怎么理解PostgreSQL事务管理中的子事务
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一、Subtransaction Handling
README
Subtransaction Handling ----------------------- 子事务处理 Subtransactions are implemented using a stack of TransactionState structures, each of which has a pointer to its parent transaction's struct. When a new subtransaction is to be opened, PushTransaction is called, which creates a new TransactionState, with its parent link pointing to the current transaction. StartSubTransaction is in charge of initializing the new TransactionState to sane values, and properly initializing other subsystems (AtSubStart routines). 子事务通过TransactionState结构体栈来实现,每一个TransactionState都有指向其父事务结构体的指针. 新的子事务即将开启时,调用PushTransaction,该函数创建TransactionState结构体,parent指向当前事务. StartSubTransaction负责完整初始化新的TransactionState结构体,并正确初始化其他子系统(AtSubStart例程实现). When closing a subtransaction, either CommitSubTransaction has to be called (if the subtransaction is committing), or AbortSubTransaction and CleanupSubTransaction (if it's aborting). In either case, PopTransaction is called so the system returns to the parent transaction. 在关闭子事务时,成功则调用CommitSubTransaction,取消/失败则调用AbortSubTransaction和CleanupSubTransaction. 无论哪种情况,都会调用PopTransaction以便返回到父事务. One important point regarding subtransaction handling is that several may need to be closed in response to a single user command. That's because savepoints have names, and we allow to commit or rollback a savepoint by name, which is not necessarily the one that was last opened. Also a COMMIT or ROLLBACK command must be able to close out the entire stack. We handle this by having the utility command subroutine mark all the state stack entries as commit- pending or abort-pending, and then when the main loop reaches CommitTransactionCommand, the real work is done. The main point of doing things this way is that if we get an error while popping state stack entries, the remaining stack entries still show what we need to do to finish up. 子事务处理一个很很重要的点是为了响应单个用户命令需要关闭多个子事务. 这是因为对于命名savepoints,PG允许通过名称提交或回滚至这些savepoint,而这些savepoint并不需要是最后才打开的那个. 同时,COMMIT/ROLLBACK命令必须可以关闭整个栈. 通过工具命令子例程来标记整个状态栈为commit-pending或者是abort-pending, 然后在主循环到达CommitTransactionCommand时,执行实际的commit/abort. 以这种方法做这些事情的主要关注点是如果在栈条目出栈时出现错误,剩余的栈条目仍然可以展示我们需要做什么才能完结. In the case of ROLLBACK TO, we abort all the subtransactions up through the one identified by the savepoint name, and then re-create that subtransaction level with the same name. So it's a completely new subtransaction as far as the internals are concerned. 在ROLLBACK TO 这种情况中,我们取消了从savepoint name到当前的所有子事务, 然后使用相同的名称重建了该子事务.因此这是内部需要关注的完全的子事务. Other subsystems are allowed to start "internal" subtransactions, which are handled by BeginInternalSubTransaction. This is to allow implementing exception handling, e.g. in PL/pgSQL. ReleaseCurrentSubTransaction and RollbackAndReleaseCurrentSubTransaction allows the subsystem to close said subtransactions. The main difference between this and the savepoint/release path is that we execute the complete state transition immediately in each subroutine, rather than deferring some work until CommitTransactionCommand. Another difference is that BeginInternalSubTransaction is allowed when no explicit transaction block has been established, while DefineSavepoint is not. 其他子系统允许通过BeginInternalSubTransaction函数启动"internal"子事务. 因此可以在诸如PL/pgSQL等编程语言中可以实现异常处理. ReleaseCurrentSubTransaction和RollbackAndReleaseCurrentSubTransaction允许子系统关闭子事务. 这种方式跟savepoint/release的不同点是在每一个子例程中我们马上执行了完整的状态变换, 而不是等到执行CommitTransactionCommand才执行某些工作. 另外一个不同点是在没有创建显式事务块的情况下允许调用BeginInternalSubTransaction, 而savepoint则需要明确的DefineSavepoint. Transaction and Subtransaction Numbering ---------------------------------------- 事务和子事务编号 Transactions and subtransactions are assigned permanent XIDs only when/if they first do something that requires one --- typically, insert/update/delete a tuple, though there are a few other places that need an XID assigned. If a subtransaction requires an XID, we always first assign one to its parent. This maintains the invariant that child transactions have XIDs later than their parents, which is assumed in a number of places. 事务和子事务在需要时才会分配持久的XIDs,典型的场景是insert/update/delete元组. 如果子事务需要XID,首先会给其父事务分配一个,这保证了子事务编号在父事务之后. The subsidiary actions of obtaining a lock on the XID and entering it into pg_subtrans and PG_PROC are done at the time it is assigned. 分配事务号还需要做的事情是在XID获取锁/写入到pg_subtrans和PG_PROC中. A transaction that has no XID still needs to be identified for various purposes, notably holding locks. For this purpose we assign a "virtual transaction ID" or VXID to each top-level transaction. VXIDs are formed from two fields, the backendID and a backend-local counter; this arrangement allows assignment of a new VXID at transaction start without any contention for shared memory. To ensure that a VXID isn't re-used too soon after backend exit, we store the last local counter value into shared memory at backend exit, and initialize it from the previous value for the same backendID slot at backend start. All these counters go back to zero at shared memory re-initialization, but that's OK because VXIDs never appear anywhere on-disk. 没有XID的事务仍然需要进行标识,特别是需要持有锁的时候. 出于这个目的,我们分配了"虚拟事务号"(即VXID)给每一个顶层事务. VXIDs由两个域组成,后台进程ID和后台进程本地计数器;这样的编号产生方法不需要共享内存争用就可以进行新VXID的分配. 为了确保在后台进程退出后VXID不会过快的被使用,我们把最后的本地计数器值存储到共享内存中, 对于同一个后台进程ID,分配先前存储的计数器值给这个新的后台进程. 在共享内存重新初始化后这些计数器会归零,由于不会出现落盘,因此这样的处理没有任何问题. Internally, a backend needs a way to identify subtransactions whether or not they have XIDs; but this need only lasts as long as the parent top transaction endures. Therefore, we have SubTransactionId, which is somewhat like CommandId in that it's generated from a counter that we reset at the start of each top transaction. The top-level transaction itself has SubTransactionId 1, and subtransactions have IDs 2 and up. (Zero is reserved for InvalidSubTransactionId.) Note that subtransactions do not have their own VXIDs; they use the parent top transaction's VXID. 在内部实现上,不论子事务是否拥有XIDs,后台进程需要标识子事务的方法;只要父顶级事务存在这种需求就好一直存在. 因此,产生了SubTransactionId,该字段类似于CommandId,在每次顶层事务都会重置的计数器. 顶层事务本身的SubTransactionId设定为1,其他子事务的ID为2或更大(0保留用于InvalidSubTransactionId). 注意子事务没有VXIDs;它们使用顶层事务的VXID.
TransactionState结构体
/*
* 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;
/*
* 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;
/*
* transaction state structure
*/
typedef struct TransactionStateData
{
FullTransactionId fullTransactionId; /* my FullTransactionId */
SubTransactionId subTransactionId; /* my subxact ID */
char *name; /* savepoint name, if any */
int savepointLevel; /* savepoint level */
TransState state; /* low-level state */
TBlockState blockState; /* high-level state */
int nestingLevel; /* transaction nesting depth */
int gucNestLevel; /* GUC context nesting depth */
MemoryContext curTransactionContext; /* my xact-lifetime context */
ResourceOwner curTransactionOwner; /* my query resources */
TransactionId *childXids; /* subcommitted child XIDs, in XID order */
int nChildXids; /* # of subcommitted child XIDs */
int maxChildXids; /* allocated size of childXids[] */
Oid prevUser; /* previous CurrentUserId setting */
int prevSecContext; /* previous SecurityRestrictionContext */
bool prevXactReadOnly; /* entry-time xact r/o state */
bool startedInRecovery; /* did we start in recovery? */
bool didLogXid; /* has xid been included in WAL record? */
int parallelModeLevel; /* Enter/ExitParallelMode counter */
bool chain; /* start a new block after this one */
struct TransactionStateData *parent; /* back link to parent */
} TransactionStateData;
typedef TransactionStateData *TransactionState;到此，相信大家对“怎么理解PostgreSQL事务管理中的子事务”有了更深的了解，不妨来实际操作一番吧！这里是创新互联网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
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