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       seq_trace - Sequential Tracing of Messages


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

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

       The  implementation  of  sequential  tracing  is  in  beta  status.  This  means  that the
       programming interface still might undergo minor adjustments (possibly incompatible)  based
       on feedback from users.


       token() = {integer(), boolean(), term(), term(), term()}

              An opaque term (a tuple) representing a trace token.


       set_token(Token) -> PreviousToken | ok


                 Token = PreviousToken = [] | token()

              Sets  the trace token for the calling process to Token. If Token == [] then tracing
              is disabled, otherwise Token should be an Erlang term returned from get_token/0  or
              set_token/1.  set_token/1  can  be used to temporarily exclude message passing from
              the trace by setting the trace token to empty like this:

              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

              Returns the previous value of the trace token.

       set_token(Component, Val) -> {Component, OldVal}


                 Component = component()
                 Val = OldVal = value()
                 component() = label | serial | flag()
                 flag() = send | 'receive' | print | timestamp
                 value() = (Integer :: integer() >= 0)
                         | {Previous :: integer() >= 0,
                            Current :: integer() >= 0}
                         | (Bool :: boolean())

              Sets the individual Component of the trace token to Val. Returns the previous value
              of the component.

                set_token(label, Integer):
                  The  label component is an integer which identifies all events belonging to the
                  same  sequential  trace.  If  several   sequential   traces   can   be   active
                  simultaneously, label is used to identify the separate traces. Default is 0.

                set_token(serial, SerialValue):
                  SerialValue = {Previous, Current}. The serial component contains counters which
                  enables the traced messages to be sorted, should never be set explicitly by the
                  user as these counters are updated automatically. Default is {0, 0}.

                set_token(send, Bool):
                  A  trace  token  flag  (true | false) which enables/disables tracing on message
                  sending. Default is false.

                set_token('receive', Bool):
                  A trace token flag (true | false) which  enables/disables  tracing  on  message
                  reception. Default is false.

                set_token(print, Bool):
                  A  trace  token  flag (true | false) which enables/disables tracing on explicit
                  calls to seq_trace:print/1. Default is false.

                set_token(timestamp, Bool):
                  A trace token flag (true | false) which  enables/disables  a  timestamp  to  be
                  generated for each traced event. Default is false.

       get_token() -> [] | token()

              Returns the value of the trace token for the calling process. If [] is returned, it
              means that tracing is not active. Any other value  returned  is  the  value  of  an
              active  trace  token.  The  value  returned can be used as input to the set_token/1

       get_token(Component) -> {Component, Val}


                 Component = component()
                 Val = value()
                 component() = label | serial | flag()
                 flag() = send | 'receive' | print | timestamp
                 value() = (Integer :: integer() >= 0)
                         | {Previous :: integer() >= 0,
                            Current :: integer() >= 0}
                         | (Bool :: boolean())

              Returns the value of the trace  token  component  Component.  See  set_token/2  for
              possible values of Component and Val.

       print(TraceInfo) -> ok


                 TraceInfo = term()

              Puts  the  Erlang  term  TraceInfo  into the sequential trace output if the calling
              process currently is executing within a sequential trace and the print flag of  the
              trace token is set.

       print(Label, TraceInfo) -> ok


                 Label = integer()
                 TraceInfo = term()

              Same  as  print/1  with  the  additional condition that TraceInfo is output only if
              Label is equal to the label component of the trace token.

       reset_trace() -> true

              Sets the trace token to empty for all processes on  the  local  node.  The  process
              internal  counters  used  to  create the serial of the trace token is set to 0. The
              trace token is set to empty for all messages in message queues. Together this  will
              effectively stop all ongoing sequential tracing in the local node.

       set_system_tracer(Tracer) -> OldTracer


                 Tracer = OldTracer = tracer()
                 tracer() = (Pid :: pid()) | port() | false

              Sets  the  system tracer. The system tracer can be either a process or port denoted
              by Tracer. Returns the previous value (which can be false if no  system  tracer  is

              Failure: {badarg, Info}} if Pid is not an existing local pid.

       get_system_tracer() -> Tracer


                 Tracer = tracer()
                 tracer() = (Pid :: pid()) | port() | false

              Returns  the  pid  or  port  identifier of the current system tracer or false if no
              system tracer is activated.


       The format of the messages are:

       {seq_trace, Label, SeqTraceInfo, TimeStamp}


       {seq_trace, Label, SeqTraceInfo}

       depending on whether the timestamp flag of the trace token is set to true or false. Where:

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

       The 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

         {'receive', Serial, From, To, Message}:
           Used  when  a  process To receives a message with a trace token that has the 'receive'
           flag 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 the print flag set to true and label set to Label.

       Serial  is  a  tuple  {PreviousSerial, ThisSerial}, where the first integer PreviousSerial
       denotes the serial counter passed in the last  received  message  which  carried  a  trace
       token. If the process is the first one in a new sequential trace, PreviousSerial is set to
       the value of the process internal "trace clock". The  second  integer  ThisSerial  is  the
       serial  counter  that  a  process sets on outgoing messages and 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  is a way to trace a sequence of messages sent between different local
       or remote processes, where the sequence is initiated by one single message.  In  short  it
       works like this:

       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.

       In  order  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 system tracer will only receive those trace  events  that  occur  locally  within  the
       Erlang  node.  To  get  the whole picture of a sequential trace that involves processes on
       several Erlang nodes, the output from the system tracer on  each  involved  node  must  be
       merged (off line).

       In  the  following  sections  Sequential  Tracing  and  its  most fundamental concepts are


       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 will be sent "invisibly"
       along with the message.

       The current token of a process is set in two ways, either

         * explicitly by the process itself, through a call to seq_trace:set_token, or

         * when a message is received.

       In both cases the current token will be set. In particular, if  the  token  of  a  message
       received 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 1 and 2 above.


       The trace token contains a component which is called serial. It consists of  two  integers
       Previous and Current. The purpose is to uniquely identify each traced event within a trace
       sequence and 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 which 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  callsseq_trace:print(Label, Info), Label matches the label part of
           the trace token and the trace token print flag is true.

           The same algorithm as for send above.

         * When a message is received and contains a nonempty 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

       The 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 it will not be  possible  to
       use  the  serial for ordering of the trace events. To prevent the counter from overflowing
       in the middle of a sequential trace the function seq_trace:reset_trace/0 can be called  to
       reset  the  prev_cnt  and curr_cnt of all processes in the Erlang node. This function will
       also set all trace tokens in processes and their message queues to  empty  and  will  thus
       stop all ongoing sequential tracing.


       The  performance  degradation  for  a  system  which  is enabled for Sequential Tracing is
       negligible as long as no tracing is activated. When tracing is  activated  there  will  of
       course be an extra cost for each traced message but all other messages will be unaffected.


       Sequential tracing is not performed across ports.

       If  the  user  for some reason wants to pass the trace token to a port this has to 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


       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 will appear as one process from the sequential  tracing  point
       of view.

       In  order  to  be able to perform sequential tracing between distributed Erlang nodes, the
       distribution protocol has been extended (in a backward compatible  way).  An  Erlang  node
       which  supports  sequential  tracing  can  communicate  with  an  older (OTP R3B) node but
       messages passed within that node can of course not be traced.


       The example shown here will give rough idea of how the new primitives can be used and what
       kind of output it will produce.

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


       loop(Port) ->
               {Port,Message} ->
                   seq_trace:print(17,"**** Trace Started ****"),
                   call_server ! {self(),the_message};
               {ack,Ack} ->

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

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

       A  possible  output  from  the  system's sequential_tracer (inspired by AXE-10 and MD-110)
       could look like:

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

       The implementation of a system tracer process that produces the printout above could  look
       like this:

       tracer() ->
               {seq_trace,Label,TraceInfo} ->
               {seq_trace,Label,TraceInfo,Ts} ->
               Other -> ignore

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

       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",
       print_trace({send,Serial,From,To,Message}) ->
           io:format("~p Sent ~p TO ~p WITH~n~p~n",

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

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

       With a function like test/0 below the whole example can be started.

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