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             <h1>Topology Validator</h1>

         <div id="preamble">
 <div class="sectionbody">
 <div class="paragraph">
 <p>Some network issues can cause the Ignite cluster to split into several isolated parts - segments. Nodes from different
 segments cannot communicate with each other, while nodes from the same segment do not experience communication problems.
 In this case, each segment marks the nodes with which the connection was lost as failed and considers itself as an
 independent Ignite cluster. Let&#8217;s call this scenario cluster segmentation.</p>
 </div>
 <div class="paragraph">
 <p>Cluster segmentation can lead to cache data inconsistency across different segments because each segment can continue to handle cache update requests independently.</p>
 </div>
 <div class="paragraph">
 <p>Apache Ignite allows the user to provide custom validation logic for Ignite caches that will be applied to
 each topology change, and if the validation fails, writes to the corresponding cache will be blocked. The mentioned
 validation logic is passed to Ignite as an TopologyValidation interface implementation. It can be done through cache
 configuration or through Ignite plugin extensions mechanism (see CacheTopologyValidatorProvider interface).</p>
 </div>
 <div class="paragraph">
 <p>This module represents an implementation of the Ignite plugin that provides the guarantee that
 after cluster segmentation, no more than one segment can process write requests to all caches. This is achieved by
 providing implementation of the TopologyValidation interface as mentioned above.</p>
 </div>
 <div class="paragraph">
 <p>The current implementation of TopologyValidation uses remaining Ignite baseline nodes in the topology to determine
 segmentation.</p>
 </div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="in-what-cases-cache-writes-will-be-blocked-for-the-segment">In what cases cache writes will be blocked for the segment?</h2>
 <div class="sectionbody">
 <div class="paragraph">
 <p>The following rules are used to determine which segment can process cache write requests after segmentation and which
 cannot:</p>
 </div>
 <div class="olist arabic">
 <ol class="arabic">
 <li>
 <p>The segment is allowed to process cache writes requests after segmentation if and only if more than configured
 fraction of the baseline nodes remain in the segment, otherwise all writes to the cache will be blocked.</p>
 </li>
 <li>
 <p>If the cluster is split into two equal segments, writing to both of them will be blocked.</p>
 </li>
 <li>
 <p>Since Ignite treats segmentation as sequential node failures, even a single node failure in an active cluster in
 which alive baseline nodes count is less or equals to segmentation threshold  is considered as segmentation and results
 in write block for all caches.</p>
 </li>
 </ol>
 </div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="configuration">Configuration</h2>
 <div class="sectionbody">
 <div class="olist arabic">
 <ol class="arabic">
 <li>
 <p>Configure CacheTopologyValidatorPluginProvider on each server node:</p>
 <div class="listingblock">
 <div class="content">
 <pre>   ```
    new IgniteConfiguration()
        ...
        .setPluginProviders(new CacheTopologyValidatorPluginProvider());
    ```</pre>
 </div>
 </div>
 </li>
 <li>
 <p>Configure baseline nodes explicitly, or configure baseline nodes auto adjustment with a timeout that significantly
 exceeds the node failure detection timeout. It can be done through Java Api or through control script.
 See [1] and [2] for more info.</p>
 </li>
 </ol>
 </div>
 <div class="paragraph">
 <p>Note that it is illegal to use baseline nodes auto adjustment with a zero timeout along with current
 TopologyValidator implementation.</p>
 </div>
 <div class="olist arabic">
 <ol class="arabic">
 <li>
 <p>Configure deactivation threshold.
 The deactivation threshold is a fraction of nodes that determines how many nodes must remain in the baseline topology in
 order to this segment was considered valid and continued to accept write requests.
 This value must be in range from 0.5 (inclusively) to 1. Default value is 0.5. If the default value suits you, nothing
 to do is required.</p>
 </li>
 </ol>
 </div>
 <div class="paragraph">
 <p>To set up custom deactivation threshold value set the <code>org.apache.ignite.topology.validator.deactivation.threshold</code>
 distributed configuration property via control script (see <a href="https://ignite.apache.org/docs/latest/tools/control-script#working-with-cluster-properties" class="bare">https://ignite.apache.org/docs/latest/tools/control-script#working-with-cluster-properties</a>)</p>
 </div>
 </div>
 </div>
 <div class="sect1">
 <h2 id="manual-segmentation-resolving">Manual segmentation resolving</h2>
 <div class="sectionbody">
 <div class="paragraph">
 <p>The state of each segment for which cache writes were blocked will be eventually switched to the READ-ONLY mode.
 Manually switching the cluster state back to ACTIVE mode will restore cache write availability. It can be done through
 Java Api or through control script. See [1] and [2] for more info.</p>
 </div>
 <div class="paragraph">
 <p>[1] - <a href="https://ignite.apache.org/docs/latest/clustering/baseline-topology" class="bare">https://ignite.apache.org/docs/latest/clustering/baseline-topology</a></p>
 </div>
 <div class="paragraph">
 <p>[2] - <a href="https://ignite.apache.org/docs/latest/tools/control-script#activation-deactivation-and-topology-management" class="bare">https://ignite.apache.org/docs/latest/tools/control-script#activation-deactivation-and-topology-management</a></p>
 </div>
 </div>
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	<h1>Topology Validator</h1>

	<div id="preamble">
	<div class="sectionbody">
	<div class="paragraph">
	<p>Some network issues can cause the Ignite cluster to split into several isolated parts - segments. Nodes from different
	segments cannot communicate with each other, while nodes from the same segment do not experience communication problems.
	In this case, each segment marks the nodes with which the connection was lost as failed and considers itself as an
	independent Ignite cluster. Let’s call this scenario cluster segmentation.</p>
	</div>
	<div class="paragraph">
	<p>Cluster segmentation can lead to cache data inconsistency across different segments because each segment can continue to handle cache update requests independently.</p>
	</div>
	<div class="paragraph">
	<p>Apache Ignite allows the user to provide custom validation logic for Ignite caches that will be applied to
	each topology change, and if the validation fails, writes to the corresponding cache will be blocked. The mentioned
	validation logic is passed to Ignite as an TopologyValidation interface implementation. It can be done through cache
	configuration or through Ignite plugin extensions mechanism (see CacheTopologyValidatorProvider interface).</p>
	</div>
	<div class="paragraph">
	<p>This module represents an implementation of the Ignite plugin that provides the guarantee that
	after cluster segmentation, no more than one segment can process write requests to all caches. This is achieved by
	providing implementation of the TopologyValidation interface as mentioned above.</p>
	</div>
	<div class="paragraph">
	<p>The current implementation of TopologyValidation uses remaining Ignite baseline nodes in the topology to determine
	segmentation.</p>
	</div>
	</div>
	</div>
	<div class="sect1">
	<h2 id="in-what-cases-cache-writes-will-be-blocked-for-the-segment">In what cases cache writes will be blocked for the segment?</h2>
	<div class="sectionbody">
	<div class="paragraph">
	<p>The following rules are used to determine which segment can process cache write requests after segmentation and which
	cannot:</p>
	</div>
	<div class="olist arabic">
	<ol class="arabic">
	<li>
	<p>The segment is allowed to process cache writes requests after segmentation if and only if more than configured
	fraction of the baseline nodes remain in the segment, otherwise all writes to the cache will be blocked.</p>
	</li>
	<li>
	<p>If the cluster is split into two equal segments, writing to both of them will be blocked.</p>
	</li>
	<li>
	<p>Since Ignite treats segmentation as sequential node failures, even a single node failure in an active cluster in
	which alive baseline nodes count is less or equals to segmentation threshold is considered as segmentation and results
	in write block for all caches.</p>
	</li>
	</ol>
	</div>
	</div>
	</div>
	<div class="sect1">
	<h2 id="configuration">Configuration</h2>
	<div class="sectionbody">
	<div class="olist arabic">
	<ol class="arabic">
	<li>
	<p>Configure CacheTopologyValidatorPluginProvider on each server node:</p>
	<div class="listingblock">
	<div class="content">
	<pre> ```
	new IgniteConfiguration()
	...
	.setPluginProviders(new CacheTopologyValidatorPluginProvider());
	```</pre>
	</div>
	</div>
	</li>
	<li>
	<p>Configure baseline nodes explicitly, or configure baseline nodes auto adjustment with a timeout that significantly
	exceeds the node failure detection timeout. It can be done through Java Api or through control script.
	See [1] and [2] for more info.</p>
	</li>
	</ol>
	</div>
	<div class="paragraph">
	<p>Note that it is illegal to use baseline nodes auto adjustment with a zero timeout along with current
	TopologyValidator implementation.</p>
	</div>
	<div class="olist arabic">
	<ol class="arabic">
	<li>
	<p>Configure deactivation threshold.
	The deactivation threshold is a fraction of nodes that determines how many nodes must remain in the baseline topology in
	order to this segment was considered valid and continued to accept write requests.
	This value must be in range from 0.5 (inclusively) to 1. Default value is 0.5. If the default value suits you, nothing
	to do is required.</p>
	</li>
	</ol>
	</div>
	<div class="paragraph">
	<p>To set up custom deactivation threshold value set the <code>org.apache.ignite.topology.validator.deactivation.threshold</code>
	distributed configuration property via control script (see <a href="https://ignite.apache.org/docs/latest/tools/control-script#working-with-cluster-properties" class="bare">https://ignite.apache.org/docs/latest/tools/control-script#working-with-cluster-properties</a>)</p>
	</div>
	</div>
	</div>
	<div class="sect1">
	<h2 id="manual-segmentation-resolving">Manual segmentation resolving</h2>
	<div class="sectionbody">
	<div class="paragraph">
	<p>The state of each segment for which cache writes were blocked will be eventually switched to the READ-ONLY mode.
	Manually switching the cluster state back to ACTIVE mode will restore cache write availability. It can be done through
	Java Api or through control script. See [1] and [2] for more info.</p>
	</div>
	<div class="paragraph">
	<p>[1] - <a href="https://ignite.apache.org/docs/latest/clustering/baseline-topology" class="bare">https://ignite.apache.org/docs/latest/clustering/baseline-topology</a></p>
	</div>
	<div class="paragraph">
	<p>[2] - <a href="https://ignite.apache.org/docs/latest/tools/control-script#activation-deactivation-and-topology-management" class="bare">https://ignite.apache.org/docs/latest/tools/control-script#activation-deactivation-and-topology-management</a></p>
	</div>
	</div>
	</div>
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