Trace Messages Sent to the System Tracer

The format of the messages is one of the following, depending on if flag timestamp of the trace token is set to true or false:

{seq_trace, Label, SeqTraceInfo, TimeStamp}

or

{seq_trace, Label, SeqTraceInfo}

Where:

Label = int()
TimeStamp = {Seconds, Milliseconds, Microseconds}  
  Seconds = Milliseconds = Microseconds = int()

SeqTraceInfo can have the following formats:

Serial is a tuple {PreviousSerial, ThisSerial}, where:

Sequential Tracing

Sequential tracing is a way to trace a sequence of messages sent between different local or remote processes, where the sequence is initiated by a single message. In short, it works as follows:

Each process has a trace token, which can be empty or not empty. When not empty, the trace token can be seen as the tuple {Label, Flags, Serial, From}. The trace token is passed invisibly with each message.

To start a sequential trace, the user must explicitly set the trace token in the process that will send the first message in a sequence.

The trace token of a process is set each time the process matches a message in a receive statement, according to the trace token carried by the received message, empty or not.

On each Erlang node, a process can be set as the system tracer. This process will receive trace messages each time a message with a trace token is sent or received (if the trace token flag send or 'receive' is set). The system tracer can then print each trace event, write it to a file, or whatever suitable.

The following sections describe sequential tracing and its most fundamental concepts.

Trace Token

Each process has a current trace token. Initially, the token is empty. When the process sends a message to another process, a copy of the current token is sent "invisibly" along with the message.

The current token of a process is set in one of the following two ways:

In both cases, the current token is set. In particular, if the token of a received message is empty, the current token of the process is set to empty.

A trace token contains a label and a set of flags. Both the label and the flags are set in both alternatives above.

Serial

The trace token contains a component called serial. It consists of two integers, Previous and Current. The purpose is to uniquely identify each traced event within a trace sequence, as well as to order the messages chronologically and in the different branches, if any.

The algorithm for updating Serial can be described as follows:

Let each process have two counters, prev_cnt and curr_cnt, both are set to 0 when a process is created. The counters are updated at the following occasions:

curr_cnt := curr_cnt + 1
tprev := prev_cnt
tcurr := curr_cnt

curr_cnt of a process is incremented each time the process is involved in a sequential trace. The counter can reach its limit (27 bits) if a process is very long-lived and is involved in much sequential tracing. If the counter overflows, the serial for ordering of the trace events cannot be used. To prevent the counter from overflowing in the middle of a sequential trace, function seq_trace:reset_trace/0 can be called to reset prev_cnt and curr_cnt of all processes in the Erlang node. This function also sets all trace tokens in processes and their message queues to empty, and thus stops all ongoing sequential tracing.

Performance Considerations

The performance degradation for a system that is enabled for sequential tracing is negligible as long as no tracing is activated. When tracing is activated, there is an extra cost for each traced message, but all other messages are unaffected.

Ports

Sequential tracing is not performed across ports.

If the user for some reason wants to pass the trace token to a port, this must be done manually in the code of the port controlling process. The port controlling processes have to check the appropriate sequential trace settings (as obtained from seq_trace:get_token/1) and include trace information in the message data sent to their respective ports.

Similarly, for messages received from a port, a port controller has to retrieve trace-specific information, and set appropriate sequential trace flags through calls to seq_trace:set_token/2.

Distribution

Sequential tracing between nodes is performed transparently. This applies to C-nodes built with Erl_Interface too. A C-node built with Erl_Interface only maintains one trace token, which means that the C-node appears as one process from the sequential tracing point of view.

Example of Use

This example gives a rough idea of how the new primitives can be used and what kind of output it produces.

Assume that you have an initiating process with Pid == <0.30.0> like this:

-module(seqex).
-compile(export_all).

loop(Port) ->
    receive 
        {Port,Message} ->
            seq_trace:set_token(label,17),
            seq_trace:set_token('receive',true),
            seq_trace:set_token(print,true),
            seq_trace:print(17,"**** Trace Started ****"),
            call_server ! {self(),the_message};
        {ack,Ack} ->
            ok
    end,
    loop(Port).

And a registered process call_server with Pid == <0.31.0> like this:

loop() ->
    receive
        {PortController,Message} ->
            Ack = {received, Message},
            seq_trace:print(17,"We are here now"),
            PortController ! {ack,Ack}
    end,
    loop().

A possible output from the system's sequential_tracer can be like this:

17:<0.30.0> Info {0,1} WITH
"**** Trace Started ****"
17:<0.31.0> Received {0,2} FROM <0.30.0> WITH
{<0.30.0>,the_message}
17:<0.31.0> Info {2,3} WITH
"We are here now"
17:<0.30.0> Received {2,4} FROM <0.31.0> WITH
{ack,{received,the_message}}

The implementation of a system tracer process that produces this printout can look like this:

tracer() ->
    receive
        {seq_trace,Label,TraceInfo} ->
           print_trace(Label,TraceInfo,false);
        {seq_trace,Label,TraceInfo,Ts} ->
           print_trace(Label,TraceInfo,Ts);
        Other -> ignore
    end,
    tracer().        

print_trace(Label,TraceInfo,false) ->
    io:format("~p:",[Label]),
    print_trace(TraceInfo);
print_trace(Label,TraceInfo,Ts) ->
    io:format("~p ~p:",[Label,Ts]),
    print_trace(TraceInfo).

print_trace({print,Serial,From,_,Info}) ->
    io:format("~p Info ~p WITH~n~p~n", [From,Serial,Info]);
print_trace({'receive',Serial,From,To,Message}) ->
    io:format("~p Received ~p FROM ~p WITH~n~p~n", 
              [To,Serial,From,Message]);
print_trace({send,Serial,From,To,Message}) ->
    io:format("~p Sent ~p TO ~p WITH~n~p~n", 
              [From,Serial,To,Message]).

The code that creates a process that runs this tracer function and sets that process as the system tracer can look like this:

start() ->
    Pid = spawn(?MODULE,tracer,[]),
    seq_trace:set_system_tracer(Pid), % set Pid as the system tracer 
    ok.

With a function like test/0, the whole example can be started:

test() ->
    P = spawn(?MODULE, loop, [port]),
    register(call_server, spawn(?MODULE, loop, [])),
    start(),
    P ! {port,message}.