int            MaxConnections;

/*

 * Primary determinants of sizes of shared-memory structures.  MaxBackends is

 * MaxConnections + autovacuum_max_workers + 1 (it is computed by the GUC

 * assign hooks for those variables):

 */

int            NBuffers = ;

int            MaxBackends = ;

int            MaxConnections = ;

/*

 * This must be called ONCE during postmaster or standalone-backend startup

 */

void

StartupXLOG(void)

{

    …

    /*

     * If any of the critical GUCs have changed, log them before we allow

     * backends to write WAL.

     */

    LocalSetXLogInsertAllowed();

    XLogReportParameters();                            

    …

}                                

/*

 * Check if any of the GUC parameters that are critical for hot standby

 * have changed, and update the value in pg_control file if necessary.

 */

static void

XLogReportParameters(void)

{

    if (wal_level != ControlFile->wal_level ||

        MaxConnections != ControlFile->MaxConnections ||

        max_prepared_xacts != ControlFile->max_prepared_xacts ||

        max_locks_per_xact != ControlFile->max_locks_per_xact)

    {

        …                        

        ControlFile->MaxConnections = MaxConnections; 

        ControlFile->max_prepared_xacts = max_prepared_xacts;

        ControlFile->max_locks_per_xact = max_locks_per_xact;

        ControlFile->wal_level = wal_level;

        UpdateControlFile();

    }

}                                

/*

 * This must be called ONCE during postmaster or standalone-backend startup

 */

void

StartupXLOG(void)

{

    …                                        

    /* REDO */

    if (InRecovery)

    {

        …

        /* Check that the GUCs used to generate the WAL allow recovery */

        CheckRequiredParameterValues();                                    

        …

        if (record != NULL)

        {

            …

            /*

             * main redo apply loop

             */

            do

            {

                …

                /*

                 * If we are attempting to enter Hot Standby mode, process

                 * XIDs we see

                 */

                if (standbyState >= STANDBY_INITIALIZED &&

                    TransactionIdIsValid(record->xl_xid))

                    RecordKnownAssignedTransactionIds(record->xl_xid);                        

                RmgrTable[record->xl_rmid].rm_redo(EndRecPtr, record);

                …                            

            } while (record != NULL && recoveryContinue);

            …

        }

        …

    }

    …

}                                            

/*

 * XLOG resource manager's routines

 *

 * Definitions of info values are in include/catalog/pg_control.h, though

 * not all record types are related to control file updates.

 */

void

xlog_redo(XLogRecPtr lsn, XLogRecord *record)

{

    …

    if (info == XLOG_NEXTOID)

    {

        …

    }

    …

    else if (info == XLOG_PARAMETER_CHANGE)

    {

        …

        /* Check to see if any changes to max_connections give problems */

        CheckRequiredParameterValues();

    }

} 

/*

 * Check to see if required parameters are set high enough on this server

 * for various aspects of recovery operation.

 */

static void

CheckRequiredParameterValues(void)

{

    ...                                      

    if (InArchiveRecovery && EnableHotStandby)

    {

        .../* We ignore autovacuum_max_workers when we make this test. */

        RecoveryRequiresIntParameter("max_connections",

                             MaxConnections,

                             ControlFile->MaxConnections);
        ...
 

    }

}