POE::Wheel::FollowTailUser Contributed Perl DocumentaPOE::Wheel::FollowTail(3)NAMEPOE::Wheel::FollowTail - follow the tail of an ever-growing file
SYNOPSIS
#!perl
use POE qw(Wheel::FollowTail);
POE::Session->create(
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::Kernel->run();
exit;
DESCRIPTIONPOE::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.
PUBLIC METHODS
new
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:
Driver
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
"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
text.
$_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
Filename => "/var/log/snort/alert",
Filter => POE::Filter::Snort->new(),
InputEvent => "got_snort_alert",
);
PollInterval
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,
);
Seek
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
complete.
"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
"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.
Handle
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.
Filename
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.
InputEvent
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
"ResetEvent" is an optional. It specifies the name of the event that
indicates file rollover or reset. Please see "PUBLIC EVENTS" for more
details.
ErrorEvent
POE::Wheel::FollowTail may emit optional "ErrorEvent"s whenever it runs
into trouble. The data that comes with this event is explained in
"PUBLIC EVENTS".
event
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".
ID
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
handlers.
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
again.
tell
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, ">", "position.save" or die $!;
print $save $_[HEAP]{tailor}->tell(), "\n";
close $save;
}
sub handle_startup {
open my $save, "<", "position.save" or die $!;
chomp(my $seek = <$save>);
$_[HEAP]{tailor} = POE::Wheel::FollowTail->new(
...,
Seek => $seek,
);
}
PUBLIC EVENTSPOE::Wheel::FollowTail emits a small number of events.
InputEvent
"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
"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
"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
file.
$_[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.
$_[ARG3] contains the wheel's ID, which has been discussed in
"InputEvent".
This error handler logs a message to STDERR and then shuts down the
wheel. It assumes that the session is watching multiple files.
sub handle_tail_error {
my ($operation, $errnum, $errstr, $wheel_id) = @_[ARG0..ARG3];
warn "Wheel $wheel_id: $operation error $errnum: $errstr\n";
delete $_[HEAP]{tailors}{$wheel_id};
}
SEE ALSO
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.
BUGS
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.
AUTHORS & COPYRIGHTS
Please see POE for more information about authors and contributors.
POD ERRORS
Hey! The above document had some coding errors, which are explained
below:
Around line 773:
A non-empty Z<>
Around line 787:
A non-empty Z<>
Around line 803:
A non-empty Z<>
perl v5.14.2 2011-12-15 POE::Wheel::FollowTail(3)
