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| <h5>YARN REST APIs</h5> |
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| <!-- CLASS: FSDataInputStreamBuilder --> |
| <!-- ============================================================= --> |
| <h1>class <tt>org.apache.hadoop.fs.FSDataInputStreamBuilder</tt></h1> |
| <ul> |
| <li><a href="#Invariants">Invariants</a></li> |
| <li><a href="#Implementation-agnostic_parameters.">Implementation-agnostic parameters.</a> |
| <ul> |
| <li><a href="#FSDataInputStreamBuilder_bufferSize.28int_bufSize.29"> FSDataInputStreamBuilder bufferSize(int bufSize)</a></li> |
| <li><a href="#FSDataInputStreamBuilder_withFileStatus.28FileStatus_status.29"> FSDataInputStreamBuilder withFileStatus(FileStatus status)</a></li> |
| <li><a href="#Set_optional_or_mandatory_parameters">Set optional or mandatory parameters</a></li></ul></li> |
| <li><a href="#Builder_interface">Builder interface</a> |
| <ul> |
| <li><a href="#CompletableFuture.3CFSDataInputStream.3E_build.28.29"> CompletableFuture<FSDataInputStream> build()</a></li></ul></li></ul> |
| |
| <p>An interface offering of the Builder pattern for creating Java <tt>Future</tt> references to <tt>FSDataInputStream</tt> and its subclasses. It is used to initate a (potentially asynchronous) operation to open an existing file for reading.</p> |
| <div class="section"> |
| <h2><a name="Invariants"></a>Invariants</h2> |
| <p>The <tt>FSDataInputStreamBuilder</tt> interface does not require parameters or or the state of <tt>FileSystem</tt> until <a href="#build"><tt>build()</tt></a> is invoked and/or during the asynchronous open operation itself.</p> |
| <p>Some aspects of the state of the filesystem, MAY be checked in the initial <tt>openFile()</tt> call, provided they are known to be invariants which will not change between <tt>openFile()</tt> and the <tt>build().get()</tt> sequence. For example, path validation.</p></div> |
| <div class="section"> |
| <h2><a name="Implementation-agnostic_parameters."></a>Implementation-agnostic parameters.</h2> |
| <div class="section"> |
| <h3><a name="FSDataInputStreamBuilder_bufferSize.28int_bufSize.29"></a><a name="Builder.bufferSize"></a> <tt>FSDataInputStreamBuilder bufferSize(int bufSize)</tt></h3> |
| <p>Set the size of the buffer to be used.</p></div> |
| <div class="section"> |
| <h3><a name="FSDataInputStreamBuilder_withFileStatus.28FileStatus_status.29"></a><a name="Builder.withFileStatus"></a> <tt>FSDataInputStreamBuilder withFileStatus(FileStatus status)</tt></h3> |
| <p>A <tt>FileStatus</tt> instance which refers to the file being opened.</p> |
| <p>This MAY be used by implementations to short-circuit checks for the file, So potentially saving on remote calls especially to object stores.</p> |
| <p>Requirements:</p> |
| <ul> |
| |
| <li><tt>status != null</tt></li> |
| <li><tt>status.getPath()</tt> == the resolved path of the file being opened.</li> |
| </ul> |
| <p>The path validation MUST take place if the store uses the <tt>FileStatus</tt> when it opens files, and MAY be performed otherwise. The validation SHOULD be postponed until the <tt>build()</tt> operation.</p> |
| <p>This operation should be considered a hint to the filesystem.</p> |
| <p>If a filesystem implementation extends the <tt>FileStatus</tt> returned in its implementation MAY use this information when opening the file.</p> |
| <p>This is relevant with those stores which return version/etag information, including the S3A and ABFS connectors -they MAY use this to guarantee that the file they opened is exactly the one returned in the listing.</p></div> |
| <div class="section"> |
| <h3><a name="Set_optional_or_mandatory_parameters"></a>Set optional or mandatory parameters</h3> |
| |
| <div> |
| <div> |
| <pre class="source">FSDataInputStreamBuilder opt(String key, ...) |
| FSDataInputStreamBuilder must(String key, ...) |
| </pre></div></div> |
| |
| <p>Set optional or mandatory parameters to the builder. Using <tt>opt()</tt> or <tt>must()</tt>, client can specify FS-specific parameters without inspecting the concrete type of <tt>FileSystem</tt>.</p> |
| |
| <div> |
| <div> |
| <pre class="source">out = fs.openFile(path) |
| .opt("fs.s3a.experimental.input.fadvise", "random") |
| .must("fs.s3a.readahead.range", 256 * 1024) |
| .withFileStatus(statusFromListing) |
| .build() |
| .get(); |
| </pre></div></div> |
| |
| <div class="section"> |
| <h4><a name="Implementation_Notes"></a>Implementation Notes</h4> |
| <p>Checking for supported options must be performed in the <tt>build()</tt> operation.</p> |
| <ol style="list-style-type: decimal"> |
| |
| <li> |
| |
| <p>If a mandatory parameter declared via <tt>must(key, value)</tt>) is not recognized, <tt>IllegalArgumentException</tt> MUST be thrown.</p> |
| </li> |
| <li> |
| |
| <p>If a mandatory parameter declared via <tt>must(key, value)</tt>) relies on a feature which is recognized but not supported in the specific Filesystem/FileContext instance <tt>UnsupportedException</tt> MUST be thrown.</p> |
| </li> |
| </ol> |
| <p>The behavior of resolving the conflicts between the parameters set by builder methods (i.e., <tt>bufferSize()</tt>) and <tt>opt()</tt>/<tt>must()</tt> is as follows:</p> |
| <blockquote> |
| |
| <p>The last option specified defines the value and its optional/mandatory state.</p> |
| </blockquote> |
| <p>If the <tt>FileStatus</tt> option passed in <tt>withFileStatus()</tt> is used, implementations MUST accept all subclasses of <tt>FileStatus</tt>, including <tt>LocatedFileStatus</tt>, rather than just any FS-specific subclass implemented by the implementation (e.g <tt>S3AFileStatus</tt>). They MAY simply ignore those which are not the custom subclasses.</p> |
| <p>This is critical to ensure safe use of the feature: directory listing/ status serialization/deserialization can result result in the <tt>withFileStatus()</tt> argumennt not being the custom subclass returned by the Filesystem instance’s own <tt>getFileStatus()</tt>, <tt>listFiles()</tt>, <tt>listLocatedStatus()</tt> calls, etc.</p> |
| <p>In such a situation the implementations must:</p> |
| <ol style="list-style-type: decimal"> |
| |
| <li>Validate the path (always).</li> |
| <li>Use the status/convert to the custom type, <i>or</i> simply discard it.</li> |
| </ol></div></div></div> |
| <div class="section"> |
| <h2><a name="Builder_interface"></a>Builder interface</h2> |
| <div class="section"> |
| <h3><a name="CompletableFuture.3CFSDataInputStream.3E_build.28.29"></a><a name="build"></a> <tt>CompletableFuture<FSDataInputStream> build()</tt></h3> |
| <p>Return an <tt>CompletableFuture<FSDataInputStream></tt> which, when successfully completed, returns an input stream which can read data from the filesystem.</p> |
| <p>The <tt>build()</tt> operation MAY perform the validation of the file’s existence, its kind, so rejecting attempts to read from a directory or non-existent file. <b>Alternatively</b>, the <tt>build()</tt> operation may delay all checks until an asynchronous operation whose outcome is provided by the <tt>Future</tt></p> |
| <p>That is, the precondition <tt>exists(FS, path)</tt> and <tt>isFile(FS, path)</tt> are only guaranteed to have been met after the <tt>get()</tt> on the returned future is successful.</p> |
| <p>Thus, if even a file does not exist, the following call will still succeed, returning a future to be evaluated.</p> |
| |
| <div> |
| <div> |
| <pre class="source">Path p = new Path("file://tmp/file-which-does-not-exist"); |
| |
| CompletableFuture<FSDataInputStream> future = p.getFileSystem(conf) |
| .openFile(p) |
| .build; |
| </pre></div></div> |
| |
| <p>The preconditions for opening the file are checked during the asynchronous evaluation, and so will surface when the future is completed:</p> |
| |
| <div> |
| <div> |
| <pre class="source">FSDataInputStream in = future.get(); |
| </pre></div></div></div></div> |
| </div> |
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