mirror of https://github.com/facebook/rocksdb.git
70 lines
2.8 KiB
C++
70 lines
2.8 KiB
C++
// Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
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// This source code is licensed under both the GPLv2 (found in the
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// COPYING file in the root directory) and Apache 2.0 License
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// (found in the LICENSE.Apache file in the root directory).
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// This file is for functions that generate unique identifiers by
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// (at least in part) by extracting novel entropy or sources of uniqueness
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// from the execution environment. (By contrast, random.h is for algorithmic
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// pseudorandomness.)
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//
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// These functions could eventually migrate to public APIs, such as in Env.
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#pragma once
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#include <atomic>
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#include <cstdint>
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#include "rocksdb/rocksdb_namespace.h"
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namespace ROCKSDB_NAMESPACE {
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// Generates a new 128-bit identifier that is universally unique
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// (with high probability) for each call. The result is split into
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// two 64-bit pieces. This function has NOT been validated for use in
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// cryptography.
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//
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// This is used in generating DB session IDs and by Env::GenerateUniqueId
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// (used for DB IDENTITY) if the platform does not provide a generator of
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// RFC 4122 UUIDs or fails somehow. (Set exclude_port_uuid=true if this
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// function is used as a fallback for GenerateRfcUuid, because no need
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// trying it again.)
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void GenerateRawUniqueId(uint64_t* a, uint64_t* b,
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bool exclude_port_uuid = false);
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#ifndef NDEBUG
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// A version of above with options for challenge testing
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void TEST_GenerateRawUniqueId(uint64_t* a, uint64_t* b, bool exclude_port_uuid,
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bool exclude_env_details,
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bool exclude_random_device);
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#endif
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// Generates globally unique ids with lower probability of any collisions
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// vs. each unique id being independently random (GenerateRawUniqueId).
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// We call this "semi-structured" because between different
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// SemiStructuredUniqueIdGen objects, the IDs are separated by random
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// intervals (unstructured), but within a single SemiStructuredUniqueIdGen
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// object, the generated IDs are trivially related (structured). See
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// https://github.com/pdillinger/unique_id for how this improves probability
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// of no collision. In short, if we have n SemiStructuredUniqueIdGen
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// objects each generating m IDs, the first collision is expected at
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// around n = sqrt(2^128 / m), equivalently n * sqrt(m) = 2^64,
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// rather than n * m = 2^64 for fully random IDs.
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class SemiStructuredUniqueIdGen {
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public:
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// Initializes with random starting state (from GenerateRawUniqueId)
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SemiStructuredUniqueIdGen();
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// Assuming no fork(), `lower` is guaranteed unique from one call
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// to the next (thread safe).
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void GenerateNext(uint64_t* upper, uint64_t* lower);
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private:
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uint64_t base_upper_;
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uint64_t base_lower_;
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std::atomic<uint64_t> counter_;
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int64_t saved_process_id_;
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};
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} // namespace ROCKSDB_NAMESPACE
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