typedef struct VariableCacheData
{
// 下一个可用的事务ID
Oid nextOid; /* next OID to assign */
uint32 oidCount; /* OIDs available before must do XLOG work */
/*
* These fields are protected by XidGenLock.
*/
FullTransactionId nextXid; /* next XID to assign */
// 集群维度最小的冻结的事务id
TransactionId oldestXid;
// 当事务ID超过这个变量的时候，事务可能执行一次vaccum，这个变量的是一个告警的作用，告诉PG事务ID的回卷已经非常靠近了。
TransactionId xidVacLimit;
// 当xidWarnLimit - xidVacLimit =1000000时候会产生告警需要手动执行vacuum,此时无法执行事务ID的申请
TransactionId xidWarnLimit;
// 当xidStopLimit - xidWarnLimit =1000000，产生告警，需要手动执行vacuum,刺水也是分配事务ID的
TransactionId xidStopLimit;
// 事务ID回卷的上限
TransactionId xidWrapLimit; /* where the world ends */
Oid oldestXidDB; /* database with minimum datfrozenxid */
/*
* These fields are protected by CommitTsLock
*/
TransactionId oldestCommitTsXid;
TransactionId newestCommitTsXid;
/*
* These fields are protected by ProcArrayLock.
*/
FullTransactionId latestCompletedXid; /* newest full XID that has
* committed or aborted */
/*
* Number of top-level transactions with xids (i.e. which may have
* modified the database) that completed in some form since the start of
* the server. This currently is solely used to check whether
* GetSnapshotData() needs to recompute the contents of the snapshot, or
* not. There are likely other users of this. Always above 1.
*/
uint64 xactCompletionCount;
/*
* These fields are protected by XactTruncationLock
*/
TransactionId oldestClogXid; /* oldest it's safe to look up in clog */
} VariableCacheData;
// 函数去除了部分代码，保留了需要说明的代码
FullTransactionId GetNewTransactionId(bool isSubXact)
{
FullTransactionId full_xid;
TransactionId xid;
// 并行操作阶段是不允许分配事务ID
if (IsInParallelMode())
elog(ERROR, "cannot assign TransactionIds during a parallel operation");
// 事务的崩溃恢复中也不能分配事务ID
if (RecoveryInProgress())
elog(ERROR, "cannot assign TransactionIds during recovery");
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
full_xid = ShmemVariableCache->nextXid;
xid = XidFromFullTransactionId(full_xid);
if (TransactionIdFollowsOrEquals(xid, ShmemVariableCache->xidVacLimit))
{
// 从共享内存中获取数据
TransactionId xidWarnLimit = ShmemVariableCache->xidWarnLimit;
TransactionId xidStopLimit = ShmemVariableCache->xidStopLimit;
TransactionId xidWrapLimit = ShmemVariableCache->xidWrapLimit;
Oid oldest_datoid = ShmemVariableCache->oldestXidDB;
LWLockRelease(XidGenLock);
// 当下一个事务ID被65536取余等于0，则开启vacuum
if (IsUnderPostmaster && (xid % 65536) == 0)
SendPostmasterSignal(PMSIGNAL_START_AUTOVAC_LAUNCHER);
// 如果事务ID 等于xidStopLimit-xid,告警事务ID不足，需要手动执行vacuum
if (IsUnderPostmaster &&
TransactionIdFollowsOrEquals(xid, xidStopLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/* complain even if that DB has disappeared */
if (oldest_datname)
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("database is not accepting commands to avoid wraparound data loss in database "%s"",
oldest_datname),
errhint("Stop the postmaster and vacuum that database in single-user mode.n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
else
ereport(ERROR,
(errcode(ERRCODE_PROGRAM_LIMIT_EXCEEDED),
errmsg("database is not accepting commands to avoid wraparound data loss in database with OID %u",
oldest_datoid),
errhint("Stop the postmaster and vacuum that database in single-user mode.n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
}
// 如果事务ID 等于xidWarnLimit-xid,告警事务ID不足，需要手动执行vacuum
else if (TransactionIdFollowsOrEquals(xid, xidWarnLimit))
{
char *oldest_datname = get_database_name(oldest_datoid);
/* complain even if that DB has disappeared */
if (oldest_datname)
ereport(WARNING,
(errmsg("database "%s" must be vacuumed within %u transactions",
oldest_datname,
xidWrapLimit - xid),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
else
ereport(WARNING,
(errmsg("database with OID %u must be vacuumed within %u transactions",
oldest_datoid,
xidWrapLimit - xid),
errhint("To avoid a database shutdown, execute a database-wide VACUUM in that database.n"
"You might also need to commit or roll back old prepared transactions, or drop stale replication slots.")));
}
/* Re-acquire lock and start over */
LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
full_xid = ShmemVariableCache->nextXid;
xid = XidFromFullTransactionId(full_xid);
}
//. 省略代码 //
LWLockRelease(XidGenLock);
return full_xid;
}
