AnyEvent-MP
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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".
=item node ID - C<[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.
=item binds - C<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 C<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.
=item 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 I<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.
=item seed IDs - C<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.
=item 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 L<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).
=back
=head1 VARIABLES/FUNCTIONS
=over 4
=cut
package AnyEvent::MP;
use AnyEvent::MP::Config ();
use AnyEvent::MP::Kernel;
use AnyEvent::MP::Kernel qw(
%NODE %PORT %PORT_DATA $UNIQ $RUNIQ $ID
add_node load_func
NODE $NODE
configure
node_of port_is_local
snd kil
db_set db_del
db_mon db_family db_keys db_values
);
use common::sense;
use Carp ();
use AnyEvent ();
use Guard ();
use base "Exporter";
our $VERSION = '2.02'; # also in MP/Config.pm
our @EXPORT = qw(
NODE $NODE
configure
node_of port_is_local
snd kil
db_set db_del
db_mon db_family db_keys db_values
*SELF
port rcv mon mon_guard psub peval spawn cal
db_set db_del db_reg
db_mon db_family db_keys db_values
after
);
our $SELF;
sub _self_die() {
my $msg = $@;
$msg =~ s/\n+$// unless ref $msg;
kil $SELF, die => $msg;
}
=item $thisnode = NODE / $NODE
The C<NODE> function returns, and the C<$NODE> variable contains, the node
ID of the node running in the current process. This value is initialised by
a call to C<configure>.
=item $nodeid = node_of $port
Extracts and returns the node ID from a port ID or a node ID.
=item $is_local = port_is_local $port
Returns true iff the port is a local port.
=item configure $profile, key => value...
=item 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
F<aemp> command line utility (sans the set/del prefix), with these additions:
=over 4
=item norc => $boolean (default false)
If true, then the rc file (e.g. F<~/.perl-anyevent-mp>) will I<not>
be consulted - all configuration options must be specified in the
C<configure> call.
=item force => $boolean (default false)
IF true, then the values specified in the C<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.
=back
=over 4
=item step 1, gathering configuration from profiles
The function first looks up a profile in the aemp configuration (see the
L<aemp> commandline utility). The profile name can be specified via the
named C<profile> parameter or can simply be the first parameter). If it is
missing, then the nodename (F<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 F<aemp> utility), then the chain of
profiles chosen by the profile name (and any C<parent> attributes).
That means that the values specified in the profile have highest priority
and the values specified directly via C<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 (C</%u>) appended.
The node ID can contain some C<%> sequences that are expanded: C<%n>
is expanded to the local nodename, C<%u> is replaced by a random
strign to make the node unique. For example, the F<aemp> commandline
utility uses C<aemp/%n/%u> as nodename, which might expand to
C<aemp/cerebro/ZQDGSIkRhEZQDGSIkRhE>.
=item 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 C<*> is
used, meaning the node will bind on a dynamically-assigned port on every
local IP address it finds.
=item step 3, connect to seed nodes
As the last step, the seed ID list from the profile is passed to the
L<AnyEvent::MP::Global> module, which will then use it to keep
connectivity with at least one node at any point in time.
=back
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)"
=item $SELF
Contains the current port id while executing C<rcv> callbacks or C<psub>
blocks.
=item *SELF, SELF, %SELF, @SELF...
Due to some quirks in how perl exports variables, it is impossible to
just export C<$SELF>, all the symbols named C<SELF> are exported by this
module, but only C<$SELF> is currently used.
=item snd $port, type => @data
=item 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 C<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.
=item $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.
=item $local_port = port { my @msg = @_ }
Creates a new local port, and returns its ID. Semantically the same as
creating a port and calling C<rcv $port, $callback> on it.
The block will be called for every message received on the port, with the
global variable C<$SELF> set to the port ID. Runtime errors will cause the
port to be C<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 C<kil> it:
my $port = port {
my @msg = @_;
...
kil $SELF;
};
=cut
sub rcv($@);
my $KILME = sub {
(my $tag = substr $_[0], 0, 30) =~ s/([^\x20-\x7e])/./g;
kil $SELF, unhandled_message => "no callback found for message '$tag'";
};
sub port(;&) {
my $id = $UNIQ . ++$ID;
my $port = "$NODE#$id";
rcv $port, shift || $KILME;
$port
}
=item rcv $local_port, $callback->(@msg)
Replaces the default callback on the specified port. There is no way to
remove the default callback: use C<sub { }> to disable it, or better
C<kil> the port when it is no longer needed.
The global C<$SELF> (exported by this module) contains C<$port> while
executing the callback. Runtime errors during callback execution will
result in the port being C<kil>ed.
The default callback receives all messages not matched by a more specific
C<tag> match.
=item rcv $local_port, tag => $callback->(@msg_without_tag), ...
Register (or replace) callbacks to be called on messages starting with the
given tag on the given port (and return the port), or unregister it (when
C<$callback> is C<$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, $otherport;
};
=cut
sub rcv($@) {
my $port = shift;
my ($nodeid, $portid) = split /#/, $port, 2;
$nodeid eq $NODE
or Carp::croak "$port: rcv can only be called on local 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->[0] = shift;
} else {
my $cb = shift;
#=item $cb2 = timeout $seconds, $cb[, @args]
=item cal $port, @msg, $callback[, $timeout]
A simple form of RPC - sends a message to the given C<$port> with the
given contents (C<@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 C<kil>ed when no longer needed.
A reply message sent to the port is passed to the C<$callback> as-is.
If an optional time-out (in seconds) is given and it is not C<undef>,
then the callback will be called without any arguments after the time-out
elapsed and the port is C<kil>ed.
If no time-out is given (or it is C<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.
=cut
sub cal(@) {
my $timeout = ref $_[-1] ? undef : pop;
my $cb = pop;
my $port = port {
undef $timeout;
kil $SELF;
&$cb;
};
if (defined $timeout) {
$timeout = AE::timer $timeout, 0, sub {
undef $timeout;
kil $port;
$cb->();
};
} else {
mon $_[0], sub {
kil $port;
$cb->();
};
}
push @_, $port;
&snd;
$port
}
=back
=head1 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 (C<[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 C<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
C<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.
=over
=item $guard = db_set $family => $subkey [=> $value]
Sets (or replaces) a key to the database - if C<$value> is omitted,
C<undef> is used instead.
When called in non-void context, C<db_set> returns a guard that
automatically calls C<db_del> when it is destroyed.
=item db_del $family => $subkey...
Deletes one or more subkeys from the database family.
=item $guard = db_reg $family => $port => $value
=item $guard = db_reg $family => $port
=item $guard = db_reg $family
Registers a port in the given family and optionally returns a guard to
remove it.
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 - 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 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.
=item * AEMP has more flexible monitoring options than Erlang.
In Erlang, you can chose to receive I<all> exit signals as messages or
I<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.
=item * 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 C<spawn_link>).
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 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.
=item 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.
=back
=head1 PORTING FROM AnyEvent::MP VERSION 1.X
AEMP version 2 has a few major incompatible changes compared to version 1:
=over 4
=item 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
C<grp_reg> is a no-brainer (just replace by C<db_reg>), but C<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
C<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.
C<grp_mon> can be replaced by C<db_mon> with minor changes - C<db_mon>
passes a hash as first argument, and an extra C<$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.
...
};
=item Nodes not longer connect to all other nodes.
In AEMP 1.x, every node automatically loads the L<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 (C<mon $node, ...>), which will attempt to
create the connection (and notify you when the connection fails).
=item Listener-less nodes (nodes without binds) are gone.
And are not coming back, at least not in their old form. If no C<binds>
are specified for a node, AnyEvent::MP assumes a default of C<*:*>.
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).
=item $AnyEvent::MP::Kernel::WARN has gone.
AnyEvent has acquired a logging framework (L<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
C<PERL_ANYEVENT_MP_WARNLEVEL>, you can get away by setting C<AE_VERBOSE> -
much less to type.
=back
=head1 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.
=head1 SEE ALSO
L<AnyEvent::MP::Intro> - a gentle introduction.
L<AnyEvent::MP::Kernel> - more, lower-level, stuff.
L<AnyEvent::MP::Global> - network maintenance and port groups, to find
your applications.
L<AnyEvent::MP::DataConn> - establish data connections between nodes.
L<AnyEvent::MP::LogCatcher> - simple service to display log messages from
all nodes.
L<AnyEvent>.
=head1 AUTHOR
Marc Lehmann <schmorp@schmorp.de>
http://home.schmorp.de/
=cut
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