PostgreSQL从库只读的实现原理

Posted 2021-10-03 PostgreSQLChina

作者：杨向博

我们接触到的数据库，基本都提供了只读实例的功能。在业务允许的情况下可以将一些读取数据的请求下发至只读实例，减缓primary的压力，以获得更佳的性能。

物理从库只读已经是一个常识问题，在PostgreSQL中具体是怎么实现的呢，一起来看看。

一、报错分析

从报错入手分析，在只读库里执行写操作，报错如下：

postgres=# select * into tbl_t from tbl_test;
ERROR:  cannot execute SELECT INTO in a read-only transaction
postgres=#

报错的函数为：

/*
 * PreventCommandIfReadOnly: throw error if XactReadOnly
 *
 * This is useful partly to ensure consistency of the error message wording;
 * some callers have checked XactReadOnly for themselves.
 */
void
PreventCommandIfReadOnly(const char *cmdname)
{
    if (XactReadOnly)
        ereport(ERROR,
                (errcode(ERRCODE_READ_ONLY_SQL_TRANSACTION),
        /* translator: %s is name of a SQL command, eg CREATE */
                 errmsg('cannot execute %s in a read-only transaction',
                        cmdname)));
}

当XactReadOnly为true时，就抛出报错，XactReadOnly是一个Bool类型的全局变量。那这里大概明白设计思路了，当执行一些操作时调用PreventCommandIfReadOnly函数，如果库是只读状态进行写操作就抛出报错。

报错堆栈：

(gdb) bt
#0  PreventCommandIfReadOnly (cmdname=0xbab6f8 'SELECT INTO') at utility.c:409
#1  0x00000000008c24a5 in standard_ProcessUtility (pstmt=0x1faef00, queryString=0x1fae098 'select * into tbl_t from tbl_test;', context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0x1fafa18, qc=0x7ffc8ac8a180)
    at utility.c:566
#2  0x00000000008c23cc in ProcessUtility (pstmt=0x1faef00, queryString=0x1fae098 'select * into tbl_t from tbl_test;', context=PROCESS_UTILITY_TOPLEVEL, params=0x0, queryEnv=0x0, dest=0x1fafa18, qc=0x7ffc8ac8a180)
    at utility.c:524
#3  0x00000000008c14e2 in PortalRunUtility (portal=0x1fffa58, pstmt=0x1faef00, isTopLevel=true, setHoldSnapshot=false, dest=0x1fafa18, qc=0x7ffc8ac8a180) at pquery.c:1157
#4  0x00000000008c16ce in PortalRunMulti (portal=0x1fffa58, isTopLevel=true, setHoldSnapshot=false, dest=0x1fafa18, altdest=0x1fafa18, qc=0x7ffc8ac8a180) at pquery.c:1303
#5  0x00000000008c0c6c in PortalRun (portal=0x1fffa58, count=9223372036854775807, isTopLevel=true, run_once=true, dest=0x1fafa18, altdest=0x1fafa18, qc=0x7ffc8ac8a180) at pquery.c:779
#6  0x00000000008bb204 in exec_simple_query (query_string=0x1fae098 'select * into tbl_t from tbl_test;') at postgres.c:1239
#7  0x00000000008bf053 in PostgresMain (argc=1, argv=0x1fd7af0, dbname=0x1fd7a28 'postgres', username=0x1fd7a08 'postgres') at postgres.c:4315
#8  0x0000000000820e1f in BackendRun (port=0x1fd13f0) at postmaster.c:4536
#9  0x0000000000820630 in BackendStartup (port=0x1fd13f0) at postmaster.c:4220
#10 0x000000000081cf62 in ServerLoop () at postmaster.c:1739
#11 0x000000000081c843 in PostmasterMain (argc=3, argv=0x1fa9d70) at postmaster.c:1412
#12 0x0000000000733286 in main (argc=3, argv=0x1fa9d70) at main.c:210
(gdb) p XactReadOnly
$1 = true

接下来追踪全局变量XactReadOnly在什么时候被赋值为true。

二、变量追踪

走读代码：postmaster守护进程fork出postgres后，在InitPostgres阶段StartTransaction()函数根据是否RecoveryInProgress来赋值，如果RecoveryInProgress()为true就设置XactReadOnly = true

/*
     * Make sure we've reset xact state variables
     *
     * If recovery is still in progress, mark this transaction as read-only.
     * We have lower level defences in XLogInsert and elsewhere to stop us
     * from modifying data during recovery, but this gives the normal
     * indication to the user that the transaction is read-only.
     */
    if (RecoveryInProgress())
    {
        s->startedInRecovery = true;
        XactReadOnly = true;
    }

使用gdb attach postmasterPID后，抓到了这个过程

Breakpoint 3, StartTransaction () at xact.c:1895
1895            s = &TopTransactionStateData;
(gdb) bt
#0  StartTransaction () at xact.c:1895
#1  0x0000000000536318 in StartTransactionCommand () at xact.c:2827
#2  0x0000000000a40778 in InitPostgres (in_dbname=0x1fd7a28 'postgres', dboid=0, username=0x1fd7a08 'postgres', useroid=0, out_dbname=0x0, overrid
e_allow_connections=false) at postinit.c:739
#3  0x00000000008beb60 in PostgresMain (argc=1, argv=0x1fd7af0, dbname=0x1fd7a28 'postgres', username=0x1fd7a08 'postgres') at postgres.c:3933
#4  0x0000000000820e1f in BackendRun (port=0x1fd13f0) at postmaster.c:4536
#5  0x0000000000820630 in BackendStartup (port=0x1fd13f0) at postmaster.c:4220
#6  0x000000000081cf62 in ServerLoop () at postmaster.c:1739
#7  0x000000000081c843 in PostmasterMain (argc=3, argv=0x1fa9d70) at postmaster.c:1412
#8  0x0000000000733286 in main (argc=3, argv=0x1fa9d70) at main.c:210
(gdb) p XactReadOnly
$1 = false
(gdb) list
1890            VirtualTransactionId vxid;
1891
1892            /*
1893             * Let's just make sure the state stack is empty
1894             */
1895            s = &TopTransactionStateData;
1896            CurrentTransactionState = s;
1897
1898            Assert(!FullTransactionIdIsValid(XactTopFullTransactionId));
1899
(gdb) n
1896            CurrentTransactionState = s;
(gdb) 
1909            s->state = TRANS_START;
(gdb) 
1910            s->fullTransactionId = InvalidFullTransactionId;        /* until assigned */
(gdb) n
1913            xact_is_sampled = log_xact_sample_rate != 0 &&
(gdb) 
1922            s->nestingLevel = 1;
(gdb) 
1923            s->gucNestLevel = 1;
(gdb) 
1924            s->childXids = NULL;
(gdb) 
1925            s->nChildXids = 0;
(gdb) 
1926            s->maxChildXids = 0;
(gdb) 
1932            GetUserIdAndSecContext(&s->prevUser, &s->prevSecContext);
(gdb) 
1945            if (RecoveryInProgress())
(gdb) 
1947                    s->startedInRecovery = true;
(gdb) p RecoveryInProgress()
$2 = true
(gdb) n
1948                    XactReadOnly = true;
(gdb) p XactReadOnly 
$3 = true

XactReadOnly赋值是在StartTransaction ()函数中完成的，首次赋值是在InitPostgres环节，除此之外，后续每次启动事务后会重新赋值。

XactReadOnly是否为ture取决于RecoveryInProgress()的返回值是否为true。对于Primary实例 startup进程不是常驻内存的，因此只有在启动过程中以及crash后进行recovery，完成后startup进程退出；对于standby 实例来说startup进程是常驻内存的，一直通过redo来回放primary发送的日志，因此RecoveryInProgress()一直为ture。

RecoveryInProgress()函数逻辑很简单，返回值取决于共享内存xlogctl->SharedRecoveryState的值，只有在StartupXlog函数退出时会修改为RECOVERY_STATE_DONE；因此在standby实例中RecoveryInProgress()函数一直返回true。

/*
 * Is the system still in recovery
 *
 * Unlike testing InRecovery, this works in any process that's connected to
 * shared memory.
 *
 * As a side-effect, we initialize the local TimeLineID and RedoRecPtr
 * variables the first time we see that recovery is finished.
 */
bool
RecoveryInProgress(void)
{
    /*
     * We check shared state each time only until we leave recovery mode. We
     * can't re-enter recovery, so there's no need to keep checking after the
     * shared variable has once been seen false.
     */
    if (!LocalRecoveryInProgress)
        return false;
    else
    {
        /*
         * use volatile pointer to make sure we make a fresh read of the
         * shared variable.
         */
        volatile XLogCtlData *xlogctl = XLogCtl;
        LocalRecoveryInProgress = (xlogctl->SharedRecoveryState != RECOVERY_STATE_DONE);
        /*
         * Initialize TimeLineID and RedoRecPtr when we discover that recovery
         * is finished. InitPostgres() relies upon this behaviour to ensure
         * that InitXLOGAccess() is called at backend startup.  (If you change
         * this, see also LocalSetXLogInsertAllowed.)
         */
        if (!LocalRecoveryInProgress)
        {
            /*
             * If we just exited recovery, make sure we read TimeLineID and
             * RedoRecPtr after SharedRecoveryState (for machines with weak
             * memory ordering).
             */
            pg_memory_barrier();
            InitXLOGAccess();
        }
        /*
         * Note: We don't need a memory barrier when we're still in recovery.
         * We might exit recovery immediately after return, so the caller
         * can't rely on 'true' meaning that we're still in recovery anyway.
         */
        return LocalRecoveryInProgress;
    }
}

三、小结

通过前边的分析，将设定只读，以及只读处理的过程通过框图来表示。

1、左边灰色框表示只读的设定，在fork出postgres之后，根据是否RecoveryInProgress来设定是否只读，实例为standby角色则设置为只读；
2、右边框图表示只读处理的过程，执行器进行判断，如果实例为standby角色，并且执行写操作，则调用PreventCommandIfReadOnly函数抛出报错终止写操作。

整体来看pg通过一个bool全局变量XactReadOnly，设定相关的逻辑，来操作实例是否只读，以及只读处理。

四、延伸

1、XactReadOnly变量对应guc参数transaction_read_only，可以使用show命令查看；但是这个参数是GUC_NO_RESET_ALL类，也就是说不能通过alter system方式修改；
2、如何设置primary实例为只读状态？
Alter system set default_transaction_read_only to on;即可

{
        {'default_transaction_read_only', PGC_USERSET, CLIENT_CONN_STATEMENT,
            gettext_noop('Sets the default read-only status of new transactions.'),
            NULL
        },
        &DefaultXactReadOnly,
        false,
        NULL, NULL, NULL
    },
    {
        {'transaction_read_only', PGC_USERSET, CLIENT_CONN_STATEMENT,
            gettext_noop('Sets the current transaction's read-only status.'),
            NULL,
            GUC_NO_RESET_ALL | GUC_NOT_IN_SAMPLE | GUC_DISALLOW_IN_FILE
        },
        &XactReadOnly,
        false,
        check_transaction_read_only, NULL, NULL
    },