This page provides details on setting up and configuring different file systems for use with Flink. We start by describing how to use and configure the different file systems that are supported by Flink out-of-the-box, before describing the necessary steps in order to add support about other/custom file system implementations.
Flink uses file systems both as sources and sinks in streaming/batch applications and as a target for checkpointing. These file systems can for example be Unix/Windows file systems, HDFS, or even object stores like S3.
The file system used for a specific file is determined by the file URI’s scheme. For example
file:///home/user/text.txt refers to
a file in the local file system, while
hdfs://namenode:50010/data/user/text.txt refers to a file in a specific HDFS cluster.
File systems are represented via the
org.apache.flink.core.fs.FileSystem class, which captures the ways to access and modify
files and objects in that file system. FileSystem instances are instantiated once per process and then cached / pooled, to
avoid configuration overhead per stream creation and to enforce certain constraints, such as connection/stream limits.
Flink ships with support for most of the popular file systems, namely local, hadoop-compatible, S3, MapR FS and OpenStack Swift FS. Each of these is identified by the scheme included in the URI of the provide file path.
Flink ships with implementations for the following file systems:
local: This file system is used when the scheme is “file://”, and it represents the file system of the local machine, including any NFS or SAN that is mounted into that local file system.
S3: Flink directly provides file systems to talk to Amazon S3. There are two alternative implementations,
flink-s3-fs-hadoop. Both implementations are self-contained with no dependency footprint. There is no need to add Hadoop to
the classpath to use them. Both internally use some Hadoop code, but “shade away” all classes to avoid any dependency conflicts.
To use those file systems when using Flink as a library, add the respective maven dependency (
org.apache.flink:flink-s3-fs-hadoop:1.8-SNAPSHOT). When starting a Flink application from the Flink binaries, copy or move
the respective jar file from the
opt folder to the
lib folder. See also AWS setup for additional details.
Attention: As described above, both Hadoop and Presto “listen” to paths with scheme set to “s3://”. This is convenient for switching between implementations (Hadoop or Presto), but it can lead to non-determinism when both implementations are required. This can happen when, for example, the job uses the StreamingFileSink which only supports Hadoop, but uses Presto for checkpointing. In this case, it is advised to use explicitly “s3a://” as a scheme for the sink (Hadoop) and “s3p://” for checkpointing (Presto).
MapR FS: The MapR file system “maprfs://” is automatically available when the MapR libraries are in the classpath.
OpenStack Swift FS: Flink directly provides a file system to talk to the OpenStack Swift file system, registered under the scheme “swift://”.
flink-swift-fs-hadoop is based on the Hadoop Project but is self-contained with no dependency footprint.
To use it when using Flink as a library, add the respective maven dependency (
When starting a Flink application from the Flink binaries, copy or move the respective jar file from the
opt folder to the
For all schemes where it cannot find a directly supported file system, Flink will try to use Hadoop to instantiate a file system for the respective scheme.
All Hadoop file systems are automatically available once
flink-runtime and the Hadoop libraries are in classpath.
That way, Flink seamlessly supports all of Hadoop file systems, and all Hadoop-compatible file systems (HCFS), for example:
The following configuration settings exist across different file systems.
If paths to files do not explicitly specify a file system scheme (and authority), a default scheme (and authority) will be used.
For example, if the default file system configured as
fs.default-scheme: hdfs://localhost:9000/, then a file path of
/user/hugo/in.txt is interpreted as
You can limit the total number of connections that a file system can concurrently open. This is useful when the file system cannot handle a large number of concurrent reads / writes or open connections at the same time.
For example, very small HDFS clusters with few RPC handlers can sometimes be overwhelmed by a large Flink job trying to build up many connections during a checkpoint.
To limit a specific file system’s connections, add the following entries to the Flink configuration. The file system to be limited is identified by its scheme.
You can limit the number if input/output connections (streams) separately (
fs.<scheme>.limit.output), as well as impose a limit on
the total number of concurrent streams (
fs.<scheme>.limit.total). If the file system tries to open more streams, the operation will block until some streams are closed.
If the opening of the stream takes longer than
fs.<scheme>.limit.timeout, the stream opening will fail.
To prevent inactive streams from taking up the complete pool (preventing new connections to be opened), you can add an inactivity timeout for streams:
fs.<scheme>.limit.stream-timeout. If a stream does not read/write any bytes for at least that amount of time, it is forcibly closed.
These limits are enforced per TaskManager, so each TaskManager in a Flink application or cluster will open up to that number of connections.
In addition, the limits are also only enforced per FileSystem instance. Because File Systems are created per scheme and authority, different
authorities will have their own connection pool. For example
hdfs://anotherhdfs:4399/ will have separate pools.
The bundled S3 file systems (
flink-s3-fs-hadoop) support entropy injection. Entropy injection is
a technique to improve scalability of AWS S3 buckets through adding some random characters near the beginning of the key.
If entropy injection is activated, a configured substring in the paths will be replaced by random characters. For example, path
s3://my-bucket/checkpoints/_entropy_/dashboard-job/ would be replaced by something like
Note that this only happens when the file creation passes the option to inject entropy!, otherwise the file path will simply remove the entropy key substring. See FileSystem.create(Path, WriteOption) for details.
Note: The Flink runtime currently passes the option to inject entropy only to checkpoint data files. All other files, including checkpoint metadata and external URI do not inject entropy, to keep checkpoint URIs predictable.
To enable entropy injection, configure the entropy key and the entropy length parameters.
s3.entropy.key: _entropy_ s3.entropy.length: 4 (default)
s3.entropy.key defines the string in paths that is replaced by the random characters. Paths that do not contain the entropy key are left unchanged.
If a file system operation does not pass the “inject entropy” write option, the entropy key substring is simply removed.
s3.entropy.length defined the number of random alphanumeric characters to replace the entropy key with.
File system implementations are discovered by Flink through Java’s service abstraction, making it easy to add additional file system implementations.
In order to add a new File System, the following steps are needed:
META-INF/services/org.apache.flink.core.fs.FileSystemFactorywhich contains the class name of your file system factory class.
See the Java Service Loader docs for more details on how service loaders work.