功能实现背景说明
我们已经有两个参数来控制长事务:statement_timeout 和 idle_in_transaction_session_timeout。但是,如果事务执行的命令足够短且不超过 statement_timeout,并且命令之间的暂停时间适合 idle_in_transaction_session_timeout,则事务可以无限期持续。
在这种情况下,transaction_timeout 可确保事务的持续时间不超过指定的超时时间。如果超过,事务和执行该事务的会话将被终止。如下:
postgres=# select version();
version
PostgreSQL 18devel on x86_64-pc-linux-gnu, compiled by gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-21), 64-bit
(1 row)
postgres=# show statement_timeout ;
statement_timeout
0
(1 row)
postgres=# show transaction_timeout ;
transaction_timeout
0
(1 row)
postgres=# set transaction_timeout = '10s';
SET
postgres=# begin ;
BEGIN
postgres=*# select pg_sleep(2);
pg_sleep
(1 row)
postgres=*# 2024-09-28 19:28:30.891 PDT [45558] FATAL: terminating connection due to transaction timeout
postgres=*#
然后看一下进程相关,如下:
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如上,45875的进程死了,会话已断开。然后随着我在psql上继续执行,又有了一个新的会话得以建立。这个问题我后面再详细解释(大家也可以看一下下图先思考思考):
在这里插入图片描述
我第一次看到这个的时候有点懵,原因如下:
在这里插入图片描述
我当时以为就像statement_timeout这样,事务超时也没必要直接断开连接 事务失败(rollback即可)。
然后带着这个疑惑,去看了一下邮件列表 如下:
Transaction timeout,点击前往
在这里插入图片描述
注1:有兴趣的小伙伴可以自行查看邮件列表
注2:接下来我们一起看一下transaction_timeout的内部实现,以及为什么不能像statement_timeout这样去实现
功能实现源码解析
首先看一下官方文档的解释,如下:
终止事务中持续时间超过指定时间的任何会话。此限制既适用于显式事务(以 BEGIN 启动),也适用于与单个语句相对应的隐式启动事务。
如果指定此值时没有单位,则以毫秒为单位。零值(默认值)将禁用超时。
如果 transaction_timeout 短于或等于 idle_in_transaction_session_timeout 或 statement_timeout,则忽略较长的超时。
不建议在 postgresql.conf 中设置 transaction_timeout,因为它会影响所有会话。
该GUC参数定义,如下:
// src/backend/utils/misc/guc_tables.c
{
{"transaction_timeout", PGC_USERSET, CLIENT_CONN_STATEMENT,
gettext_noop("Sets the maximum allowed duration of any transaction within a session (not a prepared transaction)."),
gettext_noop("A value of 0 turns off the timeout."),
GUC_UNIT_MS
},
&TransactionTimeout,
0, 0, INT_MAX,
NULL, assign_transaction_timeout, NULL
},
与 idle_in_transaction_session_timeout
// src/backend/tcop/postgres.c
...
/*
* (1) If we've reached idle state, tell the frontend we're ready for
* a new query.
*
* Note: this includes fflush()'ing the last of the prior output.
*
* This is also a good time to flush out collected statistics to the
* cumulative stats system, and to update the PS stats display. We
* avoid doing those every time through the message loop because it'd
* slow down processing of batched messages, and because we don't want
* to report uncommitted updates (that confuses autovacuum). The
* notification processor wants a call too, if we are not in a
* transaction block.
*
* Also, if an idle timeout is enabled, start the timer for that.
*/
if (send_ready_for_query)
{
if (IsAbortedTransactionBlockState())
{
set_ps_display("idle in transaction (aborted)");
pgstat_report_activity(STATE_IDLEINTRANSACTION_ABORTED, NULL);
/* Start the idle-in-transaction timer */
if (IdleInTransactionSessionTimeout > 0
&& (IdleInTransactionSessionTimeout < TransactionTimeout || TransactionTimeout == 0))
{
idle_in_transaction_timeout_enabled = true;
enable_timeout_after(IDLE_IN_TRANSACTION_SESSION_TIMEOUT,
IdleInTransactionSessionTimeout);
}
}
else if (IsTransactionOrTransactionBlock())
{
set_ps_display("idle in transaction");
pgstat_report_activity(STATE_IDLEINTRANSACTION, NULL);
/* Start the idle-in-transaction timer */
if (IdleInTransactionSessionTimeout > 0
&& (IdleInTransactionSessionTimeout < TransactionTimeout || TransactionTimeout == 0))
{
idle_in_transaction_timeout_enabled = true;
enable_timeout_after(IDLE_IN_TRANSACTION_SESSION_TIMEOUT,
IdleInTransactionSessionTimeout);
}
}
...
与 statement_timeout
/*
- Start statement timeout timer, if enabled.
* - If there's already a timeout running, don't restart the timer. That
- enables compromises between accuracy of timeouts and cost of starting a
timeout.
/
static void
enable_statement_timeout(void)
{
/ must be within an xact */
Assert(xact_started);if (StatementTimeout > 0
&& (StatementTimeout < TransactionTimeout || TransactionTimeout == 0))
{
if (!get_timeout_active(STATEMENT_TIMEOUT)) enable_timeout_after(STATEMENT_TIMEOUT, StatementTimeout);
}
else
{if (get_timeout_active(STATEMENT_TIMEOUT)) disable_timeout(STATEMENT_TIMEOUT, false);
}
}
如上,当transaction_timeout 小于或等于 idle_in_transaction_session_timeout 或 statement_timeout,则忽略较长的超时。
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transaction_timeout
// src/backend/utils/init/postinit.c
void
InitPostgres(const char in_dbname, Oid dboid,
const char username, Oid useroid,
bits32 flags,
char *out_dbname)
{
...
if (!bootstrap)
{
RegisterTimeout(DEADLOCK_TIMEOUT, CheckDeadLockAlert);
RegisterTimeout(STATEMENT_TIMEOUT, StatementTimeoutHandler);
RegisterTimeout(LOCK_TIMEOUT, LockTimeoutHandler);
RegisterTimeout(IDLE_IN_TRANSACTION_SESSION_TIMEOUT,
IdleInTransactionSessionTimeoutHandler);
RegisterTimeout(TRANSACTION_TIMEOUT, TransactionTimeoutHandler); // here
RegisterTimeout(IDLE_SESSION_TIMEOUT, IdleSessionTimeoutHandler);
RegisterTimeout(CLIENT_CONNECTION_CHECK_TIMEOUT, ClientCheckTimeoutHandler);
RegisterTimeout(IDLE_STATS_UPDATE_TIMEOUT,
IdleStatsUpdateTimeoutHandler);
}
...
}
static void
TransactionTimeoutHandler(void)
{
TransactionTimeoutPending = true;
InterruptPending = true;
SetLatch(MyLatch);
}
接下来,这里修改源码 使用ShowTransactionState函数进行打印,如下:
[postgres@localhost:~/test/bin]$ ./psql
INFO: CommitTransaction(1) name: unnamed; blockState: STARTED; state: INPROGRESS, xid/subid/cid: 0/1/0
psql (18devel)
Type "help" for help.
postgres=# set transaction_timeout = '10s';
INFO: StartTransaction(1) name: unnamed; blockState: DEFAULT; state: INPROGRESS, xid/subid/cid: 0/1/0
INFO: CommitTransaction(1) name: unnamed; blockState: STARTED; state: INPROGRESS, xid/subid/cid: 0/1/0
SET
postgres=# begin;
INFO: StartTransaction(1) name: unnamed; blockState: DEFAULT; state: INPROGRESS, xid/subid/cid: 0/1/0
BEGIN
postgres=# commit;
INFO: CommitTransaction(1) name: unnamed; blockState: END; state: INPROGRESS, xid/subid/cid: 0/1/0
COMMIT
postgres=# begin;
INFO: StartTransaction(1) name: unnamed; blockState: DEFAULT; state: INPROGRESS, xid/subid/cid: 0/1/0
BEGIN
postgres=# select pg_sleep(20);
2024-09-28 20:42:58.376 PDT [62092] FATAL: terminating connection due to transaction timeout
2024-09-28 20:42:58.376 PDT [62092] STATEMENT: select pg_sleep(20);
FATAL: terminating connection due to transaction timeout
server closed the connection unexpectedly1
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: INFO: CommitTransaction(1) name: unnamed; blockState: STARTED; state: INPROGRESS, xid/subid/cid: 0/1/0
Succeeded.
postgres=#
若是有小伙伴对父子事务有限状态机感兴趣的,可以查看本人之前的博客,如下:
PostgreSQL的学习心得和知识总结(一百四十五)|深入理解PostgreSQL数据库之ShowTransactionState的使用及父子事务有限状态机,点击前往
transaction_timeout的超时启用/禁用,如下:
// src/backend/access/transam/xact.c
/*
- StartTransaction
/
static void
StartTransaction(void)
{
...
/ Schedule transaction timeout */
if (TransactionTimeout > 0)
enable_timeout_after(TRANSACTION_TIMEOUT, TransactionTimeout);
...
}
static void
CommitTransaction(void)
{
...
/ Disable transaction timeout /
if (TransactionTimeout > 0)
disable_timeout(TRANSACTION_TIMEOUT, false);
...
}
static void
PrepareTransaction(void)
{
...
/ Disable transaction timeout /
if (TransactionTimeout > 0)
disable_timeout(TRANSACTION_TIMEOUT, false);
...
}
static void
AbortTransaction(void)
{
...
/ Disable transaction timeout /
if (TransactionTimeout > 0)
disable_timeout(TRANSACTION_TIMEOUT, false);
...
}
接下来我们调试一下transaction_timeout的相关内容,首先看一下enable_timeout_after的设置 如下:
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注意这两个时间值,以及下面的核心设置:
在这里插入图片描述
其中第一个参数:ITIMER_REAL:以系统真实的时间来计算,它送出SIGALRM信号。若是对该函数感兴趣的小伙伴可以看一下这位老哥的博客,我们这里不再赘述: