NAME
    AnyEvent::MP - erlang-style multi-processing/message-passing framework

SYNOPSIS
       use AnyEvent::MP;

       $NODE      # contains this node's node ID
       NODE       # returns this node's node ID

       $SELF      # receiving/own port id in rcv callbacks

       # initialise the node so it can send/receive messages
       configure;

       # ports are message destinations

       # sending messages
       snd $port, type => data...;
       snd $port, @msg;
       snd @msg_with_first_element_being_a_port;

       # creating/using ports, the simple way
       my $simple_port = port { my @msg = @_ };

       # creating/using ports, tagged message matching
       my $port = port;
       rcv $port, ping => sub { snd $_[0], "pong" };
       rcv $port, pong => sub { warn "pong received\n" };

       # create a port on another node
       my $port = spawn $node, $initfunc, @initdata;

       # destroy a port again
       kil $port;  # "normal" kill
       kil $port, my_error => "everything is broken"; # error kill

       # monitoring
       mon $port, $cb->(@msg)      # callback is invoked on death
       mon $port, $localport       # kill localport on abnormal death
       mon $port, $localport, @msg # send message on death

       # temporarily execute code in port context
       peval $port, sub { die "kill the port!" };

       # execute callbacks in $SELF port context
       my $timer = AE::timer 1, 0, psub {
          die "kill the port, delayed";
       };

       # distributed database - modification
       db_set $family => $subkey [=> $value]  # add a subkey
       db_del $family => $subkey...           # delete one or more subkeys
       db_reg $family => $port [=> $value]    # register a port

       # distributed database - queries
       db_family $family => $cb->(\%familyhash)
       db_keys   $family => $cb->(\@keys)
       db_values $family => $cb->(\@values)

       # distributed database - monitoring a family
       db_mon $family => $cb->(\%familyhash, \@added, \@changed, \@deleted)

DESCRIPTION
    This module (-family) implements a simple message passing framework.

    Despite its simplicity, you can securely message other processes running
    on the same or other hosts, and you can supervise entities remotely.

    For an introduction to this module family, see the AnyEvent::MP::Intro
    manual page and the examples under eg/.

CONCEPTS
    port
        Not to be confused with a TCP port, a "port" is something you can
        send messages to (with the "snd" function).

        Ports allow you to register "rcv" handlers that can match all or
        just some messages. Messages send to ports will not be queued,
        regardless of anything was listening for them or not.

        Ports are represented by (printable) strings called "port IDs".

    port ID - "nodeid#portname"
        A port ID is the concatenation of a node ID, a hash-mark ("#") as
        separator, and a port name (a printable string of unspecified format
        created by AnyEvent::MP).

    node
        A node is a single process containing at least one port - the node
        port, which enables nodes to manage each other remotely, and to
        create new ports.

        Nodes are either public (have one or more listening ports) or
        private (no listening ports). Private nodes cannot talk to other
        private nodes currently, but all nodes can talk to public nodes.

        Nodes is represented by (printable) strings called "node IDs".

    node ID - "[A-Za-z0-9_\-.:]*"
        A node ID is a string that uniquely identifies the node within a
        network. Depending on the configuration used, node IDs can look like
        a hostname, a hostname and a port, or a random string. AnyEvent::MP
        itself doesn't interpret node IDs in any way except to uniquely
        identify a node.

    binds - "ip:port"
        Nodes can only talk to each other by creating some kind of
        connection to each other. To do this, nodes should listen on one or
        more local transport endpoints - binds.

        Currently, only standard "ip:port" specifications can be used, which
        specify TCP ports to listen on. So a bind is basically just a tcp
        socket in listening mode that accepts connections from other nodes.

    seed nodes
        When a node starts, it knows nothing about the network it is in - it
        needs to connect to at least one other node that is already in the
        network. These other nodes are called "seed nodes".

        Seed nodes themselves are not special - they are seed nodes only
        because some other node *uses* them as such, but any node can be
        used as seed node for other nodes, and eahc node can use a different
        set of seed nodes.

        In addition to discovering the network, seed nodes are also used to
        maintain the network - all nodes using the same seed node are part
        of the same network. If a network is split into multiple subnets
        because e.g. the network link between the parts goes down, then
        using the same seed nodes for all nodes ensures that eventually the
        subnets get merged again.

        Seed nodes are expected to be long-running, and at least one seed
        node should always be available. They should also be relatively
        responsive - a seed node that blocks for long periods will slow down
        everybody else.

        For small networks, it's best if every node uses the same set of
        seed nodes. For large networks, it can be useful to specify
        "regional" seed nodes for most nodes in an area, and use all seed
        nodes as seed nodes for each other. What's important is that all
        seed nodes connections form a complete graph, so that the network
        cannot split into separate subnets forever.

        Seed nodes are represented by seed IDs.

    seed IDs - "host:port"
        Seed IDs are transport endpoint(s) (usually a hostname/IP address
        and a TCP port) of nodes that should be used as seed nodes.

    global nodes
        An AEMP network needs a discovery service - nodes need to know how
        to connect to other nodes they only know by name. In addition, AEMP
        offers a distributed "group database", which maps group names to a
        list of strings - for example, to register worker ports.

        A network needs at least one global node to work, and allows every
        node to be a global node.

        Any node that loads the AnyEvent::MP::Global module becomes a global
        node and tries to keep connections to all other nodes. So while it
        can make sense to make every node "global" in small networks, it
        usually makes sense to only make seed nodes into global nodes in
        large networks (nodes keep connections to seed nodes and global
        nodes, so making them the same reduces overhead).

VARIABLES/FUNCTIONS
    $thisnode = NODE / $NODE
        The "NODE" function returns, and the $NODE variable contains, the
        node ID of the node running in the current process. This value is
        initialised by a call to "configure".

    $nodeid = node_of $port
        Extracts and returns the node ID from a port ID or a node ID.

    $is_local = port_is_local $port
        Returns true iff the port is a local port.

    configure $profile, key => value...
    configure key => value...
        Before a node can talk to other nodes on the network (i.e. enter
        "distributed mode") it has to configure itself - the minimum a node
        needs to know is its own name, and optionally it should know the
        addresses of some other nodes in the network to discover other
        nodes.

        This function configures a node - it must be called exactly once (or
        never) before calling other AnyEvent::MP functions.

        The key/value pairs are basically the same ones as documented for
        the aemp command line utility (sans the set/del prefix), with these
        additions:

        norc => $boolean (default false)
            If true, then the rc file (e.g. ~/.perl-anyevent-mp) will *not*
            be consulted - all configuration options must be specified in
            the "configure" call.

        force => $boolean (default false)
            IF true, then the values specified in the "configure" will take
            precedence over any values configured via the rc file. The
            default is for the rc file to override any options specified in
            the program.

        step 1, gathering configuration from profiles
            The function first looks up a profile in the aemp configuration
            (see the aemp commandline utility). The profile name can be
            specified via the named "profile" parameter or can simply be the
            first parameter). If it is missing, then the nodename (uname -n)
            will be used as profile name.

            The profile data is then gathered as follows:

            First, all remaining key => value pairs (all of which are
            conveniently undocumented at the moment) will be interpreted as
            configuration data. Then they will be overwritten by any values
            specified in the global default configuration (see the aemp
            utility), then the chain of profiles chosen by the profile name
            (and any "parent" attributes).

            That means that the values specified in the profile have highest
            priority and the values specified directly via "configure" have
            lowest priority, and can only be used to specify defaults.

            If the profile specifies a node ID, then this will become the
            node ID of this process. If not, then the profile name will be
            used as node ID, with a unique randoms tring ("/%u") appended.

            The node ID can contain some "%" sequences that are expanded: %n
            is expanded to the local nodename, %u is replaced by a random
            strign to make the node unique. For example, the aemp
            commandline utility uses "aemp/%n/%u" as nodename, which might
            expand to "aemp/cerebro/ZQDGSIkRhEZQDGSIkRhE".

        step 2, bind listener sockets
            The next step is to look up the binds in the profile, followed
            by binding aemp protocol listeners on all binds specified (it is
            possible and valid to have no binds, meaning that the node
            cannot be contacted from the outside. This means the node cannot
            talk to other nodes that also have no binds, but it can still
            talk to all "normal" nodes).

            If the profile does not specify a binds list, then a default of
            "*" is used, meaning the node will bind on a
            dynamically-assigned port on every local IP address it finds.

        step 3, connect to seed nodes
            As the last step, the seed ID list from the profile is passed to
            the AnyEvent::MP::Global module, which will then use it to keep
            connectivity with at least one node at any point in time.

        Example: become a distributed node using the local node name as
        profile. This should be the most common form of invocation for
        "daemon"-type nodes.

           configure

        Example: become a semi-anonymous node. This form is often used for
        commandline clients.

           configure nodeid => "myscript/%n/%u";

        Example: configure a node using a profile called seed, which is
        suitable for a seed node as it binds on all local addresses on a
        fixed port (4040, customary for aemp).

           # use the aemp commandline utility
           # aemp profile seed binds '*:4040'

           # then use it
           configure profile => "seed";

           # or simply use aemp from the shell again:
           # aemp run profile seed

           # or provide a nicer-to-remember nodeid
           # aemp run profile seed nodeid "$(hostname)"

    $SELF
        Contains the current port id while executing "rcv" callbacks or
        "psub" blocks.

    *SELF, SELF, %SELF, @SELF...
        Due to some quirks in how perl exports variables, it is impossible
        to just export $SELF, all the symbols named "SELF" are exported by
        this module, but only $SELF is currently used.

    snd $port, type => @data
    snd $port, @msg
        Send the given message to the given port, which can identify either
        a local or a remote port, and must be a port ID.

        While the message can be almost anything, it is highly recommended
        to use a string as first element (a port ID, or some word that
        indicates a request type etc.) and to consist if only simple perl
        values (scalars, arrays, hashes) - if you think you need to pass an
        object, think again.

        The message data logically becomes read-only after a call to this
        function: modifying any argument (or values referenced by them) is
        forbidden, as there can be considerable time between the call to
        "snd" and the time the message is actually being serialised - in
        fact, it might never be copied as within the same process it is
        simply handed to the receiving port.

        The type of data you can transfer depends on the transport protocol:
        when JSON is used, then only strings, numbers and arrays and hashes
        consisting of those are allowed (no objects). When Storable is used,
        then anything that Storable can serialise and deserialise is
        allowed, and for the local node, anything can be passed. Best rely
        only on the common denominator of these.

    $local_port = port
        Create a new local port object and returns its port ID. Initially it
        has no callbacks set and will throw an error when it receives
        messages.

    $local_port = port { my @msg = @_ }
        Creates a new local port, and returns its ID. Semantically the same
        as creating a port and calling "rcv $port, $callback" on it.

        The block will be called for every message received on the port,
        with the global variable $SELF set to the port ID. Runtime errors
        will cause the port to be "kil"ed. The message will be passed as-is,
        no extra argument (i.e. no port ID) will be passed to the callback.

        If you want to stop/destroy the port, simply "kil" it:

           my $port = port {
              my @msg = @_;
              ...
              kil $SELF;
           };

    rcv $local_port, $callback->(@msg)
        Replaces the default callback on the specified port. There is no way
        to remove the default callback: use "sub { }" to disable it, or
        better "kil" the port when it is no longer needed.

        The global $SELF (exported by this module) contains $port while
        executing the callback. Runtime errors during callback execution
        will result in the port being "kil"ed.

        The default callback receives all messages not matched by a more
        specific "tag" match.

    rcv $local_port, tag => $callback->(@msg_without_tag), ...
        Register (or replace) callbacks to be called on messages starting
        with the given tag on the given port (and return the port), or
        unregister it (when $callback is $undef or missing). There can only
        be one callback registered for each tag.

        The original message will be passed to the callback, after the first
        element (the tag) has been removed. The callback will use the same
        environment as the default callback (see above).

        Example: create a port and bind receivers on it in one go.

          my $port = rcv port,
             msg1 => sub { ... },
             msg2 => sub { ... },
          ;

        Example: create a port, bind receivers and send it in a message
        elsewhere in one go:

           snd $otherport, reply =>
              rcv port,
                 msg1 => sub { ... },
                 ...
           ;

        Example: temporarily register a rcv callback for a tag matching some
        port (e.g. for an rpc reply) and unregister it after a message was
        received.

           rcv $port, $otherport => sub {
              my @reply = @_;

              rcv $SELF, $otherport;
           };

    peval $port, $coderef[, @args]
        Evaluates the given $codref within the context of $port, that is,
        when the code throws an exception the $port will be killed.

        Any remaining args will be passed to the callback. Any return values
        will be returned to the caller.

        This is useful when you temporarily want to execute code in the
        context of a port.

        Example: create a port and run some initialisation code in it's
        context.

           my $port = port { ... };

           peval $port, sub {
              init
                 or die "unable to init";
           };

    $closure = psub { BLOCK }
        Remembers $SELF and creates a closure out of the BLOCK. When the
        closure is executed, sets up the environment in the same way as in
        "rcv" callbacks, i.e. runtime errors will cause the port to get
        "kil"ed.

        The effect is basically as if it returned "sub { peval $SELF, sub {
        BLOCK }, @_ }".

        This is useful when you register callbacks from "rcv" callbacks:

           rcv delayed_reply => sub {
              my ($delay, @reply) = @_;
              my $timer = AE::timer $delay, 0, psub {
                 snd @reply, $SELF;
              };
           };

    $guard = mon $port, $rcvport # kill $rcvport when $port dies
    $guard = mon $port # kill $SELF when $port dies
    $guard = mon $port, $cb->(@reason) # call $cb when $port dies
    $guard = mon $port, $rcvport, @msg # send a message when $port dies
        Monitor the given port and do something when the port is killed or
        messages to it were lost, and optionally return a guard that can be
        used to stop monitoring again.

        The first two forms distinguish between "normal" and "abnormal"
        kil's:

        In the first form (another port given), if the $port is "kil"'ed
        with a non-empty reason, the other port ($rcvport) will be kil'ed
        with the same reason. That is, on "normal" kil's nothing happens,
        while under all other conditions, the other port is killed with the
        same reason.

        The second form (kill self) is the same as the first form, except
        that $rvport defaults to $SELF.

        The remaining forms don't distinguish between "normal" and
        "abnormal" kil's - it's up to the callback or receiver to check
        whether the @reason is empty and act accordingly.

        In the third form (callback), the callback is simply called with any
        number of @reason elements (empty @reason means that the port was
        deleted "normally"). Note also that *the callback must never die*,
        so use "eval" if unsure.

        In the last form (message), a message of the form "$rcvport, @msg,
        @reason" will be "snd".

        Monitoring-actions are one-shot: once messages are lost (and a
        monitoring alert was raised), they are removed and will not trigger
        again, even if it turns out that the port is still alive.

        As a rule of thumb, monitoring requests should always monitor a
        remote port locally (using a local $rcvport or a callback). The
        reason is that kill messages might get lost, just like any other
        message. Another less obvious reason is that even monitoring
        requests can get lost (for example, when the connection to the other
        node goes down permanently). When monitoring a port locally these
        problems do not exist.

        "mon" effectively guarantees that, in the absence of hardware
        failures, after starting the monitor, either all messages sent to
        the port will arrive, or the monitoring action will be invoked after
        possible message loss has been detected. No messages will be lost
        "in between" (after the first lost message no further messages will
        be received by the port). After the monitoring action was invoked,
        further messages might get delivered again.

        Inter-host-connection timeouts and monitoring depend on the
        transport used. The only transport currently implemented is TCP, and
        AnyEvent::MP relies on TCP to detect node-downs (this can take 10-15
        minutes on a non-idle connection, and usually around two hours for
        idle connections).

        This means that monitoring is good for program errors and cleaning
        up stuff eventually, but they are no replacement for a timeout when
        you need to ensure some maximum latency.

        Example: call a given callback when $port is killed.

           mon $port, sub { warn "port died because of <@_>\n" };

        Example: kill ourselves when $port is killed abnormally.

           mon $port;

        Example: send us a restart message when another $port is killed.

           mon $port, $self => "restart";

    $guard = mon_guard $port, $ref, $ref...
        Monitors the given $port and keeps the passed references. When the
        port is killed, the references will be freed.

        Optionally returns a guard that will stop the monitoring.

        This function is useful when you create e.g. timers or other
        watchers and want to free them when the port gets killed (note the
        use of "psub"):

          $port->rcv (start => sub {
             my $timer; $timer = mon_guard $port, AE::timer 1, 1, psub {
                undef $timer if 0.9 < rand;
             });
          });

    kil $port[, @reason]
        Kill the specified port with the given @reason.

        If no @reason is specified, then the port is killed "normally" -
        monitor callback will be invoked, but the kil will not cause linked
        ports ("mon $mport, $lport" form) to get killed.

        If a @reason is specified, then linked ports ("mon $mport, $lport"
        form) get killed with the same reason.

        Runtime errors while evaluating "rcv" callbacks or inside "psub"
        blocks will be reported as reason "die => $@".

        Transport/communication errors are reported as "transport_error =>
        $message".

        Common idioms:

           # silently remove yourself, do not kill linked ports
           kil $SELF;

           # report a failure in some detail
           kil $SELF, failure_mode_1 => "it failed with too high temperature";

           # do not waste much time with killing, just die when something goes wrong
           open my $fh, "<file"
              or die "file: $!";

    $port = spawn $node, $initfunc[, @initdata]
        Creates a port on the node $node (which can also be a port ID, in
        which case it's the node where that port resides).

        The port ID of the newly created port is returned immediately, and
        it is possible to immediately start sending messages or to monitor
        the port.

        After the port has been created, the init function is called on the
        remote node, in the same context as a "rcv" callback. This function
        must be a fully-qualified function name (e.g.
        "MyApp::Chat::Server::init"). To specify a function in the main
        program, use "::name".

        If the function doesn't exist, then the node tries to "require" the
        package, then the package above the package and so on (e.g.
        "MyApp::Chat::Server", "MyApp::Chat", "MyApp") until the function
        exists or it runs out of package names.

        The init function is then called with the newly-created port as
        context object ($SELF) and the @initdata values as arguments. It
        *must* call one of the "rcv" functions to set callbacks on $SELF,
        otherwise the port might not get created.

        A common idiom is to pass a local port, immediately monitor the
        spawned port, and in the remote init function, immediately monitor
        the passed local port. This two-way monitoring ensures that both
        ports get cleaned up when there is a problem.

        "spawn" guarantees that the $initfunc has no visible effects on the
        caller before "spawn" returns (by delaying invocation when spawn is
        called for the local node).

        Example: spawn a chat server port on $othernode.

           # this node, executed from within a port context:
           my $server = spawn $othernode, "MyApp::Chat::Server::connect", $SELF;
           mon $server;

           # init function on C<$othernode>
           sub connect {
              my ($srcport) = @_;

              mon $srcport;

              rcv $SELF, sub {
                 ...
              };
           }

    after $timeout, @msg
    after $timeout, $callback
        Either sends the given message, or call the given callback, after
        the specified number of seconds.

        This is simply a utility function that comes in handy at times - the
        AnyEvent::MP author is not convinced of the wisdom of having it,
        though, so it may go away in the future.

    cal $port, @msg, $callback[, $timeout]
        A simple form of RPC - sends a message to the given $port with the
        given contents (@msg), but adds a reply port to the message.

        The reply port is created temporarily just for the purpose of
        receiving the reply, and will be "kil"ed when no longer needed.

        A reply message sent to the port is passed to the $callback as-is.

        If an optional time-out (in seconds) is given and it is not "undef",
        then the callback will be called without any arguments after the
        time-out elapsed and the port is "kil"ed.

        If no time-out is given (or it is "undef"), then the local port will
        monitor the remote port instead, so it eventually gets cleaned-up.

        Currently this function returns the temporary port, but this
        "feature" might go in future versions unless you can make a
        convincing case that this is indeed useful for something.

DISTRIBUTED DATABASE
    AnyEvent::MP comes with a simple distributed database. The database will
    be mirrored asynchronously on all global nodes. Other nodes bind to one
    of the global nodes for their needs. Every node has a "local database"
    which contains all the values that are set locally. All local databases
    are merged together to form the global database, which can be queried.

    The database structure is that of a two-level hash - the database hash
    contains hashes which contain values, similarly to a perl hash of
    hashes, i.e.:

      $DATABASE{$family}{$subkey} = $value

    The top level hash key is called "family", and the second-level hash key
    is called "subkey" or simply "key".

    The family must be alphanumeric, i.e. start with a letter and consist of
    letters, digits, underscores and colons ("[A-Za-z][A-Za-z0-9_:]*",
    pretty much like Perl module names.

    As the family namespace is global, it is recommended to prefix family
    names with the name of the application or module using it.

    The subkeys must be non-empty strings, with no further restrictions.

    The values should preferably be strings, but other perl scalars should
    work as well (such as "undef", arrays and hashes).

    Every database entry is owned by one node - adding the same
    family/subkey combination on multiple nodes will not cause discomfort
    for AnyEvent::MP, but the result might be nondeterministic, i.e. the key
    might have different values on different nodes.

    Different subkeys in the same family can be owned by different nodes
    without problems, and in fact, this is the common method to create
    worker pools. For example, a worker port for image scaling might do
    this:

       db_set my_image_scalers => $port;

    And clients looking for an image scaler will want to get the
    "my_image_scalers" keys from time to time:

       db_keys my_image_scalers => sub {
          @ports = @{ $_[0] };
       };

    Or better yet, they want to monitor the database family, so they always
    have a reasonable up-to-date copy:

       db_mon my_image_scalers => sub {
          @ports = keys %{ $_[0] };
       };

    In general, you can set or delete single subkeys, but query and monitor
    whole families only.

    If you feel the need to monitor or query a single subkey, try giving it
    it's own family.

    $guard = db_set $family => $subkey [=> $value]
        Sets (or replaces) a key to the database - if $value is omitted,
        "undef" is used instead.

        When called in non-void context, "db_set" returns a guard that
        automatically calls "db_del" when it is destroyed.

    db_del $family => $subkey...
        Deletes one or more subkeys from the database family.

    $guard = db_reg $family => $port => $value
    $guard = db_reg $family => $port
    $guard = db_reg $family
        Registers a port in the given family and optionally returns a guard
        to remove it.

        This function basically does the same as:

           db_set $family => $port => $value

        Except that the port is monitored and automatically removed from the
        database family when it is kil'ed.

        If $value is missing, "undef" is used. If $port is missing, then
        $SELF is used.

        This function is most useful to register a port in some port group
        (which is just another name for a database family), and have it
        removed when the port is gone. This works best when the port is a
        local port.

    db_family $family => $cb->(\%familyhash)
        Queries the named database $family and call the callback with the
        family represented as a hash. You can keep and freely modify the
        hash.

    db_keys $family => $cb->(\@keys)
        Same as "db_family", except it only queries the family *subkeys* and
        passes them as array reference to the callback.

    db_values $family => $cb->(\@values)
        Same as "db_family", except it only queries the family *values* and
        passes them as array reference to the callback.

    $guard = db_mon $family => $cb->(\%familyhash, \@added, \@changed,
    \@deleted)
        Creates a monitor on the given database family. Each time a key is
        set or is deleted the callback is called with a hash containing the
        database family and three lists of added, changed and deleted
        subkeys, respectively. If no keys have changed then the array
        reference might be "undef" or even missing.

        If not called in void context, a guard object is returned that, when
        destroyed, stops the monitor.

        The family hash reference and the key arrays belong to AnyEvent::MP
        and must not be modified or stored by the callback. When in doubt,
        make a copy.

        As soon as possible after the monitoring starts, the callback will
        be called with the intiial contents of the family, even if it is
        empty, i.e. there will always be a timely call to the callback with
        the current contents.

        It is possible that the callback is called with a change event even
        though the subkey is already present and the value has not changed.

        The monitoring stops when the guard object is destroyed.

        Example: on every change to the family "mygroup", print out all
        keys.

           my $guard = db_mon mygroup => sub {
              my ($family, $a, $c, $d) = @_;
              print "mygroup members: ", (join " ", keys %$family), "\n";
           };

        Exmaple: wait until the family "My::Module::workers" is non-empty.

           my $guard; $guard = db_mon My::Module::workers => sub {
              my ($family, $a, $c, $d) = @_;
              return unless %$family;
              undef $guard;
              print "My::Module::workers now nonempty\n";
           };

        Example: print all changes to the family
        "AnyEvent::Fantasy::Module".

           my $guard = db_mon AnyEvent::Fantasy::Module => sub {
              my ($family, $a, $c, $d) = @_;

              print "+$_=$family->{$_}\n" for @$a;
              print "*$_=$family->{$_}\n" for @$c;
              print "-$_=$family->{$_}\n" for @$d;
           };

AnyEvent::MP vs. Distributed Erlang
    AnyEvent::MP got lots of its ideas from distributed Erlang (Erlang node
    == aemp node, Erlang process == aemp port), so many of the documents and
    programming techniques employed by Erlang apply to AnyEvent::MP. Here is
    a sample:

       http://www.erlang.se/doc/programming_rules.shtml
       http://erlang.org/doc/getting_started/part_frame.html # chapters 3 and 4
       http://erlang.org/download/erlang-book-part1.pdf      # chapters 5 and 6
       http://erlang.org/download/armstrong_thesis_2003.pdf  # chapters 4 and 5

    Despite the similarities, there are also some important differences:

    *   Node IDs are arbitrary strings in AEMP.

        Erlang relies on special naming and DNS to work everywhere in the
        same way. AEMP relies on each node somehow knowing its own
        address(es) (e.g. by configuration or DNS), and possibly the
        addresses of some seed nodes, but will otherwise discover other
        nodes (and their IDs) itself.

    *   Erlang has a "remote ports are like local ports" philosophy, AEMP
        uses "local ports are like remote ports".

        The failure modes for local ports are quite different (runtime
        errors only) then for remote ports - when a local port dies, you
        *know* it dies, when a connection to another node dies, you know
        nothing about the other port.

        Erlang pretends remote ports are as reliable as local ports, even
        when they are not.

        AEMP encourages a "treat remote ports differently" philosophy, with
        local ports being the special case/exception, where transport errors
        cannot occur.

    *   Erlang uses processes and a mailbox, AEMP does not queue.

        Erlang uses processes that selectively receive messages out of
        order, and therefore needs a queue. AEMP is event based, queuing
        messages would serve no useful purpose. For the same reason the
        pattern-matching abilities of AnyEvent::MP are more limited, as
        there is little need to be able to filter messages without dequeuing
        them.

        This is not a philosophical difference, but simply stems from
        AnyEvent::MP being event-based, while Erlang is process-based.

        You can have a look at Coro::MP for a more Erlang-like process model
        on top of AEMP and Coro threads.

    *   Erlang sends are synchronous, AEMP sends are asynchronous.

        Sending messages in Erlang is synchronous and blocks the process
        until a connection has been established and the message sent (and so
        does not need a queue that can overflow). AEMP sends return
        immediately, connection establishment is handled in the background.

    *   Erlang suffers from silent message loss, AEMP does not.

        Erlang implements few guarantees on messages delivery - messages can
        get lost without any of the processes realising it (i.e. you send
        messages a, b, and c, and the other side only receives messages a
        and c).

        AEMP guarantees (modulo hardware errors) correct ordering, and the
        guarantee that after one message is lost, all following ones sent to
        the same port are lost as well, until monitoring raises an error, so
        there are no silent "holes" in the message sequence.

        If you want your software to be very reliable, you have to cope with
        corrupted and even out-of-order messages in both Erlang and AEMP.
        AEMP simply tries to work better in common error cases, such as when
        a network link goes down.

    *   Erlang can send messages to the wrong port, AEMP does not.

        In Erlang it is quite likely that a node that restarts reuses an
        Erlang process ID known to other nodes for a completely different
        process, causing messages destined for that process to end up in an
        unrelated process.

        AEMP does not reuse port IDs, so old messages or old port IDs
        floating around in the network will not be sent to an unrelated
        port.

    *   Erlang uses unprotected connections, AEMP uses secure authentication
        and can use TLS.

        AEMP can use a proven protocol - TLS - to protect connections and
        securely authenticate nodes.

    *   The AEMP protocol is optimised for both text-based and binary
        communications.

        The AEMP protocol, unlike the Erlang protocol, supports both
        programming language independent text-only protocols (good for
        debugging), and binary, language-specific serialisers (e.g.
        Storable). By default, unless TLS is used, the protocol is actually
        completely text-based.

        It has also been carefully designed to be implementable in other
        languages with a minimum of work while gracefully degrading
        functionality to make the protocol simple.

    *   AEMP has more flexible monitoring options than Erlang.

        In Erlang, you can chose to receive *all* exit signals as messages
        or *none*, there is no in-between, so monitoring single Erlang
        processes is difficult to implement.

        Monitoring in AEMP is more flexible than in Erlang, as one can
        choose between automatic kill, exit message or callback on a
        per-port basis.

    *   Erlang tries to hide remote/local connections, AEMP does not.

        Monitoring in Erlang is not an indicator of process death/crashes,
        in the same way as linking is (except linking is unreliable in
        Erlang).

        In AEMP, you don't "look up" registered port names or send to named
        ports that might or might not be persistent. Instead, you normally
        spawn a port on the remote node. The init function monitors you, and
        you monitor the remote port. Since both monitors are local to the
        node, they are much more reliable (no need for "spawn_link").

        This also saves round-trips and avoids sending messages to the wrong
        port (hard to do in Erlang).

RATIONALE
    Why strings for port and node IDs, why not objects?
        We considered "objects", but found that the actual number of methods
        that can be called are quite low. Since port and node IDs travel
        over the network frequently, the serialising/deserialising would add
        lots of overhead, as well as having to keep a proxy object
        everywhere.

        Strings can easily be printed, easily serialised etc. and need no
        special procedures to be "valid".

        And as a result, a port with just a default receiver consists of a
        single code reference stored in a global hash - it can't become much
        cheaper.

    Why favour JSON, why not a real serialising format such as Storable?
        In fact, any AnyEvent::MP node will happily accept Storable as
        framing format, but currently there is no way to make a node use
        Storable by default (although all nodes will accept it).

        The default framing protocol is JSON because a) JSON::XS is many
        times faster for small messages and b) most importantly, after years
        of experience we found that object serialisation is causing more
        problems than it solves: Just like function calls, objects simply do
        not travel easily over the network, mostly because they will always
        be a copy, so you always have to re-think your design.

        Keeping your messages simple, concentrating on data structures
        rather than objects, will keep your messages clean, tidy and
        efficient.

PORTING FROM AnyEvent::MP VERSION 1.X
    AEMP version 2 has a few major incompatible changes compared to version
    1:

    AnyEvent::MP::Global no longer has group management functions.
        At least not officially - the grp_* functions are still exported and
        might work, but they will be removed in some later release.

        AnyEvent::MP now comes with a distributed database that is more
        powerful. Its database families map closely to port groups, but the
        API has changed (the functions are also now exported by
        AnyEvent::MP). Here is a rough porting guide:

          grp_reg $group, $port                      # old
          db_reg $group, $port                       # new

          $list = grp_get $group                     # old
          db_keys $group, sub { my $list = shift }   # new

          grp_mon $group, $cb->(\@ports, $add, $del) # old
          db_mon $group, $cb->(\%ports, $add, $change, $del) # new

        "grp_reg" is a no-brainer (just replace by "db_reg"), but "grp_get"
        is no longer instant, because the local node might not have a copy
        of the group. You can either modify your code to allow for a
        callback, or use "db_mon" to keep an updated copy of the group:

          my $local_group_copy;
          db_mon $group => sub { $local_group_copy = $_[0] };

          # now "keys %$local_group_copy" always returns the most up-to-date
          # list of ports in the group.

        "grp_mon" can be replaced by "db_mon" with minor changes - "db_mon"
        passes a hash as first argument, and an extra $chg argument that can
        be ignored:

          db_mon $group => sub {
             my ($ports, $add, $chg, $del) = @_;
             $ports = [keys %$ports];

             # now $ports, $add and $del are the same as
             # were originally passed by grp_mon.
             ...
          };

    Nodes not longer connect to all other nodes.
        In AEMP 1.x, every node automatically loads the AnyEvent::MP::Global
        module, which in turn would create connections to all other nodes in
        the network (helped by the seed nodes).

        In version 2.x, global nodes still connect to all other global
        nodes, but other nodes don't - now every node either is a global
        node itself, or attaches itself to another global node.

        If a node isn't a global node itself, then it attaches itself to one
        of its seed nodes. If that seed node isn't a global node yet, it
        will automatically be upgraded to a global node.

        So in many cases, nothing needs to be changed - one just has to make
        sure that all seed nodes are meshed together with the other seed
        nodes (as with AEMP 1.x), and other nodes specify them as seed
        nodes. This is most easily achieved by specifying the same set of
        seed nodes for all nodes in the network.

        Not opening a connection to every other node is usually an
        advantage, except when you need the lower latency of an already
        established connection. To ensure a node establishes a connection to
        another node, you can monitor the node port ("mon $node, ..."),
        which will attempt to create the connection (and notify you when the
        connection fails).

    Listener-less nodes (nodes without binds) are gone.
        And are not coming back, at least not in their old form. If no
        "binds" are specified for a node, AnyEvent::MP assumes a default of
        "*:*".

        There are vague plans to implement some form of routing domains,
        which might or might not bring back listener-less nodes, but don't
        count on it.

        The fact that most connections are now optional somewhat mitigates
        this, as a node can be effectively unreachable from the outside
        without any problems, as long as it isn't a global node and only
        reaches out to other nodes (as opposed to being contacted from other
        nodes).

    $AnyEvent::MP::Kernel::WARN has gone.
        AnyEvent has acquired a logging framework (AnyEvent::Log), and AEMP
        now uses this, and so should your programs.

        Every module now documents what kinds of messages it generates, with
        AnyEvent::MP acting as a catch all.

        On the positive side, this means that instead of setting
        "PERL_ANYEVENT_MP_WARNLEVEL", you can get away by setting
        "AE_VERBOSE" - much less to type.

LOGGING
    AnyEvent::MP does not normally log anything by itself, but since it is
    the root of the context hierarchy for AnyEvent::MP modules, it will
    receive all log messages by submodules.

SEE ALSO
    AnyEvent::MP::Intro - a gentle introduction.

    AnyEvent::MP::Kernel - more, lower-level, stuff.

    AnyEvent::MP::Global - network maintenance and port groups, to find your
    applications.

    AnyEvent::MP::DataConn - establish data connections between nodes.

    AnyEvent::MP::LogCatcher - simple service to display log messages from
    all nodes.

    AnyEvent.

AUTHOR
     Marc Lehmann <schmorp@schmorp.de>
     http://home.schmorp.de/

