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<img alt="Erlang logo" src="../../../../doc/erlang-logo.png"><br><small><a href="index.html">Reference Manual</a><br><a href="release_notes.html">Release Notes</a><br><a href="../pdf/kernel-2.15.3.pdf">PDF</a><br><a href="../../../../doc/index.html">Top</a></small><p><strong>Kernel</strong><br><strong>Reference Manual</strong><br><small>Version 2.15.3</small></p>
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<li title="start-1"><a href="erl_boot_server.html#start-1">start/1</a></li>
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<!-- refpage --><center><h1>seq_trace</h1></center>
<h3>MODULE</h3>
<div class="REFBODY">seq_trace</div>
<h3>MODULE SUMMARY</h3>
<div class="REFBODY">Sequential Tracing of Messages</div>
<h3>DESCRIPTION</h3>
<div class="REFBODY"><p>
<p>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 <span class="code">erlang:trace/3</span> BIF. See the chapter
<span class="bold_code"><a href="#whatis">What is Sequential Tracing</a></span>
below for more information about what sequential tracing is and
how it can be used.</p>
<p><span class="code">seq_trace</span> provides functions which control all aspects of
sequential tracing. There are functions for activation,
deactivation, inspection and for collection of the trace output.</p>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>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.</p>
</p></div>
</div>
</p></div>
<h3>DATA TYPES</h3>
<p>
<span class="bold_code"><a name="type-token">token()</a> = {integer(), boolean(), term(), term(), term()}</span><br></p>
<div class="REFBODY"><p>
<p>An opaque term (a tuple) representing a trace token.</p>
</p></div>
<h3>EXPORTS</h3>
<p><a name="set_token-1"></a><span class="bold_code">set_token(Token) -> PreviousToken | ok</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Token = PreviousToken = [] | <span class="bold_code"><a href="#type-token">token()</a></span></span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
</div></p>
<div class="REFBODY"><p>
<p>Sets the trace token for the calling process to <span class="code">Token</span>.
If <span class="code">Token == []</span> then tracing is disabled, otherwise
<span class="code">Token</span> should be an Erlang term returned from
<span class="code">get_token/0</span> or <span class="code">set_token/1</span>. <span class="code">set_token/1</span>
can be used to temporarily exclude message passing from
the trace by setting the trace token to empty like this:</p>
<div class="example"><pre>
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
... </pre></div>
<p>Returns the previous value of the trace token.</p>
</p></div>
<p><a name="set_token-2"></a><span class="bold_code">set_token(Component, Val) -> {Component, OldVal}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Component = component()</span></div>
<div class="REFTYPES"><span class="bold_code">Val = OldVal = value()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES">
<span class="bold_code">component() = label | serial | flag()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">flag() = send | 'receive' | print | timestamp</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">value() = (Integer :: integer() >= 0)<br> | {Previous :: integer() >= 0,<br> Current :: integer() >= 0}<br> | (Bool :: boolean())</span><br>
</div>
</div></p>
<div class="REFBODY"><p>
<p>Sets the individual <span class="code">Component</span> of the trace token to
<span class="code">Val</span>. Returns the previous value of the component.</p>
<dl>
<dt><strong><span class="code">set_token(label, Integer)</span></strong></dt>
<dd>
<p>The <span class="code">label</span> component is an integer which
identifies all events belonging to the same sequential
trace. If several sequential traces can be active
simultaneously, <span class="code">label</span> is used to identify
the separate traces. Default is 0.</p>
</dd>
<dt><strong><span class="code">set_token(serial, SerialValue)</span></strong></dt>
<dd>
<p><span class="code">SerialValue = {Previous, Current}</span>.
The <span class="code">serial</span> 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 <span class="code">{0, 0}</span>.</p>
</dd>
<dt><strong><span class="code">set_token(send, Bool)</span></strong></dt>
<dd>
<p>A trace token flag (<span class="code">true | false</span>) which
enables/disables tracing on message sending. Default is
<span class="code">false</span>.</p>
</dd>
<dt><strong><span class="code">set_token('receive', Bool)</span></strong></dt>
<dd>
<p>A trace token flag (<span class="code">true | false</span>) which
enables/disables tracing on message reception. Default is
<span class="code">false</span>.</p>
</dd>
<dt><strong><span class="code">set_token(print, Bool)</span></strong></dt>
<dd>
<p>A trace token flag (<span class="code">true | false</span>) which
enables/disables tracing on explicit calls to
<span class="code">seq_trace:print/1</span>. Default is <span class="code">false</span>.</p>
</dd>
<dt><strong><span class="code">set_token(timestamp, Bool)</span></strong></dt>
<dd>
<p>A trace token flag (<span class="code">true | false</span>) which
enables/disables a timestamp to be generated for each
traced event. Default is <span class="code">false</span>.</p>
</dd>
</dl>
</p></div>
<p><a name="get_token-0"></a><span class="bold_code">get_token() -> [] | <span class="bold_code"><a href="#type-token">token()</a></span></span><br></p>
<div class="REFBODY"><p>
<p>Returns the value of the trace token for the calling process.
If <span class="code">[]</span> 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 <span class="code">set_token/1</span> function.</p>
</p></div>
<p><a name="get_token-1"></a><span class="bold_code">get_token(Component) -> {Component, Val}</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Component = component()</span></div>
<div class="REFTYPES"><span class="bold_code">Val = value()</span></div>
<div class="REFTYPES">
<span class="bold_code">component() = label | serial | flag()</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">flag() = send | 'receive' | print | timestamp</span><br>
</div>
<div class="REFTYPES">
<span class="bold_code">value() = (Integer :: integer() >= 0)<br> | {Previous :: integer() >= 0,<br> Current :: integer() >= 0}<br> | (Bool :: boolean())</span><br>
</div>
</div></p>
<div class="REFBODY"><p>
<p>Returns the value of the trace token component
<span class="code">Component</span>. See
<span class="bold_code"><a href="#set_token-2">set_token/2</a></span> for
possible values of <span class="code">Component</span> and <span class="code">Val</span>.</p>
</p></div>
<p><a name="print-1"></a><span class="bold_code">print(TraceInfo) -> ok</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">TraceInfo = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Puts the Erlang term <span class="code">TraceInfo</span> into the sequential
trace output if the calling process currently is executing
within a sequential trace and the <span class="code">print</span> flag of
the trace token is set.</p>
</p></div>
<p><a name="print-2"></a><span class="bold_code">print(Label, TraceInfo) -> ok</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Label = integer()</span></div>
<div class="REFTYPES"><span class="bold_code">TraceInfo = term()</span></div>
</div></p>
<div class="REFBODY"><p>
<p>Same as <span class="code">print/1</span> with the additional condition that
<span class="code">TraceInfo</span> is output only if <span class="code">Label</span> is equal to
the label component of the trace token.</p>
</p></div>
<p><a name="reset_trace-0"></a><span class="bold_code">reset_trace() -> true</span><br></p>
<div class="REFBODY"><p>
<p>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.</p>
</p></div>
<p><a name="set_system_tracer-1"></a><span class="bold_code">set_system_tracer(Tracer) -> OldTracer</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Tracer = OldTracer = tracer()</span></div>
<div class="REFTYPES"><span class="bold_code"></span></div>
<div class="REFTYPES">
<span class="bold_code">tracer() = (Pid :: pid()) | port() | false</span><br>
</div>
</div></p>
<div class="REFBODY"><p>
<p>Sets the system tracer. The system tracer can be either a
process or port denoted by <span class="code">Tracer</span>. Returns the previous
value (which can be <span class="code">false</span> if no system tracer is
active).</p>
<p>Failure: <span class="code">{badarg, Info}}</span> if <span class="code">Pid</span> is not an
existing local pid.</p>
</p></div>
<p><a name="get_system_tracer-0"></a><span class="bold_code">get_system_tracer() -> Tracer</span><br><div class="REFBODY">
<p>Types:</p>
<div class="REFTYPES"><span class="bold_code">Tracer = tracer()</span></div>
<div class="REFTYPES">
<span class="bold_code">tracer() = (Pid :: pid()) | port() | false</span><br>
</div>
</div></p>
<div class="REFBODY"><p>
<p>Returns the pid or port identifier of the current system
tracer or <span class="code">false</span> if no system tracer is activated.</p>
</p></div>
<h3><a name="id135249">Trace Messages Sent To the System Tracer</a></h3>
<div class="REFBODY">
<p>The format of the messages are:</p>
<div class="example"><pre>
{seq_trace, Label, SeqTraceInfo, TimeStamp}</pre></div>
<p>or</p>
<div class="example"><pre>
{seq_trace, Label, SeqTraceInfo}</pre></div>
<p>depending on whether the <span class="code">timestamp</span> flag of the trace
token is set to <span class="code">true</span> or <span class="code">false</span>. Where:</p>
<div class="example"><pre>
Label = int()
TimeStamp = {Seconds, Milliseconds, Microseconds}
Seconds = Milliseconds = Microseconds = int()</pre></div>
<p>The <span class="code">SeqTraceInfo</span> can have the following formats:</p>
<dl>
<dt><strong><span class="code">{send, Serial, From, To, Message}</span></strong></dt>
<dd>
<p>Used when a process <span class="code">From</span> with its trace token flag
<span class="code">print</span> set to <span class="code">true</span> has sent a message.</p>
</dd>
<dt><strong><span class="code">{'receive', Serial, From, To, Message}</span></strong></dt>
<dd>
<p>Used when a process <span class="code">To</span> receives a message with a
trace token that has the <span class="code">'receive'</span> flag set to
<span class="code">true</span>.</p>
</dd>
<dt><strong><span class="code">{print, Serial, From, _, Info}</span></strong></dt>
<dd>
<p>Used when a process <span class="code">From</span> has called
<span class="code">seq_trace:print(Label, TraceInfo)</span> and has a trace
token with the <span class="code">print</span> flag set to <span class="code">true</span> and
<span class="code">label</span> set to <span class="code">Label</span>.</p>
</dd>
</dl>
<p><span class="code">Serial</span> is a tuple <span class="code">{PreviousSerial, ThisSerial}</span>,
where the first integer <span class="code">PreviousSerial</span> 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,
<span class="code">PreviousSerial</span> is set to the value of the process internal
"trace clock". The second integer <span class="code">ThisSerial</span> 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.</p>
</div>
<h3><a name="id135423">What is Sequential Tracing</a></h3>
<div class="REFBODY">
<a name="whatis"></a>
<p>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:</p>
<p>Each process has a <strong>trace token</strong>, which can be empty or
not empty. When not empty the trace token can be seen as
the tuple <span class="code">{Label, Flags, Serial, From}</span>. The trace token is
passed invisibly with each message.</p>
<p>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.</p>
<p>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.</p>
<p>On each Erlang node a process can be set as the <strong>system tracer</strong>. This process will receive trace messages each time
a message with a trace token is sent or received (if the trace
token flag <span class="code">send</span> or <span class="code">'receive'</span> is set). The system
tracer can then print each trace event, write it to a file or
whatever suitable.</p>
<div class="note">
<div class="label">Note</div>
<div class="content"><p>
<p>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).</p>
</p></div>
</div>
<p>In the following sections Sequential Tracing and its most
fundamental concepts are described.</p>
</div>
<h3><a name="id135490">Trace Token</a></h3>
<div class="REFBODY">
<p>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.</p>
<p>The current token of a process is set in two ways, either</p>
<ul>
<li>
<p>explicitly by the process itself, through a call to
<span class="code">seq_trace:set_token</span>, or</p>
</li>
<li>
<p>when a message is received.</p>
</li>
</ul>
<p>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.</p>
<p>A trace token contains a label, and a set of flags. Both
the label and the flags are set in 1 and 2 above.</p>
</div>
<h3><a name="id135541">Serial</a></h3>
<div class="REFBODY">
<p>The trace token contains a component which is called
<span class="code">serial</span>. It consists of two integers <span class="code">Previous</span> and
<span class="code">Current</span>. 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.</p>
<p>The algorithm for updating <span class="code">Serial</span> can be described as
follows:</p>
<p>Let each process have two counters <span class="code">prev_cnt</span> and
<span class="code">curr_cnt</span> which both are set to 0 when a process is created.
The counters are updated at the following occasions:</p>
<ul>
<li>
<p><strong>When the process is about to send a message and the trace token is not empty.</strong></p>
<p>Let the serial of the trace token be <span class="code">tprev</span> and
<span class="code">tcurr</span>. <br>
<span class="code">curr_cnt := curr_cnt + 1</span> <br>
<span class="code">tprev := prev_cnt</span> <br>
<span class="code">tcurr := curr_cnt</span></p>
<p>The trace token with <span class="code">tprev</span> and <span class="code">tcurr</span> is then
passed along with the message.</p>
</li>
<li>
<p><strong>When the process calls</strong><span class="code">seq_trace:print(Label, Info)</span>, <strong>Label matches the label part of the trace token and the trace token print flag is true.</strong></p>
<p>The same algorithm as for send above.</p>
</li>
<li>
<p><strong>When a message is received and contains a nonempty trace token.</strong></p>
<p>The process trace token is set to the trace token from
the message.</p>
<p>Let the serial of the trace token be <span class="code">tprev</span> and
<span class="code">tcurr</span>. <br>
<span class="code">if (curr_cnt < tcurr )</span> <br>
<span class="code">curr_cnt := tcurr</span> <br>
<span class="code">prev_cnt := tcurr</span></p>
</li>
</ul>
<p>The <span class="code">curr_cnt</span> 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 <span class="code">seq_trace:reset_trace/0</span>
can be called to reset the <span class="code">prev_cnt</span> and <span class="code">curr_cnt</span> 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.</p>
</div>
<h3><a name="id135725">Performance considerations</a></h3>
<div class="REFBODY">
<p>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.</p>
</div>
<h3><a name="id135739">Ports</a></h3>
<div class="REFBODY">
<p>Sequential tracing is not performed across ports.</p>
<p>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
<span class="code">seq_trace:get_token/1</span> and include trace information in
the message data sent to their respective ports.</p>
<p>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
<span class="code">seq_trace:set_token/2</span>.</p>
</div>
<h3><a name="id135769">Distribution</a></h3>
<div class="REFBODY">
<p>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.</p>
<p>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.</p>
</div>
<h3><a name="id135790">Example of Usage</a></h3>
<div class="REFBODY">
<p>The example shown here will give rough idea of how the new
primitives can be used and what kind of output it will produce.</p>
<p>Assume that we have an initiating process with
<span class="code">Pid == <0.30.0></span> like this:</p>
<div class="example"><pre>
-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).</pre></div>
<p>And a registered process <span class="code">call_server</span> with
<span class="code">Pid == <0.31.0></span> like this:</p>
<div class="example"><pre>
loop() ->
receive
{PortController,Message} ->
Ack = {received, Message},
seq_trace:print(17,"We are here now"),
PortController ! {ack,Ack}
end,
loop().</pre></div>
<p>A possible output from the system's sequential_tracer (inspired
by AXE-10 and MD-110) could look like:</p>
<div class="example"><pre>
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}}</pre></div>
<p>The implementation of a system tracer process that produces
the printout above could look like this:</p>
<div class="example"><pre>
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]).</pre></div>
<p>The code that creates a process that runs the tracer function
above and sets that process as the system tracer could look like
this:</p>
<div class="example"><pre>
start() ->
Pid = spawn(?MODULE,tracer,[]),
seq_trace:set_system_tracer(Pid), % set Pid as the system tracer
ok.</pre></div>
<p>With a function like <span class="code">test/0</span> below the whole example can be
started.</p>
<div class="example"><pre>
test() ->
P = spawn(?MODULE, loop, [port]),
register(call_server, spawn(?MODULE, loop, [])),
start(),
P ! {port,message}.</pre></div>
</div>
</div>
<div class="footer">
<hr>
<p>Copyright © 1997-2012 Ericsson AB. All Rights Reserved.</p>
</div>
</div>
</div></body>
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