NAME
    AnyEvent::MP - 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;

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

CURRENT STATUS
       bin/aemp                - stable.
       AnyEvent::MP            - stable API, should work.
       AnyEvent::MP::Intro     - epxlains most concepts.
       AnyEvent::MP::Kernel    - mostly stable.
       AnyEvent::MP::Global    - stable API, protocol not yet final.

       stay tuned.

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
        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.

    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).

    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.

    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.

    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.

    seeds - "host:port"
        When a node starts, it knows nothing about the network. To teach the
        node about the network it first has to contact some other node
        within the network. This node is called a seed.

        Seeds are transport endpoint(s) of as many nodes as one wants. Those
        nodes are expected to be long-running, and at least one of those
        should always be available. When nodes run out of connections (e.g.
        due to a network error), they try to re-establish connections to
        some seednodes again to join the network.

        Apart from being sued for seeding, seednodes are not special in any
        way - every public node can be a seednode.

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.

    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.

        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. 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
            conviniently 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. The special node ID of "anon/" will be replaced
            by a random node ID.

        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 form 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 seeds 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 locla node name as
        profile. This should be the most common form of invocation for
        "daemon"-type nodes.

           configure

        Example: become an anonymous node. This form is often used for
        commandline clients.

           configure nodeid => "anon/";

        Example: configure a node using a profile called seed, which si
        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 nodeid anon/ 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 received 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 a rpc reply) and unregister it after a message was
        received.

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

              rcv $SELF, $otherport;
           };

    $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.

        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, $cb->(@reason) # call $cb when $port dies
    $guard = mon $port, $rcvport # kill $rcvport when $port dies
    $guard = mon $port # kill $SELF 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.

        "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.

        Note that monitoring-actions are one-shot: once messages are lost
        (and a monitoring alert was raised), they are removed and will not
        trigger again.

        In the first form (callback), the callback is simply called with any
        number of @reason elements (no @reason means that the port was
        deleted "normally"). Note also that *the callback must never die*,
        so use "eval" if unsure.

        In the second form (another port given), the other port ($rcvport)
        will be "kil"'ed with @reason, iff a @reason was specified, i.e. on
        "normal" kils nothing happens, while under all other conditions, the
        other port is killed with the same reason.

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

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

        As a rule of thumb, monitoring requests should always monitor a port
        from a local port (or 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 exmaple,
        when the connection to the other node goes down permanently). When
        monitoring a port locally these problems do not exist.

        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"
        (ports monitoring other ports will not necessarily die because a
        port dies "normally").

        Otherwise, linked ports get killed with the same reason (second form
        of "mon", see above).

        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".

    $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.

        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.

        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.

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 configuraiton or DNS), but will otherwise
        discover other odes 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, 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 dequeing them.

        (But see Coro::MP for a more Erlang-like process model on top of
        AEMP).

    *   Erlang sends are synchronous, AEMP sends are asynchronous.

        Sending messages in Erlang is synchronous and blocks the process
        (and so does not need a queue that can overflow). AEMP sends are
        immediate, connection establishment is handled in the background.

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

        Erlang makes 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 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.

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

        In Erlang it is quite likely that a node that restarts reuses a
        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 never reuses 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 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-process 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 miniport consists of a single closure 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.

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

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

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

    AnyEvent.

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

