Sequential tracing of messages.

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.


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() = label | serial | flag()
  • flag() =
        send |
        'receive' |
        print |
        timestamp |
        monotonic_timestamp |
  • value() =
        (Label :: term()) |
        {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, Label)

The label component is a term 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.


Labels were restricted to small signed integers (28 bits) prior to OTP 21. The trace token will be silenty dropped if it crosses over to a node that does not support the label.

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.

set_token(strict_monotonic_timestamp, Bool)

A trace token flag (true | false) which enables/disables a strict monotonic timestamp to be generated for each traced event. Default is false. Timestamps will consist of Erlang monotonic time and a monotonically increasing integer. The time-stamp has the same format and value as produced by {erlang:monotonic_time(nanosecond), erlang:unique_integer([monotonic])}.

set_token(monotonic_timestamp, Bool)

A trace token flag (true | false) which enables/disables a strict monotonic timestamp to be generated for each traced event. Default is false. Timestamps will use Erlang monotonic time. The time-stamp has the same format and value as produced by erlang:monotonic_time(nanosecond).

If multiple timestamp flags are passed, timestamp has precedence over strict_monotonic_timestamp which in turn has precedence over monotonic_timestamp. All timestamp flags are remembered, so if two are passed and the one with highest precedence later is disabled the other one will become active.

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 function.

get_token(Component) -> {Component, Val}

  • component() = label | serial | flag()
  • flag() =
        send |
        'receive' |
        print |
        timestamp |
        monotonic_timestamp |
  • value() =
        (Label :: term()) |
        {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() =
        (Pid :: pid()) |
        port() |
        (TracerModule :: {module(), term()}) |

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

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

get_system_tracer() -> Tracer

  • tracer() =
        (Pid :: pid()) |
        port() |
        (TracerModule :: {module(), term()}) |

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

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}


{seq_trace, Label, SeqTraceInfo}


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 system tracer only receives those trace events that occur locally within the Erlang node. To get the whole picture of a sequential trace, involving processes on many Erlang nodes, the output from the system tracer on each involved node must be merged (offline).

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.


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.


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.


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:


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 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
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 this printout can 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 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 

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

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