Github user alpinegizmo commented on a diff in the pull request:
https://github.com/apache/flink/pull/3301#discussion_r101012059
--- Diff: docs/internals/filesystems.md ---
@@ -0,0 +1,138 @@
+---
+title: "File Systems"
+nav-parent_id: internals
+nav-pos: 10
+---
+<!--
+Licensed to the Apache Software Foundation (ASF) under one
+or more contributor license agreements. See the NOTICE file
+distributed with this work for additional information
+regarding copyright ownership. The ASF licenses this file
+to you under the Apache License, Version 2.0 (the
+"License"); you may not use this file except in compliance
+with the License. You may obtain a copy of the License at
+
+ http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing,
+software distributed under the License is distributed on an
+"AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, either express or implied. See the License for the
+specific language governing permissions and limitations
+under the License.
+-->
+
+* Replaced by the TOC
+{:toc}
+
+Flink has its own file system abstraction via the
`org.apache.flink.core.fs.FileSystem` class.
+This abstraction provides a common set of operations and minimal
guarantees across various types
+of file system implementations.
+
+The `FileSystem`'s set of available operations is quite limited, in order
to suport a wide
+range of file systems. For example, appending to or mutating existing
files is not supported.
+
+File systems are identified by a *file system scheme*, such as `file://`,
`hdfs://`, etc.
+
+# Implementations
+
+Flink implements the file systems directly, with the following file system
schemes:
+
+ - `file`, which represents the machines local file system.
+
+Other file system types are accessed by an implementation that bridges to
the suite of file systems supported by
+[Apache Hadoop](https://hadoop.apache.org/). The following is an
incomplete list of examples:
+
+ - `hdfs`: Hadoop Distributed File System
+ - `s3`, `s3n`, and `s3a`: Amazon S3 file system
+ - `gcs`: Google Cloud Storage
+ - `maprfs`: The MapR distributed file system
+ - ...
+
+Flink loads Hadoop's file systems transparently if it finds the Hadoop
File System classes in the class path and finds a valid
+Hadoop configuration. By default, it looks for the Hadoop configuration in
the class path. Alternatively, one can specify a
+custom location via the configuration entry `fs.hdfs.hadoopconf`.
+
+
+# Persistence Guarantees
+
+These `FileSystem` and its `FsDataOutputStream` instances are used to
persistently store data, both for results of applications
+and for fault tolerance and recovery. It is therefore crucial that the
persistence semantics of these streams are well defined.
+
+## Definition of Persistence Guarantees
+
+Data written to an output stream is considered persistent, if two
requirements are met:
+
+ 1. **Visibility Requirement:** It must be guaranteed that all other
processes, machines,
+ virtual machines, containers, etc. that are able to access the file
see the data consistently
+ when given the absolute file path. This requirement is similar to the
*close-to-open*
+ semantics defined by POSIX, but restricted to the file itself (by its
absolute path).
+
+ 2. **Durability Requirement:** The file system's specific
durability/persistence requirements
+ must be met. These are specific to the particular file system. For
example the
+ {@link LocalFileSystem} does not provide any durability guarantees
for crashes of both
+ hardware and operating system, while replicated distributed file
systems (like HDFS)
+ guarantee typically durability in the presence of up to concurrent
failure or *n*
+ nodes, where *n* is the replication factor.
+
+Updates to the file's parent directory (such as that the file shows up when
+listing the directory contents) are not required to be complete for the
data in the file stream
+to be considered persistent. This relaxation is important for file systems
where updates to
+directory contents are only eventually consistent.
+
+The `FSDataOutputStream` has to guarantee data persistence for the written
bytes once the call to
+`FSDataOutputStream.close()` returns.
+
+## Examples
+
+ - For **fault-tolerant distributed file systems**, data is considered
persistent once
+ it has been received and acknowledged by the file system, typically by
having been replicated
+ to a quorum of machines (*durability requirement*). In addition the
absolute file path
+ must be visible to all other machines that will potentially access the
file (*visibility requirement*).
+
+ Whether data has hit non-volatile storage on the storage nodes depends
on the specific
+ guarantees of the particular file system.
+
+ The metadata updates to the file's parent directory are not required
to have reached
+ a consistent state. It is permissible that some machines see the file
when listing the parent
+ directory's contents while other do not, as long as access to the file
by its absolute path
+ is possible on all nodes.
+
+ - A **local file system** must support the POSIX *close-to-open*
semantics.
+ Because the local file system does not have any fault tolerance
guarantees, no further
+ requirements exist.
+
+ The above implies specifically that data may still be in the OS cache
when considered
+ persistent from the local file system's perspective. Crashes that
cause the OS cache loose
+ data are considered fatal to the local machine and not covered by the
local file system's
+ guarantees as defined by Flink.
+
+ That means that computed results, checkpoints, and savepoints that are
written only to
+ the local filesystem are not guaranteed to be recoverable from the
local machine's failure,
+ making local file systems unsuitable for production setups.
+
+# Updating File Contents
+
+Many file systems either do not support overwriting contents of existing
files at all, or do not support consistent visibility of the
+updated contents in that case. For that reason, Flink's FileSystem does
not support appending to existing files, or seeking output streams
--- End diff --
or seeking within output streams
---
If your project is set up for it, you can reply to this email and have your
reply appear on GitHub as well. If your project does not have this feature
enabled and wishes so, or if the feature is enabled but not working, please
contact infrastructure at [email protected] or file a JIRA ticket
with INFRA.
---
