building_data_management_systems.Xuanzhou.2024Fall.DaSE
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levelDB-LAB1-pzy_ckx


								// 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.


								#ifndef STORAGE_LEVELDB_INCLUDE_OPTIONS_H_

								#define STORAGE_LEVELDB_INCLUDE_OPTIONS_H_


								#include <stddef.h>


								namespace leveldb {


								class Cache;

								class Comparator;

								class Env;

								class Snapshot;

								class WritableFile;


								// DB contents are stored in a set of blocks, each of which holds a

								// sequence of key,value pairs.  Each block may be compressed before

								// being stored in a file.  The following enum describes which

								// compression method (if any) is used to compress a block.

								enum CompressionType {

								  // NOTE: do not change the values of existing entries, as these are

								  // part of the persistent format on disk.

								  kNoCompression           = 0x0,

								  kLightweightCompression  = 0x1,

								};


								// Options to control the behavior of a database (passed to DB::Open)

								struct Options {

								  // -------------------

								  // Parameters that affect behavior


								  // Comparator used to define the order of keys in the table.

								  // Default: a comparator that uses lexicographic byte-wise ordering

								  //

								  // REQUIRES: The client must ensure that the comparator supplied

								  // here has the same name and orders keys *exactly* the same as the

								  // comparator provided to previous open calls on the same DB.

								  const Comparator* comparator;


								  // If true, the database will be created if it is missing.

								  // Default: false

								  bool create_if_missing;


								  // If true, an error is raised if the database already exists.

								  // Default: false

								  bool error_if_exists;


								  // If true, the implementation will do aggressive checking of the

								  // data it is processing and will stop early if it detects any

								  // errors.  This may have unforeseen ramifications: for example, a

								  // corruption of one DB entry may cause a large number of entries to

								  // become unreadable or for the entire DB to become unopenable.

								  // Default: false

								  bool paranoid_checks;


								  // Use the specified object to interact with the environment,

								  // e.g. to read/write files, schedule background work, etc.

								  // Default: Env::Default()

								  Env* env;


								  // Any internal progress/error information generated by the db will

								  // be to written to info_log if it is non-NULL, or to a file stored

								  // in the same directory as the DB contents if info_log is NULL.

								  // Default: NULL

								  WritableFile* info_log;


								  // -------------------

								  // Parameters that affect performance


								  // Amount of data to build up in memory before converting to an

								  // on-disk file.

								  //

								  // Some DB operations may encounter a delay proportional to the size

								  // of this parameter.  Therefore we recommend against increasing

								  // this parameter unless you are willing to live with an occasional

								  // slow operation in exchange for faster bulk loading throughput.

								  //

								  // Default: 1MB

								  size_t write_buffer_size;


								  // Number of open files that can be used by the DB.  You may need to

								  // increase this if your database has a large working set (budget

								  // one open file per 2MB of working set).

								  //

								  // Default: 1000

								  int max_open_files;


								  // Handle values larger than "large_value_threshold" bytes

								  // specially, by writing them into their own files (to avoid

								  // compaction overhead) and doing content-based elimination of

								  // duplicate values to save space.

								  //

								  // We recommend against changing this value.

								  //

								  // Default: 64K

								  size_t large_value_threshold;


								  // Control over blocks (user data is stored in a set of blocks, and

								  // a block is the unit of reading from disk).


								  // Use the specified cache for blocks (if non-NULL).

								  // Default: NULL

								  Cache* block_cache;


								  // Approximate size of user data packed per block.  Note that the

								  // block size specified here corresponds to uncompressed data.  The

								  // actual size of the unit read from disk may be smaller if

								  // compression is enabled.  This parameter can be changed dynamically.

								  //

								  // Default: 8K

								  int block_size;


								  // Number of keys between restart points for delta encoding of keys.

								  // This parameter can be changed dynamically.  Most clients should

								  // leave this parameter alone.

								  //

								  // Default: 16

								  int block_restart_interval;


								  // Compress blocks using the specified compression algorithm.  This

								  // parameter can be changed dynamically.

								  //

								  // Default: kLightweightCompression, which gives lightweight but fast

								  // compression.

								  //

								  // Typical speeds of kLightweightCompression on an Intel(R) Core(TM)2 2.4GHz:

								  //    ~200-500MB/s compression

								  //    ~400-800MB/s decompression

								  // Note that these speeds are significantly faster than most

								  // persistent storage speeds, and therefore it is typically never

								  // worth switching to kNoCompression.  Even if the input data is

								  // incompressible, the kLightweightCompression implementation will

								  // efficiently detect that and will switch to uncompressed mode.

								  CompressionType compression;


								  // Create an Options object with default values for all fields.

								  Options();

								};


								// Options that control read operations

								struct ReadOptions {

								  // If true, all data read from underlying storage will be

								  // verified against corresponding checksums.

								  // Default: false

								  bool verify_checksums;


								  // Should the data read for this iteration be cached in memory?

								  // Callers may wish to set this field to false for bulk scans.

								  // Default: true

								  bool fill_cache;


								  // If "snapshot" is non-NULL, read as of the supplied snapshot

								  // (which must belong to the DB that is being read and which must

								  // not have been released).  If "snapshot" is NULL, use an impliicit

								  // snapshot of the state at the beginning of this read operation.

								  // Default: NULL

								  const Snapshot* snapshot;


								  ReadOptions()

								      : verify_checksums(false),

								        fill_cache(true),

								        snapshot(NULL) {

								  }

								};


								// Options that control write operations

								struct WriteOptions {

								  // If true, the write will be flushed from the operating system

								  // buffer cache (by calling WritableFile::Sync()) before the write

								  // is considered complete.  If this flag is true, writes will be

								  // slower.

								  //

								  // If this flag is false, and the machine crashes, some recent

								  // writes may be lost.  Note that if it is just the process that

								  // crashes (i.e., the machine does not reboot), no writes will be

								  // lost even if sync==false.

								  //

								  // Default: true

								  bool sync;


								  // If "post_write_snapshot" is non-NULL, and the write succeeds,

								  // *post_write_snapshot will be modified to point to a snapshot of

								  // the DB state immediately after this write.  The caller must call

								  // DB::ReleaseSnapshot(*post_write_snapshotsnapshot) when the

								  // snapshot is no longer needed.

								  //

								  // If "post_write_snapshot" is non-NULL, and the write fails,

								  // *post_write_snapshot will be set to NULL.

								  //

								  // Default: NULL

								  const Snapshot** post_write_snapshot;


								  WriteOptions()

								      : sync(true),

								        post_write_snapshot(NULL) {

								  }

								};


								}


								#endif  // STORAGE_LEVELDB_INCLUDE_OPTIONS_H_