--- AnyEvent-MP/MP.pm 2009/08/04 18:33:30 1.22 +++ AnyEvent-MP/MP.pm 2009/08/14 23:17:17 1.55 @@ -10,20 +10,44 @@ NODE # returns this node's noderef NODE $port # returns the noderef of the port + $SELF # receiving/own port id in rcv callbacks + + # initialise the node so it can send/receive messages + initialise_node; # -OR- + initialise_node "localhost:4040"; # -OR- + initialise_node "slave/", "localhost:4040" + + # ports are message endpoints + + # sending messages snd $port, type => data...; + snd $port, @msg; + snd @msg_with_first_element_being_a_port; - $SELF # receiving/own port id in rcv callbacks + # creating/using ports, the simple way + my $simple_port = port { my @msg = @_; 0 }; + + # creating/using ports, tagged message matching + my $port = port; + rcv $port, ping => sub { snd $_[0], "pong"; 0 }; + rcv $port, pong => sub { warn "pong received\n"; 0 }; + + # 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 + +=head1 CURRENT STATUS - rcv $port, smartmatch => $cb->($port, @msg); + AnyEvent::MP - stable API, should work + AnyEvent::MP::Intro - outdated + AnyEvent::MP::Kernel - WIP + AnyEvent::MP::Transport - mostly stable - # examples: - rcv $port2, ping => sub { snd $_[0], "pong"; 0 }; - rcv $port1, pong => sub { warn "pong received\n" }; - snd $port2, ping => $port1; - - # more, smarter, matches (_any_ is exported by this module) - rcv $port, [child_died => $pid] => sub { ... - rcv $port, [_any_, _any_, 3] => sub { .. $_[2] is 3 + stay tuned. =head1 DESCRIPTION @@ -32,7 +56,10 @@ Despite its simplicity, you can securely message other processes running on the same or other hosts. -At the moment, this module family is severly brokena nd underdocumented, +For an introduction to this module family, see the L +manual page. + +At the moment, this module family is severly broken and underdocumented, so do not use. This was uploaded mainly to reserve the CPAN namespace - stay tuned! @@ -42,31 +69,42 @@ =item port -A port is something you can send messages to with the C function, and -you can register C handlers with. All C handlers will receive -messages they match, messages will not be queued. +A port is something you can send messages to (with the C function). + +Ports allow you to register C handlers that can match all or just +some messages. Messages will not be queued. =item port id - C -A port id is always the noderef, a hash-mark (C<#>) as separator, followed -by a port name (a printable string of unspecified format). +A port ID is the concatenation of a noderef, a hash-mark (C<#>) as +separator, and a port name (a printable string of unspecified format). An +exception is the the node port, whose ID is identical to its node +reference. =item node -A node is a single process containing at least one port - the node -port. You can send messages to node ports to let them create new ports, -among other things. - -Initially, nodes are either private (single-process only) or hidden -(connected to a master node only). Only when they epxlicitly "become -public" can you send them messages from unrelated other nodes. +A node is a single process containing at least one port - the node port, +which provides nodes to manage each other remotely, and to create new +ports. + +Nodes are either private (single-process only), slaves (connected to a +master node only) or public nodes (connectable from unrelated nodes). =item noderef - C, C, C -A noderef is a string that either uniquely identifies a given node (for -private and hidden nodes), or contains a recipe on how to reach a given +A node reference is a string that either simply identifies the node (for +private and slave nodes), or contains a recipe on how to reach a given node (for public nodes). +This recipe is simply a comma-separated list of C pairs (for +TCP/IP, other protocols might look different). + +Node references come in two flavours: resolved (containing only numerical +addresses) or unresolved (where hostnames are used instead of addresses). + +Before using an unresolved node reference in a message you first have to +resolve it. + =back =head1 VARIABLES/FUNCTIONS @@ -77,7 +115,7 @@ package AnyEvent::MP; -use AnyEvent::MP::Base; +use AnyEvent::MP::Kernel; use common::sense; @@ -87,11 +125,12 @@ use base "Exporter"; -our $VERSION = '0.02'; +our $VERSION = $AnyEvent::MP::Kernel::VERSION; + our @EXPORT = qw( NODE $NODE *SELF node_of _any_ - become_slave become_public - snd rcv mon kil reg psub + resolve_node initialise_node + snd rcv mon kil reg psub spawn port ); @@ -105,291 +144,301 @@ =item $thisnode = NODE / $NODE -The C function returns, and the C<$NODE> variable contains -the noderef of the local node. The value is initialised by a call -to C or C, after which all local port -identifiers become invalid. +The C function returns, and the C<$NODE> variable contains the +noderef of the local node. The value is initialised by a call to +C. -=item $noderef = node_of $portid +=item $noderef = node_of $port -Extracts and returns the noderef from a portid or a noderef. +Extracts and returns the noderef from a port ID or a noderef. -=item $SELF +=item initialise_node $noderef, $seednode, $seednode... -Contains the current port id while executing C callbacks or C -blocks. +=item initialise_node "slave/", $master, $master... -=item SELF, %SELF, @SELF... +Before a node can talk to other nodes on the network it has to initialise +itself - the minimum a node needs to know is it's own name, and optionally +it should know the noderefs of some other nodes in the network. -Due to some quirks in how perl exports variables, it is impossible to -just export C<$SELF>, all the symbols called C are exported by this -module, but only C<$SELF> is currently used. +This function initialises a node - it must be called exactly once (or +never) before calling other AnyEvent::MP functions. -=item snd $portid, type => @data +All arguments (optionally except for the first) are noderefs, which can be +either resolved or unresolved. -=item snd $portid, @msg +The first argument will be looked up in the configuration database first +(if it is C then the current nodename will be used instead) to find +the relevant configuration profile (see L). If none is found then +the default configuration is used. The configuration supplies additional +seed/master nodes and can override the actual noderef. -Send the given message to the given port ID, which can identify either -a local or a remote port, and can be either a string or soemthignt hat -stringifies a sa port ID (such as a port object :). +There are two types of networked nodes, public nodes and slave nodes: -While the message can be about anything, it is highly recommended to use a -string as first element (a portid, or some word that indicates a request -type etc.). +=over 4 -The message data effectively becomes read-only after a call to this -function: modifying any argument is not allowed and can cause many -problems. +=item public nodes -The type of data you can transfer depends on the transport protocol: when -JSON is used, then only strings, numbers and arrays and hashes consisting -of those are allowed (no objects). When Storable is used, then anything -that Storable can serialise and deserialise is allowed, and for the local -node, anything can be passed. +For public nodes, C<$noderef> (supplied either directly to +C or indirectly via a profile or the nodename) must be a +noderef (possibly unresolved, in which case it will be resolved). + +After resolving, the node will bind itself on all endpoints and try to +connect to all additional C<$seednodes> that are specified. Seednodes are +optional and can be used to quickly bootstrap the node into an existing +network. + +=item slave nodes + +When the C<$noderef> (either as given or overriden by the config file) +is the special string C, then the node will become a slave +node. Slave nodes cannot be contacted from outside and will route most of +their traffic to the master node that they attach to. + +At least one additional noderef is required (either by specifying it +directly or because it is part of the configuration profile): The node +will try to connect to all of them and will become a slave attached to the +first node it can successfully connect to. -=item kil $portid[, @reason] +=back -Kill the specified port with the given C<@reason>. +This function will block until all nodes have been resolved and, for slave +nodes, until it has successfully established a connection to a master +server. -If no C<@reason> is specified, then the port is killed "normally" (linked -ports will not be kileld, or even notified). +Example: become a public node listening on the guessed noderef, or the one +specified via C for the current node. This should be the most common +form of invocation for "daemon"-type nodes. -Otherwise, linked ports get killed with the same reason (second form of -C, see below). + initialise_node; -Runtime errors while evaluating C callbacks or inside C blocks -will be reported as reason C<< die => $@ >>. +Example: become a slave node to any of the the seednodes specified via +C. This form is often used for commandline clients. -Transport/communication errors are reported as C<< transport_error => -$message >>. + initialise_node "slave/"; -=item $guard = mon $portid, $cb->(@reason) +Example: become a slave node to any of the specified master servers. This +form is also often used for commandline clients. -=item $guard = mon $portid, $otherport + initialise_node "slave/", "master1", "192.168.13.17", "mp.example.net"; -=item $guard = mon $portid, $otherport, @msg +Example: become a public node, and try to contact some well-known master +servers to become part of the network. -Monitor the given port and do something when the port is killed. + initialise_node undef, "master1", "master2"; -In the first form, the callback is simply called with any number -of C<@reason> elements (no @reason means that the port was deleted -"normally"). Note also that I<< the callback B never die >>, so use -C if unsure. +Example: become a public node listening on port C<4041>. -In the second form, the other port will be C'ed with C<@reason>, iff -a @reason was specified, i.e. on "normal" kils nothing happens, while -under all other conditions, the other port is killed with the same reason. + initialise_node 4041; -In the last form, a message of the form C<@msg, @reason> will be C. +Example: become a public node, only visible on localhost port 4044. -Example: call a given callback when C<$port> is killed. + initialise_node "localhost:4044"; - mon $port, sub { warn "port died because of <@_>\n" }; +=item $cv = resolve_node $noderef -Example: kill ourselves when C<$port> is killed abnormally. +Takes an unresolved node reference that may contain hostnames and +abbreviated IDs, resolves all of them and returns a resolved node +reference. - mon $port, $self; +In addition to C pairs allowed in resolved noderefs, the +following forms are supported: -Example: send us a restart message another C<$port> is killed. +=over 4 - mon $port, $self => "restart"; +=item the empty string -=cut +An empty-string component gets resolved as if the default port (4040) was +specified. -sub mon { - my ($noderef, $port, $cb) = ((split /#/, shift, 2), shift); +=item naked port numbers (e.g. C<1234>) - my $node = $NODE{$noderef} || add_node $noderef; +These are resolved by prepending the local nodename and a colon, to be +further resolved. - #TODO: ports must not be references - if (!ref $cb or "AnyEvent::MP::Port" eq ref $cb) { - if (@_) { - # send a kill info message - my (@msg) = ($cb, @_); - $cb = sub { snd @msg, @_ }; - } else { - # simply kill other port - my $port = $cb; - $cb = sub { kil $port, @_ if @_ }; - } - } +=item hostnames (e.g. C, C) - $node->monitor ($port, $cb); +These are resolved by using AnyEvent::DNS to resolve them, optionally +looking up SRV records for the C port, if no port was +specified. - defined wantarray - and AnyEvent::Util::guard { $node->unmonitor ($port, $cb) } -} +=back -=item $guard = mon_guard $port, $ref, $ref... +=item $SELF -Monitors the given C<$port> and keeps the passed references. When the port -is killed, the references will be freed. +Contains the current port id while executing C callbacks or C +blocks. -Optionally returns a guard that will stop the monitoring. +=item SELF, %SELF, @SELF... -This function is useful when you create e.g. timers or other watchers and -want to free them when the port gets killed: +Due to some quirks in how perl exports variables, it is impossible to +just export C<$SELF>, all the symbols called C are exported by this +module, but only C<$SELF> is currently used. - $port->rcv (start => sub { - my $timer; $timer = mon_guard $port, AE::timer 1, 1, sub { - undef $timer if 0.9 < rand; - }); - }); +=item snd $port, type => @data -=cut +=item snd $port, @msg -sub mon_guard { - my ($port, @refs) = @_; +Send the given message to the given port ID, which can identify either +a local or a remote port, and must be a port ID. - mon $port, sub { 0 && @refs } -} +While the message can be about anything, it is highly recommended to use a +string as first element (a port ID, or some word that indicates a request +type etc.). -=item $local_port = port +The message data effectively becomes read-only after a call to this +function: modifying any argument is not allowed and can cause many +problems. -Create a new local port object that supports message matching. +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. + +=item $local_port = port -=item $portid = port { my @msg = @_; $finished } +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. -Creates a "mini port", that is, a very lightweight port without any -pattern matching behind it, and returns its ID. +=item $local_port = port { my @msg = @_ } -The block will be called for every message received on the port. When the -callback returns a true value its job is considered "done" and the port -will be destroyed. Otherwise it will stay alive. +Creates a new local port, and returns its ID. Semantically the same as +creating a port and calling C on it. -The message will be passed as-is, no extra argument (i.e. no port id) will -be passed to the callback. +The block will be called for every message received on the port, with the +global variable C<$SELF> set to the port ID. Runtime errors will cause the +port to be Ced. The message will be passed as-is, no extra argument +(i.e. no port ID) will be passed to the callback. -If you need the local port id in the callback, this works nicely: +If you want to stop/destroy the port, simply C it: - my $port; $port = miniport { - snd $otherport, reply => $port; + my $port = port { + my @msg = @_; + ... + kil $SELF; }; =cut +sub rcv($@); + +sub _kilme { + die "received message on port without callback"; +} + sub port(;&) { my $id = "$UNIQ." . $ID++; my $port = "$NODE#$id"; - if (@_) { - my $cb = shift; - $PORT{$id} = sub { - local $SELF = $port; - eval { - &$cb - and kil $id; - }; - _self_die if $@; - }; - } else { - my $self = bless { - id => "$NODE#$id", - }, "AnyEvent::MP::Port"; - - $PORT_DATA{$id} = $self; - $PORT{$id} = sub { - local $SELF = $port; - - eval { - for (@{ $self->{rc0}{$_[0]} }) { - $_ && &{$_->[0]} - && undef $_; - } - - for (@{ $self->{rcv}{$_[0]} }) { - $_ && [@_[1 .. @{$_->[1]}]] ~~ $_->[1] - && &{$_->[0]} - && undef $_; - } - - for (@{ $self->{any} }) { - $_ && [@_[0 .. $#{$_->[1]}]] ~~ $_->[1] - && &{$_->[0]} - && undef $_; - } - }; - _self_die if $@; - }; - } + rcv $port, shift || \&_kilme; $port } -=item reg $portid, $name +=item rcv $local_port, $callback->(@msg) -Registers the given port under the name C<$name>. If the name already -exists it is replaced. +Replaces the default callback on the specified port. There is no way to +remove the default callback: use C to disable it, or better +C the port when it is no longer needed. -A port can only be registered under one well known name. +The global C<$SELF> (exported by this module) contains C<$port> while +executing the callback. Runtime errors during callback execution will +result in the port being Ced. -A port automatically becomes unregistered when it is killed. +The default callback received all messages not matched by a more specific +C match. -=cut - -sub reg(@) { - my ($portid, $name) = @_; - - $REG{$name} = $portid; -} - -=item rcv $portid, tagstring => $callback->(@msg), ... +=item rcv $local_port, tag => $callback->(@msg_without_tag), ... -=item rcv $portid, $smartmatch => $callback->(@msg), ... +Register (or replace) callbacks to be called on messages starting with the +given tag on the given port (and return the port), or unregister it (when +C<$callback> is C<$undef> or missing). There can only be one callback +registered for each tag. -=item rcv $portid, [$smartmatch...] => $callback->(@msg), ... +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). -Register callbacks to be called on matching messages on the given port. +Example: create a port and bind receivers on it in one go. -The callback has to return a true value when its work is done, after -which is will be removed, or a false value in which case it will stay -registered. + my $port = rcv port, + msg1 => sub { ... }, + msg2 => sub { ... }, + ; -The global C<$SELF> (exported by this module) contains C<$portid> while -executing the callback. +Example: create a port, bind receivers and send it in a message elsewhere +in one go: -Runtime errors wdurign callback execution will result in the port being -Ced. + snd $otherport, reply => + rcv port, + msg1 => sub { ... }, + ... + ; -If the match is an array reference, then it will be matched against the -first elements of the message, otherwise only the first element is being -matched. +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. -Any element in the match that is specified as C<_any_> (a function -exported by this module) matches any single element of the message. + rcv $port, $otherport => sub { + my @reply = @_; -While not required, it is highly recommended that the first matching -element is a string identifying the message. The one-string-only match is -also the most efficient match (by far). + rcv $SELF, $otherport; + }; =cut sub rcv($@) { - my ($noderef, $port) = split /#/, shift, 2; + my $port = shift; + my ($noderef, $portid) = split /#/, $port, 2; ($NODE{$noderef} || add_node $noderef) == $NODE{""} - or Carp::croak "$noderef#$port: rcv can only be called on local ports, caught"; + or Carp::croak "$port: rcv can only be called on local ports, caught"; - my $self = $PORT_DATA{$port} - or Carp::croak "$noderef#$port: rcv can only be called on message matching ports, caught"; + while (@_) { + if (ref $_[0]) { + if (my $self = $PORT_DATA{$portid}) { + "AnyEvent::MP::Port" eq ref $self + or Carp::croak "$port: rcv can only be called on message matching ports, caught"; + + $self->[2] = shift; + } else { + my $cb = shift; + $PORT{$portid} = sub { + local $SELF = $port; + eval { &$cb }; _self_die if $@; + }; + } + } elsif (defined $_[0]) { + my $self = $PORT_DATA{$portid} ||= do { + my $self = bless [$PORT{$port} || sub { }, { }, $port], "AnyEvent::MP::Port"; + + $PORT{$portid} = sub { + local $SELF = $port; + + if (my $cb = $self->[1]{$_[0]}) { + shift; + eval { &$cb }; _self_die if $@; + } else { + &{ $self->[0] }; + } + }; - "AnyEvent::MP::Port" eq ref $self - or Carp::croak "$noderef#$port: rcv can only be called on message matching ports, caught"; + $self + }; - while (@_) { - my ($match, $cb) = splice @_, 0, 2; + "AnyEvent::MP::Port" eq ref $self + or Carp::croak "$port: rcv can only be called on message matching ports, caught"; - if (!ref $match) { - push @{ $self->{rc0}{$match} }, [$cb]; - } elsif (("ARRAY" eq ref $match && !ref $match->[0])) { - my ($type, @match) = @$match; - @match - ? push @{ $self->{rcv}{$match->[0]} }, [$cb, \@match] - : push @{ $self->{rc0}{$match->[0]} }, [$cb]; - } else { - push @{ $self->{any} }, [$cb, $match]; + my ($tag, $cb) = splice @_, 0, 2; + + if (defined $cb) { + $self->[1]{$tag} = $cb; + } else { + delete $self->[1]{$tag}; + } } } + + $port } =item $closure = psub { BLOCK } @@ -430,27 +479,200 @@ } } -=back +=item $guard = mon $port, $cb->(@reason) -=head1 FUNCTIONS FOR NODES +=item $guard = mon $port, $rcvport -=over 4 +=item $guard = mon $port -=item become_public endpoint... +=item $guard = mon $port, $rcvport, @msg -Tells the node to become a public node, i.e. reachable from other nodes. +Monitor the given port and do something when the port is killed or +messages to it were lost, and optionally return a guard that can be used +to stop monitoring again. -If no arguments are given, or the first argument is C, then -AnyEvent::MP tries to bind on port C<4040> on all IP addresses that the -local nodename resolves to. - -Otherwise the first argument must be an array-reference with transport -endpoints ("ip:port", "hostname:port") or port numbers (in which case the -local nodename is used as hostname). The endpoints are all resolved and -will become the node reference. +C effectively guarantees that, in the absence of hardware failures, +that after starting the monitor, either all messages sent to the port +will arrive, or the monitoring action will be invoked after possible +message loss has been detected. No messages will be lost "in between" +(after the first lost message no further messages will be received by the +port). After the monitoring action was invoked, further messages might get +delivered again. + +In the first form (callback), the callback is simply called with any +number of C<@reason> elements (no @reason means that the port was deleted +"normally"). Note also that I<< the callback B never die >>, so use +C if unsure. + +In the second form (another port given), the other port (C<$rcvport>) +will be C'ed with C<@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 +C<$rvport> defaults to C<$SELF>. + +In the last form (message), a message of the form C<@msg, @reason> will be +C. + +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 C<$port> is killed. + + mon $port, sub { warn "port died because of <@_>\n" }; + +Example: kill ourselves when C<$port> is killed abnormally. + + mon $port; + +Example: send us a restart message when another C<$port> is killed. + + mon $port, $self => "restart"; + +=cut + +sub mon { + my ($noderef, $port) = split /#/, shift, 2; + + my $node = $NODE{$noderef} || add_node $noderef; + + my $cb = @_ ? shift : $SELF || Carp::croak 'mon: called with one argument only, but $SELF not set,'; + + unless (ref $cb) { + if (@_) { + # send a kill info message + my (@msg) = ($cb, @_); + $cb = sub { snd @msg, @_ }; + } else { + # simply kill other port + my $port = $cb; + $cb = sub { kil $port, @_ if @_ }; + } + } + + $node->monitor ($port, $cb); + + defined wantarray + and AnyEvent::Util::guard { $node->unmonitor ($port, $cb) } +} + +=item $guard = mon_guard $port, $ref, $ref... + +Monitors the given C<$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: + + $port->rcv (start => sub { + my $timer; $timer = mon_guard $port, AE::timer 1, 1, sub { + undef $timer if 0.9 < rand; + }); + }); + +=cut + +sub mon_guard { + my ($port, @refs) = @_; + + #TODO: mon-less form? + + mon $port, sub { 0 && @refs } +} + +=item kil $port[, @reason] + +Kill the specified port with the given C<@reason>. + +If no C<@reason> is specified, then the port is killed "normally" (linked +ports will not be kileld, or even notified). + +Otherwise, linked ports get killed with the same reason (second form of +C, see below). + +Runtime errors while evaluating C callbacks or inside C blocks +will be reported as reason C<< die => $@ >>. + +Transport/communication errors are reported as C<< transport_error => +$message >>. + +=cut + +=item $port = spawn $node, $initfunc[, @initdata] + +Creates a port on the node C<$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 return immediately, and it is +permissible to immediately start sending messages or monitor the port. + +After the port has been created, the init function is +called. This function must be a fully-qualified function name +(e.g. C). To specify a function in the main +program, use C<::name>. + +If the function doesn't exist, then the node tries to C +the package, then the package above the package and so on (e.g. +C, C, C) 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 (C<$SELF>) and the C<@initdata> values as arguments. + +A common idiom is to pass your own port, monitor the spawned port, and +in the init function, monitor the original port. This two-way monitoring +ensures that both ports get cleaned up when there is a problem. + +Example: spawn a chat server port on C<$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 { + ... + }; + } =cut +sub _spawn { + my $port = shift; + my $init = shift; + + local $SELF = "$NODE#$port"; + eval { + &{ load_func $init } + }; + _self_die if $@; +} + +sub spawn(@) { + my ($noderef, undef) = split /#/, shift, 2; + + my $id = "$RUNIQ." . $ID++; + + $_[0] =~ /::/ + or Carp::croak "spawn init function must be a fully-qualified name, caught"; + + snd_to_func $noderef, "AnyEvent::MP::_spawn" => $id, @_; + + "$noderef#$id" +} + =back =head1 NODE MESSAGES @@ -460,6 +682,8 @@ message - C<$reply[0]> is the port to reply to, C<$reply[1]> the type and the remaining arguments are simply the message data. +While other messages exist, they are not public and subject to change. + =over 4 =cut @@ -497,6 +721,167 @@ =back +=head1 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: + +=over 4 + +=item * Node references contain the recipe on how to contact them. + +Erlang relies on special naming and DNS to work everywhere in the +same way. AEMP relies on each node knowing it's own address(es), with +convenience functionality. + +This means that AEMP requires a less tightly controlled environment at the +cost of longer node references and a slightly higher management overhead. + +=item * 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 I 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. + +=item * 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 L for a more Erlang-like process model on top of AEMP). + +=item * 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. + +=item * 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 there are no +holes in the message sequence. + +=item * In Erlang, processes can be declared dead and later be found to be +alive. + +In Erlang it can happen that a monitored process is declared dead and +linked processes get killed, but later it turns out that the process is +still alive - and can receive messages. + +In AEMP, when port monitoring detects a port as dead, then that port will +eventually be killed - it cannot happen that a node detects a port as dead +and then later sends messages to it, finding it is still alive. + +=item * 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. + +=item * Erlang uses unprotected connections, AEMP uses secure +authentication and can use TLS. + +AEMP can use a proven protocol - SSL/TLS - to protect connections and +securely authenticate nodes. + +=item * The AEMP protocol is optimised for both text-based and binary +communications. + +The AEMP protocol, unlike the Erlang protocol, supports both +language-independent text-only protocols (good for debugging) and binary, +language-specific serialisers (e.g. Storable). + +It has also been carefully designed to be implementable in other languages +with a minimum of work while gracefully degrading fucntionality to make the +protocol simple. + +=item * AEMP has more flexible monitoring options than Erlang. + +In Erlang, you can chose to receive I exit signals as messages +or I, 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. + +=item * Erlang tries to hide remote/local connections, AEMP does not. + +Monitoring in Erlang is not an indicator of process death/crashes, +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 the you, and you monitor +the remote port. Since both monitors are local to the node, they are much +more reliable. + +This also saves round-trips and avoids sending messages to the wrong port +(hard to do in Erlang). + +=back + +=head1 RATIONALE + +=over 4 + +=item Why strings for ports and noderefs, why not objects? + +We considered "objects", but found that the actual number of methods +thatc an be called are very low. Since port IDs and noderefs travel over +the network frequently, the serialising/deserialising would add lots of +overhead, as well as having to keep a proxy object. + +Strings can easily be printed, easily serialised etc. and need no special +procedures to be "valid". + +And a a miniport consists of a single closure stored in a global hash - it +can't become much cheaper. + +=item Why favour JSON, why not 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. + +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 gains: 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. + +=back + =head1 SEE ALSO L.