xact就是事务,只不过有些地方用xact,有些地方用trans,有些用transaction。
从内部看,事务状态分为:
/*
* transaction states - transaction state from server perspective
*/
typedef enum TransState
{
TRANS_DEFAULT, /* idle */
TRANS_START, /* transaction starting */
TRANS_INPROGRESS, /* inside a valid transaction */
TRANS_COMMIT, /* commit in progress */
TRANS_ABORT, /* abort in progress */
TRANS_PREPARE /* prepare in progress */
} TransState;
从用户角度看,事务状态分为:
/* * transaction block states - transaction state of client queries * * Note: the subtransaction states are used only for non-topmost * transactions; the others appear only in the topmost transaction. */ typedef enum TBlockState { /* not-in-transaction-block states */ TBLOCK_DEFAULT, /* idle */ TBLOCK_STARTED, /* running single-query transaction */ /* transaction block states */ TBLOCK_BEGIN, /* starting transaction block */ TBLOCK_INPROGRESS, /* live transaction */ TBLOCK_IMPLICIT_INPROGRESS, /* live transaction after implicit BEGIN */ TBLOCK_PARALLEL_INPROGRESS, /* live transaction inside parallel worker */ TBLOCK_END, /* COMMIT received */ TBLOCK_ABORT, /* failed xact, awaiting ROLLBACK */ TBLOCK_ABORT_END, /* failed xact, ROLLBACK received */ TBLOCK_ABORT_PENDING, /* live xact, ROLLBACK received */ TBLOCK_PREPARE, /* live xact, PREPARE received */ /* subtransaction states */ TBLOCK_SUBBEGIN, /* starting a subtransaction */ TBLOCK_SUBINPROGRESS, /* live subtransaction */ TBLOCK_SUBRELEASE, /* RELEASE received */ TBLOCK_SUBCOMMIT, /* COMMIT received while TBLOCK_SUBINPROGRESS */ TBLOCK_SUBABORT, /* failed subxact, awaiting ROLLBACK */ TBLOCK_SUBABORT_END, /* failed subxact, ROLLBACK received */ TBLOCK_SUBABORT_PENDING, /* live subxact, ROLLBACK received */ TBLOCK_SUBRESTART, /* live subxact, ROLLBACK TO received */ TBLOCK_SUBABORT_RESTART /* failed subxact, ROLLBACK TO received */ } TBlockState;
默认都是空闲,由全局变量TopTransactionStateData定义:
/* * CurrentTransactionState always points to the current transaction state * block. It will point to TopTransactionStateData when not in a * transaction at all, or when in a top-level transaction. */ static TransactionStateData TopTransactionStateData = { .state = TRANS_DEFAULT, .blockState = TBLOCK_DEFAULT, }; static TransactionState CurrentTransactionState = &TopTransactionStateData;
PS:在PG中,事务是支持超时的(还有其他很多逻辑比如死锁检测)。超时包括下列类型:
/* * Identifiers for timeout reasons. Note that in case multiple timeouts * trigger at the same time, they are serviced in the order of this enum. */ typedef enum TimeoutId { /* Predefined timeout reasons */ STARTUP_PACKET_TIMEOUT, DEADLOCK_TIMEOUT, LOCK_TIMEOUT, STATEMENT_TIMEOUT, STANDBY_DEADLOCK_TIMEOUT, STANDBY_TIMEOUT, STANDBY_LOCK_TIMEOUT, IDLE_IN_TRANSACTION_SESSION_TIMEOUT, /* First user-definable timeout reason */ USER_TIMEOUT, /* Maximum number of timeout reasons */ MAX_TIMEOUTS = 16 } TimeoutId;
好奇一下,超时是什么机制做的?是基于SIGALRM信号触发的,超时管理实现在src/backend/utils/misc/timeout.c中,超时后是可以回调函数的。前3个事务ID保留:
#define InvalidTransactionId ((TransactionId) 0)
#define BootstrapTransactionId ((TransactionId) 1)
#define FrozenTransactionId ((TransactionId) 2)
#define FirstNormalTransactionId ((TransactionId) 3)
#define MaxTransactionId ((TransactionId) 0xFFFFFFFF)
事务ID包括:
LocalTransactionId:进程本地计数器生成,主要是为了优化。
VirtualTransactionId:分为两部分,前半部分为BackendId,后半部分为LocalTransactionId。
TransactionId/MultiXactId:主要是非专用区别场景使用,就是不需要明显区分,不需要特别的区分上下文。
SubTransactionId:子事务ID。