Deploy Alluxio on a Cluster with HA
- Basic Setup
- Start an Alluxio Cluster with HA
- Access an Alluxio Cluster with HA
- Common Operations
An Alluxio service with High availability (or HA) achieved by running multiple Alluxio master processes on different nodes in the system. One of these masters is elected the leading master which serves all workers and clients as the primary point of contact. The other master processes act as standby masters which maintain the same file system state with the leading master by trailing a shared journal. Note that, standby masters do not serve any client or worker requests; however, if the leading master fails, one standby master will automatically be chosen to take over and become the new leading master. Once the new leading master starts serving, Alluxio clients and workers proceed as usual. During the failover to a standby master, clients may experience brief delays or transient errors.
The major challenges here to achieve HA are maintaining the shared file system state across service restarts and maintain consensus among masters on leading master after failover. In Alluxio 2.0, there are two different ways to achieve these two goals:
- Option1: Use an internal replicated state machine based on RAFT to both store the file system journal and select a leading master. This approach is introduced in Alluxio 2.0 and requires no dependency on external services.
- Option2: Leverage an external Zookeeper service for leader election on the leading master and a shared storage (e.g., the root UFS) for shared journal. See journal management documentation for more information about choosing and configuring Alluxio journal system.
- To deploy a Alluxio cluster, first download the pre-compiled Alluxio binary file, extract the tarball and copy the extracted directory to all nodes (including nodes running masters and workers).
- Enable SSH login without password from master node to worker nodes. You can add a public SSH key for
the host into
~/.ssh/authorized_keys. See this tutorial for more details.
- TCP traffic across all nodes is allowed. For basic functionality make sure RPC port (default :19998) is open on all nodes.
Option1: Raft-based Embedded Journal
The minimal configuration to set up a HA cluster is to give the embedded journal addresses to
all nodes inside the cluster. On each Alluxio node, create the
conf/alluxio-site.properties configuration file from the template.
cp conf/alluxio-site.properties.template conf/alluxio-site.properties
Add the following properties to the
alluxio.master.hostname=<MASTER_HOSTNAME> # Only needed on master node alluxio.master.mount.table.root.ufs=<STORAGE_URI> alluxio.master.embedded.journal.addresses=<EMBEDDED_JOURNAL_ADDRESS>
- The first property
alluxio.master.hostname=<MASTER_HOSTNAME>is required on each master node to be its own externally visible hostname. This is required on each individual component of the master quorum to have its own address set. On worker nodes, this parameter will be ignored. Examples include
- The second property
alluxio.master.mount.table.root.ufs=<STORAGE_URI>sets the URI of the shared under store to mount to the Alluxio root. This shared under store must be accessible by all master nodes and all worker nodes. Examples include
- The third property
alluxio.master.embedded.journal.addressessets the sets of masters to participate Alluxio’s internal leader election and determine the leading master. The default embedded journal port is
19200. An example:
Note that embedded journal feature relies on Copycat which has built-in leader election. The built-in leader election cannot work with Zookeeper since we cannot have two leaders which might not match. Enabling embedded journal enables Alluxio’s internal leader election. See embedded journal configuration documentation for more details and alternative ways to set up HA cluster with internal leader election.
Option2: Zookeeper and Shared Journal Storage
The additional prerequisites of setting up Zookeeper HA cluster are:
- A ZooKeeper cluster. Alluxio masters use ZooKeeper for leader election, and Alluxio clients and workers use ZooKeeper to inquire about the identity of the current leading master.
- A shared storage system on which to place the journal (accessible by all Alluxio masters). The leading
master writes to the journal on this shared storage system, while the standby masters continually
replay the journal entries to stay up-to-date.
The journal storage system is recommended to be:
- Highly available. All metadata modifications on the master requires writing to the journal, so any downtime of the journal storage system will directly impact the Alluxio master availability.
- Filesystem, not an object store. The Alluxio master writes to journal files to this storage system, and utilizes filesystem operations such as rename and flush. Object stores do not support these operations, and/or perform them slowly, so when the journal is stored on an object store, the Alluxio master operation throughput is significantly reduced.
The minimal configuration parameters which must be set are:
alluxio.zookeeper.enabled=true alluxio.zookeeper.address=<ZOOKEEPER_ADDRESS> alluxio.master.journal.type=UFS alluxio.master.journal.folder=<JOURNAL_URI>
alluxio.zookeeper.enabled=trueenables the HA mode for the masters, and informs workers that HA mode is enabled.
alluxio.zookeeper.address=<ZOOKEEPER_ADDRESS>sets the ZooKeeper address when
alluxio.zookeeper.enabledis enabled. The HA masters use ZooKeeper for leader election, and the workers use ZooKeeper to discover the leading master. Multiple ZooKeeper addresses can be specified by delimiting with commas. Examples include
alluxio.master.journal.type=UFSindicates UFS is used as the journal place. Note that Zookeeper cannot work with journal type
EMBEDDED(use a journal embedded in the masters).
alluxio.master.journal.folder=<JOURNAL_URI>sets the URI of the shared journal location for the Alluxio leading master to write the journal to, and for standby masters to replay journal entries from. This shared shared storage system must be accessible by all master nodes. Examples include
For clusters with large namespaces, increased CPU overhead on leader could cause delays on Zookeeper client heartbeats. For this reason, we recommend setting Zookeeper client session timeout to at least 2 minutes on large clusters with namespace size more than several hundred millions of files.
- Zookeeper server’s tick time must also be configured as such to allow this timeout. The current implementation requires that the timeout be a minimum of 2 times the tickTime (as set in the server configuration) and a maximum of 20 times the tickTime.
Make sure all master nodes and all worker nodes have configured their respective
conf/alluxio-site.properties configuration file appropriately.
Once all the Alluxio masters and workers are configured in this way, Alluxio is ready to be formatted started.
Start an Alluxio Cluster with HA
Before Alluxio can be started for the first time, the journal must be formatted.
Formatting the journal will delete all metadata from Alluxio. However, the data in under storage will be untouched.
On any master node, format Alluxio with the following command:
To start the Alluxio cluster with the provided scripts, on any master node, list all the
worker hostnames in the
conf/workers file, and list all the masters in the
conf/masters file. This
will allow the start scripts to start the appropriate processes on the appropriate nodes.
On the master node, start the Alluxio cluster with the following command:
./bin/alluxio-start.sh all SudoMount
This will start Alluxio masters on all the nodes specified in
conf/masters, and start the workers on all the
nodes specified in
SudoMount indicates to mount the RamFS on each worker
sudo privilege, if it is not already mounted.
Verify Alluxio Cluster
To verify that Alluxio is running, you can visit the web UI of the leading master. To determine the leading master, run:
./bin/alluxio fs leader
http://<LEADER_HOSTNAME>:19999 to see the status page of the Alluxio leading master.
Alluxio comes with a simple program that writes and reads sample files in Alluxio. Run the sample program with:
Access an Alluxio Cluster with HA
When an application interacts with Alluxio in HA mode, the client must know about the Alluxio HA cluster, so that the client knows how to discover the Alluxio leading master. There are two ways to specify the HA Alluxio service address on the client side:
Specify Alluxio Service in Configuration Parameters
Users can also pre-configure the service address of an Alluxio HA cluster in environment variables
or site properties, and then connect to the service using Alluxio URI like to
alluxio:///path where the required connection details to the HA cluster information is already
configured with these parameters.
For example, if using Hadoop, you can configure the properties in
core-site.xml, and then use the
Hadoop CLI with an Alluxio URI.
hadoop fs -ls alluxio:///directory
Depending on the different approach to achieve HA, different properties are required to set:
- When connecting to an Alluxio HA cluster using embedded journal,
alluxio.master.rpc.addressesto decide the node addresses to query. For examples,
- When connecting to an Alluxio HA cluster using Zookeeper,
the following properties are needed to connect to Zookeeper to get the leading master information.
Note that, the ZooKeeper address (
alluxio.zookeeper.address) must be specified when
alluxio.zookeeper.enabledis enabled and vise versa. Multiple ZooKeeper addresses can be specified by delimiting with commas
Specify Alluxio Service with URL Authority
Users can also fully specify the HA cluster information in the URI to connect to an Alluxio HA cluster. Configuration derived from the HA authority takes precedence over all other forms of configuration, e.g. site properties or environment variables.
- When using embedded journal, use
- When using Zookeeper leader election, use
For many applications (e.g., Hadoop, HBase, Hive and Flink), you can use a comma as the
delimiter for multiple addresses in the URI, like
For some other applications (e.g., Spark) where comma is not accepted inside a URL authority, you
need to use semicolons as the delimiter for multiple addresses,
Below are common operations to perform on an Alluxio cluster.
To stop an Alluxio service, run:
This will stop all the processes on all nodes listed in
You can stop just the masters and just the workers with the following commands:
./bin/alluxio-stop.sh masters # stops all masters in conf/masters ./bin/alluxio-stop.sh workers # stops all workers in conf/workers
If you do not want to use
ssh to login to all the nodes and stop all the processes, you can run
commands on each node individually to stop each component. For any node, you can stop a master or worker with:
./bin/alluxio-stop.sh master # stops the local master ./bin/alluxio-stop.sh worker # stops the local worker
Starting Alluxio is similar. If
conf/masters are both populated, you can start
the cluster with:
You can start just the masters and just the workers with the following commands:
./bin/alluxio-start.sh masters # starts all masters in conf/masters ./bin/alluxio-start.sh workers # starts all workers in conf/workers
If you do not want to use
ssh to login to all the nodes and start all the processes, you can run
commands on each node individually to start each component. For any node, you can start a master or worker with:
./bin/alluxio-start.sh master # starts the local master ./bin/alluxio-start.sh worker # starts the local worker
Format the Journal
On any master node, format the Alluxio journal with the following command:
Formatting the journal will delete all metadata from Alluxio. However, the data in under storage will be untouched.
Add/Remove Workers Dynamically
Adding a worker to an Alluxio cluster dynamically is as simple as starting a new Alluxio worker process, with the appropriate configuration. In most cases, the new worker’s configuration should be the same as all the other workers’ configuration. Run the following command on the new worker to add
./bin/alluxio-start.sh worker SudoMount # starts the local worker
Once the worker is started, it will register itself with the Alluxio master, and become part of the Alluxio cluster.
Removing a worker is as simple as stopping the worker process.
./bin/alluxio-stop.sh worker # stops the local worker
Once the worker is stopped, and after
a timeout on the master (configured by master parameter
alluxio.master.worker.timeout), the master
will consider the worker as “lost”, and no longer consider it as part of the cluster.
In order to add a master, the Alluxio cluster must operate in HA mode. If you are running the cluster as a single master cluster, you must configure it to be an HA cluster before having more than one master.
Alluxio HA cluster with embedded journal
When internal leader election is used, Alluxio masters are determined. Adding or removing master nodes requires:
- Shut down the whole cluster
- Add/remove one or more Alluxio master
- Format the whole cluster
- Update the cluster-wide embedded journal configuration
- Start the whole cluster
Note that all previously stored data and metadata in Alluxio filesystem will be erased. If you are using embedded journal, you should not plan to add new masters.
Alluxio HA cluster with Zookeeper and Shared Journal
To add a master to an HA Alluxio cluster, you can simply start a new Alluxio master process, with
the appropriate configuration. The configuration for the new master should be the same as other masters,
except that the parameter
alluxio.master.hostname=<MASTER_HOSTNAME> should reflect the new hostname.
Once the new master is started, it will start interacting with ZooKeeper to participate in leader election.
Removing a master is as simple as stopping the master process. If the cluster is a single master cluster, stopping the master will essentially shutdown the cluster, since the single master is down. If the Alluxio cluster is an HA cluster, stopping the leading master will force ZooKeeper to elect a new leading master and failover to that new leader. If a standby master is stopped, then the operation of the cluster is unaffected. Keep in mind, Alluxio masters high availability depends on the availability on standby masters. If there are not enough standby masters, the availability of the leading master will be affected. It is recommended to have at least 3 masters for an HA Alluxio cluster.
Update Master-side Configuration
In order to update the master-side configuration, you can first stop the service,
conf/alluxio-site.properties file on master node,
and then restart the service.
Note that, this approach introduces downtime of the Alluxio service.
Alternatively, one benefit of running Alluxio in HA mode is to use rolling restarts to minimize downtime when updating configurations:
- Update the master configuration on all the master nodes without restarting any master.
- Restart the leading master (can be determined by running
bin/alluxio leader). Note that, a new leading master will be elected to continue the service.
- Wait for the previous leading master to come up successfully but as a standby master.
- Update and restart all remaining standby masters.
- Verify the configuration update
Update Worker-side Configuration
If you only need to update some local configuration for a worker (e.g., change the mount
of storage capacity allocated to this worker or update the storage directory), the master node does
not need to be stopped and restarted. One can simply stop the local worker, update the configuration
conf/alluxio-site.properties) file on this worker, and then restart the worker.