perldoc > POE::Wheel::FollowTail

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📖 NAME

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

🚀 Quick Reference

Use Case	Command	Description
📄 Watch a log file for new lines	POE::Wheel::FollowTail->new( Filename => "/var/log/system.log", InputEvent => "got_log_line" )	Creates a wheel that emits InputEvent for each new line appended to the file
🔄 Handle log rotation / reset	... ResetEvent => "got_log_rollover"	Add ResetEvent to detect when the file is replaced or rolled over
⏱️ Adjust polling frequency	... PollInterval => 10	Check for new data every 10 seconds instead of the default 1
🎯 Seek to a specific position	... Seek => $position	Start reading from an arbitrary byte offset (positive = from start, negative = from end)
💾 Save & restore file position	$wheel->tell() then Seek => $saved_pos	Preserve position across program restarts
🔧 Custom data parsing	... Filter => POE::Filter::Snort->new()	Override the default newline filter with a custom parser
📌 Watch an already-open file handle	... Handle => $fh	Follow a file via its handle (cannot detect resets this way)
🛑 React to errors	... ErrorEvent => "handle_error"	Get notified when read operations fail

📝 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 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 ErrorEvents 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.

📖 NAME 🚀 Quick Reference 📝 SYNOPSIS 📚 DESCRIPTION 🔧 PUBLIC METHODS 📢 PUBLIC EVENTS 📎 SEE ALSO 🐛 BUGS 👥 AUTHORS & COPYRIGHTS