Add Package Level JavaDoc on Snapshots (#38108)

* Add Package Level JavaDoc on Snapshots

server/src/main/java/org/elasticsearch/snapshots/package-info.java

@@ -0,0 +1,107 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch licenses this file to you under
+ * the Apache License, Version 2.0 (the "License"); you may
+ * not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+/**
+ * <p>This package exposes the Elasticsearch Snapshot functionality.</p>
+ *
+ * <h1>Preliminaries</h1>
+ *
+ * <p>There are two communication channels between all nodes and master in the snapshot functionality:</p>
+ * <ul>
+ * <li>The master updates the cluster state by adding, removing or altering the contents of its custom entry
+ * {@link org.elasticsearch.cluster.SnapshotsInProgress}. All nodes consume the state of the {@code SnapshotsInProgress} and will start or
+ * abort relevant shard snapshot tasks accordingly.</li>
+ * <li>Nodes that are executing shard snapshot tasks report either success or failure of their snapshot task by submitting a
+ * {@link org.elasticsearch.snapshots.SnapshotShardsService.UpdateIndexShardSnapshotStatusRequest} to the master node that will update the
+ * snapshot's entry in the cluster state accordingly.</li>
+ * </ul>
+ *
+ * <h1>Snapshot Creation</h1>
+ * <p>Snapshots are created by the following sequence of events:</p>
+ * <ol>
+ * <li>An invocation of {@link org.elasticsearch.snapshots.SnapshotsService#createSnapshot} enqueues a cluster state update to create
+ * a {@link org.elasticsearch.cluster.SnapshotsInProgress.Entry} in the cluster state's {@code SnapshotsInProgress}. This initial snapshot
+ * entry has its state set to {@code INIT} and an empty map set for the state of the individual shard's snapshots.</li>
+ *
+ * <li>After the snapshot's entry with state {@code INIT} is in the cluster state, {@link org.elasticsearch.snapshots.SnapshotsService}
+ * determines the primary shards' assignments for all indices that are being snapshotted and updates the existing
+ * {@code SnapshotsInProgress.Entry} with state {@code STARTED} and adds the map of {@link org.elasticsearch.index.shard.ShardId} to
+ * {@link org.elasticsearch.cluster.SnapshotsInProgress.ShardSnapshotStatus} that tracks the assignment of which node is to snapshot which
+ * shard. All shard snapshots are executed on the shard's primary node. Thus all shards for which the primary node was found to have a
+ * healthy copy of the shard are marked as being in state {@code INIT} in this map. If the primary for a shard is unassigned, it is marked
+ * as {@code MISSING} in this map. In case the primary is initializing at this point, it is marked as in state {@code WAITING}. In case a
+ * shard's primary is relocated at any point after its {@code SnapshotsInProgress.Entry} has moved to state {@code STARTED} and thus been
+ * assigned to a specific cluster node, that shard's snapshot will fail and move to state {@code FAILED}.</li>
+ *
+ * <li>The new {@code SnapshotsInProgress.Entry} is then observed by
+ * {@link org.elasticsearch.snapshots.SnapshotShardsService#clusterChanged} on all nodes and since the entry is in state {@code STARTED}
+ * the {@code SnapshotShardsService} will check if any local primary shards are to be snapshotted (signaled by the shard's snapshot state
+ * being {@code INIT}). For those local primary shards found in state {@code INIT}) the snapshot process of writing the shard's data files
+ * to the snapshot's {@link org.elasticsearch.repositories.Repository} is executed. Once the snapshot execution finishes for a shard an
+ * {@code UpdateIndexShardSnapshotStatusRequest} is sent to the master node signaling either status {@code SUCCESS} or {@code FAILED}.
+ * The master node will then update a shard's state in the snapshots {@code SnapshotsInProgress.Entry} whenever it receives such a
+ * {@code UpdateIndexShardSnapshotStatusRequest}.</li>
+ *
+ * <li>If as a result of the received status update requests, all shards in the cluster state are in a completed state, i.e are marked as
+ * either {@code SUCCESS}, {@code FAILED} or {@code MISSING}, the {@code SnapshotShardsService} will update the state of the {@code Entry}
+ * itself and mark it as {@code SUCCESS}. At the same time {@link org.elasticsearch.snapshots.SnapshotsService#endSnapshot} is executed,
+ * writing the metadata necessary to finalize the snapshot in the repository to the repository.</li>
+ *
+ * <li>After writing the final metadata to the repository, a cluster state update to remove the snapshot from the cluster state is
+ * submitted and the removal of the snapshot's {@code SnapshotsInProgress.Entry} from the cluster state completes the snapshot process.
+ * </li>
+ * </ol>
+ *
+ * <h1>Deleting a Snapshot</h1>
+ *
+ * <p>Deleting a snapshot can take the form of either simply deleting it from the repository or (if it has not completed yet) aborting it
+ * and subsequently deleting it from the repository.</p>
+ *
+ * <h2>Aborting a Snapshot</h2>
+ *
+ * <ol>
+ * <li>Aborting a snapshot starts by updating the state of the snapshot's {@code SnapshotsInProgress.Entry} to {@code ABORTED}.</li>
+ *
+ * <li>The snapshot's state change to {@code ABORTED} in cluster state is then picked up by the {@code SnapshotShardsService} on all nodes.
+ * Those nodes that have shard snapshot actions for the snapshot assigned to them, will abort them and notify master about the shards
+ * snapshot status accordingly. If the shard snapshot action completed or was in state {@code FINALIZE} when the abort was registered by
+ * the {@code SnapshotShardsService}, then the shard's state will be reported to master as {@code SUCCESS}.
+ * Otherwise, it will be reported as {@code FAILED}.</li>
+ *
+ * <li>Once all the shards are reported to master as either {@code SUCCESS} or {@code FAILED} the {@code SnapshotsService} on the master
+ * will finish the snapshot process as all shard's states are now completed and hence the snapshot can be completed as explained in point 4
+ * of the snapshot creation section above.</li>
+ * </ol>
+ *
+ * <h2>Deleting a Snapshot from a Repository</h2>
+ *
+ * <ol>
+ * <li>Assuming there are no entries in the cluster state's {@code SnapshotsInProgress}, deleting a snapshot starts by the
+ * {@code SnapshotsService} creating an entry for deleting the snapshot in the cluster state's
+ * {@link org.elasticsearch.cluster.SnapshotDeletionsInProgress}.</li>
+ *
+ * <li>Once the cluster state contains the deletion entry in {@code SnapshotDeletionsInProgress} the {@code SnapshotsService} will invoke
+ * {@link org.elasticsearch.repositories.Repository#deleteSnapshot} for the given snapshot, which will remove files associated with the
+ * snapshot from the repository as well as update its meta-data to reflect the deletion of the snapshot.</li>
+ *
+ * <li>After the deletion of the snapshot's data from the repository finishes, the {@code SnapshotsService} will submit a cluster state
+ * update to remove the deletion's entry in {@code SnapshotDeletionsInProgress} which concludes the process of deleting a snapshot.</li>
+ * </ol>
+ */
+package org.elasticsearch.snapshots;