Add comments in file_reader_writer.h (#5355)

Summary:
Add file and class level comments in file_reader_writer.h
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5355

Differential Revision: D15499020

Pulled By: sagar0

fbshipit-source-id: 925b2326885cdb4357e6a139ac65ee5e2ce1d613

util/file_reader_writer.h

@@ -6,6 +6,7 @@
 // Copyright (c) 2011 The LevelDB Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
+
 #pragma once
 #include <atomic>
 #include <sstream>
@@ -22,9 +23,22 @@ namespace rocksdb {
 class Statistics;
 class HistogramImpl;
 
+// This file provides the following main abstractions:
+// SequentialFileReader : wrapper over Env::SequentialFile
+// RandomAccessFileReader : wrapper over Env::RandomAccessFile
+// WritableFileWriter : wrapper over Env::WritableFile
+// In addition, it also exposed NewReadaheadRandomAccessFile, NewWritableFile,
+// and ReadOneLine primitives.
+
+// NewReadaheadRandomAccessFile provides a wrapper over RandomAccessFile to
+// always prefetch additional data with every read. This is mainly used in
+// Compaction Table Readers.
 std::unique_ptr<RandomAccessFile> NewReadaheadRandomAccessFile(
   std::unique_ptr<RandomAccessFile>&& file, size_t readahead_size);
 
+// SequentialFileReader is a wrapper on top of Env::SequentialFile. It handles
+// Buffered (i.e when page cache is enabled) and Direct (with O_DIRECT / page
+// cache disabled) reads appropriately, and also updates the IO stats.
 class SequentialFileReader {
  private:
   std::unique_ptr<SequentialFile> file_;
@@ -61,6 +75,12 @@ class SequentialFileReader {
   bool use_direct_io() const { return file_->use_direct_io(); }
 };
 
+// RandomAccessFileReader is a wrapper on top of Env::RnadomAccessFile. It is
+// responsible for:
+// - Handling Buffered and Direct reads appropriately.
+// - Rate limiting compaction reads.
+// - Notifying any interested listeners on the completion of a read.
+// - Updating IO stats.
 class RandomAccessFileReader {
  private:
 #ifndef ROCKSDB_LITE
@@ -151,7 +171,13 @@ class RandomAccessFileReader {
   bool use_direct_io() const { return file_->use_direct_io(); }
 };
 
-// Use posix write to write data to a file.
+// WritableFileWriter is a wrapper on top of Env::WritableFile. It provides
+// facilities to:
+// - Handle Buffered and Direct writes.
+// - Rate limit writes.
+// - Flush and Sync the data to the underlying filesystem.
+// - Notify any interested listeners on the completion of a write.
+// - Update IO stats.
 class WritableFileWriter {
  private:
 #ifndef ROCKSDB_LITE
@@ -277,13 +303,31 @@ class WritableFileWriter {
   Status SyncInternal(bool use_fsync);
 };
 
-// FilePrefetchBuffer can automatically do the readahead if file_reader,
+// FilePrefetchBuffer is a smart buffer to store and read data from a file.
-// readahead_size, and max_readahead_size are passed in.
-// max_readahead_size should be greater than or equal to readahead_size.
-// readahead_size will be doubled on every IO, until max_readahead_size.
 class FilePrefetchBuffer {
  public:
-  // If `track_min_offset` is true, track minimum offset ever read.
+  // Constructor.
+  //
+  // All arguments are optional.
+  // file_reader        : the file reader to use. Can be a nullptr.
+  // readahead_size     : the initial readahead size.
+  // max_readahead_size : the maximum readahead size.
+  //   If max_readahead_size > readahead_size, the readahead size will be
+  //   doubled on every IO until max_readahead_size is hit.
+  //   Typically this is set as a multiple of readahead_size.
+  //   max_readahead_size should be greater than equal to readahead_size.
+  // enable : controls whether reading from the buffer is enabled.
+  //   If false, TryReadFromCache() always return false, and we only take stats
+  //   for the minimum offset if track_min_offset = true.
+  // track_min_offset : Track the minimum offset ever read and collect stats on
+  //   it. Used for adaptable readahead of the file footer/metadata.
+  //
+  // Automatic readhead is enabled for a file if file_reader, readahead_size,
+  // and max_readahead_size are passed in.
+  // If file_reader is a nullptr, setting readadhead_size and max_readahead_size
+  // does not make any sense. So it does nothing.
+  // A user can construct a FilePrefetchBuffer without any arguments, but use
+  // `Prefetch` to load data into the buffer.
   FilePrefetchBuffer(RandomAccessFileReader* file_reader = nullptr,
                      size_t readadhead_size = 0, size_t max_readahead_size = 0,
                      bool enable = true, bool track_min_offset = false)
@@ -294,11 +338,26 @@ class FilePrefetchBuffer {
         min_offset_read_(port::kMaxSizet),
         enable_(enable),
         track_min_offset_(track_min_offset) {}
+
+  // Load data into the buffer from a file.
+  // reader : the file reader.
+  // offset : the file offset to start reading from.
+  // n      : the number of bytes to read.
   Status Prefetch(RandomAccessFileReader* reader, uint64_t offset, size_t n);
+
+  // Tries returning the data for a file raed from this buffer, if that data is
+  // in the buffer.
+  // It handles tracking the minimum read offset if track_min_offset = true.
+  // It also does the exponential readahead when readadhead_size is set as part
+  // of the constructor.
+  //
+  // offset : the file offset.
+  // n      : the number of bytes.
+  // result : output buffer to put the data into.
   bool TryReadFromCache(uint64_t offset, size_t n, Slice* result);
 
-  // The minimum `offset` ever passed to TryReadFromCache(). Only be tracked
+  // The minimum `offset` ever passed to TryReadFromCache(). This will nly be
-  // if track_min_offset = true.
+  // tracked if track_min_offset = true.
   size_t min_offset_read() const { return min_offset_read_; }
 
  private:
@@ -317,9 +376,17 @@ class FilePrefetchBuffer {
   bool track_min_offset_;
 };
 
+// Returns a WritableFile.
+//
+// env     : the Env.
+// fname   : the file name.
+// result  : output arg. A WritableFile based on `fname` returned.
+// options : the Env Options.
 extern Status NewWritableFile(Env* env, const std::string& fname,
                               std::unique_ptr<WritableFile>* result,
                               const EnvOptions& options);
+
+// Read a single line from a file.
 bool ReadOneLine(std::istringstream* iss, SequentialFile* seq_file,
                  std::string* output, bool* has_data, Status* result);