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.
Note!
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.
Functions
set_token(Token) -> PreviousToken
Token = PreviousToken = term() | []
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
Val = OldVal -- see below
Sets the individual Component
of the trace token to
Val
. Returns the previous value of the component.
set_token(label, Int)
-
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}
. Theserial
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 isfalse
. set_token('receive', Bool)
-
A trace token flag (
true | false
) which enables/disables tracing on message reception. Default isfalse
. set_token(print, Bool)
-
A trace token flag (
true | false
) which enables/disables tracing on explicit calls toseq_trace:print/1
. Default isfalse
. set_token(timestamp, Bool)
-
A trace token flag (
true | false
) which enables/disables a timestamp to be generated for each traced event. Default isfalse
.
get_token() -> TraceToken
TraceToken = term() | []
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
Val -- see set_token/2
Returns the value of the trace token component
Component
. See
set_token/2 for
possible values of Component
and Val
.
print(TraceInfo) -> void()
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) -> void()
Label = int()
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() -> void()
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 = 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
active).
Failure: {badarg, Info}}
if Pid
is not an
existing local pid.
get_system_tracer() -> Tracer
Tracer = pid() | port() | false
Returns the pid or port identifier 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 are:
{seq_trace, Label, SeqTraceInfo, TimeStamp}
or
{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 flagprint
set totrue
has sent a message. {'receive', Serial, From, To, Message}
-
Used when a process
To
receives a message with a trace token that has the'receive'
flag set totrue
. {print, Serial, From, _, Info}
-
Used when a process
From
has calledseq_trace:print(Label, TraceInfo)
and has a trace token with theprint
flag set totrue
andlabel
set toLabel
.
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.
What is 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 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.
Note!
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 described.
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 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.
Serial
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
andtcurr
.
curr_cnt := curr_cnt + 1
tprev := prev_cnt
tcurr := curr_cnt
The trace token with
tprev
andtcurr
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 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
andtcurr
.
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.
Performance considerations
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.
Ports
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
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 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.
Example of Usage
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:
-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 (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 {<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 the printout above could 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 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 ok.
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, [])), start(), P ! {port,message}.