HADOOP-18177. Document prefetching architecture. (apache#4205)

Contributed by Ahmar Suhail

Author: ahmarsuhail

hadoop-tools/hadoop-aws/src/site/markdown/tools/hadoop-aws/prefetching.md

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+  Licensed under the Apache License, Version 2.0 (the "License");
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+
+   http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
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+
+# S3A Prefetching
+
+This document explains the `S3PrefetchingInputStream` and the various components it uses.
+
+This input stream implements prefetching and caching to improve read performance of the input
+stream.
+A high level overview of this feature was published in
+[Pinterest Engineering's blog post titled "Improving efficiency and reducing runtime using S3 read optimization"](https://medium.com/pinterest-engineering/improving-efficiency-and-reducing-runtime-using-s3-read-optimization-b31da4b60fa0).
+
+With prefetching, the input stream divides the remote file into blocks of a fixed size, associates
+buffers to these blocks and then reads data into these buffers asynchronously.
+It also potentially caches these blocks.
+
+### Basic Concepts
+
+* **Remote File**: A binary blob of data stored on some storage device.
+* **Block File**: Local file containing a block of the remote file.
+* **Block**: A file is divided into a number of blocks.
+The size of the first n-1 blocks is same, and the size of the last block may be same or smaller.
+* **Block based reading**: The granularity of read is one block.
+That is, either an entire block is read and returned or none at all.
+Multiple blocks may be read in parallel.
+
+### Configuring the stream
+
+|Property    |Meaning    |Default    |
+|---|---|---|
+|`fs.s3a.prefetch.enabled`    |Enable the prefetch input stream    |`true` |
+|`fs.s3a.prefetch.block.size`    |Size of a block    |`8M`    |
+|`fs.s3a.prefetch.block.count`    |Number of blocks to prefetch    |`8`    |
+
+### Key Components
+
+`S3PrefetchingInputStream` - When prefetching is enabled, S3AFileSystem will return an instance of
+this class as the input stream.
+Depending on the remote file size, it will either use
+the `S3InMemoryInputStream` or the `S3CachingInputStream` as the underlying input stream.
+
+`S3InMemoryInputStream` - Underlying input stream used when the remote file size < configured block
+size.
+Will read the entire remote file into memory.
+
+`S3CachingInputStream` - Underlying input stream used when remote file size > configured block size.
+Uses asynchronous prefetching of blocks and caching to improve performance.
+
+`BlockData` - Holds information about the blocks in a remote file, such as:
+
+* Number of blocks in the remote file
+* Block size
+* State of each block (initially all blocks have state *NOT_READY*).
+Other states are: Queued, Ready, Cached.
+
+`BufferData` - Holds the buffer and additional information about it such as:
+
+* The block number this buffer is for
+* State of the buffer (Unknown, Blank, Prefetching, Caching, Ready, Done).
+Initial state of a buffer is blank.
+
+`CachingBlockManager` - Implements reading data into the buffer, prefetching and caching.
+
+`BufferPool` - Manages a fixed sized pool of buffers.
+It's used by `CachingBlockManager` to acquire buffers.
+
+`S3File` - Implements operations to interact with S3 such as opening and closing the input stream to
+the remote file in S3.
+
+`S3Reader` - Implements reading from the stream opened by `S3File`.
+Reads from this input stream in blocks of 64KB.
+
+`FilePosition` - Provides functionality related to tracking the position in the file.
+Also gives access to the current buffer in use.
+
+`SingleFilePerBlockCache` - Responsible for caching blocks to the local file system.
+Each cache block is stored on the local disk as a separate block file.
+
+### Operation
+
+#### S3InMemoryInputStream
+
+For a remote file with size 5MB, and block size = 8MB, since file size is less than the block size,
+the `S3InMemoryInputStream` will be used.
+
+If the caller makes the following read calls:
+
+```
+in.read(buffer, 0, 3MB);
+in.read(buffer, 0, 2MB);
+```
+
+When the first read is issued, there is no buffer in use yet.
+The `S3InMemoryInputStream` gets the data in this remote file by calling the `ensureCurrentBuffer()`
+method, which ensures that a buffer with data is available to be read from.
+
+The `ensureCurrentBuffer()` then:
+
+* Reads data into a buffer by calling `S3Reader.read(ByteBuffer buffer, long offset, int size)`.
+* `S3Reader` uses `S3File` to open an input stream to the remote file in S3 by making
+  a `getObject()` request with range as `(0, filesize)`.
+* The `S3Reader` reads the entire remote file into the provided buffer, and once reading is complete
+  closes the S3 stream and frees all underlying resources.
+* Now the entire remote file is in a buffer, set this data in `FilePosition` so it can be accessed
+  by the input stream.
+
+The read operation now just gets the required bytes from the buffer in `FilePosition`.
+
+When the second read is issued, there is already a valid buffer which can be used.
+Don't do anything else, just read the required bytes from this buffer.
+
+#### S3CachingInputStream
+
+If there is a remote file with size 40MB and block size = 8MB, the `S3CachingInputStream` will be
+used.
+
+##### Sequential Reads
+
+If the caller makes the following calls:
+
+```
+in.read(buffer, 0, 5MB)
+in.read(buffer, 0, 8MB)
+```
+
+For the first read call, there is no valid buffer yet.
+`ensureCurrentBuffer()` is called, and for the first `read()`, prefetch count is set as 1.
+
+The current block (block 0) is read synchronously, while the blocks to be prefetched (block 1) is
+read asynchronously.
+
+The `CachingBlockManager` is responsible for getting buffers from the buffer pool and reading data
+into them. This process of acquiring the buffer pool works as follows:
+
+* The buffer pool keeps a map of allocated buffers and a pool of available buffers.
+The size of this pool is = prefetch block count + 1.
+If the prefetch block count is 8, the buffer pool has a size of 9.
+* If the pool is not yet at capacity, create a new buffer and add it to the pool.
+* If it's at capacity, check if any buffers with state = done can be released.
+Releasing a buffer means removing it from allocated and returning it back to the pool of available
+buffers.
+* If there are no buffers with state = done currently then nothing will be released, so retry the
+  above step at a fixed interval a few times till a buffer becomes available.
+* If after multiple retries there are still no available buffers, release a buffer in the ready state.
+The buffer for the block furthest from the current block is released.
+
+Once a buffer has been acquired by `CachingBlockManager`, if the buffer is in a *READY* state, it is
+returned.
+This means that data was already read into this buffer asynchronously by a prefetch.
+If it's state is *BLANK* then data is read into it using
+`S3Reader.read(ByteBuffer buffer, long offset, int size).`
+
+For the second read call, `in.read(buffer, 0, 8MB)`, since the block sizes are of 8MB and only 5MB
+of block 0 has been read so far, 3MB of the required data will be read from the current block 0.
+Once all data has been read from this block, `S3CachingInputStream` requests the next block (
+block 1), which will already have been prefetched and so it can just start reading from it.
+Also, while reading from block 1 it will also issue prefetch requests for the next blocks.
+The number of blocks to be prefetched is determined by `fs.s3a.prefetch.block.count`.
+
+##### Random Reads
+
+If the caller makes the following calls:
+
+```
+in.read(buffer, 0, 5MB)
+in.seek(10MB)
+in.read(buffer, 0, 4MB)
+in.seek(2MB)
+in.read(buffer, 0, 4MB)
+```
+
+The `CachingInputStream` also caches prefetched blocks.
+This happens when a `seek()` is issued for outside the current block and the current block still has
+not been fully read.
+
+For the above read sequence, when the `seek(10MB)` call is issued, block 0 has not been read
+completely so cache it as the caller will probably want to read from it again.
+
+When `seek(2MB)` is called, the position is back inside block 0.
+The next read can now be satisfied from the locally cached block file, which is typically orders of
+magnitude faster than a network based read.