Native Java Client
- Getting a Filesystem Client
- Creating a File
- Specifying Operation Options
- IO Options
- Location policy
- Write Tier
- Accessing an existing file in Alluxio
- Reading Data
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, a native API and a Hadoop compatible API. The native 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 native 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:
FileSystem fs = FileSystem.Factory.get();
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:
FileSystem fs = FileSystem.Factory.get(); AlluxioURI path = new AlluxioURI("/myFile"); // Create a file and get its output stream FileOutStream out = fs.createFile(path); // Write data out.write(...); // Close and complete file out.close();
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:
FileSystem fs = FileSystem.Factory.get(); AlluxioURI path = new AlluxioURI("/myFile"); // Generate options to set a custom blocksize of 128 MB CreateFileOptions options = CreateFileOptions.defaults().setBlockSize(128 * Constants.MB); FileOutStream out = fs.createFile(path, 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 the Alluxio’s native storage and under storage through
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.
|CACHE_PROMOTE||Data is moved to the highest tier in the worker where the data was read. If the data was not in the Alluxio storage of the local worker, a replica will be added to the local Alluxio worker. If `alluxio.user.file.cache.partially.read.block` is set to true, the entire block will be cached by Alluxio space even if the client only reads a part of this block. Otherwise, a block will be cached only if it is fully read.|
|CACHE||If the data was not in the Alluxio storage of the local worker, a replica will be added to the local Alluxio worker. If `alluxio.user.file.cache.partially.read.block` is set to true, the entire block will be cached by Alluxio space even if the client only reads a part of this block. Otherwise, a block will be cached only if it is fully read.|
|NO_CACHE||Data is read without storing a copy of it in Alluxio.|
Below is a table of the expected behaviors of
|CACHE_THROUGH||Data is written synchronously to a Alluxio worker and the under storage system.|
|MUST_CACHE||Data is written synchronously to a Alluxio worker. No data will be written to the under storage. This is the default write type.|
|THROUGH||Data is written synchronously to the under storage. No data will be written to Alluxio.|
|ASYNC_THROUGH||Data is written synchronously to a Alluxio worker and asynchronously to the under storage system. Experimental.|
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:
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.
Returns the worker with the most available bytes.
Chooses the worker for the next block in a round-robin manner and skips workers that do not have enough capacity.
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.policyFileWriteLocationPolicy. 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.
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
With the AlluxioURI, the user may use any of the methods of
FileSystem to access the resource.
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:
FileSystem fs = FileSystem.Factory.get(); AlluxioURI path = new AlluxioURI("/myFile"); // Open the file for reading FileInStream in = fs.openFile(path); // Read data in.read(...); // Close file relinquishing the lock in.close();
For additional API information, please refer to the Alluxio javadocs.