Hi, I posted a bit ago about session locking. I had a half-witted idea
then for how to do it, and now I have something even worse. Now, I'll
share the worse idea :)
Basically, I'm working on a fault-tolerant/HA server that is
multi-master replicated between two peers. Also, their data is
replicated into an SQL db real-time for reporting purposes. There
should never be a high volume of data, so I can easily trade bandwidth
for predictability and reliability.
There is a "main" session, a "server" sesssion, a "client" session. The
server session is a PoCo::Server::TCP, while the client session is a
PoCo::Client::TCP. Each of these works only in half-duplex. This way,
each daemon behaves identically (peer-peer vs. master-slave).
DAEMON A DAEMON B
|---------------| |---------------|
| client(20001) |--->| server(20002) |<--
| | | |
-->| server(20002) |<---| client(20001) |
|---------------| |---------------|
\ /
----------
| RDBMS |
----------
The server takes data from both the peer and from various clients
throughout the network. The client is only for sending replication
events to a peer server.
Now, onto the locking mechanism. Basically, I create a new POE::Session
for managing the locking, and most importantly, causing a call to the
lock to block. Postbacks play a large part in this. Here's the package
.... it's ugly, but currently working.
package Lock;
use POE::Session;
sub new {
my( $type, $resources ) = @_;
our %value = map { $_ => 1 } @$resources; # initialize all resources to 1 (locked)
my $self = { resources => $resources, value => \%value };
$self->{poe_session} = POE::Session->create(
inline_states => {
_start => sub { $_[KERNEL]->alias_set( 'LOCK' ) },
_stop => sub { 1 },
wait => sub {
my($kernel,$heap,$postback,$what) = @_[KERNEL,HEAP,ARG0,ARG1];
if ( $heap->{value}{$what} == 0 ) {
$postback->(); # call the postback since lock is unset
}
# wait for the lock to clear
else { $kernel->delay_set( 'wait', 1, $postback, $what ); }
}
},
heap => { value => \%value }
);
print "new session with id ", $self->{poe_session}->ID(), "\n";
$self->{start_wait} = $self->{poe_session}->postback( 'wait' );
return bless( $self, $type );
}
sub lock {
my( $self, $what ) = @_;
if ( $what eq 'all' ) {
foreach my $part ( keys(%{$self->{value}}) ) {
print "locking $part\n";
$self->{value}{$part}++;
}
}
elsif ( exists($self->{value}{$what}) ) {
$self->{value}{$what}++;
}
else {
die "invalid call to lock( '$what' ) from ".join(' ',caller());
}
print "lock(): $what\n";
}
sub unlock {
my( $self, $what ) = @_;
if ( $what eq 'all' ) {
foreach my $part ( keys(%{$self->{value}}) ) {
print "unlocking $part\n";
die "unlock when not '$part' not locked"
unless ( $self->{value}{$part} > 0 );
$self->{value}{$part}--;
}
}
elsif ( exists($self->{value}{$what}) ) {
die "unlock when not '$what' not locked"
unless ( $self->{value}{$what} > 0 );
$self->{value}{$what}--;
}
else {
die "invalid call to lock( '$what' )";
}
print "unlock(): $what\n";
}
# returns undef if not locked
sub locked {
my( $self, $what ) = @_;
die "invalid call to Lock::locked - improper what '$what'"
if ( !defined($what) || !exists($self->{value}{$what}) );
if ( $self->{value}{$what} == 0 ) {
return undef;
}
else {
return 1;
}
}
# usage: $lock->wait( [postback], [resource name] );
sub wait {
my( $self, $postback, $what ) = @_;
# add a check for valid $what in $self->{value} here
$POE::Kernel::poe_kernel->post( 'LOCK', 'wait', $postback, $what );
}
1;
# EOF
my $lock = Lock->new();
To check if a value is locked, just call $lock->locked( 'name' );. To
have an event block on a lock (without holding up the whole kernel) ,
you'd use $lock->wait( $_[SESSION]->postback( 'my_event', @arguments ),
'client' );.
Of course, if this process were running in threads, it would have to use
lock() or sem(get|op|set|ctl) to protect the %value hash, but since I'm
using a monolithic process, I get away with it.
Anyways, just posting this to see if anybody thinks its stupid or has
use for something like this. Maybe I should look into IKC ...?
-Al Tobey
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