calculate the time spent by the garbage collector:

    fun(_Fd,{trace_ts,P,gc_start,_Info,StartTs},_TraceInfo,State) ->
          [{P,StartTs}|State];
       (Fd,{trace_ts,P,gc_end,_Info,EndTs},_TraceInfo,State) ->
          {value,{P,StartTs}} = lists:keysearch(P,1,State),
          Time = diff(StartTs,EndTs),
          io:format("GC in process ~w: ~w milliseconds~n", [P,Time]),
          State -- [{P,StartTs}]
    end

A more refined version of this format handler is function `handle_gc/4` in module `multitrace.erl` included in directory `src` of the Observer application.

The trace message is passed as the second argument (`Trace`). The possible values of `Trace` are the following:

- All trace messages described in `erlang:trace/3`
- `{drop, N}` if IP tracer is used (see `dbg:trace_port/2`)
- `end_of_trace` received once when all trace messages are processed

By giving the format handler `ttb:get_et_handler()`, you can have the trace log presented graphically with `et_viewer` in the ET application (see section Presenting Trace Logs with Event Tracer).

You can always decide not to format the whole trace data contained in the fetch directory, but analyze single files instead. To do so, a single file (or list of files) must be passed as the first argument to `format/1,2`.

Wrap logs can be formatted one by one or all at once. To format one of the wrap logs in a set, specify the exact file name. To format the whole set of wrap logs, specify the name with `*` instead of the wrap count.

Example:

Start tracing:

    (tiger@durin)1> ttb:tracer(node(),{file,{wrap,"trace"}}).
    {ok,[tiger@durin]}
    (tiger@durin)2> ttb:p(...)
    ...

This gives a set of binary logs, for example:

    tiger@durin-trace.0.wrp
    tiger@durin-trace.1.wrp
    tiger@durin-trace.2.wrp
    ...

Format the whole set of logs:

    1> ttb:format("tiger@durin-trace.*.wrp").
    ....
    ok
    2>

Format only the first log:

    1> ttb:format("tiger@durin-trace.0.wrp").
    ....
    ok
    2>

To merge all wrap logs from two nodes:

    1> ttb:format(["tiger@durin-trace.*.wrp","lion@durin-trace.*.wrp"]).
    ....
    ok
    2>

### Presenting Trace Logs with Event Tracer

For detailed information about the Event Tracer, see the ET application.

By giving the format handler `ttb:get_et_handler()`, you can have the trace log presented graphically with `et_viewer` in the ET application. `ttb` provides filters that can be selected from the menu Filter in the `et_viewer` window. The filters are names according to the type of actors they present (that is, what each vertical line in the sequence diagram represents). Interaction between actors is shown as red arrows between two vertical lines, and activities within an actor are shown as blue text to the right of the actors line.

The `processes` filter is the only filter showing all trace messages from a trace log. Each vertical line in the sequence diagram represents a process. Erlang messages, spawn, and link/unlink are typical interactions between processes. Function calls, scheduling, and garbage collection, are typical activities within a process. `processes` is the default filter.

The remaining filters only show function calls and function returns. All other trace message are discarded. To get the most out of these filters, `et_viewer` must know the caller of each function and the time of return. This can be obtained using both the `call` and `return_to` flags when tracing. Notice that flag `return_to` only works with local call trace, that is, when trace patterns are set with `ttb:tpl`.

The same result can be obtained by using the flag `call` only and setting a match specification on local or global function calls as follows:

    1> dbg:fun2ms(fun(_) -> return_trace(),message(caller()) end).
    [{'_',[],[{return_trace},{message,{caller}}]}]

This must however be done with care, as function `{return_trace}` in the match specification destroys tail recursiveness.

The `modules` filter shows each module as a vertical line in the sequence diagram. External function calls/returns are shown as interactions between modules, and internal function calls/returns are shown as activities within a module.

The `functions` filter shows each function as a vertical line in the sequence diagram. A function calling itself is shown as an activity within a function, and all other function calls are shown as interactions between functions.

The `mods_and_procs` and `funcs_and_procs` filters are equivalent to the `modules` and `functions` filters respectively, except that each module or function can have many vertical lines, one for each process it resides on.

In the following example, modules `foo` and `bar` are used:

    -module(foo).
    -export([start/0,go/0]).

    start() ->
        spawn(?MODULE, go, []).

    go() ->
        receive
            stop ->
                ok;
            go ->
                bar:f1(),
                go()
        end.

    -module(bar).
    -export([f1/0,f3/0]).
    f1() ->
        f2(),
        ok.
    f2() ->
        spawn(?MODULE,f3,[]).
    f3() ->
        ok.

Setting up the trace:

    (tiger@durin)1> %%First we retrieve the Pid to limit traced processes set
    (tiger@durin)1> Pid = foo:start().
    (tiger@durin)2> %%Now we set up tracing
    (tiger@durin)2> ttb:tracer().
    (tiger@durin)3> ttb:p(Pid, [call, return_to, procs, set_on_spawn]).
    (tiger@durin)4> ttb:tpl(bar, []).
    (tiger@durin)5> %%Invoke our test function and see output with et viewer
    (tiger@durin)5> Pid ! go.
    (tiger@durin)6> ttb:stop({format, {handler, ttb:get_et_handler()}}).

This renders a result similar to the following:

Notice that function `ttb:start_trace/4` can be used as help as follows:

    (tiger@durin)1> Pid = foo:start().
    (tiger@durin)2> ttb:start_trace([node()],
                                    [{bar,[]}],
                                    {Pid, [call, return_to, procs, set_on_spawn]}
                                    {handler, ttb:get_et_handler()}).
    (tiger@durin)3> Pid ! go.
    (tiger@durin)4> ttb:stop(format).

## Automatically Collect and Format Logs from All Nodes

By default, `ttb:stop/1` fetches trace logs and trace information files from all nodes. The logs are stored in a new directory named `ttb_upload-Filename-Timestamp` under the working directory of the trace control node. Fetching can be disabled by providing option `nofetch` to `ttb:stop/1`. The user can specify a fetch directory by passing option `{fetch_dir, Dir}`.

If option `format` is specified to `ttb:stop/1`, the trace logs are automatically formatted after tracing is stopped.

## History and Configuration Files

For the tracing functionality, `dbg` can be used instead of `ttb` for setting trace flags on processes and trace patterns for call trace, that is, the functions `p`, `tp`, `tpl`, `ctp`, `ctpl`, and `ctpg`. Only the following two things are added by `ttb` for these functions:

- All calls are stored in the history buffer and can be recalled and stored in a configuration file. This makes it easy to set up the same trace environment, for example, if you want to compare two test runs. It also reduces the amount of typing when using `ttb` from the Erlang shell.
- Shortcuts are provided for the most common match specifications (to not force you to use `dbg:fun2ms` continually).

Use `ttb:list_history/0` to see the content of the history buffer and `ttb:run_history/1` to re-execute one of the entries.

The main purpose of the history buffer is the possibility to create configuration files. Any function stored in the history buffer can be written to a configuration file and used for creating a specific configuration at any time with a single function call.

A configuration file is created or extended with `ttb:write_config/2,3`. Configuration files are binary files and can therefore only be read and written with functions provided by `ttb`.

The complete content of the history buffer can be written to a configuration file by calling `ttb:write_config(ConfigFile,all)`. Selected entries from the history can be written by calling `ttb:write_config(ConfigFile,NumList)`, where `NumList` is a list of integers pointing out the history entries to write. Moreover, the history buffer is always dumped to `ttb_last_config` when `ttb:stop/0,1` is called.

User-defined entries can also be written to a configuration file by calling function `ttb:write_config(ConfigFile,ConfigList)`, where `ConfigList` is a list of `{Module,Function,Args}`.

Any existing file `ConfigFile` is deleted and a new file is created when `write_config/2` is called. Option `append` can be used to add something at the end of an existing configuration file, for example, `ttb:write_config(ConfigFile,What,[append])`.

Example:

See the content of the history buffer:

    (tiger@durin)191> ttb:tracer().
    {ok,[tiger@durin]}
    (tiger@durin)192> ttb:p(self(),[garbage_collection,call]).
    {ok,{[<0.1244.0>],[garbage_collection,call]}}
    (tiger@durin)193> ttb:tp(ets,new,2,[]).
    {ok,[{matched,1}]}
    (tiger@durin)194> ttb:list_history().
    [{1,{ttb,tracer,[tiger@durin,[]]}},
     {2,{ttb,p,[<0.1244.0>,[garbage_collection,call]]}},
     {3,{ttb,tp,[ets,new,2,[]]}}]

Execute an entry from the history buffer:

    (tiger@durin)195> ttb:ctp(ets,new,2).
    {ok,[{matched,1}]}
    (tiger@durin)196> ttb:list_history().
    [{1,{ttb,tracer,[tiger@durin,[]]}},
     {2,{ttb,p,[<0.1244.0>,[garbage_collection,call]]}},
     {3,{ttb,tp,[ets,new,2,[]]}},
     {4,{ttb,ctp,[ets,new,2]}}]
    (tiger@durin)197> ttb:run_history(3).
    ttb:tp(ets,new,2,[]) ->
    {ok,[{matched,1}]}

Write the content of the history buffer to a configuration file:

    (tiger@durin)198> ttb:write_config("myconfig",all).
    ok
    (tiger@durin)199> ttb:list_config("myconfig").
    [{1,{ttb,tracer,[tiger@durin,[]]}},
     {2,{ttb,p,[<0.1244.0>,[garbage_collection,call]]}},
     {3,{ttb,tp,[ets,new,2,[]]}},
     {4,{ttb,ctp,[ets,new,2]}},
     {5,{ttb,tp,[ets,new,2,[]]}}]

Extend an existing configuration:

    (tiger@durin)200> ttb:write_config("myconfig",[{ttb,tp,[ets,delete,1,[]]}],
    [append]).
    ok
    (tiger@durin)201> ttb:list_config("myconfig").
    [{1,{ttb,tracer,[tiger@durin,[]]}},
     {2,{ttb,p,[<0.1244.0>,[garbage_collection,call]]}},
     {3,{ttb,tp,[ets,new,2,[]]}},
     {4,{ttb,ctp,[ets,new,2]}},
     {5,{ttb,tp,[ets,new,2,[]]}},
     {6,{ttb,tp,[ets,delete,1,[]]}}]

Go back to a previous configuration after stopping Trace Tool Builder:

    (tiger@durin)202> ttb:stop().
    ok
    (tiger@durin)203> ttb:run_config("myconfig").
    ttb:tracer(tiger@durin,[]) ->
    {ok,[tiger@durin]}

    ttb:p(<0.1244.0>,[garbage_collection,call]) ->
    {ok,{[<0.1244.0>],[garbage_collection,call]}}

    ttb:tp(ets,new,2,[]) ->
    {ok,[{matched,1}]}

    ttb:ctp(ets,new,2) ->
    {ok,[{matched,1}]}

    ttb:tp(ets,new,2,[]) ->
    {ok,[{matched,1}]}

    ttb:tp(ets,delete,1,[]) ->
    {ok,[{matched,1}]}

    ok

Write selected entries from the history buffer to a configuration file:

    (tiger@durin)204> ttb:list_history().
    [{1,{ttb,tracer,[tiger@durin,[]]}},
     {2,{ttb,p,[<0.1244.0>,[garbage_collection,call]]}},
     {3,{ttb,tp,[ets,new,2,[]]}},
     {4,{ttb,ctp,[ets,new,2]}},
     {5,{ttb,tp,[ets,new,2,[]]}},
     {6,{ttb,tp,[ets,delete,1,[]]}}]
    (tiger@durin)205> ttb:write_config("myconfig",[1,2,3,6]).
    ok
    (tiger@durin)206> ttb:list_config("myconfig").
    [{1,{ttb,tracer,[tiger@durin,[]]}},
     {2,{ttb,p,[<0.1244.0>,[garbage_collection,call]]}},
     {3,{ttb,tp,[ets,new,2,[]]}},
     {4,{ttb,tp,[ets,delete,1,[]]}}]
    (tiger@durin)207>

## Sequential Tracing

To learn what sequential tracing is and how it can be used, see the Reference Manual for `seq_trace`.

The support for sequential tracing provided by Trace Tool Builder includes the following:

- Initiation of the system tracer. This is automatically done when a trace port is started with `ttb:tracer/0,1,2`.
- Creation of match specifications that activates sequential tracing.

Starting sequential tracing requires that a tracer is started with function `ttb:tracer/0,1,2`. Sequential tracing can then be started in either of the following ways:

- Through a trigger function with a match specification created with `ttb:seq_trigger_ms/0,1`.
- Directly by using module `seq_trace`.

Example 1:

In the following example, function `dbg:get_tracer/0` is used as trigger for sequential tracing:

    (tiger@durin)110> ttb:tracer().
    {ok,[tiger@durin]}
    (tiger@durin)111> ttb:p(self(),call).
    {ok,{[<0.158.0>],[call]}}
    (tiger@durin)112> ttb:tp(dbg,get_tracer,0,ttb:seq_trigger_ms(send)).
    {ok,[{matched,1},{saved,1}]}
    (tiger@durin)113> dbg:get_tracer(), seq_trace:reset_trace().
    true
    (tiger@durin)114> ttb:stop(format).
    ({<0.158.0>,{shell,evaluator,3},tiger@durin}) call dbg:get_tracer()
    SeqTrace [0]: ({<0.158.0>,{shell,evaluator,3},tiger@durin})
    {<0.237.0>,dbg,tiger@durin} ! {<0.158.0>,{get_tracer,tiger@durin}}
    [Serial: {0,1}]
    SeqTrace [0]: ({<0.237.0>,dbg,tiger@durin})
    {<0.158.0>,{shell,evaluator,3},tiger@durin} ! {dbg,{ok,#Port<0.222>}}
    [Serial: {1,2}]
    ok
    (tiger@durin)116>

Example 2:

Starting sequential tracing with a trigger is more useful if the trigger function is not called directly from the shell, but rather implicitly within a larger system. When calling a function from the shell, it is simpler to start sequential tracing directly, for example, as follows:

    (tiger@durin)116> ttb:tracer().
    {ok,[tiger@durin]}
    (tiger@durin)117> seq_trace:set_token(send,true), dbg:get_tracer(),
    seq_trace:reset_trace().
    true
    (tiger@durin)118> ttb:stop(format).
    SeqTrace [0]: ({<0.158.0>,{shell,evaluator,3},tiger@durin})
    {<0.246.0>,dbg,tiger@durin} ! {<0.158.0>,{get_tracer,tiger@durin}}
    [Serial: {0,1}]
    SeqTrace [0]: ({<0.246.0>,dbg,tiger@durin})
    {<0.158.0>,{shell,evaluator,3},tiger@durin} ! {dbg,{ok,#Port<0.229>}}
    [Serial: {1,2}]
    ok
    (tiger@durin)120>

In both previous examples, `seq_trace:reset_trace/0` resets the trace token immediately after the traced function to avoid many trace messages because of the printouts in the Erlang shell.

All functions in module `seq_trace`, except `set_system_tracer/1`, can be used after the trace port is started with `ttb:tracer/0,1,2`.

## Multipurpose Trace Tool

Module `multitrace` in directory `src` of the Observer application provides a small tool with three possible trace settings. The trace messages are written to binary files, which can be formatted with function `multitrace:format/1,2`:

- `multitrace:debug(What)` - Start calltrace on all processes and trace the specified function(s). The format handler used is `multitrace:handle_debug/4` that prints each call and returns. `What` must be an item or a list of items to trace, specified on the format `{Module,Function,Arity}`, `{Module,Function}`, or only `Module`.

- `multitrace:gc(Procs)` - Trace garbage collection on the specified process(es). The format handler used is `multitrace:handle_gc/4` that prints start, stop, and the time spent for each garbage collection.

- `multitrace:schedule(Procs)` - Trace in-scheduling and out-scheduling on the specified process(es). The format handler used is `multitrace:handle_schedule/4` that prints each in-scheduling and out-scheduling with process, time-stamp, and current function. It also prints the total time each traced process was scheduled in.

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