WRITESET_SESSION特殊点
/*
This object encapsulates the state kept between transactions of the same
client in order to compute logical timestamps based on WRITESET_SESSION.
*/
class Dependency_tracker_ctx {
public:
Dependency_tracker_ctx() : m_last_session_sequence_number(0) {}
void set_last_session_sequence_number(int64 sequence_number) {
m_last_session_sequence_number = sequence_number;
}
int64 get_last_session_sequence_number() {
return m_last_session_sequence_number;
}
private:
int64 m_last_session_sequence_number;
};
计算依赖入口
/**
Get the dependencies in a transaction, the main entry point for the
dependency tracking work.
*/
void Transaction_dependency_tracker::get_dependency(THD *thd,
int64 &sequence_number,
int64 &commit_parent) {
sequence_number = commit_parent = 0;
switch (m_opt_tracking_mode) {
case DEPENDENCY_TRACKING_COMMIT_ORDER:
m_commit_order.get_dependency(thd, sequence_number, commit_parent);
break;
case DEPENDENCY_TRACKING_WRITESET:
m_commit_order.get_dependency(thd, sequence_number, commit_parent);
m_writeset.get_dependency(thd, sequence_number, commit_parent);
break;
case DEPENDENCY_TRACKING_WRITESET_SESSION:
m_commit_order.get_dependency(thd, sequence_number, commit_parent);
m_writeset.get_dependency(thd, sequence_number, commit_parent);
m_writeset_session.get_dependency(thd, sequence_number, commit_parent);
break;
default:
DBUG_ASSERT(0); // blow up on debug
/*
Fallback to commit order on production builds.
*/
m_commit_order.get_dependency(thd, sequence_number, commit_parent);
}
}
可以发现WRITESET和WRITESET_SESSION都调用到COMMIT_ORDER的get_dependency函数
COMMIT_ORDER计算依赖
/**
Get the sequence_number for a transaction, and get the last_commit based
on parallel committing transactions.
@param[in] thd Current THD from which to extract trx context.
@param[in,out] sequence_number Sequence number of current transaction.
@param[in,out] commit_parent Commit_parent of current transaction,
pre-filled with the commit_parent calculated
by the logical clock logic.
*/
void Commit_order_trx_dependency_tracker::get_dependency(THD *thd,
int64 &sequence_number,
int64 &commit_parent) {
Transaction_ctx *trn_ctx = thd->get_transaction();
DBUG_ASSERT(trn_ctx->sequence_number >
m_max_committed_transaction.get_offset());
/*
Prepare sequence_number and commit_parent relative to the current
binlog. This is done by subtracting the binlog's clock offset
from the values.
A transaction that commits after the binlog is rotated, can have a
commit parent in the previous binlog. In this case, subtracting
the offset from the sequence number results in a negative
number. The commit parent dependency gets lost in such
case. Therefore, we log the value SEQ_UNINIT in this case.
*/
sequence_number =
trn_ctx->sequence_number - m_max_committed_transaction.get_offset();
if (trn_ctx->last_committed <= m_max_committed_transaction.get_offset())
commit_parent = SEQ_UNINIT;
else
commit_parent =
std::max(trn_ctx->last_committed, m_last_blocking_transaction) -
m_max_committed_transaction.get_offset();
if (is_trx_unsafe_for_parallel_slave(thd))
m_last_blocking_transaction = trn_ctx->sequence_number;
}
WRITESET计算依赖
/**
Get the writeset dependencies of a transaction.
This takes the commit_parent that must be previously set using
Commit_order_trx_dependency_tracker and tries to make the commit_parent as
low as possible, using the writesets of each transaction.
The commit_parent returned depends on how many row hashes are stored in the
writeset_history, which is cleared once it reaches the user-defined maximum.
@param[in] thd Current THD from which to extract trx context.
@param[in,out] sequence_number Sequence number of current transaction.
@param[in,out] commit_parent Commit_parent of current transaction,
pre-filled with the commit_parent calculated by
Commit_order_trx_dependency_tracker to use when
the writeset commit_parent is not valid.
*/
void Writeset_trx_dependency_tracker::get_dependency(THD *thd,
int64 &sequence_number,
int64 &commit_parent) {
Rpl_transaction_write_set_ctx *write_set_ctx =
thd->get_transaction()->get_transaction_write_set_ctx();
std::vector<uint64> *writeset = write_set_ctx->get_write_set();
#ifndef DBUG_OFF
/* The writeset of an empty transaction must be empty. */
if (is_empty_transaction_in_binlog_cache(thd))
DBUG_ASSERT(writeset->size() == 0);
#endif
/*
Check if this transaction has a writeset, if the writeset will overflow the
history size, if the transaction_write_set_extraction is consistent
between session and global or if changes in the tables referenced in this
transaction cascade to other tables. If that happens revert to using the
COMMIT_ORDER and clear the history to keep data consistent.
*/
bool can_use_writesets =
// empty writeset implies DDL or similar, except if there are missing keys
(writeset->size() != 0 || write_set_ctx->get_has_missing_keys() ||
/*
The empty transactions do not need to clear the writeset history, since
they can be executed in parallel.
*/
is_empty_transaction_in_binlog_cache(thd)) &&
// hashing algorithm for the session must be the same as used by other
// rows in history
(global_system_variables.transaction_write_set_extraction ==
thd->variables.transaction_write_set_extraction) &&
// must not use foreign keys
!write_set_ctx->get_has_related_foreign_keys() &&
// it did not broke past the capacity already
!write_set_ctx->was_write_set_limit_reached();
bool exceeds_capacity = false;
if (can_use_writesets) {
/*
Check if adding this transaction exceeds the capacity of the writeset
history. If that happens, m_writeset_history will be cleared only after
using its information for current transaction.
*/
exceeds_capacity =
m_writeset_history.size() + writeset->size() > m_opt_max_history_size;
/*
Compute the greatest sequence_number among all conflicts and add the
transaction's row hashes to the history.
*/
int64 last_parent = m_writeset_history_start;
for (std::vector<uint64>::iterator it = writeset->begin();
it != writeset->end(); ++it) {
Writeset_history::iterator hst = m_writeset_history.find(*it);
if (hst != m_writeset_history.end()) {
if (hst->second > last_parent && hst->second < sequence_number)
last_parent = hst->second;
hst->second = sequence_number;
} else {
if (!exceeds_capacity)
m_writeset_history.insert(
std::pair<uint64, int64>(*it, sequence_number));
}
}
/*
If the transaction references tables with missing primary keys revert to
COMMIT_ORDER, update and not reset history, as it is unnecessary because
any transaction that refers this table will also revert to COMMIT_ORDER.
*/
if (!write_set_ctx->get_has_missing_keys()) {
/*
The WRITESET commit_parent then becomes the minimum of largest parent
found using the hashes of the row touched by the transaction and the
commit parent calculated with COMMIT_ORDER.
*/
commit_parent = std::min(last_parent, commit_parent);
}
}
if (exceeds_capacity || !can_use_writesets) {
m_writeset_history_start = sequence_number;
m_writeset_history.clear();
}
}
WRITESET_SESSION计算依赖
/**
Get the writeset commit parent of transactions using the session dependencies.
@param[in] thd Current THD from which to extract trx context.
@param[in,out] sequence_number Sequence number of current transaction.
@param[in,out] commit_parent Commit_parent of current transaction,
pre-filled with the commit_parent calculated
by the Write_set_trx_dependency_tracker as a
fall-back.
*/
void Writeset_session_trx_dependency_tracker::get_dependency(
THD *thd, int64 &sequence_number, int64 &commit_parent) {
int64 session_parent = thd->rpl_thd_ctx.dependency_tracker_ctx()
.get_last_session_sequence_number();
if (session_parent != 0 && session_parent < sequence_number)
commit_parent = std::max(commit_parent, session_parent);
thd->rpl_thd_ctx.dependency_tracker_ctx().set_last_session_sequence_number(
sequence_number);
}