Algorithm 2 Bridging joint $Subscribe$ topic channels among broker clusters.

Function: coordinateJointSubscribeTopic($tp\_Id$, $brk\_Id$, $scr\_Tb$, $pls\_Tb$, $pd\_Tb$, $cls\_Tb$):   1:if (! $tp\_Id$is in$pls\_Tb$ ) then   2:    addSubscriber($tp\_Id$, $brk\_Id$, $pd\_Tb$);   3:else   4:    $cls\_Id$, $clsH\_Id$, $rlCB\_Id$ = findClusterInfo($tp\_Id$, $brk\_Id$, $scr\_Tb$, $cls\_Tb$);   5:    if ( $tp\_Id$is in$cls\_Id$’s $scr\_Tb$ ) then   6:       addSubscriber($tp\_Id$, $brk\_Id$, $scr\_Tb$);   7:       $p\_Brk$ = findIntraBrokers($tp\_Id$, $cls\_Id$, $pls\_Tb$);   8:       inform_Bridging($tp\_Id$, $brk\_Id$, $p\_Brk$);   9:    else 10:       addSubscriber($tp\_Id$, $brk\_Id$, $clsH\_Id$, $scr\_Tb$); 11:       $rl\_Brk$ = findInterBrokers($tp\_Id$, $scr\_Tb$, $cls\_Tb$); 12:       inform_Bridging($tp\_Id$, $rlCB\_Id$, $rl\_Brk$); 13:       inform_Bridging($tp\_Id$, $clsH\_Id$, $rlCB\_Id$); 14:       inform_Bridging($tp\_Id$, $brk\_Id$, $clsH\_ID$); 15:    end if 16:end if