镜像切换Logreader Agent报错：分发数据库中可能存在不一致的状态

软件环境：Windows Server 2008 R2 sp1 SQL Server 2008 R2 sp2

架构：ServerA(主体）+ServerB(镜像)+ServerSub(订阅)+ServerDist(分发)

特别注意：ServerDist分发服务器为单机，故开启了Sync_With_Backup选项。

关于Sync_With_Backup：

初衷：简单来说，开启该选项，使得单机分发库在宕机时，利用最近的系统数据库和用户数据库备份，还原到一台新的服务器上，即可使复制继续工作成为可能。

需要注意的是：在分发库开启该选项，意味着发布库的日志只有在分发库被备份后才会截断。

查看该选项是否开启：

     SELECT name,is_sync_with_backup FROM sys.databases WHERE name LIKE 'distribution%'

开启该选项：

sp_replicationdboption 'distribution','sync with backup','true'

场景：

在生产环境中，分发库备份为半小时一次。

将主体从ServerA切换至ServerB时，Logreader Agent报错：

直观上可以看到，Logreader 认为dist_backup_lsn和dist_last_sn不一致，导致其无法继续工作。下面我们实际去看一下这两者到底是否一致。

下面是Logreader、Distribution Agent工作的步骤列表，为了方便，我都贴出来：

Log Reader Agent (logread.exe) – Sequence of Steps1.Calls sp_MSadd_LogReader_History to write to MSLogReader_History – “Starting Agent”2.sp_MShelp_logreader_agentid – Obtain log reader agent specific information for that publication 3.sp_MShelp_PRofile – Obtains profile information for the Log Reader4.MSadd_logreader_history to write MSlogreader_history – “Initializing” 5.sp_MSget_last_transaction – determine where the log reader agent left off reading the log. 6.Read the transaction log – sp_replcmds 7.Processes the returned commands from the sp_replcmds in batches by calling sp_MSadd_repl_commands 8.Marks this transaction as committed in distribution database by using sp_repldone procedure 9.Adjusts the identity range if necessary and if you are using Automatic Identity Range Management y calling sp_MSpub_adjust_identity 10.Calls sp_MSget_last_transaction to check the last transaction read and stored in MSReplication_transactions table 11.When all transactions are read, LogRead.exe calls sp_MSAdd_logreader_history and writes message to MSLogReader_history “1 transactions with 9 commands were delivered”•Distribution Agent (distrib.exe) - Sequence of Steps1.master.db.sp_msget_jobstate – get the status of the job (if it is already started) 2.sp_Msadd_distribution_history – MSDistribution_history – Starting agent 3.sp_MSSubscription_Status – whether subscription has expired or the snapshot is ready 4.sp_server_info- determines the collation 5.sp_mshelp_subscriber_info – retrieve subscriber information 6.sp_mshelp_subscription_agentid – determine the name of the distribution agent 7.sp_Msadd_distribution_history – Initializing message – Msrepl_distribution_history 8.sp_Msadd_distribution_history – Connecting to Subscriber - Msrepl_distribution_history 9.so_datatype_info – to determine the data type mapping necessary to create the tracking table necessary for the Distribution agent 10.sp_MScheck_subscribe on subscription database – verifies that SQL Server Agent account is in sysadmin and db_owner role in subscription database 11.sp_mscreate_sub_tables on subscriber in subscription database – creates MSSusbcription_agents and MSreplication_subscriptions tables 12.Sp_MSinit_Subscription_agent – updates the Subscription agent information on subscription database 13.Retrieves transaction_timestamp and subscription_guid to determine what Distribution agent has already replicated to the Subscriber. Transaction_timestamp correlates to xact_seqno column in MSReplication_transactions table in distribution database. All values large than the xact_seqno will be replicated 14.If we are doing initial sync, Distribution Agent calls sp_MSupdatelastsyncinfo which updates MSreplication_susbcriptions and MSSusbcription_agents table 15.Starts to retrieve all transactions and their corresponding commands from MSReplication_transactions and MSreplication_commands table where transaction_timestamp column in subscription database < xact_seqno column in MSreplication_transactions table. Applies the transaction using sp_MS_get_repl_commands procedure 16.Issues dynamic SQL to update the MSreplication_subscriptions table with the last delivered transaction ID 17.sp_MSDistribution_history to write the MSrepl_distribution_history table with status message “nn transaction(S) with nn command(s) were delivered”

在步骤5中，我们得知，Logreader用存储过程sp_Msget_last_transaction来定位发布库日志中最后一个被写入到分发库中的事务，我们找到这个存储过程的源码：

CREATE PROCEDURE sys.sp_MSget_last_transaction(    @publisher_id int = NULL,    @publisher_db sysname,    @publisher sysname = NULL,    @max_xact_seqno varbinary(16) = NULL output    ,@for_truncate bit = 0)ASbegin    declare @publisher_database_id int    declare @max_xact_id varbinary(16)    declare @sync_bit int    declare @sync_with_backup bit    set nocount on        -- security check    -- only db_owner can execute this        if (is_member ('db_owner') != 1)    begin        raiserror(14260, 16, -1)        return (1)    end    SELECT @sync_bit = 32    if @publisher_id is NULL        select @publisher_id = srvid from master.dbo.sysservers where            UPPER(srvname) = UPPER(@publisher)    -- Get publisher database id.    SELECT @publisher_database_id = id from MSpublisher_databases where publisher_id = @publisher_id and        publisher_db = @publisher_db    if exists ( select * from master.dbo.sysdatabases where            name = db_name() and            category & @sync_bit = 0)        select @sync_with_backup = 0    else        select @sync_with_backup = 1    if @for_truncate = 0    begin        select top 1 @max_xact_id = rt.xact_id, @max_xact_seqno = rt.xact_seqno          from          MSrepl_transactions rt          where             rt.publisher_database_id = @publisher_database_id and             not xact_id = 0x0             order by xact_seqno desc    end    -- If (1) requesting truncate lsn (distbackuplsn), (2) sync with backup is set    -- query the values from MSrepl_backup_lsn    else if    @sync_with_backup = 1    begin        -- Get the last backed up lsn if available.        select top 1 @max_xact_id = valid_xact_id, @max_xact_seqno = valid_xact_seqno          from          MSrepl_backup_lsns          where             publisher_database_id = @publisher_database_id    end        -- If @publisher is not null, we are calling this sp from sp_replrestart    -- Don't return result set.    if @publisher is null        select @max_xact_id, @max_xact_seqno, @publisher_database_id            -- Don't return any result when requsting a truncate lsn and            -- the database is not in 'sync with backup' mode, which signal the            -- distribution agent to use last dist lsn to call sp_repldone.            where not (@sync_with_backup = 0 and @for_truncate = 1)end

存储过程中，代码

else if    @sync_with_backup = 1begin    -- Get the last backed up lsn if available.    select top 1 @max_xact_id = valid_xact_id, @max_xact_seqno = valid_xact_seqno      from      MSrepl_backup_lsns      where         publisher_database_id = @publisher_database_idend

可见，报错中的dist_backup_lsn取自表MSrepl_backup_lsns

我们来查询一下该表：

select * from MSrepl_backup_lsns

列valid_xact_id为最后备份的事务ID，valid_xact_seqno为该事务中的LSN，即valid_xact_seqno为报错中显示的dist_backup_lsn，（请勿与图中贴出的错误进行对比，上图仅为错误示例截图）。

我们再去分发库MSrepl_transactions表中找到当前已经被读取到分发库的最大事务的LSN

SELECT TOP 1 * from     [dbo].[MSrepl_transactions] as twhere t.publisher_database_id = 5ORDER BY t.entry_time DESC