YARN-2960. Add documentation for the YARN shared cache.
(cherry picked from commit 7e76f85bc6)
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Chris Trezzo
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<item name="Graceful Decommission" href="hadoop-yarn/hadoop-yarn-site/GracefulDecommission.html"/>
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<item name="Opportunistic Containers" href="hadoop-yarn/hadoop-yarn-site/OpportunisticContainers.html"/>
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<item name="YARN Federation" href="hadoop-yarn/hadoop-yarn-site/Federation.html"/>
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<item name="Resource Profiles" href="hadoop-yarn/hadoop-yarn-site/ResourceProfiles.html"/>
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<item name="Shared Cache" href="hadoop-yarn/hadoop-yarn-site/SharedCache.html"/>
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</menu>
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<menu name="YARN REST APIs" inherit="top">
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<!---
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Licensed under the Apache License, Version 2.0 (the "License");
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you may not use this file except in compliance with the License.
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You may obtain a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS,
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WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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See the License for the specific language governing permissions and
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limitations under the License. See accompanying LICENSE file.
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-->
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YARN Shared Cache
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===================
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<!-- MACRO{toc|fromDepth=0|toDepth=3} -->
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Overview
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--------
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The YARN Shared Cache provides the facility to upload and manage shared
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application resources to HDFS in a safe and scalable manner. YARN applications
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can leverage resources uploaded by other applications or previous
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runs of the same application without having to reupload and localize identical files
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multiple times. This will save network resources and reduce YARN application
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startup time.
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Current Status and Future Plans
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------------
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Currently the YARN Shared Cache is released and ready to use. The major
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components are implemented and have been deployed in a large-scale
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production setting. There are still some pieces missing (i.e. strong
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authentication). These missing features will be implemented as part of a
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follow-up phase 2 effort. Please see
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[YARN-7282](https://issues.apache.org/jira/browse/YARN-7282) for more information.
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Architecture
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------------
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The shared cache feature consists of 4 major components:
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1. The shared cache client.
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2. The HDFS directory that acts as a cache.
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3. The shared cache manager (aka. SCM).
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4. The localization service and uploader.
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### The Shared Cache Client
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YARN application developers and users, should interact with the shared cache using
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the shared cache client. This client is responsible for interacting with the
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shared cache manager, computing the checksum of application resources, and
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claiming application resources in the shared cache. Once an application has claimed
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a resource, it is free to use that resource for the life-cycle of the application.
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Please see the SharedCacheClient.java javadoc for further documentation.
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### The Shared Cache HDFS Directory
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The shared cache HDFS directory stores all of the shared cache resources. It is protected
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by HDFS permissions and is globally readable, but writing is restricted to a trusted user.
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This HDFS directory is only modified by the shared cache manager and the resource uploader
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on the node manager. Resources are spread across a set of subdirectories using the resources's
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checksum:
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```
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/sharedcache/a/8/9/a896857d078/foo.jar
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/sharedcache/5/0/f/50f11b09f87/bar.jar
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/sharedcache/a/6/7/a678cb1aa8f/job.jar
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```
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### Shared Cache Manager (SCM)
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The shared cache manager is responsible for serving requests from the client and
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managing the contents of the shared cache. It looks after both the meta data as
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well as the persisted resources in HDFS. It is made up of two major components,
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a back end store and a cleaner service. The SCM runs as a separate daemon
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process that can be placed on any node in the cluster. This allows for
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administrators to start/stop/upgrade the SCM without affecting other YARN
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components (i.e. the resource manager or node managers).
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The back end store is responsible for maintaining and persisting metadata about
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the shared cache. This includes the resources in the cache, when a resource was
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last used and a list of applications that are currently using the resource. The
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implementation for the backing store is pluggable and it currently uses an
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in-memory store that recreates its state after a restart.
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The cleaner service maintains the persisted resources in HDFS by ensuring that
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resources that are no longer used are removed from the cache. It scans the
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resources in the cache periodically and evicts resources if they are both stale
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and there are no live applications currently using the application.
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### The Shared Cache uploader and localization
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The shared cache uploader is a service that runs on the node manager and adds
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resources to the shared cache. It is responsible for verifying a resources
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checksum, uploading the resource to HDFS and notifying the shared cache
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manager that a resource has been added to the cache. It is important to note
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that the uploader service is asynchronous from the container launch and does not
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block the startup of a yarn application. In addition adding things to the cache
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is done in a best effort way and does not impact running applications. Once the
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uploader has placed a resource in the shared cache, YARN uses the normal node
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manager localization mechanism to make resources available to the application.
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Developing YARN applications with the Shared Cache
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------------
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To support the YARN shared cache, an application must use the shared cache
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client during application submission. The shared cache client returns a
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URL corresponding to a resource if it is in the shared cache. To use the cached
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resource, a YARN application simply uses the cached URL to create a
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LocalResource object and sets setShouldBeUploadedToSharedCache to true during
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application submission.
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For example, here is how you would create a LocalResource using a cached URL:
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```
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String localPathChecksum = sharedCacheClient.getFileChecksum(localPath);
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URL cachedResource = sharedCacheClient.use(appId, localPathChecksum);
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LocalResource resource = LocalResource.newInstance(cachedResource,
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LocalResourceType.FILE, LocalResourceVisibility.PUBLIC
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size, timestamp, null, true);
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```
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Administrating the Shared Cache
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------------
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### Setting up the shared cache
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An administrator can initially set up the shared cache by following these steps:
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1. Create an HDFS directory for the shared cache (default: /sharedcache).
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2. Set the shared cache directory permissions to 0755.
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3. Ensure that the shared cache directory is owned by the user that runs the
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shared cache manager daemon and the node manager.
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4. In the yarn-site.xml file, set *yarn.sharedcache.enabled* to true and
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*yarn.sharedcache.root-dir* to the directory specified in step 1. For more configuration
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parameters, see the configuration parameters section.
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5. Start the shared cache manager:
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```
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/hadoop/bin/yarn --daemon start sharedcachemanager
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```
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### Configuration parameters
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The configuration parameters can be found in yarn-default.xml and should be set
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in the yarn-site.xml file. Here are a list of configuration parameters and their defaults:
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Name | Description | Default value
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--- | --- | ---
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yarn.sharedcache.enabled | Whether the shared cache is enabled | false
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yarn.sharedcache.root-dir | The root directory for the shared cache | /sharedcache
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yarn.sharedcache.nested-level | The level of nested directories before getting to the checksum directories. It must be non-negative. | 3
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yarn.sharedcache.store.class | The implementation to be used for the SCM store | org.apache.hadoop.yarn.server.sharedcachemanager.store.InMemorySCMStore
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yarn.sharedcache.app-checker.class | The implementation to be used for the SCM app-checker | org.apache.hadoop.yarn.server.sharedcachemanager.RemoteAppChecker
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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
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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
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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
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yarn.sharedcache.admin.address | The address of the admin interface in the SCM (shared cache manager) | 0.0.0.0:8047
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yarn.sharedcache.admin.thread-count | The number of threads used to handle SCM admin interface (1 by default) | 1
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yarn.sharedcache.webapp.address | The address of the web application in the SCM (shared cache manager) | 0.0.0.0:8788
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yarn.sharedcache.cleaner.period-mins | The frequency at which a cleaner task runs. Specified in minutes. | 1440
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yarn.sharedcache.cleaner.initial-delay-mins | Initial delay before the first cleaner task is scheduled. Specified in minutes. | 10
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yarn.sharedcache.cleaner.resource-sleep-ms | The time to sleep between processing each shared cache resource. Specified in milliseconds. | 0
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yarn.sharedcache.uploader.server.address | The address of the node manager interface in the SCM (shared cache manager) | 0.0.0.0:8046
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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
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yarn.sharedcache.client-server.address | The address of the client interface in the SCM (shared cache manager) | 0.0.0.0:8045
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yarn.sharedcache.client-server.thread-count | The number of threads used to handle shared cache manager requests from clients (50 by default) | 50
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yarn.sharedcache.checksum.algo.impl | The algorithm used to compute checksums of files (SHA-256 by default) | org.apache.hadoop.yarn.sharedcache.ChecksumSHA256Impl
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yarn.sharedcache.nm.uploader.replication.factor | The replication factor for the node manager uploader for the shared cache (10 by default) | 10
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yarn.sharedcache.nm.uploader.thread-count | The number of threads used to upload files from a node manager instance (20 by default) | 20
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