<div id="docs-body">
 <div class="section">
  <div class="titlepage">
   <div>
    <div>
     <h4 class="title">
      <a name="group-replication-probes-and-statistics">
      </a>
      20.7.2.1 Probes and Statistics
     </h4>
    </div>
   </div>
  </div>
  <a class="indexterm" name="idm46045132204592">
  </a>
  <p>
   The monitoring mechanism works by having each member deploying a
        set of probes to collect information about its work queues and
        throughput. It then propagates that information to the group
        periodically to share that data with the other members.
  </p>
  <p>
   Such probes are scattered throughout the plugin stack and allow
        one to establish metrics, such as:
  </p>
  <div class="itemizedlist">
   <ul class="itemizedlist" style="list-style-type: disc; ">
    <li class="listitem">
     <p>
      the certifier queue size;
     </p>
    </li>
    <li class="listitem">
     <p>
      the replication applier queue size;
     </p>
    </li>
    <li class="listitem">
     <p>
      the total number of transactions certified;
     </p>
    </li>
    <li class="listitem">
     <p>
      the total number of remote transactions applied in the
            member;
     </p>
    </li>
    <li class="listitem">
     <p>
      the total number of local transactions.
     </p>
    </li>
   </ul>
  </div>
  <p>
   Once a member receives a message with statistics from another
        member, it calculates additional metrics regarding how many
        transactions were certified, applied and locally executed in the
        last monitoring period.
  </p>
  <p>
   Monitoring data is shared with others in the group periodically.
        The monitoring period must be high enough to allow the other
        members to decide on the current write requests, but low enough
        that it has minimal impact on group bandwidth. The information
        is shared every second, and this period is sufficient to address
        both concerns.
  </p>
 </div>
 <br/>
</div>
