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 POE::Session - POE State Machine


POE::Session - POE State Machine


  • Linux
  • Solaris
  • Windows
This module is not included with the standard ActivePerl distribution. It is available as a separate download using PPM.


  # Original inline session constructor:
  new POE::Session(
    name1 => \&name1_handler, # \&name1_handler is the "name1" state
    name2 => sub { ... },     # anonymous is the "name2" state
    \@start_args,             # ARG0..ARGn for the the _start state
  # Original package session constructor:
  new POE::Session(
    $package, [ 'name1',      # $package->name1() is the "name1" state
                'name2',      # $package->name2() is the "name2" state
    \@start_args,             # ARG0..ARGn for the start _start state
  # Original object session constructor:
  my $object1 = new SomeObject(...);
  my $object2 = new SomeOtherObject(...);
  new POE::Session(
    # $object1->name1() is the "name1" state
    # $object1->name2() is the "name2" state
    $object1 => [ 'name1', 'name2' ],
    # $object2->name1() is the "name3" state
    # $object2->name2() is the "name3" state
    $object2 => [ 'name3', 'name4' ],
    \@start_args,             # ARG0..ARGn for the _start state
  # New constructor:
  create POE::Session(
    # ARG0..ARGn for the session's _start handler
    args => \@args,
    inline_states  => { state1 => \&handler1,
                        state2 => \&handler2,
    object_states  => [ $objref1 => \@methods1,
                        $objref2 => { state_name_1 => 'method_name_1',
                                      state_name_2 => 'method_name_2',
    package_states => [ $package1 => \@function_names_1,
                        $package2 => { state_name_1 => 'method_name_1',
                                       state_name_2 => 'method_name_2',
    options => \%options,
  # Set or clear some session options:
  $session->option( trace => 1, default => 1 );


(Note: Session constructors were changed in version 0.06. Processes no longer support multiple kernels. This made the $kernel parameter to session constructors obsolete, so it was removed.)

POE::Session is a generic state machine class. Session instances are driven by state transition events, dispatched by POE::Kernel.

Sessions are POE's basic, self-contained units of program execution. They are equivalent to operating system processes or threads. As part of their creation, sessions register themselves with the process' Kernel instance. The kernel will keep track of their resources, and perform garbage collection at appropriate times.


POE::Session exports constants for states' parameters. The constants are discussed down in the STATE PARAMETERS section.


  • new

    POE::Session::new() is the original, highly overloaded constructor style. It creates a new POE::Session instance, populated with states given as its parameters.

    A reference to the new session will be given to the process' Kernel instance. The kernel will manage the session and its resources, and it expects perl to destroy the session when it releases its reference.

    The constructor will also return a reference to the new session. This reference may be used directly, but keeping a copy of it will prevent perl from garbage collecting the session when the kernel is done with it. Some uses for the session reference include posting ``bootstrap'' events to the session or manipulating the session's options with its option() method.

    Some people consider the new() constructor awkward, or ``action at a distance''. POE provides a semantically ``sweeter'' Kernel method, POE::Kernel::session_create() for these people. Please note, however, that session_create is depreciated as of version 0.06_09, since POE::Session has become a proper object.

    POE::Session::new() accepts pairs of parameters, with one exception. The first parameter in the pair determines the pair's type. The pairs may be used interchangeably:

    Inline states are described by a scalar and a coderef. The scalar is a string containing the state's name, which is also the name of the event that will trigger the state. The coderef is the Perl subroutine that will be called to handle the event.

      new POE::Session( event_name => \&state_handler );

    Object states are described by an object reference and a reference to an array of method names. The named methods will be invoked to handle similarly named events.

      my $object = new Object;
      new POE::Session( $object => [ qw(method1 method2 method3) ] );

    Package states are described by a package name and a reference to an array of subroutine names. The subroutines will handle events with the same names. If two or more packages are listed in the constructor, and the packages have matching subroutine names, then the last one wins.

      new POE::Session( 'Package' => [ 'sub1', 'sub2', 'sub3' ] );

    Sessions may use any combination of Inline, Object and Package states:

      my $object = new Object;
      new POE::Session( event_name => \&state_handler,
                        $object   => [ qw(method1 method2 method3) ],
                        'Package' => [ 'sub1', 'sub2', 'sub3' ]

    There is one parameter that isn't part of a pair. It is a stand-alone array reference. The contents of this arrayref are sent as arguments to the session's _start state.

  • create

    POE::Session::create() is a new constructor style. It does not use parameter overloading and DWIM to discern different session types. It also supports the ability to set options in the constructor, unlike POE::Session::new().

    Please see the SYNOPSIS for create() usage.

  • option

    POE::Session::option() stores, fetches or removes a session's option. Options are similar to environment variables.

    The option() method's behavior changed in version 0.06_09. It now supports fetching option values without changing or deleting the option.

      $session->option( 'name1' );         # Fetches option 'name1'
      $session->option( name2 => undef );  # Deletes option 'name2'
      $session->option( name3 => 1,        # Sets name3...
                        name4 => 2         # ... and name4.

    Actually, option() always returns the values of the options its passed. If more than one option is supplied in the parameters, then option() returns a reference to a hash containing names and previous values. If a single option is specified, then option() returns its value as a scalar.

    The option() method can only accept more than one option name while storing or deleting. POE::Session::option() only changes the options that are present as parameters. Unspecified options are left alone.

    For example:

      $session->option( trace => 1, default => 0 );

    Logical values may be sent as either 1, 0, 'on', 'off', 'yes', 'no', 'true' or 'false'. Stick with 1 and 0, though, because somebody somewhere won't like the value translation and will request that it be removed.

    These are the options that POE currently uses internally. Others may be added later.

    • trace

      Accepts a logical true/false value. This option enables or disables a trace of events as they're dispatched to states.

    • default

      Accepts a logical true/false value. When the ``default'' option is enabled, POE will warn and confess about events that arrive but can't be dispatched. Note: The ``default'' option will not do anything if the session has a _default state, because then every event can be dispatched.

  • ID

    POE::Session::ID() returns this session's unique ID, as maintained by POE::Kernel. It's a shortcut for $kernel->ID_session_to_id($session).


State parameters changed in version 0.06. Before 0.06, inline handlers received different parameters than object and package handlers. The call signatures have been unified in version 0.06. This breaks programs written with POE 0.05 or earlier. Thankfully, there aren't many.

To prevent future breakage, POE::Session now exports constants for parameters' offsets into @_. Programs that use the constants are guaranteed not to break whenever states' call signatures change. Or, if parameters are removed, programs will break at compile time rather than mysteriously failing at runtime.

Parameters may be used discretely:


If several parameters are needed multiple times, it may be easier (and faster) to assign them to lexicals all at once with an array slice:

  my ($kernel, $operation, $errnum, $errstr) =
     @_[KERNEL, ARG0, ARG1, ARG2];

The parameter constants are:


    The value in $_[OBJECT] is dependent on how the state was defined. It is undef for inline states. For object states, it contains a reference to the object that owns the method being called. For package states, it contains the name of the package the subroutine exists in.


    $_[KERNEL] is a reference to the kernel that is managing this session. It exists for times when $poe_kernel isn't available. $_[KERNEL] is recommended over $poe_kernel in states. They may be different at some point.


    $_[SESSION] is a reference to the current POE session. It is included mainly as a parameter to POE::Kernel methods, and for manipulating session options.

  • HEAP

    $_[HEAP] is a reference to a hash set aside for each session to store its global data. Information stored in the heap will be persistent between states, for the life of the session.

    POE will destroy the heap when its session stops, but it will not walk the heap and make sure that circular references are broken. Developers are expected to do any special heap cleanup in the session's _stop state.

    Support for using $_[HEAP] (formerly known as $me or $namespace) as an alias for $_[SESSION] in Kernel method calls is depreciated, starting in version 0.06. It will be removed after version 0.07.


    $_[STATE] is the name of the state being invoked. In most cases, this will be the name of the event that caused this handler to be called. In some cases though, most notably with _default and _signal, the state being invoked may not match the event being dispatched. (Predictably enough, it will be _default or _signal). You can find out the original event name for _default (see the _default event's description). The _signal event includes the signal name that caused it to be posted.


    $_[SENDER] is a reference to the session that sent the event. It is suitable as a destination for responses. Please be careful about deadlocks is using POE::Kernel::call() in both directions.

  • ARG0..ARG9

    @_[ARG0..ARG9] are the first ten elements of @args, as passed to the POE::Kernel post(), call(), yield(), alarm() and delay() methods. If more than ten items are needed, they may be referenced as $_[ARG9+1..], but it would be more efficient to pass them all as an array reference in $_[ARG0].

    Another way to grab the arguments, no matter how many there are, is:

      my @args = @_[ARG0..$#_];


Events that aren't prefixed with leading underscores may have been defined by the state machines themselves or by Wheel instances the machines are using.

In almost all these cases, the event name should be mapped to a state in the POE::Session constructor. Finding the event's source may be more difficult. It could come from a Wheel in the same session, or one of the &kernel calls. In the case of inter-session communication, it may even come from outside the session.


POE reserves some event names for internal and standard use. All its predefined events begin with an underscore, and future ones will too. It may be wise to avoid leading underscores in your own event names.

Every predefined event is accompanied by the standard OBJECT, KERNEL, SESSION, HEAP, STATE and SENDER parameters.

  • _start

    Sessions can't start running until the kernel knows they exist. After the kernel registers a session, it sends a _start event to let it know it's okay to begin. POE requires every state machine to have a special _start state. Otherwise, how would they know when to start?

    SENDER contains a reference to the new session's parent session.

    ARG0..ARG9 contain parameters as they were given to the session's constructor. See POE::Session::new() and POE::Session::create() for more information.

  • _stop

    Sessions receive _stop events when it is time for them to stop. _stop is dispatched to sessions just before the kernel destroys them. The _stop state commonly contains special destructor code, possibly to clean up things that the kernel doesn't know about.

    Sessions stop when they run out of pending state transition events and don't hold resources to create new ones. Event-generating resources include selects (filehandle monitors), child sessions, and aliases.

    The kernel's run() method will return if all its sessions stop.

    SENDER is the session that posted the _stop event. In the case of resource starvation, this is the KERNEL.

    ARG0..ARG9 are empty in the case of resource starvation.

  • _signal

    POE sets handlers for most of the signals in %SIG. The only exceptions are things which might exist in %SIG but probably shouldn't. POE will not register signal handlers for SIGRTMIN, for example, because doing that breaks Perl on some HP-UX systems.

    Signals are propagated to child sessions first. Since every session is a descendent of the kernel, posting signals to the kernel guarantees that every session receives them.

    POE does not yet magically solve Perl's problems with signals. Namely, perl tends to dump core if it keeps receiving signals. That has a detrimental effect on programs that expect long uptimes.

    There are a few kinds of signals. The kernel processes each kind differently:

    SIGPIPE causes a _signal event to be posted directly to the session that is running when the signal was received. ARG0 contains the signal name, as it appears in %SIG.

    The handler for SIGCHLD and SIGCLD calls wait() to acquire the dying child's process ID and result code. If the child PID is valid, a _signal event will be posted to all sessions. ARG0 will contain CHLD regardless of the actual signal name. ARG1 contains the child PID, and ARG2 contains the contents of $? just after the wait() call.

    All other signals cause a _signal event to be posted to all sessions. ARG0 contains the signal name as it appears in %SIG.

    SIGWINCH is ignored. Resizing an xterm causes a bunch of these, quickly killing perl.

  • _garbage_collect

    The _garbage_collect event tells the kernel to check a session's resources and stop it if none are left. It never is dispatched to a session. This was added to delay garbage collection checking for new sessions. This delayed garbage collection gives parent sessions a chance to interact with their newly-created children.

  • _parent

    The _parent event lets child sessions know that they are about to be orphaned and adopted. It tells each child session who their old parent was and who their new parent is.

    SENDER should always equal KERNEL. If not, the event was spoofed by another session. ARG0 is the session's old parent; ARG1 is the session's new one.

  • _child

    The _child event is sent to parent sessions when they acquire or lose child sessions. _child is dispatched to parents after the children receive _start or before they receive _stop.

    SENDER should always equal KERNEL. If not, the event was spoofed by another session.

    ARG0 indicates what is happening to the child. It is 'gain' if the session is a grandchild being given by a dying child. It is 'lose' if the session is itself a dying child. It is 'create' if the child was created by the current session.

    ARG1 is a reference to the child session. It will still be valid, even if the child is in its death throes.

    ARG2 is only valid when ARG0 is 'create'. It contains the return value of the child's _start state. See ABOUT STATES' RETURN VALUES for more information about states' return values.

  • Select Events

    Select events are generated by POE::Kernel when selected file handles become active. They have no default names.

    ARG0 is the file handle that had activity.

  • _default

    The _default state is invoked whenever a session receives an event for which it does not have a registered state. If the session doesn't have a _default state, then the event will be discarded. If the session's default option is true, then POE will carp and confess about the discarded event.

    ARG0 holds the original event's name. ARG1 holds a reference to the original event's parameters.

    If _default catches a _signal event, its return value will be used to determine if the signal was handled. This may make some programs difficult to stop. Please see the description for the _signal event for more information.

    The _default state can be used to catch misspelled events, but $session->option('default',1) may be better.


States are evaluated in a scalar context. States that must return more than one value should return an arrayref instead.

Signal handlers tell POE whether or not a signal was handled by returning a logical true or false value. See the description for the _signal state for more information.

POE::Kernel::call() will return whatever a called state returns. See the description of POE::Kernel::call() for more information.

If a state returns a reference to an object in the POE namespace (or any namespace starting with POE::), then that reference is immediately stringified. This is done to prevent ``blessing bleed'' (see the Changes file) from interfering with POE's and Perl's garbage collection. The code that checks for POE objects does not look inside data passed by reference-- it's just there to catch accidents, like:

  sub _stop {
    delete $_[HEAP]->{'readwrite wheel'};
    # reference to the readwrite wheel is implicitly returned

That accidentally returns a reference to a POE::Wheel::ReadWrite object. If the reference was not stringified, it would delay the wheel's destruction until after the session stopped. The wheel would try to remove its states from the nonexistent session, and the program would crash.


POE; POE::Kernel


The documentation for POE::Session::create() is fairly nonexistent.


Please see the POE manpage.

 POE::Session - POE State Machine