HDFS-14131. [SBN read] Create user guide for Consistent Reads from Observer feature. Contributed by Chao Sun.
This commit is contained in:
Erik Krogen
Chen Liang
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@ -508,6 +508,7 @@ Usage:
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hdfs haadmin -transitionToActive <serviceId> [--forceactive]
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hdfs haadmin -transitionToStandby <serviceId>
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hdfs haadmin -transitionToObserver <serviceId>
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hdfs haadmin -failover [--forcefence] [--forceactive] <serviceId> <serviceId>
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hdfs haadmin -getServiceState <serviceId>
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hdfs haadmin -getAllServiceState
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@ -523,6 +524,7 @@ Usage:
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| `-getAllServiceState` | returns the state of all the NameNodes | |
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| `-transitionToActive` | transition the state of the given NameNode to Active (Warning: No fencing is done) |
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| `-transitionToStandby` | transition the state of the given NameNode to Standby (Warning: No fencing is done) |
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| `-transitionToObserver` | transition the state of the given NameNode to Observer (Warning: No fencing is done) |
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| `-help` [cmd] | Displays help for the given command or all commands if none is specified. |
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See [HDFS HA with NFS](./HDFSHighAvailabilityWithNFS.html#Administrative_commands) or [HDFS HA with QJM](./HDFSHighAvailabilityWithQJM.html#Administrative_commands) for more information on this command.
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@ -0,0 +1,173 @@
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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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Consistent Reads from HDFS Observer NameNode
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=============================================================
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<!-- MACRO{toc|fromDepth=0|toDepth=3} -->
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Purpose
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--------
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This guide provides an overview of the HDFS Observer NameNode feature
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and how to configure/install it in a typical HA-enabled cluster. For a
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detailed technical design overview, please check the doc attached to
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HDFS-12943.
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Background
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-----------
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In a HA-enabled HDFS cluster (for more information, check
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[HDFSHighAvailabilityWithQJM](./HDFSHighAvailabilityWithQJM.md)), there
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is a single Active NameNode and one or more Standby NameNode(s). The
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Active NameNode is responsible for serving all client requests, while
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Standby NameNode just keep the up-to-date information regarding the
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namespace, by tailing edit logs from JournalNodes, as well as block
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location information, by receiving block reports from all the DataNodes.
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One drawback of this architecture is that the Active NameNode could be a
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single bottle-neck and be overloaded with client requests, especially in
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a busy cluster.
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The Consistent Reads from HDFS Observer NameNode feature addresses the
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above by introducing a new type of NameNode called **Observer
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NameNode**. Similar to Standby NameNode, Observer NameNode keeps itself
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up-to-date regarding the namespace and block location information.
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In addition, it also has the ability to serve consistent reads, like
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Active NameNode. Since read requests are the majority in a typical
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environment, this can help to load balancing the NameNode traffic and
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improve overall throughput.
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Architecture
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--------------
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In the new architecture, a HA cluster could consists of namenodes in 3
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different states: active, standby and observer. State transition can
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happen between active and standby, standby and observer, but not
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directly between active and observer.
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To ensure read-after-write consistency within a single client, a state
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ID, which is implemented using transaction ID within NameNode, is
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introduced in RPC headers. When a client performs write through Active
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NameNode, it updates its state ID using the latest transaction ID from
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the NameNode. When performing a subsequent read, the client passes this
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state ID to Observe NameNode, which will then check against its own
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transaction ID, and will ensure its own transaction ID has caught up
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with the request's state ID, before serving the read request.
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Edit log tailing is critical for Observer NameNode as it directly affects
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the latency between when a transaction is applied in Active NameNode and
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when it is applied in the Observer NameNode. A new edit log tailing
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mechanism, named "Edit Tailing Fast-Path", is introduced to
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significantly reduce this latency. This is built on top of the existing
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in-progress edit log tailing feature, with further improvements such as
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RPC-based tailing instead of HTTP, a in-memory cache on the JournalNode,
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etc. For more details, please see the design doc attached to HDFS-13150.
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New client-side proxy providers are also introduced.
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ObserverReadProxyProvider, which inherits the existing
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ConfiguredFailoverProxyProvider, should be used to replace the latter to
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enable reads from Observer NameNode. When submitting a client read
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request, the proxy provider will first try each Observer NameNode
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available in the cluster, and only fall back to Active NameNode if all
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of the former failed. Similarly, ObserverReadProxyProviderWithIPFailover
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is introduced to replace IPFailoverProxyProvider in a IP failover setup.
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Deployment
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-----------
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### Configurations
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To enable consistent reads from Observer NameNode, you'll need to add a
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few configurations to your **hdfs-site.xml**:
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* **dfs.ha.tail-edits.in-progress** - to enable fast tailing on
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in-progress edit logs.
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This enables fast edit log tailing through in-progress edit logs and
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also other mechanisms such as RPC-based edit log fetching, in-memory
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cache in JournalNodes, and so on. It is disabled by default, but is
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**required to be turned on** for the Observer NameNode feature.
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<property>
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<name>dfs.ha.tail-edits.in-progress</name>
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<value>true</value>
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</property>
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* **dfs.journalnode.edit-cache-size.bytes** - the in-memory cache size,
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in bytes, on the JournalNodes.
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This is the size, in bytes, of the in-memory cache for storing edits
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on the JournalNode side. The cache is used for serving edits via
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RPC-based tailing. This is only effective when
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dfs.ha.tail-edits.in-progress is turned on.
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<property>
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<name>dfs.journalnode.edit-cache-size.bytes</name>
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<value>1048576</value>
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</property>
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### New administrative command
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A new HA admin command is introduced to transition a Standby NameNode
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into observer state:
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haadmin -transitionToObserver
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Note this can only be executed on Standby NameNode. Exception will be
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thrown when invoking this on Active NameNode.
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Similarly, existing **transitionToStandby** can also be run on an
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Observer NameNode, which transition it to the standby state.
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**NOTE**: the feature for Observer NameNode to participate in failover
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is not implemented yet. Therefore, as described in the next section, you
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should only use **transitionToObserver** to bring up an observer and put
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it outside the ZooKeeper controlled failover group. You should not use
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**transitionToStandby** since the host for the Observer NameNode cannot
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have ZKFC running.
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### Deployment details
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To enable observer support, first you'll need a HA-enabled HDFS cluster
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with more than 2 namenodes. Then, you need to transition Standby
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NameNode(s) into the observer state. An minimum setup would be running 3
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namenodes in the cluster, one active, one standby and one observer. For
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large HDFS clusters we recommend running two or more Observers depending
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on the intensity of read requests and HA requirements.
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Note that currently Observer NameNode doesn't integrate fully when
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automatic failover is enabled. If the
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**dfs.ha.automatic-failover.enabled** is turned on, you'll also need to
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disable ZKFC on the namenode for observer. In addition to that, you'll
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also need to add **forcemanual** flag to the **transitionToObserver**
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command:
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haadmin -transitionToObserver -forcemanual
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In future, this restriction will be lifted.
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### Client configuration
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Clients who wish to use Observer NameNode for read accesses can
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specify the ObserverReadProxyProvider class for proxy provider
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implementation, in the client-side **hdfs-site.xml** configuration file:
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<property>
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<name>dfs.client.failover.proxy.provider.<nameservice></name>
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<value>org.apache.hadoop.hdfs.server.namenode.ha.ObserverReadProxyProvider</value>
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</property>
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Clients who do not wish to use Observer NameNode can still use the
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existing ConfiguredFailoverProxyProvider and should not see any behavior
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change.
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