103 lines
3.8 KiB
C++
103 lines
3.8 KiB
C++
#ifndef MEMGRAPH_STORAGE_MODEL_UTILS_MVCC_HPP
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#define MEMGRAPH_STORAGE_MODEL_UTILS_MVCC_HPP
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#include <atomic>
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#include "transaction.hpp"
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#include "minmax.hpp"
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#include "version.hpp"
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// the mvcc implementation used here is very much like postgresql's
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// more info: https://momjian.us/main/writings/pgsql/mvcc.pdf
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namespace mvcc
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{
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template <class T>
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class Mvcc : public Version<T>
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{
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public:
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Mvcc() {}
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// tx.min is the id of the transaction that created the record
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// and tx.max is the id of the transaction that deleted the record
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// these values are used to determine the visibility of the record
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// to the current transaction
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MinMax<uint64_t> tx;
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// cmd.min is the id of the command in this transaction that created the
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// record and cmd.max is the id of the command in this transaction that
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// deleted the record. these values are used to determine the visibility
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// of the record to the current command in the running transaction
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MinMax<uint8_t> cmd;
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// check if this record is visible to the transaction t
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bool visible(const Transaction& t)
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{
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// TODO check if the record was created by a transaction that has been
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// aborted. one might implement this by checking the hints in mvcc
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// anc/or consulting the commit log
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// Mike Olson says 17 march 1993: the tests in this routine are correct;
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// if you think they're not, you're wrong, and you should think about it
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// again. i know, it happened to me.
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return ((tx.min() == t.id && // inserted by the current transaction
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cmd.min() < t.cid && // before this command, and
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(tx.max() == 0 || // the row has not been deleted, or
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(tx.max() == t.id && // it was deleted by the current
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// transaction
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cmd.max() >= t.cid))) // but not before this command,
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|| // or
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(t.committed(tx.min()) && // the record was inserted by a
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// committed transaction, and
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(tx.max() == 0 || // the record has not been deleted, or
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(tx.max() == t.id && // the row is being deleted by this
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// transaction
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cmd.max() >= t.cid) || // but it's not deleted "yet", or
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(tx.max() != t.id && // the row was deleted by another
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// transaction
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!t.committed(tx.max()) // that has not been committed
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))));
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}
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// inspects the record change history and returns the record version visible
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// to the current transaction if it exists, otherwise it returns nullptr
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T* latest_visible(const Transaction& t)
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{
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T* record = this, newer = this->newer();
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// move down through the versions of the nodes until you find the first
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// one visible to this transaction. if no visible records are found,
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// the function returns a nullptr
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while(newer != nullptr && !newer->visible(t))
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record = newer, newer = record->newer();
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return record;
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}
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void mark_created(const Transaction& t)
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{
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tx.min(t.id);
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cmd.min(t.cid);
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}
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void mark_deleted(const Transaction& t)
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{
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tx.max(t.id);
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cmd.max(t.cid);
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}
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protected:
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// known committed and known aborted for both xmax and xmin
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// this hints are used to quickly check the commit/abort status of the
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// transaction that created this record. if these are not set, one should
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// consult the commit log to find the status and update the status here
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// more info https://wiki.postgresql.org/wiki/Hint_Bits
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std::atomic<uint8_t> hints;
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};
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}
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#endif
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