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85eac7c34e
Data-plane requests typically hold a strong pointer to the effective_replication_map (ERM) to protect against tablet migrations and other topology operations. This works because major steps in the topology coordinator use global barriers. These barriers install a new token_metadata version on each shard and wait for all references to the old one to be dropped. Since the ERM holds a strong pointer to token_metadata, it effectively blocks these operations until it's released. For LWT, we usually deal with a single token within a single tablet. In such cases, it's enough to block topology changes for just that one tablet. The existing tablet_metadata_guard class already supports this: it tracks tablet-specific changes and updates the ERM pointer automatically, unless the change affects the guarded tablet. However, this only works for tablet-aware tables. To support LWT with vnodes (i.e., non-tablet-aware tables), this commit introduces a new token_metadata_guard class. It wraps tablet_metadata_guard when the table uses tablets, and falls back to holding a plain strong ERM pointer otherwise. In the next commits, we’ll migrate LWT to use token_metadata_guard in paxos_response_handler instead of erm.
100 lines
3.8 KiB
C++
100 lines
3.8 KiB
C++
/*
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* Copyright (C) 2023-present ScyllaDB
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*/
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/*
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* SPDX-License-Identifier: LicenseRef-ScyllaDB-Source-Available-1.0
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*/
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#pragma once
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#include "replica/database_fwd.hh"
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#include "locator/tablets.hh"
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#include "locator/abstract_replication_strategy.hh"
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namespace locator {
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/// A holder for table's effective_replication_map which
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/// automatically switches to the latest one as long as it
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/// does not affect the tablet associated with this guard.
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///
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/// This is useful for tracking long-running operations in
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/// the context of a single tablet which we don't want to
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/// block topology barriers for other tablets, only barriers for
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/// this tablet.
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class tablet_metadata_guard {
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lw_shared_ptr<replica::table> _table;
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global_tablet_id _tablet;
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effective_replication_map_ptr _erm;
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std::optional<tablet_transition_stage> _stage;
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seastar::abort_source _abort_source;
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optimized_optional<seastar::abort_source::subscription> _callback;
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private:
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void subscribe();
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void check() noexcept;
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public:
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tablet_metadata_guard(replica::table& table, global_tablet_id tablet);
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~tablet_metadata_guard();
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tablet_metadata_guard(tablet_metadata_guard&&) = delete;
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tablet_metadata_guard(const tablet_metadata_guard&) = delete;
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tablet_metadata_guard& operator=(tablet_metadata_guard&&) = delete;
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tablet_metadata_guard& operator=(const tablet_metadata_guard&) = delete;
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/// Returns an abort_source which is signaled when effective_replication_map changes
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/// in a way which is relevant for the tablet associated with this guard.
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/// When this happens, the guard stops refreshing the effective_replication_map,
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/// which will block topology coordinator barriers until the guard is destroyed.
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///
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/// The abort_source is valid as long as this instance is alive.
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seastar::abort_source& get_abort_source() {
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return _abort_source;
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}
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locator::token_metadata_ptr get_token_metadata() {
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return _erm->get_token_metadata_ptr();
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}
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const effective_replication_map_ptr& get_erm() const {
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return _erm;
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}
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/// Returns tablet_map for the table of the tablet associated with this guard.
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/// The result is valid until the next deferring point.
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const locator::tablet_map& get_tablet_map() {
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return get_token_metadata()->tablets().get_tablet_map(_tablet.table);
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}
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};
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// A topology guard for a single token.
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//
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// Data-plane requests typically hold a strong pointer to the
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// effective_replication_map (ERM) to guard against tablet migrations and
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// other topology changes. This works because topology coordinator steps
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// use global barriers that install a new token_metadata version on each shard
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// and wait for all references to the old one to be dropped. Since ERM holds
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// a strong pointer to token_metadata, it blocks these operations until released.
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//
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// When operating on a single token, it is sufficient to block topology changes
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// only for the relevant tablet. The tablet_metadata_guard class implements
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// this behavior for tablet-aware tables by updating the ERM pointer automatically,
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// unless the change affects the guarded tablet.
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//
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// The token_metadata_guard wraps tablet_metadata_guard for tablet-aware tables,
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// and falls back to a plain strong ERM pointer for vnode-based (non-tablet-aware) tables.
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//
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// Unlike tablet_metadata_guard, token_metadata_guard is copyable and movable, since
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// it contains only pointers.
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class token_metadata_guard {
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// Holds tablet_metadata_guard for tablet-aware tables and erm otherwise.
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using guard_type = std::variant<lw_shared_ptr<tablet_metadata_guard>, effective_replication_map_ptr>;
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guard_type _guard;
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public:
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token_metadata_guard(replica::table& table, dht::token token);
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const effective_replication_map_ptr& get_erm() const;
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};
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} // namespace locator
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