bgaborg commented on a change in pull request #575: HADOOP-13327 Output Stream Specification URL: https://github.com/apache/hadoop/pull/575#discussion_r264215533
########## File path: hadoop-common-project/hadoop-common/src/site/markdown/filesystem/outputstream.md ########## @@ -0,0 +1,857 @@ +<!--- + 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. +--> + +<!-- MACRO{toc|fromDepth=1|toDepth=3} --> + +# Output: `OutputStream`, `Syncable` and `StreamCapabilities` + + +With the exception of `FileSystem.copyFromLocalFile()`, +all API operations which write data to a filesystem in Hadoop do so +through the Java "OutputStreams" API. More specifically, they do +so through `OutputStream` subclasses obtained through calls to +`FileSystem.create()`, `FileSystem.append()`, +or `FSDataOutputStreamBuilder.build()`. + +These all return instances of `FSDataOutputStream`, through which data +can be written through various `write()` methods. +After a stream's `close()` method is called, all data written to the +stream MUST BE persisted to the fileysystem and visible to oll other +clients attempting to read data from that path via `FileSystem.open()`. + +As well as operations to write the data, Hadoop's Output Streams +provide methods to flush buffered data back to the filesystem, +so as to ensure that the data is reliably persisted and/or visible +to other callers. This is done via the `Syncable` interface. It was +originally intended that the presence of this interface could be interpreted +as a guarantee that the stream supported it's methods, but this has proven +impossible to guarantee as the static nature of the interface is incompatible +with filesystems whose syncability semantics may vary on a store/path basis. +As an example, erasure coded files in HDFS have +A new interface, `StreamCapabilities` has been implemented to allow callers to probe the exact capabilities of a stream, even transitively +through a chain of streams. + +* HDFS's primary stream implementation is +`org.apache.hadoop.hdfs.DFSOutputStream`. +* The subclass `org.apache.hadoop.hdfs.DFSStripedOutputStream` supports erasure +coding: it removes the `Syncable` behaviors from the base class. +* The output streams `org.apache.hadoop.fs.FSOutputSummer` and +`org.apache.hadoop.fs.ChecksumFileSystem.ChecksumFSOutputSummer` +contain the underlying checksummed output stream used by +both HDFS and the "file" filesystems. + + +## Output Stream Model + +For this specification, an output stream can be viewed as a list of bytes +stored in in the client + +```python +buffer: List[byte]` +``` + +A flag, `open` tracks whether the stream is open: after the stream +is closed no more data may be written to it: + +```python +open: bool +buffer: List[byte] +``` + +The destination path of the stream, `path` can be tracked to form a triple +`Path, open, buffer` + + +```python +Stream = (path, open, buffer) +``` + +(Immediately) after `Syncable` operations which flush data to the filesystem, +the data at the stream's destination path must match that of +`buffer`. That is, the following condition holds: + +```python +FS'.Files(path) == buffer +``` + +Any client reading the data at the path will see the new data. +The two sync operations, `hflush()` and `hsync()` differ in their durability +guarantees, not visibility of data. + + +### State of Stream and filesystem after `Filesystem.create()` + + +The output stream returned by a `FileSystem.create(path)` call contains no +data: + +```python +Stream' = (path, true, []) +``` + +The filesystem `FS'` must contain a 0-byte file at the path: + +```python +data(FS', path) == [] +``` + +Accordingly, the the initial state of `Stream'.buffer` is implicitly +consistent with the data at the filesystem. + + +*Object Stores*: the 0-byte empty file may not exist in the filesystem. + +### State of Stream and filesystem after `Filesystem.append()` + +The output stream returned from a call of + `FileSystem.append(path, buffersize, progress)`, +can be modelled as a stream whose `buffer` is intialized to that of +the original file: + +```python +Stream' = (path, true, data(FS, path)) +``` + +#### Persisting data + +When the stream writes data back to its store, be it in any +supported flush operation, in the `close()` operation, or at any other +time the stream chooses to do so, the contents of the file +are replaced with the current buffer + +```python +Stream' = (path, true, buffer) +FS' = FS where data(FS', path) == buffer +``` + +After a call to `close()`, the stream is closed for all operations other +than `close()`; they MAY fail with `IOException` or `RuntimeException`. + +```python +Stream' = (path, false, []) +``` + +The `close()` operation must be idempotent with the sole attempt to write the +data made in the first invocation. + +1. If `close()` succeeds, subsequent calls are no-ops. +1. If `close()` fails, again, subsequent calls are no-ops. They MAY rethrow +the previous exception, but they MUST NOT retry the write. + +<!-- ============================================================= --> +<!-- CLASS: FSDataOutputStream --> +<!-- ============================================================= --> + +# Class `FSDataOutputStream` + + +```java +public class FSDataOutputStream + extends DataOutputStream + implements Syncable, CanSetDropBehind, StreamCapabilities { + // ... +} +``` + +The `FileSystem.create()`, `FileSystem.append()` and +`FSDataOutputStreamBuilder.build()` calls return an instance +of a class `FSDataOutputStream`, a subclass of `java.io.OutputStream`. + +The base class wraps an `OutputStream` instance, one which may implement `Streamable`, +`CanSetDropBehind` and `StreamCapabilities`. + +This document covers the requirements of such implementations. + +HDFS's `FileSystem` implementation, `DistributedFileSystem`, returns an instance +of `HdfsDataOutputStream`. This implementation has at least two behaviors +which are not explicitly declared by the base Java implmentation + +1. Writes are synchronized: more than one thread can write to the same +output stream. This is a use pattern which HBase relies on. + +1. `OutputStream.flush()` is a no-op when the file is closed. Apache Druid +has made such a call on this in the past +[HADOOP-14346](https://issues.apache.org/jira/browse/HADOOP-14346). + + +As the HDFS implementation is considered the de-facto specification of +the FileSystem APIs, the fact that `write()` is thread-safe is significant. + +For compatibility, not only must other FS clients be thread-safe, +but new HDFS featues, such as encryption and Erasure Coding must also +implement consistent behavior with the core HDFS output stream. + +Put differently: + +*It isn't enough for Output Streams to implement the core semantics +of `java.io.OutputStream`: they need to implement the extra semantics +of `HdfsDataOutputStream`. Failure to do so must be considered regressions* + +The concurrent `write()` call is the most significant tightening of +the Java specification. + + +## Class `java.io.OutputStream` + + +A Java `OutputStream` allows applications to write a sequence of bytes to a destination. +In a Hadoop filesystem, that destination is the data under a path in the filesystem. + + +```java +public abstract class OutputStream implements Closeable, Flushable { + public abstract void write(int b) throws IOException; + public void write(byte b[]) throws IOException; + public void write(byte b[], int off, int len) throws IOException; + public void flush() throws IOException ; + public void close() throws IOException; +} +``` + +### <a name="write(data: int)"></a>`write(Stream, data)` + +Writes a byte of data to the stream. + +#### Preconditions + +```python +Stream.open else raise ClosedChannelException, PathIOException, IOException +``` + +The exception `java.nio.channels.ClosedChannelExceptionn` is +raised in the HDFS output streams when trying to write to a closed file. +Ths exception does not include the destination path; and +`Exception.getMessage()` is `null`. It is therefore of limited value in stack +traces. Implementors may wish to raise exceptions with more detail, such +as a `PathIOException`. + + +#### Postconditions + +The buffer has the lower 8 bits of the data argument appended to it. + +```python +Stream'.buffer = Stream.buffer + [data & 0xff] +``` + +There may be an explicit limit on the size of cached data, or an implicit +limit based by the available capacity of the destination filesystem. +When a limit is reached, `write()` SHOULD fail with an `IOException`. + +### <a name="write(buffer,offset,len)"></a>`write(Stream, byte[] data, int offset, int len)` + + +#### Preconditions + +The preconditions are all defined in `OutputStream.write()` + +```python +Stream.open else raise ClosedChannelException, PathIOException, IOException +data != null else raise NullPointerException +offset >= 0 else raise IndexOutOfBoundsException +len >= 0 else raise IndexOutOfBoundsException +offset < data.length else raise IndexOutOfBoundsException +offset + len < data.length else raise IndexOutOfBoundsException +``` + +There may be an explicit limit on the size of cached data, or an implicit +limit based by the available capacity of the destination filesystem. +When a limit is reached, `write()` SHOULD fail with an `IOException`. + +After the operation has returned, the buffer may be re-used. The outcome +of updates to the buffer while the `write()` operation is in progress is undefined. + +#### Postconditions + +```python +Stream'.buffer = Stream.buffer + data[offset...(offset + len)] +``` + +### <a name="write(buffer)"></a>`write(byte[] data)` + +This is defined as the equivalent of: + +```python +write(data, 0, data.length) +``` + +### <a name="flush()"></a>`flush()` + +Requests that the data is flushed. The specification of `ObjectStream.flush()` +declares that this SHOULD write data to the "intended destination". + +It explicitly precludes any guarantees about durability. + +For that reason, this document doesn't provide any normative +specifications of behaviour. + + +#### Preconditions + + +```python +Stream.open else raise IOException +``` + +#### Postconditions + +None. + +If the implementation chooses to implement a stream-flushing operation, +the data may be saved to the file system such that it becomes visible to +others" + +```python +FS' = FS where data(FS, path) == buffer +``` + +Some applications have been known to call `flush()` on a closed stream +on the assumption that it is harmless. Implementations MAY choose to +support this behaviour. + +### <a name="close"></a>`close()` + +The `close()` operation saves all data to the filesystem and +releases any resources used for writing data. + +The `close()` call is expected to block +until the write has completed (as with `Syncable.hflush()`), possibly +until it has been written to durable storage (as HDFS does). + +After `close()` completes, the data in a file MUST be visible and consistent +with the data most recently written. The metadata of the file MUST be consistent +with the data and the write history itself (i.e. any modification time fields +updated). + +After `close()` is invoked, all subsequent `write()` calls on the stream +MUST fail with an `IOException`. + + +Any locking/leaseholding mechanism is also required to release its lock/lease. + +```python +Stream'.open = false +FS' = FS where data(FS, path) == buffer +``` + +The `close()` call MAY fail during its operation. + +1. Callers of the API MUST expect for some calls to fail and SHOULD code appropriately. +Catching and swallowing exceptions, while common, is not always the ideal solution. +1. Even after a failure, `close()` MUST place the stream into a closed state. +Follow-on calls to `close()` are ignored, and calls to other methods +rejected. That is: caller's cannot be expected to call `close()` repeatedly +until it succeeds. +1. The duration of the `call()` operation is undefined. Operations which rely +on acknowledgements from remote systems to meet the persistence guarantees +implicitly have to await these acknowledgements. Some Object Store output streams +upload the entire data file in the `close()` operation. This can take a large amount +of time. The fact that many user applications assume that `close()` is both fast +and does not fail means that this behavior is dangerous. + +Recommendations for safe use by callers + +* Do plan for exceptions being raised, either in catching and logging or +by throwing the exception further up. Catching and silently swallowing exceptions +may hide serious problems. +* Heartbeat operations SHOULD take place on a separate thread, so that a long +delay in `close()` does not block the thread so long that the heartbeat times +out. + + +## Interface `StreamCapabilities` + +```java +@InterfaceAudience.Public +@InterfaceStability.Evolving +``` + +The `StreamCapabilities` interface exists to allow callers to dynamically +determine the behavior of a stream. + + +The reference Implementation of this interface is + `org.apache.hadoop.hdfs.DFSOutputStream` + +```java +public boolean hasCapability(String capability) { Review comment: In the implementation of DFSOutputStream there's ``` @Override public boolean hasCapability(String capability) { switch (StringUtils.toLowerCase(capability)) { case StreamCapabilities.HSYNC: case StreamCapabilities.HFLUSH: return true; default: return false; } } ``` Why do you use **if** here? ---------------------------------------------------------------- This is an automated message from the Apache Git Service. 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