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| <h5>YARN REST APIs</h5> |
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| <!-- CLASS: FileSystem --> |
| <!-- ============================================================= --> |
| <h1>interface <tt>org.apache.hadoop.fs.Abortable</tt></h1> |
| <ul> |
| <li><a href="#Method_abort.28.29">Method abort()</a></li> |
| <li><a href="#Method_abort.28.29_on_an_output_stream">Method abort() on an output stream</a></li> |
| <li><a href="#Preconditions">Preconditions</a></li> |
| <li><a href="#Postconditions">Postconditions</a> |
| <ul> |
| <li><a href="#Cleanup">Cleanup</a></li> |
| <li><a href="#Thread_safety_and_atomicity">Thread safety and atomicity</a></li></ul></li> |
| <li><a href="#Path.2FStream_capability_.E2.80.9Cfs.capability.outputstream.abortable.E2.80.9D">Path/Stream capability “fs.capability.outputstream.abortable”</a></li></ul> |
| |
| <p>Abort the active operation such that the output does not become manifest.</p> |
| <p>Specifically, if supported on an <a href="outputstream.html">output stream</a>, a successful <tt>abort()</tt> MUST guarantee that the stream will not be made visible in the <tt>close()</tt> operation.</p> |
| |
| <div> |
| <div> |
| <pre class="source">@InterfaceAudience.Public |
| @InterfaceStability.Unstable |
| public interface Abortable { |
| |
| /** |
| * Abort the active operation without the output becoming visible. |
| * |
| * This is to provide ability to cancel the write on stream; once |
| * a stream is aborted, the write MUST NOT become visible. |
| * |
| * @throws UnsupportedOperationException if the operation is not supported. |
| * @return the result. |
| */ |
| AbortableResult abort(); |
| |
| /** |
| * Interface for the result of aborts; allows subclasses to extend |
| * (IOStatistics etc) or for future enhancements if ever needed. |
| */ |
| interface AbortableResult { |
| |
| /** |
| * Was the stream already closed/aborted? |
| * @return true if a close/abort operation had already |
| * taken place. |
| */ |
| boolean alreadyClosed(); |
| |
| /** |
| * Any exception caught during cleanup operations, |
| * exceptions whose raising/catching does not change |
| * the semantics of the abort. |
| * @return an exception or null. |
| */ |
| IOException anyCleanupException(); |
| } |
| } |
| </pre></div></div> |
| |
| <div class="section"> |
| <h2><a name="Method_abort.28.29"></a>Method <tt>abort()</tt></h2> |
| <p>Aborts the ongoing operation such that no output SHALL become visible when the operation is completed.</p> |
| <p>Unless and until other File System classes implement <tt>Abortable</tt>, the interface is specified purely for output streams.</p></div> |
| <div class="section"> |
| <h2><a name="Method_abort.28.29_on_an_output_stream"></a>Method <tt>abort()</tt> on an output stream</h2> |
| <p><tt>Abortable.abort()</tt> MUST only be supported on output streams whose output is only made visible when <tt>close()</tt> is called, for example. output streams returned by the S3A FileSystem.</p></div> |
| <div class="section"> |
| <h2><a name="Preconditions"></a>Preconditions</h2> |
| <p>The stream MUST implement <tt>Abortable</tt> and <tt>StreamCapabilities</tt>.</p> |
| |
| <div> |
| <div> |
| <pre class="source"> if unsupported: |
| throw UnsupportedException |
| |
| if not isOpen(stream): |
| no-op |
| |
| StreamCapabilities.hasCapability("fs.capability.outputstream.abortable") == True |
| |
| </pre></div></div> |
| </div> |
| <div class="section"> |
| <h2><a name="Postconditions"></a>Postconditions</h2> |
| <p>After <tt>abort()</tt> returns, the filesystem MUST be unchanged:</p> |
| |
| <div> |
| <div> |
| <pre class="source">FS' = FS |
| </pre></div></div> |
| |
| <p>A successful <tt>abort()</tt> operation MUST guarantee that when the stream<tt>close()</tt> is invoked no output shall be manifest.</p> |
| <ul> |
| |
| <li>The stream MUST retry any remote calls needed to force the abort outcome.</li> |
| <li>If any file was present at the destination path, it MUST remain unchanged.</li> |
| </ul> |
| <p>Strictly then:</p> |
| <blockquote> |
| |
| <p>if <tt>Abortable.abort()</tt> does not raise <tt>UnsupportedOperationException</tt> then returns, then it guarantees that the write SHALL NOT become visible and that any existing data in the filesystem at the destination path SHALL continue to be available.</p> |
| </blockquote> |
| <ol style="list-style-type: decimal"> |
| |
| <li>Calls to <tt>write()</tt> methods MUST fail.</li> |
| <li>Calls to <tt>flush()</tt> MUST be no-ops (applications sometimes call this on closed streams)</li> |
| <li>Subsequent calls to <tt>abort()</tt> MUST be no-ops.</li> |
| <li><tt>close()</tt> MUST NOT manifest the file, and MUST NOT raise an exception</li> |
| </ol> |
| <p>That is, the postconditions of <tt>close()</tt> becomes:</p> |
| |
| <div> |
| <div> |
| <pre class="source">FS' = FS |
| </pre></div></div> |
| |
| <div class="section"> |
| <h3><a name="Cleanup"></a>Cleanup</h3> |
| <ul> |
| |
| <li> |
| |
| <p>If temporary data is stored in the local filesystem or in the store’s upload infrastructure then this MAY be cleaned up; best-effort is expected here.</p> |
| </li> |
| <li> |
| |
| <p>The stream SHOULD NOT retry cleanup operations; any failure there MUST be caught and added to <tt>AbortResult</tt></p> |
| </li> |
| </ul> |
| <div class="section"> |
| <h4><a name="Returned_AbortResult"></a>Returned <tt>AbortResult</tt></h4> |
| <p>The <tt>AbortResult</tt> value returned is primarily for testing and logging.</p> |
| <p><tt>alreadyClosed()</tt>: MUST return <tt>true</tt> if the write had already been aborted or closed;</p> |
| <p><tt>anyCleanupException();</tt>: SHOULD return any IOException raised during any optional cleanup operations.</p></div></div> |
| <div class="section"> |
| <h3><a name="Thread_safety_and_atomicity"></a>Thread safety and atomicity</h3> |
| <p>Output streams themselves aren’t formally required to be thread safe, but as applications do sometimes assume they are, this call MUST be thread safe.</p></div></div> |
| <div class="section"> |
| <h2><a name="Path.2FStream_capability_.E2.80.9Cfs.capability.outputstream.abortable.E2.80.9D"></a>Path/Stream capability “fs.capability.outputstream.abortable”</h2> |
| <p>An application MUST be able to verify that a stream supports the <tt>Abortable.abort()</tt> operation without actually calling it. This is done through the <tt>StreamCapabilities</tt> interface.</p> |
| <ol style="list-style-type: decimal"> |
| |
| <li> |
| |
| <p>If a stream instance supports <tt>Abortable</tt> then it MUST return <tt>true</tt> in the probe <tt>hasCapability("fs.capability.outputstream.abortable")</tt></p> |
| </li> |
| <li> |
| |
| <p>If a stream instance does not support <tt>Abortable</tt> then it MUST return <tt>false</tt> in the probe <tt>hasCapability("fs.capability.outputstream.abortable")</tt></p> |
| </li> |
| </ol> |
| <p>That is: if a stream declares its support for the feature, a call to <tt>abort()</tt> SHALL meet the defined semantics of the operation.</p> |
| <p>FileSystem/FileContext implementations SHOULD declare support similarly, to allow for applications to probe for the feature in the destination directory/path.</p> |
| <p>If a filesystem supports <tt>Abortable</tt> under a path <tt>P</tt> then it SHOULD return <tt>true</tt> to <tt>PathCababilities.hasPathCapability(path, "fs.capability.outputstream.abortable")</tt> This is to allow applications to verify that the store supports the feature.</p> |
| <p>If a filesystem does not support <tt>Abortable</tt> under a path <tt>P</tt> then it MUST return <tt>false</tt> to <tt>PathCababilities.hasPathCapability(path, "fs.capability.outputstream.abortable")</tt></p></div> |
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