my $max_mongodb_connections = 4;
our @mongodb_pool;
our %used_connections;

sub mongodb() {
  my $mongodb;
  if(@mongodb_pool){
    $log->info("Using existing DB connection from pool");
    $mongodb = shift @mongodb_pool;

  } elsif(@mongodb_pool and keys %used_connections < $max_mongodb_connections){
    $log->info("Creating new DB connection");
    $mongodb =  MongoDB::MongoClient->new;
  } else{
    $log->info("Need to wait until a mongodb connection becomes available");
  }

  $used_connections{ $mongodb } = 1 if $mongodb;
  return $mongodb
};

sub return_connection {
  my( $mongodb )= @_;
  delete $used_connections{ $mongodb };
  unshift @mongodb_pool, $mongodb; # keep the hot connections hot
}