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Clients-Alluxio-Java.md

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Alluxio Java Client
Alluxio Java Client
Clients
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  • Table of Contents {:toc}

Alluxio provides access to data through a filesystem interface. Files in Alluxio offer write-once semantics: they become immutable after they have been written in their entirety and cannot be read before being completed. Alluxio provides two different Filesystem APIs, the Alluxio API and a Hadoop compatible API. The Alluxio API provides additional functionality, while the Hadoop compatible API gives users the flexibility of leveraging Alluxio without having to modify existing code written using Hadoop's API.

All resources with the Alluxio Java API are specified through a AlluxioURI which represents the path to the resource.

Getting a Filesystem Client

To obtain an Alluxio filesystem client in Java code, use:

{% include File-System-API/get-fileSystem.md %}

Creating a File

All metadata operations as well as opening a file for reading or creating a file for writing are executed through the FileSystem object. Since Alluxio files are immutable once written, the idiomatic way to create files is to use FileSystem#createFile(AlluxioURI), which returns a stream object that can be used to write the file. For example:

{% include File-System-API/write-file.md %}

Specifying Operation Options

For all FileSystem operations, an additional options field may be specified, which allows users to specify non-default settings for the operation. For example:

{% include File-System-API/specify-options.md %}

IO Options

Alluxio uses two different storage types: Alluxio managed storage and under storage. Alluxio managed storage is the memory, SSD, and/or HDD allocated to Alluxio workers. Under storage is the storage resource managed by the underlying storage system, such as S3, Swift or HDFS. Users can specify the interaction with Alluxio managed storage and under storage through ReadType and WriteType. ReadType specifies the data read behavior when reading a file. WriteType specifies the data write behavior when writing a new file, ie. whether the data should be written in Alluxio Storage.

Below is a table of the expected behaviors of ReadType. Reads will always prefer Alluxio storage over the under storage system.

{% for readtype in site.data.table.ReadType %} {% endfor %}
Read TypeBehavior
{{readtype.readtype}} {{site.data.table.en.ReadType[readtype.readtype]}}

Below is a table of the expected behaviors of WriteType

{% for writetype in site.data.table.WriteType %} {% endfor %}
Write TypeBehavior
{{writetype.writetype}} {{site.data.table.en.WriteType[writetype.writetype]}}

Location policy

Alluxio provides location policy to choose which workers to store the blocks of a file.

Using Alluxio's Java API, users can set the policy in CreateFileOptions for writing files and OpenFileOptions for reading files into Alluxio.

Users can simply override the default policy class in the configuration file at property alluxio.user.file.write.location.policy.class. The built-in policies include:

  • LocalFirstPolicy (alluxio.client.file.policy.LocalFirstPolicy)

    Returns the local host first, and if the local worker doesn't have enough capacity of a block, it randomly picks a worker from the active workers list. This is the default policy.

  • MostAvailableFirstPolicy (alluxio.client.file.policy.MostAvailableFirstPolicy)

    Returns the worker with the most available bytes.

  • RoundRobinPolicy (alluxio.client.file.policy.RoundRobinPolicy)

    Chooses the worker for the next block in a round-robin manner and skips workers that do not have enough capacity.

  • SpecificHostPolicy (alluxio.client.file.policy.SpecificHostPolicy)

    Returns a worker with the specified host name. This policy cannot be set as default policy.

Alluxio supports custom policies, so you can also develop your own policy appropriate for your workload by implementing interface alluxio.client.file.policy.FileWriteLocationPolicy. Note that a default policy must have an empty constructor. And to use ASYNC_THROUGH write type, all the blocks of a file must be written to the same worker.

Write Tier

Alluxio allows a client to select a tier preference when writing blocks to a local worker. Currently this policy preference exists only for local workers, not remote workers; remote workers will write blocks to the highest tier.

By default, data is written to the top tier. Users can modify the default setting through the alluxio.user.file.write.tier.default configuration property or override it through an option to the FileSystem#createFile(AlluxioURI) API call.

Accessing an existing file in Alluxio

All operations on existing files or directories require the user to specify the AlluxioURI. With the AlluxioURI, the user may use any of the methods of FileSystem to access the resource.

Reading Data

A AlluxioURI can be used to perform Alluxio FileSystem operations, such as modifying the file metadata, ie. ttl or pin state, or getting an input stream to read the file.

For example, to read a file:

{% include File-System-API/read-file.md %}

Javadoc

For additional API information, please refer to the Alluxio javadocs.