Github user alpinegizmo commented on a diff in the pull request:
https://github.com/apache/flink/pull/3301#discussion_r101011358
--- Diff: docs/internals/filesystems.md ---
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+---
+title: "File Systems"
+nav-parent_id: internals
+nav-pos: 10
+---
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+
+* 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*
--- End diff --
typically guarantee durability in the presence of at most *n* concurrent
node failures,
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