seq_trace

Module

seq_trace

Module Summary

Sequential tracing of messages.

Description

Sequential tracing makes it possible to trace all messages resulting from one initial message. Sequential tracing is independent of the ordinary tracing in Erlang, which is controlled by the erlang:trace/3 BIF. For more information about what sequential tracing is and how it can be used, see section Sequential Tracing.

seq_trace provides functions that control all aspects of sequential tracing. There are functions for activation, deactivation, inspection, and for collection of the trace output.

Data Types

Exports

OldToken = seq_trace:set_token([]), % set to empty and save 
                                    % old value
% do something that should not be part of the trace
io:format("Exclude the signalling caused by this~n"),
seq_trace:set_token(OldToken), % activate the trace token again
...  

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:

{send, Serial, From, To, Message}

Used when a process From with its trace token flag print set to true has sent a message.

{'receive', Serial, From, To, Message}

Used when a process To receives a message with a trace token that has flag 'receive' set to true.

{print, Serial, From, _, Info}

Used when a process From has called seq_trace:print(Label, TraceInfo) and has a trace token with flag print set to true, and label set to Label.

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

• Integer PreviousSerial denotes the serial counter passed in the last received message that carried a trace token. If the process is the first in a new sequential trace, PreviousSerial is set to the value of the process internal "trace clock".

• Integer ThisSerial is the serial counter that a process sets on outgoing messages. It is based on the process internal "trace clock", which is incremented by one before it is attached to the trace token in the message.

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:

• Explicitly by the process itself, through a call to seq_trace:set_token/1,2

• When a message is received

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:

• When the process is about to send a message and the trace token is not empty.

  Let the serial of the trace token be tprev and tcurr.

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

  The trace token with tprev and tcurr is then passed along with the message.

• When the process calls seq_trace:print(Label, Info), Label matches the label part of the trace token and the trace token print flag is true.

  The algorithm is the same as for send above.

• When a message is received and contains a non-empty trace token.

  The process trace token is set to the trace token from the message.

  Let the serial of the trace token be tprev and tcurr.

  if (curr_cnt < tcurr )
     curr_cnt := tcurr
  prev_cnt := tcurr

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}.