YARN-2960. Add documentation for the YARN shared cache.

(cherry picked from commit 7e76f85bc68166b01b51fcf6ba4b3fd9281d4a03)

hadoop-project/src/site/site.xml

@@ -138,6 +138,7 @@
       <item name="Graceful Decommission" href="hadoop-yarn/hadoop-yarn-site/GracefulDecommission.html"/>
       <item name="Opportunistic Containers" href="hadoop-yarn/hadoop-yarn-site/OpportunisticContainers.html"/>
       <item name="YARN Federation" href="hadoop-yarn/hadoop-yarn-site/Federation.html"/>
+      <item name="Shared Cache" href="hadoop-yarn/hadoop-yarn-site/SharedCache.html"/>
     </menu>
 
     <menu name="YARN REST APIs" inherit="top">

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-site/src/site/markdown/SharedCache.md

@@ -0,0 +1,168 @@
+<!---
+  Licensed 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. See accompanying LICENSE file.
+-->
+
+YARN Shared Cache
+===================
+
+<!-- MACRO{toc|fromDepth=0|toDepth=3} -->
+
+Overview
+--------
+
+The YARN Shared Cache provides the facility to upload and manage shared
+application resources to HDFS in a safe and scalable manner. YARN applications
+can leverage resources uploaded by other applications or previous
+runs of the same application without having to re­upload and localize identical files
+multiple times. This will save network resources and reduce YARN application
+startup time.
+
+Current Status and Future Plans
+------------
+
+Currently the YARN Shared Cache is released and ready to use. The major
+components are implemented and have been deployed in a large-scale
+production setting. There are still some pieces missing (i.e. strong
+authentication). These missing features will be implemented as part of a
+follow-up phase 2 effort. Please see
+[YARN-7282](https://issues.apache.org/jira/browse/YARN-7282) for more information.
+
+Architecture
+------------
+
+The shared cache feature consists of 4 major components:
+
+1. The shared cache client.
+2. The HDFS directory that acts as a cache.
+3. The shared cache manager (aka. SCM).
+4. The localization service and uploader.
+
+### The Shared Cache Client
+
+YARN application developers and users, should interact with the shared cache using
+the shared cache client. This client is responsible for interacting with the
+shared cache manager, computing the checksum of application resources, and
+claiming application resources in the shared cache. Once an application has claimed
+a resource, it is free to use that resource for the life-cycle of the application.
+Please see the SharedCacheClient.java javadoc for further documentation.
+
+### The Shared Cache HDFS Directory
+
+The shared cache HDFS directory stores all of the shared cache resources. It is protected
+by HDFS permissions and is globally readable, but writing is restricted to a trusted user.
+This HDFS directory is only modified by the shared cache manager and the resource uploader
+on the node manager. Resources are spread across a set of subdirectories using the resources's
+checksum:
+```
+/sharedcache/a/8/9/a896857d078/foo.jar
+/sharedcache/5/0/f/50f11b09f87/bar.jar
+/sharedcache/a/6/7/a678cb1aa8f/job.jar
+```
+
+### Shared Cache Manager (SCM)
+
+The shared cache manager is responsible for serving requests from the client and
+managing the contents of the shared cache. It looks after both the meta data as
+well as the persisted resources in HDFS. It is made up of two major components,
+a back end store and a cleaner service. The SCM runs as a separate daemon
+process that can be placed on any node in the cluster. This allows for
+administrators to start/stop/upgrade the SCM without affecting other YARN
+components (i.e. the resource manager or node managers).
+
+The back end store is responsible for maintaining and persisting metadata about
+the shared cache. This includes the resources in the cache, when a resource was
+last used and a list of applications that are currently using the resource. The
+implementation for the backing store is pluggable and it currently uses an
+in-memory store that recreates its state after a restart.
+
+The cleaner service maintains the persisted resources in HDFS by ensuring that
+resources that are no longer used are removed from the cache. It scans the
+resources in the cache periodically and evicts resources if they are both stale
+and there are no live applications currently using the application.
+
+### The Shared Cache uploader and localization
+
+The shared cache uploader is a service that runs on the node manager and adds
+resources to the shared cache. It is responsible for verifying a resources
+checksum, uploading the resource to HDFS and notifying the shared cache
+manager that a resource has been added to the cache. It is important to note
+that the uploader service is asynchronous from the container launch and does not
+block the startup of a yarn application. In addition adding things to the cache
+is done in a best effort way and does not impact running applications. Once the
+uploader has placed a resource in the shared cache, YARN uses the normal node
+manager localization mechanism to make resources available to the application.
+
+Developing YARN applications with the Shared Cache
+------------
+
+To support the YARN shared cache, an application must use the shared cache
+client during application submission. The shared cache client returns a
+URL corresponding to a resource if it is in the shared cache. To use the cached
+resource, a YARN application simply uses the cached URL to create a
+LocalResource object and sets setShouldBeUploadedToSharedCache to true during
+application submission.
+
+For example, here is how you would create a LocalResource using a cached URL:
+```
+String localPathChecksum = sharedCacheClient.getFileChecksum(localPath);
+URL cachedResource = sharedCacheClient.use(appId, localPathChecksum);
+LocalResource resource = LocalResource.newInstance(cachedResource,
+      LocalResourceType.FILE, LocalResourceVisibility.PUBLIC
+      size, timestamp, null, true);
+```
+
+Administrating the Shared Cache
+------------
+
+### Setting up the shared cache
+An administrator can initially set up the shared cache by following these steps:
+
+1. Create an HDFS directory for the shared cache (default: /sharedcache).
+2. Set the shared cache directory permissions to 0755.
+3. Ensure that the shared cache directory is owned by the user that runs the
+shared cache manager daemon and the node manager.
+4. In the yarn-site.xml file, set *yarn.sharedcache.enabled* to true and
+*yarn.sharedcache.root-dir* to the directory specified in step 1. For more configuration
+parameters, see the configuration parameters section.
+5. Start the shared cache manager:
+```
+/hadoop/bin/yarn --daemon start sharedcachemanager
+```
+
+### Configuration parameters
+The configuration parameters can be found in yarn-default.xml and should be set
+in the yarn-site.xml file. Here are a list of configuration parameters and their defaults:
+
+Name | Description | Default value
+--- | --- | ---
+yarn.sharedcache.enabled | Whether the shared cache is enabled | false
+yarn.sharedcache.root-dir | The root directory for the shared cache | /sharedcache
+yarn.sharedcache.nested-level | The level of nested directories before getting to the checksum directories. It must be non-negative. | 3
+yarn.sharedcache.store.class | The implementation to be used for the SCM store | org.apache.hadoop.yarn.server.sharedcachemanager.store.InMemorySCMStore
+yarn.sharedcache.app-checker.class | The implementation to be used for the SCM app-checker | org.apache.hadoop.yarn.server.sharedcachemanager.RemoteAppChecker
+yarn.sharedcache.store.in-memory.staleness-period-mins | A resource in the in-memory store is considered stale if the time since the last reference exceeds the staleness period. This value is specified in minutes. | 10080
+yarn.sharedcache.store.in-memory.initial-delay-mins | Initial delay before the in-memory store runs its first check to remove dead initial applications. Specified in minutes. | 10
+yarn.sharedcache.store.in-memory.check-period-mins | The frequency at which the in-memory store checks to remove dead initial applications. Specified in minutes. | 720
+yarn.sharedcache.admin.address | The address of the admin interface in the SCM (shared cache manager) | 0.0.0.0:8047
+yarn.sharedcache.admin.thread-count | The number of threads used to handle SCM admin interface (1 by default) | 1
+yarn.sharedcache.webapp.address | The address of the web application in the SCM (shared cache manager) | 0.0.0.0:8788
+yarn.sharedcache.cleaner.period-mins | The frequency at which a cleaner task runs. Specified in minutes. | 1440
+yarn.sharedcache.cleaner.initial-delay-mins | Initial delay before the first cleaner task is scheduled. Specified in minutes. | 10
+yarn.sharedcache.cleaner.resource-sleep-ms | The time to sleep between processing each shared cache resource. Specified in milliseconds. | 0
+yarn.sharedcache.uploader.server.address | The address of the node manager interface in the SCM (shared cache manager) | 0.0.0.0:8046
+yarn.sharedcache.uploader.server.thread-count | The number of threads used to handle shared cache manager requests from the node manager (50 by default) | 50
+yarn.sharedcache.client-server.address | The address of the client interface in the SCM (shared cache manager) | 0.0.0.0:8045
+yarn.sharedcache.client-server.thread-count | The number of threads used to handle shared cache manager requests from clients (50 by default) | 50
+yarn.sharedcache.checksum.algo.impl | The algorithm used to compute checksums of files (SHA-256 by default) | org.apache.hadoop.yarn.sharedcache.ChecksumSHA256Impl
+yarn.sharedcache.nm.uploader.replication.factor | The replication factor for the node manager uploader for the shared cache (10 by default) | 10
+yarn.sharedcache.nm.uploader.thread-count | The number of threads used to upload files from a node manager instance (20 by default) | 20