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rocksdb/include/rocksdb/slice_transform.h

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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
// Copyright (c) 2012 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.
//
// Class for specifying user-defined functions which perform a
// transformation on a slice. It is not required that every slice
// belong to the domain and/or range of a function. Subclasses should
// define InDomain and InRange to determine which slices are in either
// of these sets respectively.
#pragma once
#include <memory>
#include <string>
#include "rocksdb/customizable.h"
#include "rocksdb/rocksdb_namespace.h"
namespace ROCKSDB_NAMESPACE {
class Slice;
struct ConfigOptions;
// A SliceTransform is a generic pluggable way of transforming one string
// to another. Its primary use-case is in configuring RocksDB prefix Bloom
// filters, by setting prefix_extractor in ColumnFamilyOptions.
//
// Exceptions MUST NOT propagate out of overridden functions into RocksDB,
// because RocksDB is not exception-safe. This could cause undefined behavior
// including data loss, unreported corruption, deadlocks, and more.
class SliceTransform : public Customizable {
public:
virtual ~SliceTransform(){};
// Return the name of this transformation.
virtual const char* Name() const override = 0;
static const char* Type() { return "SliceTransform"; }
// Creates and configures a new SliceTransform from the input options and id.
static Status CreateFromString(const ConfigOptions& config_options,
const std::string& id,
std::shared_ptr<const SliceTransform>* result);
// Returns a string representation of this SliceTransform, representing the ID
// and any additional properties.
std::string AsString() const;
// Extract a prefix from a specified key, partial key, iterator upper bound,
// etc. This is normally used for building and checking prefix Bloom filters
// but should accept any string for which InDomain() returns true.
// See ColumnFamilyOptions::prefix_extractor for specific properties that
// must be satisfied by prefix extractors.
virtual Slice Transform(const Slice& key) const = 0;
// Determine whether the specified key is compatible with the logic
// specified in the Transform method. Keys for which InDomain returns
// false will not be added to or queried against prefix Bloom filters.
//
// For example, if the Transform method returns a fixed length
// prefix of size 4, then an invocation to InDomain("abc") returns
// false because the specified key length(3) is shorter than the
// prefix size of 4.
//
// Wiki documentation here:
// https://github.com/facebook/rocksdb/wiki/Prefix-Seek
//
virtual bool InDomain(const Slice& key) const = 0;
// DEPRECATED: This is currently not used and remains here for backward
// compatibility.
virtual bool InRange(const Slice& /*dst*/) const { return false; }
// Returns information on maximum prefix length, if there is one.
// If Transform(x).size() == n for some keys and otherwise < n,
// should return true and set *len = n. Returning false is safe but
// currently disables some auto_prefix_mode filtering.
// Specifically, if the iterate_upper_bound is the immediate successor (see
// Comparator::IsSameLengthImmediateSuccessor) of the seek key's prefix,
// we require this function return true and iterate_upper_bound.size() == n
// to recognize and optimize the prefix seek.
// Otherwise (including FullLengthEnabled returns false, or prefix length is
// less than maximum), Seek with auto_prefix_mode is only optimized if the
// iterate_upper_bound and seek key have the same prefix.
// BUG: Despite all these conditions and even with the extra condition on
// IsSameLengthImmediateSuccessor (see it's "BUG" section), it is not
// sufficient to ensure auto_prefix_mode returns all entries that
// total_order_seek would return. See auto_prefix_mode "BUG" section.
virtual bool FullLengthEnabled(size_t* /*len*/) const { return false; }
// Transform(s)=Transform(`prefix`) for any s with `prefix` as a prefix.
//
// This function is not used by RocksDB, but for users. If users pass
// Options by string to RocksDB, they might not know what prefix extractor
// they are using. This function is to help users can determine:
// if they want to iterate all keys prefixing `prefix`, whether it is
// safe to use prefix bloom filter and seek to key `prefix`.
// If this function returns true, this means a user can Seek() to a prefix
// using the bloom filter. Otherwise, user needs to skip the bloom filter
// by setting ReadOptions.total_order_seek = true.
//
// Here is an example: Suppose we implement a slice transform that returns
// the first part of the string up to and including first ",":
// 1. SameResultWhenAppended("abc,") should return true. If applying prefix
// bloom filter using it, all slices matching "abc,.*" will be extracted
// to "abc,", so any SST file or memtable containing any of those key
// will not be filtered out.
// 2. SameResultWhenAppended("abc") should return false. A user will not be
// guaranteed to see all the keys matching "abc.*" if a user prefix
// seeks to "abc" against a DB with the same setting. If one SST file
// only contains "abcd,e", the file can be filtered out and the key will
// be invisible, because the prefix according to the configured extractor
// is "abcd,".
//
// i.e., an implementation always returning false is safe.
virtual bool SameResultWhenAppended(const Slice& /*prefix*/) const {
return false;
}
};
// The prefix is the first `prefix_len` bytes of the key, and keys shorter
// then `prefix_len` are not InDomain.
extern const SliceTransform* NewFixedPrefixTransform(size_t prefix_len);
// The prefix is the first min(length(key),`cap_len`) bytes of the key, and
// all keys are InDomain.
extern const SliceTransform* NewCappedPrefixTransform(size_t cap_len);
// Prefix is equal to key. All keys are InDomain.
extern const SliceTransform* NewNoopTransform();
} // namespace ROCKSDB_NAMESPACE