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<h2 class="title"><a id="_pending_tasks"></a>Pending Tasks<a class="edit_me edit_me_private" rel="nofollow" title="Editing on GitHub is available to Elastic" href="https://github.com/elastic/elasticsearch-definitive-guide/edit/2.x/500_Cluster_Admin/40_other_stats.asciidoc">edit</a></h2>
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<p>There are certain tasks that only the master can perform, such as creating a new
index or moving shards around the cluster. Since a cluster can have only one
master, only one node can ever process cluster-level metadata changes. For
99.9999% of the time, this is never a problem. The queue of metadata changes
remains essentially zero.</p>
<p>In some <em>rare</em> clusters, the number of metadata changes occurs faster than
the master can process them. This leads to a buildup of pending actions that
are queued.</p>
<p>The <code class="literal">pending-tasks</code> API will show you what (if any) cluster-level metadata changes
are pending in the queue:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">GET _cluster/pending_tasks</pre>
</div>
<p>Usually, the response will look like this:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
"tasks": []
}</pre>
</div>
<p>This means there are no pending tasks. If you have one of the rare clusters that
bottlenecks on the master node, your pending task list may look like this:</p>
<div class="pre_wrapper lang-js">
<pre class="programlisting prettyprint lang-js">{
"tasks": [
{
"insert_order": 101,
"priority": "URGENT",
"source": "create-index [foo_9], cause [api]",
"time_in_queue_millis": 86,
"time_in_queue": "86ms"
},
{
"insert_order": 46,
"priority": "HIGH",
"source": "shard-started ([foo_2][1], node[tMTocMvQQgGCkj7QDHl3OA], [P],
s[INITIALIZING]), reason [after recovery from gateway]",
"time_in_queue_millis": 842,
"time_in_queue": "842ms"
},
{
"insert_order": 45,
"priority": "HIGH",
"source": "shard-started ([foo_2][0], node[tMTocMvQQgGCkj7QDHl3OA], [P],
s[INITIALIZING]), reason [after recovery from gateway]",
"time_in_queue_millis": 858,
"time_in_queue": "858ms"
}
]
}</pre>
</div>
<p>You can see that tasks are assigned a priority (<code class="literal">URGENT</code> is processed before <code class="literal">HIGH</code>,
for example), the order it was inserted, how long the action has been queued and
what the action is trying to perform. In the preceding list, there is a <code class="literal">create-index</code>
action and two <code class="literal">shard-started</code> actions pending.</p>
<div class="sidebar">
<div class="titlepage"><div><div>
<p class="title"><strong>When Should I Worry About Pending Tasks?</strong></p>
</div></div></div>
<p>As mentioned, the master node is rarely the bottleneck for clusters. The only
time it could bottleneck is if the cluster state is both very large
<em>and</em> updated frequently.</p>
<p>For example, if you allow customers to create as many dynamic fields as they wish,
and have a unique index for each customer every day, your cluster state will grow
very large. The cluster state includes (among other things) a list of all indices,
their types, and the fields for each index.</p>
<p>So if you have 100,000 customers, and each customer averages 1,000 fields and 90
days of retention—​that’s nine billion fields to keep in the cluster state.
Whenever this changes, the nodes must be notified.</p>
<p>The master must process these changes, which requires nontrivial CPU overhead,
plus the network overhead of pushing the updated cluster state to all nodes.</p>
<p>It is these clusters that may begin to see cluster-state actions queuing up.
There is no easy solution to this problem, however. You have three options:</p>
<div class="ulist itemizedlist">
<ul class="itemizedlist">
<li class="listitem">
Obtain a beefier master node. Vertical scaling just delays the inevitable,
unfortunately.
</li>
<li class="listitem">
Restrict the dynamic nature of the documents in some way, so as to limit the
cluster-state size.
</li>
<li class="listitem">
Spin up another cluster after a certain threshold has been crossed.
</li>
</ul>
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