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POE::Wheel::FollowTailUser Contributed Perl DocumentaPOE::Wheel::FollowTail(3)

       POE::Wheel::FollowTail -	follow the tail	of an ever-growing file


	 use POE qw(Wheel::FollowTail);

	   inline_states => {
	     _start => sub {
	       $_[HEAP]{tailor}	= POE::Wheel::FollowTail->new(
		 Filename => "/var/log/system.log",
		 InputEvent => "got_log_line",
		 ResetEvent => "got_log_rollover",
	     got_log_line => sub {
	       print "Log: $_[ARG0]\n";
	     got_log_rollover => sub {
	       print "Log rolled over.\n";


       POE::Wheel::FollowTail objects watch for	new data at the	end of a file
       and generate new	events when things happen to the file. Its "Filter"
       parameter defines how to	parse data from	the file. Each new item	is
       sent to the creator's session as	an "InputEvent"	event. Log rotation
       will trigger a "ResetEvent".

       POE::Wheel::FollowTail only reads from a	file, so it doesn't implement
       a put() method.

       new() returns a new POE::Wheel::FollowTail object.  As long as this
       object exists, it will generate events when the corresponding file's
       status changes.

       new() accepts a small set of named parameters:


       The optional "Driver" parameter specifies which driver to use when
       reading from the	tailed file.  If omitted, POE::Wheel::FollowTail will
       use POE::Driver::SysRW.	This is	almost always the right	thing to do.


       "Filter"	is an optional constructor parameter that specifies how	to
       parse data from the followed file.  By default, POE::Wheel::FollowTail
       will use	POE::Filter::Line to parse files as plain, newline-separated

	 $_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
	   Filename => "/var/log/snort/alert",
	   Filter => POE::Filter::Snort->new(),
	   InputEvent => "got_snort_alert",


       POE::Wheel::FollowTail needs to periodically check for new data on the
       followed	file.  "PollInterval" specifies	the number of seconds to wait
       between checks.	Applications that need to poll once per	second may
       omit "PollInterval", as it defaults to 1.

       Longer poll intervals may be used to reduce the polling overhead	for
       infrequently updated files.

	 $_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
	   PollInterval	=> 10,


       If specified, "Seek" instructs POE::Wheel::FollowTail to	seek to	a
       specific	spot in	the tailed file	before beginning to read from it.  A
       positive	"Seek" value is	interpreted as the number of octets to seek
       from the	start of the file.  Negative "Seek" will, like negative	array
       indices,	seek backwards from the	end of the file.  Zero "Seek" starts
       reading from the	beginning of the file.

       Be careful when using "Seek", as	it's quite easy	to seek	into the
       middle of a record.  When in doubt, and when beginning at the end of
       the file, omit "Seek" entirely.	POE::Wheel::FollowTail will seek 4
       kilobytes back from the end of the file,	then parse and discard all
       records unto EOF.  As long as the file's	records	are smaller than 4
       kilobytes, this will guarantee that the first record returned will be

       "Seek" may also be used with the	wheel's	tell() method to restore the
       file position after a program restart.  Save the	tell() value prior to
       exiting,	and load and "Seek" back to it on subsequent start-up.


       "SeekBack" behaves like the inverse of "Seek".  A positive value	acts
       like a negative "Seek".	A negative value acts like a positive "Seek".
       A zero "SeekBack" instructs POE::Wheel::FollowTail to begin at the very
       end of the file.

       "Seek" and "SeekBack" are mutually exclusive.

       See "Seek" for caveats, techniques, and an explanation of the magic
       that happens when neither "Seek"	nor "SeekBack" is specified.


       POE::Wheel::FollowTail may follow a previously opened file "Handle".
       Unfortunately it	cannot follow log resets this way, as it won't be able
       to reopen the file once it has been reset.  Applications	that must
       follow resets should use	"Filename" instead.

       "Handle"	is still useful	for files that will never be reset, or for
       devices that require setup outside of POE::Wheel::FollowTail's purview.

       "Handle"	and "Filename" are mutually exclusive.	One of them is
       required, however.


       Specify the "Filename" to watch.	 POE::Wheel::FollowTail	will wait for
       the file	to appear if it	doesn't	exist.	The wheel will also reopen the
       file if it disappears, such as when it has been reset or	rolled over.
       In the case of a	reset, POE::Wheel::FollowTail will also	emit a
       "ResetEvent", if	one has	been requested.

       "Handle"	and "Filename" are mutually exclusive.	One of them is
       required, however.

       See the "SYNOPSIS" for an example.


       "IdleEvent" is an optional event.  If specified,	it will	fire whenever
       POE::Wheel::FollowTail checks for activity but sees nothing.  It	was
       added in	POE 1.362 as a way to advance certain test programs without
       needing to wait conservatively large amounts of time.

       "IdleEvent" is described	in "PUBLIC EVENTS".


       The "InputEvent"	parameter is required, and it specifies	the event to
       emit when new data arrives in the watched file.	"InputEvent" is
       described in detail in "PUBLIC EVENTS".


       "ResetEvent" is an optional.  It	specifies the name of the event	that
       indicates file rollover or reset.  Please see "PUBLIC EVENTS" for more


       POE::Wheel::FollowTail may emit optional	"ErrorEvent"s whenever it runs
       into trouble.  The data that comes with this event is explained in

       event() allows a	session	to change the events emitted by	a wheel
       without destroying and re-creating the object.  It accepts one or more
       of the events listed in "PUBLIC EVENTS".	 Undefined event names disable
       those events.

       Stop handling log resets:

	 sub some_event_handler	{
	   $_[HEAP]{tailor}->event( ResetEvent => undef	);

       The events are described	in more	detail in "PUBLIC EVENTS".

       The ID()	method returns the wheel's unique ID.  It's useful for storing
       the wheel in a hash.  All POE::Wheel events should be accompanied by a
       wheel ID, which allows the wheel	to be referenced in their event

	 sub setup_tailor {
	   my $wheel = POE::Wheel::FollowTail->new(... incomplete ...);
	   $_[HEAP]{tailors}{$wheel->ID} = $wheel;

       See the example in "ErrorEvent" for a handler that will find this wheel

       tell() returns the current position for the file	being watched by
       POE::Wheel::FollowTail.	It may be useful for saving the	position
       program termination.  new()'s "Seek" parameter may be used to resume
       watching	the file where tell() left off.

	 sub handle_shutdown {
	   # Not robust.  Do better in production.
	   open	my $save, ">", "" or die $!;
	   print $save $_[HEAP]{tailor}->tell(), "\n";
	   close $save;

	 sub handle_startup {
	   open	my $save, "<", "" or die $!;
	   chomp(my $seek = <$save>);
	   $_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
	     Seek => $seek,

       POE::Wheel::FollowTail emits a small number of events.

       "IdleEvent" specifies the name of an event to be	fired when
       POE::Wheel::FollowTail doesn't detect activity on the watched file.

       $_[ARG0]	contains the ID	of the POE::Wheel::FollowTail object that
       fired the event.

       "InputEvent" sets the name of the event to emit when new	data arrives
       into the	tailed file.  The event	will be	accompanied by two parameters:

       $_[ARG0]	contains the data that was read	from the file, after being
       parsed by the current "Filter".

       $_[ARG1]	contains the wheel's ID, which may be used as a	key into a
       data structure tracking multiple	wheels.	 No assumption should be made
       about the nature	or format of this ID, as it may	change at any time.
       Therefore, track	your wheels in a hash.

       See the "SYNOPSIS" for an example.

       "ResetEvent" names the event to be emitted whenever the wheel detects
       that the	followed file has been reset.  It's only available when
       watching	files by name, as POE::Wheel::FollowTail must reopen the file
       after it	has been reset.

       "ResetEvent" comes with only one	parameter, $_[ARG0], which contains
       the wheel's ID.	See "InputEvent" for some notes	about what may be done
       with wheel IDs.

       See the "SYNOPSIS" for an example.

       "ErrorEvent" names the event emitted when POE::Wheel::FollowTail
       encounters a problem.  Every "ErrorEvent" comes with four parameters
       that describe the error and its situation:

       $_[ARG0]	describes the operation	that failed.  This is usually "read",
       since POE::Wheel::FollowTail spends most	of its time reading from a

       $_[ARG1]	and $_[ARG2] contain the numeric and stringified values	of $!,
       respectively.  They will	never contain EAGAIN (or its local equivalent)
       since POE::Wheel::FollowTail handles that error itself.

       $_[ARG3]	contains the wheel's ID, which has been	discussed in

       This error handler logs a message to STDERR and then shuts down the
       wheel.  It assumes that the session is watching multiple	files.

	 sub handle_tail_error {
	   my ($operation, $errnum, $errstr, $wheel_id)	= @_[ARG0..ARG3];
	   warn	"Wheel $wheel_id: $operation error $errnum: $errstr\n";
	   delete $_[HEAP]{tailors}{$wheel_id};

       POE::Wheel describes the	basic operations of all	wheels in more depth.
       You need	to know	this.

       The SEE ALSO section in POE contains a table of contents	covering the
       entire POE distribution.

       This wheel can't	tail pipes and consoles	on some	operating systems.

       POE::Wheel::FollowTail generally	reads ahead of the data	it returns, so
       the tell() position may be later	in the file than the data an
       application has already received.

       Please see POE for more information about authors and contributors.

perl v5.32.0			  2020-02-01	     POE::Wheel::FollowTail(3)


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