Advanced examples — et v1.7.1

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# Advanced examples

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## A simulated Mnesia transaction

The Erlang code for running the simulated `Mnesia` transaction example in the previous chapter is included in the `et/examples/et_demo.erl` file:

    sim_trans() ->
        sim_trans([]).

    sim_trans(ExtraOptions) ->
        Options = [{dict_insert, {filter, mgr_actors}, fun mgr_actors/1}],
        {ok, Viewer} = et_viewer:start_link(Options ++ ExtraOptions),
        Collector = et_viewer:get_collector_pid(Viewer),
        et_collector:report_event(Collector, 60, my_shell, mnesia_tm, start_outer,
                                  "Start outer transaction"),
        et_collector:report_event(Collector, 40, mnesia_tm, my_shell, new_tid,
                                  "New transaction id is 4711"),
        et_collector:report_event(Collector, 20, my_shell, mnesia_locker, try_write_lock,
                                  "Acquire write lock for {my_tab, key}"),
        et_collector:report_event(Collector, 10, mnesia_locker, my_shell, granted,
                                  "You got the write lock for {my_tab, key}"),
        et_collector:report_event(Collector, 60, my_shell, do_commit,
                                  "Perform  transaction commit"),
        et_collector:report_event(Collector, 40, my_shell, mnesia_locker, release_tid,
                                  "Release all locks for transaction 4711"),
        et_collector:report_event(Collector, 60, my_shell, mnesia_tm, delete_transaction,
                                  "End of outer transaction"),
        et_collector:report_event(Collector, 20, my_shell, end_outer,
                                  "Transaction returned {atomic, ok}"),
        {collector, Collector}.

    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 you invoke the `et_demo:sim_trans()` function, a `Viewer` window will pop up and the sequence trace will be almost the same as if the following `Mnesia` transaction would have been run:

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

And the viewer window will look like:

    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>;}
    2> et_demo:sim_trans().
    {ok,{table_handle,<0.45.0>,24596,trace_ts,
         #Fun<et_collector.0.62831470>}}

## Some convenient functions used in the Mnesia transaction example

The `module_as_actor` filter converts the `Event Records` so the module names becomes actors and the invoked functions becomes labels. If the information about who the caller was it will be displayed as an arrow directed from the caller to the callee. The `[{message, {caller}}, {return_trace}]` options to `dbg:tpl/2` function will imply the necessary information in the Erlang traces. Here follows the `module_as_actor` filter:

    module_as_actor(E) when is_record(E, event) ->
        case lists:keysearch(mfa, 1, E#event.contents) of
            {value, {mfa, {M, F, _A}}} ->
                case lists:keysearch(pam_result, 1, E#event.contents) of
                    {value, {pam_result, {M2, _F2, _A2}}} ->
                        {true, E#event{label = F, from = M2, to = M}};
                    _ ->
                        {true, E#event{label = F, from = M, to = M}}
                end;
            _ ->
                false
        end.

The `plain_process_info` filter does not alter the `Event Records`. It merely ensures that the event not related to processes are skipped:

    plain_process_info(E) when is_record(E, event) ->
        case E#event.label of
            send                          -> true;
            send_to_non_existing_process  -> true;
            'receive'                     -> true;
            spawn                         -> true;
            exit                          -> true;
            link                          -> true;
            unlink                        -> true;
            getting_linked                -> true;
            {seq_send, _Label}            -> true;
            {seq_receive, _Label}         -> true;
            {seq_print, _Label}           -> true;
            {drop, _N}                    -> true;
            _                             -> false
        end.

The `plain_process_info_nolink` filter does not alter the `Event Records`. It do makes use of the `plain_process_info` , but do also ensure that the process info related to linking and unlinking is skipped:

    plain_process_info_nolink(E) when is_record(E, event) ->
        (E#event.label /= link) and
        (E#event.label /= unlink) and
        (E#event.label /= getting_linked) and
        plain_process_info(E).

In order to simplify the startup of an `et_viewer` process with the filters mentioned above, plus some others (that also are found in `et/examples/et_demo.erl` src/et_collector.erl the `et_demo:start/0,1` functions can be used:

    start() ->
        start([]).

    start(ExtraOptions) ->
        Options = [{trace_global, true},
                   {parent_pid, undefined},
                   {max_actors, infinity},
                   {max_events, 1000},
                   {active_filter, module_as_actor}],
        et_viewer:start_link(filters() ++ Options ++ ExtraOptions).

A simple one-liner starts the tool:

              erl -pa ../examples -s et_demo

The filters are included by the following parameters:

    filters() ->
        [{dict_insert, {filter, module_as_actor},
                       fun module_as_actor/1},
         {dict_insert, {filter, plain_process_info},
                       fun plain_process_info/1},
         {dict_insert, {filter, plain_process_info_nolink},
                       fun plain_process_info_nolink/1},
         {dict_insert, {filter, named_process_info},
                       fun named_process_info/1},
         {dict_insert, {filter, named_process_info_nolink},
                       fun named_process_info_nolink/1},
         {dict_insert, {filter, node_process_info},
                       fun node_process_info/1},
         {dict_insert, {filter, node_process_info_nolink},
                       fun node_process_info_nolink/1},
         {dict_insert, {filter, application_as_actor},
                       fun application_as_actor/1}
        ].

## Erlang trace of a real Mnesia transaction

The following piece of code `et_demo:trace_mnesia/0` activates call tracing of both local and external function calls for all modules in the `Mnesia` application. The call traces are configured cover all processes (both existing and those that are spawned in the future) and include timestamps for trace data. It do also activate tracing of process related events for `Mnesia`'s static processes plus the calling process (that is your shell). Please, observe that the `whereis/1` call in the following code requires that both the traced `Mnesia` application and the `et_viewer` is running on the same node:

    trace_mnesia() ->
        Modules = mnesia:ms(),
        Spec = [{message, {caller}}, {return_trace}],
        Flags = [send, 'receive', procs, timestamp],
        dbg:p(all, [call, timestamp]),
        [dbg:tpl(M, [{'_', [], Spec}]) || M <- Modules],
        LocallyRunningServers = [M || M <- Modules, whereis(M) /= undefined],
        [dbg:p(whereis(RS), Flags) || RS <- LocallyRunningServers],
        dbg:p(self(), Flags),
        LocallyRunningServers.

The `et_demo:live_trans/0` function starts the global `Collector`, starts a `Viewer`, starts `Mnesia`, creates a local table, activates tracing (as described above) and registers the shell process is as 'my_shell' for clarity. Finally a simple `Mnesia` transaction that writes a single record is run:

    live_trans() ->
        live_trans([]).

    live_trans(ExtraOptions) ->
        Options = [{title, "Mnesia tracer"},
    	       {hide_actions, true},
    	       {active_filter, named_process_info_nolink}],
        et_demo:start(Options ++ ExtraOptions),
        mnesia:start(),
        mnesia:create_table(my_tab, [{ram_copies, [node()]}]),
        et_demo:trace_mnesia(),
        register(my_shell, self()),

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

Now we run the `et_demo:live_trans/0` function:

    erl -pa ../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> et_demo:live_trans().
    {atomic,ok}

Please, explore the different filters in order to see how the traced transaction can be seen from different point of views:

## Erlang trace of Megaco startup

The `Event Tracer (ET)` tool was initially written in order to demonstrate how messages where sent over the `Megaco` protocol. This were back in the old days before the standard bodies of `IETF` and `ITU` had approved `Megaco` (also called `H.248`) as an international standard.

In the `Megaco` application of Erlang/OTP, the code is carefully instrumented with calls to `et:trace_me/5`. For each call a detail level is given in order to enable dynamic control of the trace level in a simple manner.

The `megaco_filter` module implements a customized filter for `Megaco` messages. It does also make use of `trace_global` combined with usage of the `trace_pattern`:

    -module(megaco_filter).
    -export([start/0]).

    start() ->
        Options =
            [{event_order, event_ts},
             {scale, 3},
             {max_actors, infinity},
             {trace_pattern, {megaco, max}},
             {trace_global, true},
             {dict_insert, {filter, megaco_filter}, fun filter/1},
             {active_filter, megaco_filter},
             {title, "Megaco tracer - Erlang/OTP"}],
        et_viewer:start(Options).

First we start an Erlang node with a global `Collector` and its `Viewer`.

    erl -sname observer
    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)
    (observer@falco)1> megaco_filter:start().
    {ok,<0.48.0>}

Secondly we start another Erlang node which we connect the observer node, before we start the application that we want to trace. In this case we start a Media Gateway Controller that listens for both TCP and UDP on the text and binary ports for Megaco:

    erl -sname mgc -pa ../../megaco/examples/simple
    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)
    (mgc@falco)1> net:ping(observer@falco).
    pong
    (mgc@falco)2> megaco:start().
    ok
    (mgc@falco)3> megaco_simple_mgc:start().
    {ok,[{ok,2944,
             {megaco_receive_handle,{deviceName,"controller"},
                                    megaco_pretty_text_encoder,[],megaco_tcp,dynamic}},
         {ok,2944,
             {megaco_receive_handle,{deviceName,"controller"},
                                    megaco_pretty_text_encoder,[],megaco_udp,dynamic}},
         {ok,2945,
             {megaco_receive_handle,{deviceName,"controller"},
                                    megaco_binary_encoder,[],megaco_tcp,dynamic}},
         {ok,2945,
             {megaco_receive_handle,{deviceName,"controller"},
                                    megaco_binary_encoder,[],megaco_udp,dynamic}}]}

And finally we start an Erlang node for the Media Gateways and connect to the observer node. Each Media Gateway connects to the controller and sends an initial Service Change message. The controller accepts the gateways and sends a reply to each one using the same transport mechanism and message encoding according to the preference of each gateway. That is all combinations of TCP/IP transport, UDP/IP transport, text encoding and ASN.1 BER encoding:

    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)
    (mg@falco)1> net:ping(observer@falco).
    pong
    (mg@falco)2> megaco_simple_mg:start().
    [{{deviceName,"gateway_tt"},
      {error,{start_user,megaco_not_started}}},
     {{deviceName,"gateway_tb"},
      {error,{start_user,megaco_not_started}}},
     {{deviceName,"gateway_ut"},
      {error,{start_user,megaco_not_started}}},
     {{deviceName,"gateway_ub"},
      {error,{start_user,megaco_not_started}}}]
    (mg@falco)3> megaco:start().
    ok
    (mg@falco)4> megaco_simple_mg:start().
    [{{deviceName,"gateway_tt"},
      {1,
       {ok,[{'ActionReply',0,asn1_NOVALUE,asn1_NOVALUE,
                [{serviceChangeReply,
                     {'ServiceChangeReply',
                         [{megaco_term_id,false,["root"]}],
                         {serviceChangeResParms,
                             {'ServiceChangeResParm',
                                 {deviceName,"controller"},
                                 asn1_NOVALUE,asn1_NOVALUE,asn1_NOVALUE,
                                 asn1_NOVALUE}}}}]}]}}},
     {{deviceName,"gateway_tb"},
      {1,
       {ok,[{'ActionReply',0,asn1_NOVALUE,asn1_NOVALUE,
                [{serviceChangeReply,
                     {'ServiceChangeReply',
                         [{megaco_term_id,false,["root"]}],
                         {serviceChangeResParms,
                             {'ServiceChangeResParm',
                                 {deviceName,"controller"},
                                 asn1_NOVALUE,asn1_NOVALUE,asn1_NOVALUE,
                                 asn1_NOVALUE}}}}]}]}}},
     {{deviceName,"gateway_ut"},
      {1,
       {ok,[{'ActionReply',0,asn1_NOVALUE,asn1_NOVALUE,
                [{serviceChangeReply,
                     {'ServiceChangeReply',
                         [{megaco_term_id,false,["root"]}],
                         {serviceChangeResParms,
                             {'ServiceChangeResParm',
                                 {deviceName,"controller"},
                                 asn1_NOVALUE,asn1_NOVALUE,asn1_NOVALUE,
                                 asn1_NOVALUE}}}}]}]}}},
     {{deviceName,"gateway_ub"},
      {1,
       {ok,[{'ActionReply',0,asn1_NOVALUE,asn1_NOVALUE,
                [{serviceChangeReply,
                     {'ServiceChangeReply',
                         [{megaco_term_id,false,["root"]}],
                         {serviceChangeResParms,
                             {'ServiceChangeResParm',
                                 {deviceName,"controller"},
                                 asn1_NOVALUE,asn1_NOVALUE,
                                 asn1_NOVALUE,...}}}}]}]}}}]

The `Megaco` adopted viewer looks like this, when we have clicked on the [gateway_tt] actor name in order to only display the events regarding that actor:

A pretty printed `Megaco` message looks like this:

And the corresponding internal form for the same `Megaco` message looks like this:

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