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https://issues.apache.org/jira/browse/HADOOP-16202?focusedWorklogId=751636&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-751636
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ASF GitHub Bot logged work on HADOOP-16202:
-------------------------------------------
Author: ASF GitHub Bot
Created on: 01/Apr/22 15:47
Start Date: 01/Apr/22 15:47
Worklog Time Spent: 10m
Work Description: steveloughran commented on a change in pull request
#2584:
URL: https://github.com/apache/hadoop/pull/2584#discussion_r840714093
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File path:
hadoop-common-project/hadoop-common/src/site/markdown/filesystem/openfile.md
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@@ -0,0 +1,522 @@
+<!---
+ Licensed 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. See accompanying LICENSE file.
+-->
+
+# `FileSystem.openFile()`/`FileContext.openFile()`
+
+This is a method provided by both FileSystem and FileContext for
+advanced file opening options and, where implemented,
+an asynchrounous/lazy opening of a file.
+
+Creates a builder to open a file, supporting options
+both standard and filesystem specific. The return
+value of the `build()` call is a `Future<FSDataInputStream>`,
+which must be waited on. The file opening may be
+asynchronous, and it may actually be postponed (including
+permission/existence checks) until reads are actually
+performed.
+
+This API call was added to `FileSystem` and `FileContext` in
+Hadoop 3.3.0; it was tuned in Hadoop 3.3.1 as follows.
+
+* Added `opt(key, long)` and `must(key, long)`.
+* Declared that `withFileStatus(null)` is allowed.
+* Declared that `withFileStatus(status)` only checks
+ the filename of the path, not the full path.
+ This is needed to support passthrough/mounted filesystems.
+* Added standard option keys.
+
+### <a name="openfile(path)"></a> `FutureDataInputStreamBuilder openFile(Path
path)`
+
+Creates a [`FutureDataInputStreamBuilder`](fsdatainputstreambuilder.html)
+to construct a operation to open the file at `path` for reading.
+
+When `build()` is invoked on the returned `FutureDataInputStreamBuilder`
instance,
+the builder parameters are verified and
+`FileSystem.openFileWithOptions(Path, OpenFileParameters)` or
+`AbstractFileSystem.openFileWithOptions(Path, OpenFileParameters)` invoked.
+
+These protected methods returns a `CompletableFuture<FSDataInputStream>`
+which, when its `get()` method is called, either returns an input
+stream of the contents of opened file, or raises an exception.
+
+The base implementation of the `FileSystem.openFileWithOptions(PathHandle,
OpenFileParameters)`
+ultimately invokes `FileSystem.open(Path, int)`.
+
+Thus the chain `FileSystem.openFile(path).build().get()` has the same
preconditions
+and postconditions as `FileSystem.open(Path p, int bufferSize)`
+
+However, there is one difference which implementations are free to
+take advantage of:
+
+The returned stream MAY implement a lazy open where file non-existence or
+access permission failures may not surface until the first `read()` of the
+actual data.
+
+The `openFile()` operation MAY check the state of the filesystem during its
+invocation, but as the state of the filesystem may change betwen this call and
+the actual `build()` and `get()` operations, this file-specific
+preconditions (file exists, file is readable, etc) MUST NOT be checked here.
+
+FileSystem implementations which do not implement `open(Path, int)`
+MAY postpone raising an `UnsupportedOperationException` until either the
+`FutureDataInputStreamBuilder.build()` or the subsequent `get()` call,
+else they MAY fail fast in the `openFile()` call.
+
+### Implementors notes
+
+The base implementation of `openFileWithOptions()` actually executes
+the `open(path)` operation synchronously, yet still returns the result
+or any failures in the `CompletableFuture<>`, so as to ensure that users
+code expecting this.
+
+Any filesystem where the time to open a file may be significant SHOULD
+execute it asynchronously by submitting the operation in some executor/thread
+pool. This is particularly recommended for object stores and other filesystems
+likely to be accessed over long-haul connections.
+
+Arbitrary filesystem-specific options MAY be supported; these MUST
+be prefixed with either the filesystem schema, e.g. `hdfs.`
+or in the `fs.SCHEMA` format as normal configuration settings `fs.hdfs`. The
+latter style allows the same configuration option to be used for both
+filesystem configuration and file-specific configuration.
+
+It SHOULD be possible to always open a file without specifying any options,
+so as to present a consistent model to users. However, an implementation MAY
+opt to require one or more mandatory options to be set.
+
+The returned stream may perform "lazy" evaluation of file access. This is
+relevant for object stores where the probes for existence are expensive, and,
+even with an asynchronous open, may be considered needless.
+
+### <a name="openfile(pathhandle)"></a> `FutureDataInputStreamBuilder
openFile(PathHandle)`
+
+Creates a [`FutureDataInputStreamBuilder`](fsdatainputstreambuilder.html)
+to construct a operation to open the file identified by the given `PathHandle`
for reading.
+
+When `build()` is invoked on the returned `FutureDataInputStreamBuilder`
instance,
+the builder parameters are verified and
+`openFileWithOptions(PathHandle, OpenFileParameters)` invoked.
+
+This (protected) operation returns a `CompletableFuture<FSDataInputStream>`
+which, when its `get()` method is called, either returns an input
+stream of the contents of opened file, or raises an exception.
+
+The base implementation of the `openFileWithOptions(PathHandle,
OpenFileParameters)` method
+returns a future which invokes `open(Path, int)`.
+
+Thus the chain `openFile(pathhandle).build().get()` has the same preconditions
+and postconditions as `open(Pathhandle, int)`
+
+As with `FutureDataInputStreamBuilder openFile(PathHandle)`, the `openFile()`
+call must not be where path-specific preconditions are checked -that
+is postponed to the `build()` and `get()` calls.
+
+FileSystem implementations which do not implement `open(PathHandle handle, int
bufferSize)`
+MAY postpone raising an `UnsupportedOperationException` until either the
+`FutureDataInputStreamBuilder.build()` or the subsequent `get()` call,
+else they MAY fail fast in the `openFile()` call.
+
+The base implementation raises this exception in the `build()` operation;
+other implementations SHOULD copy this.
+
+
+## <a name="options"></a> Standard `openFile()` options since Hadoop 3.3.1
+
+These are options which FileSystem implementations are expected to
+recognise -and ideally, support by changing the behavior of the input streams
+as a appropriate.
+
+Hadoop 3.3.0 added the `openFile()` API; these standard options were
+defined in Hadoop 3.3.1. Therefore, although they are "well known", unless
+confident that the application will only be executed against releases
+of Hadoop which knows of the options -set the options via `opt()`
+calls rather than `must()`.
+
+When opening a file through the `openFile()` builder API, callers
+MAY use both `.opt(key, value)` and `.must(key, value)` calls to set
+standard and filesystem-specific options.
+
+If set as an `opt()` parameter, unsupported "standard" options MUST be ignored,
+as MUST unrecognized standard options.
+
+If set as an `must()` parameter, unsupported "standard" options MUST be
ignored.
+unrecognized standard options MUST be rejected.
+
+The standard `openFile()` options are defined in
`org.apache.hadoop.fs.OpenFileOptions`;
+they all SHALL start with `fs.option.openfile.`.
+
+Note that while all `FileSystem`/`FileContext` instances SHALL support these
options
+to the extent that `must()` declarations SHALL NOT fail, the implementations
+MAY support them to the extent of interpreting the values.
+This means that it is not a requirement for the stores to actually read the
+the read policy or file length values and use them when opening files.
Review comment:
i'm trying to say that the read and length policies must be understood,
but may not be used, if the implementation chooses not to
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Issue Time Tracking
-------------------
Worklog Id: (was: 751636)
Time Spent: 15.5h (was: 15h 20m)
> Stabilize openFile() and adopt internally
> -----------------------------------------
>
> Key: HADOOP-16202
> URL: https://issues.apache.org/jira/browse/HADOOP-16202
> Project: Hadoop Common
> Issue Type: Bug
> Components: fs, fs/s3, tools/distcp
> Affects Versions: 3.3.0
> Reporter: Steve Loughran
> Assignee: Steve Loughran
> Priority: Major
> Labels: pull-request-available
> Time Spent: 15.5h
> Remaining Estimate: 0h
>
> The {{openFile()}} builder API lets us add new options when reading a file
> Add an option {{"fs.s3a.open.option.length"}} which takes a long and allows
> the length of the file to be declared. If set, *no check for the existence of
> the file is issued when opening the file*
> Also: withFileStatus() to take any FileStatus implementation, rather than
> only S3AFileStatus -and not check that the path matches the path being
> opened. Needed to support viewFS-style wrapping and mounting.
> and Adopt where appropriate to stop clusters with S3A reads switched to
> random IO from killing download/localization
> * fs shell copyToLocal
> * distcp
> * IOUtils.copy
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