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NAME
seq_trace - Sequential tracing of information transfers.
DESCRIPTION
Sequential tracing makes it possible to trace information flows between processes
resulting from one initial transfer of information. 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
token() = {integer(), boolean(), term(), term(), term()}
An opaque term (a tuple) representing a trace token.
EXPORTS
set_token(Token) -> PreviousToken | ok
Types:
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) -> OldVal
Types:
Component = component()
Val = OldVal = value()
component() = label | serial | flag()
flag() =
send | 'receive' | print | timestamp | monotonic_timestamp |
strict_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.
Warning:
Labels were restricted to small signed integers (28 bits) prior to OTP 21. The
trace token will be silently 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 information
sending. Default is false.
set_token('receive', Bool):
A trace token flag (true | false) which enables/disables tracing on information
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}
Types:
Component = component()
Val = value()
component() = label | serial | flag()
flag() =
send | 'receive' | print | timestamp | monotonic_timestamp |
strict_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
Types:
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
Types:
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
Types:
Tracer = OldTracer = tracer()
tracer() =
(Pid :: pid()) |
port() |
(TracerModule :: {module(), term()}) |
false
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
Types:
Tracer = tracer()
tracer() =
(Pid :: pid()) |
port() |
(TracerModule :: {module(), term()}) |
false
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}
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 send set to true has sent
information. To may be a process identifier, a registered name on a node represented
as {NameAtom, NodeAtom}, or a node name represented as an atom. From may be a process
identifier or a node name represented as an atom. Message contains the information
passed along in this information transfer. If the transfer is done via message
passing, it is the actual message.
{'receive', Serial, From, To, Message}:
Used when a process To receives information with a trace token that has flag 'receive'
set to true. To may be a process identifier, or a node name represented as an atom.
From may be a process identifier or a node name represented as an atom. Message
contains the information passed along in this information transfer. If the transfer is
done via message passing, it is the actual message.
{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
information 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 information transfers between different
local or remote processes, where the sequence is initiated by a single transfer. The
typical information transfer is an ordinary Erlang message passed between two processes,
but information is transferred also in other ways. 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 when information is passed between processes. In most cases the information is
passed in ordinary messages between processes, but information is also passed between
processes by other means. For example, by spawning a new process. An information transfer
between two processes is represented by a send event and a receive event regardless of how
it is passed.
To start a sequential trace, the user must explicitly set the trace token in the process
that will send the first information in a sequence.
The trace token of a process is set each time the process receives information. This is
typically when 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 information 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 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.
DIFFERENT INFORMATION TRANSFERS
Information flows between processes in a lot of different ways. Not all flows of
information will be covered by sequential tracing. One example is information passed via
ETS tables. Below is a list of information paths that are covered by sequential tracing:
Message Passing:
All ordinary messages passed between Erlang processes.
Exit signals:
An exit signal is represented as an {'EXIT', Pid, Reason} tuple.
Process Spawn:
A process spawn is represented as multiple information transfers. At least one spawn
request and one spawn reply. The actual amount of information transfers depends on
what type of spawn it is and may also change in future implementations. Note that this
is more or less an internal protocol that you are peeking at. The spawn request will
be represented as a tuple with the first element containing the atom spawn_request,
but this is more or less all that you can depend on.
Note:
If you do ordinary send or receive trace on the system, you will only see ordinary message
passing, not the other information transfers listed above.
Note:
When a send event and corresponding receive event do not both correspond to ordinary
Erlang messages, the Message part of the trace messages may not be identical. This since
all information not necessarily are available when generating the trace messages.
TRACE TOKEN
Each process has a current trace token which is "invisibly" passed from the parent process
on creation of the process.
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 information is received. This is typically when a received message is matched out
in a receive expression, but also when information is received in other ways.
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 outside of a trace sequence. The counters are updated at the following
occasions:
* When the process is about to pass along information to another process and the trace
token is not empty. This typically occurs when sending a message, but also, for
example, when spawning another process.
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 information passed
to the other process.
* 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 information is received that also contains a non-empty trace token. For example,
when a message is matched out in a receive expression, or when a new process is
spawned.
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}.