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https://github.com/scylladb/scylladb.git
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94 lines
3.8 KiB
C++
94 lines
3.8 KiB
C++
/*
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* Copyright (C) 2026-present ScyllaDB
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*/
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/*
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* SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.1
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*/
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#pragma once
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#include <seastar/core/sstring.hh>
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#include "dht/i_partitioner.hh"
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#include "dht/token-sharding.hh"
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class schema;
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namespace sstables {
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class key_view;
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}
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namespace dht {
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/// A partitioner mainly for Raft metadata tables for strongly consistent tables
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/// (raft_groups, raft_groups_snapshots, raft_groups_snapshot_config).
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///
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/// These tables have partition keys with the shard as the first column.
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/// The partitioner creates tokens that will be assigned to the shard specified
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/// in the partition key's first column.
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///
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/// Token encoding:
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/// [shard: 16 bits][hash: 48 bits]
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///
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/// To skip converting between signed and unsigned tokens (biasing), the top bit
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/// is always 0, so we can effectively use only 15 bits for the shard.
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/// This correlates with the limit enforced by the Raft tables' schema which uses
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/// a signed int (smallint) for the shard column, where allowing only positive
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/// shards allows up to 32767 (1 << 15 - 1) shards.
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///
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/// The lower 48 bits is a hash of the entire partition key.
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///
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/// This encoding is shard-count independent - the shard can be extracted by simple
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/// bit shifting regardless of how many shards exist in the cluster.
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struct fixed_shard_partitioner final : public dht::i_partitioner {
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static constexpr unsigned shard_bits = 16;
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static constexpr unsigned shard_shift = 64 - shard_bits;
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static constexpr uint16_t max_shard = std::numeric_limits<int16_t>::max();
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static constexpr uint64_t hash_mask = (uint64_t(1) << shard_shift) - 1;
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static const sstring classname;
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fixed_shard_partitioner() = default;
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virtual const sstring name() const override;
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virtual dht::token get_token(const schema& s, partition_key_view key) const override;
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virtual dht::token get_token(const sstables::key_view& key) const override;
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static dht::token token_for_shard(uint16_t shard, uint64_t hash_bits);
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static unsigned shard_of(dht::token token);
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};
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/// A sharder for Raft metadata tables for strongly consistent tables (raft_groups,
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/// raft_groups_snapshots, raft_groups_snapshot_config).
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///
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/// These tables store raft group state for all tablets of strongly consistent tables.
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/// The sharder allows specifying the shard where the metadata should be located by
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/// including the shard id in the partition key.
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///
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/// The shard is encoded in the token by fixed_shard_partitioner. The sharder extracts
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/// the shard by decoding the token bits used for shard encoding.
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///
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/// We inherit from static_sharder because that's what we use for system tables.
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class fixed_shard_sharder : public dht::static_sharder {
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public:
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/// Singleton instance for the raft tablet sharder.
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static fixed_shard_sharder& instance();
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fixed_shard_sharder();
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virtual ~fixed_shard_sharder() = default;
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/// Returns the shard for a token by extracting it from the token's high bits.
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/// This overrides static_sharder::shard_of to use the bit-based encoding.
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virtual unsigned shard_of(const dht::token& t) const override;
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virtual std::optional<unsigned> try_get_shard_for_reads(const dht::token& t) const override;
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virtual dht::shard_replica_set shard_for_writes(const dht::token& t, std::optional<dht::write_replica_set_selector> sel) const override;
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virtual dht::token token_for_next_shard(const dht::token& t, shard_id shard, unsigned spans = 1) const override;
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virtual dht::token token_for_next_shard_for_reads(const dht::token& t, shard_id shard, unsigned spans = 1) const override;
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virtual std::optional<dht::shard_and_token> next_shard(const dht::token& t) const override;
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virtual std::optional<dht::shard_and_token> next_shard_for_reads(const dht::token& t) const override;
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};
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} // namespace dht
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