POE::Wheel::FollowTail - phpMan

NAME
    POE::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;

DESCRIPTION
    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.

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.

   IdleEvent
    "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".

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

  IdleEvent
    "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
    "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.