Pythian Blog: Technical Track

Merge Replication Identity Range Management - An Identity Disorder? Part 2

by Pythian Marketing on Feb 4, 2014 12:00:00 AM

In Part 1 of this series, we explored Merge Replication Identity Range Management of tables with identity columns. We outlined how important it is to select the right datatype for identity columns and assign suitable ranges for your identity to reduce the overhead of allocating new ranges frequently. There are some cases that can affect the range management due to the changes they make to the identity values allocation , this includes batches and transactions! The case of batches and transactions on publisher I had worked recently with a case where the client’s apps fail intermittently with this error: The insert failed. It conflicted with an identity range check constraint in database %, replicated table %, column %. If the identity column is automatically managed by replication, update the range as follows: for the Publisher, execute sp_adjustpublisheridentityrange; for the Subscriber, run the Distribution Agent or the Merge Agent. The client just followed the advice and executed sp_adjustpublisheridentityrange to assign a new range on publisher but that was not convenient. The user was a db_owner so it should assign a new range by itself, but the error kept appearing. Initially, as a best practice, we advised changing the range to something larger in order to accommodate new rows -- but this did not work. We began looking at processes inserting the data, and we found few stored procedures that insert single rows -- nothing abnormal. It wasn't until we ran a trace to capture the load for few hours -- we found a job running SSIS package on another instance. The ETL SSIS package calls a stored procedure on publisher to load data from another system and dumps thousands of rows inside a transaction. Worse, the job never failed because the SSIS package used conditional constraints to suppress the stored procedure failure through conditional constraints, it also did not include logic to email anyone if the SP step failed. The code was something like this, logic wise: [sourcecode language="sql"] SELECT DISTINCT rows INTO #temp from staging_instance.staging_database.dbo.table BEGIN TRAN T1 INSERT INTO dbo.table1 (col1, col2,....., colZ) SELECT * FROM #temp INSERT INTO dbo.table1_details (col1, col2,....., colZ) SELECT t.* , stag.col1, stag.col2 From table1 t1 join #temp t on t1.id = t.id and t.date_ < gedate() -2 join stag on t.Detail_id = stag.detail_id Insert into tracking select ‘Complete_ETL’, getdate() , @@rowcount Commit Tran1 [/sourcecode] The problem here became obvious: Triggers fire once per statement, not once per row. It is possible to create triggers to cope with multiple rows using INSERTED but a replication trigger is not the place we want to mess with! Let us test this on our existing simple schema , please refer to Part 1 of this series for the schema. We will insert 400 rows into a temp table and insert them at once to the published table.
[sourcecode language="sql"] use pub Create table #tmp (col2 datetime) Insert into #tmp (col2) select getdate(); Go 400 select count(*) from #tmp 400 [/sourcecode] Let’s record the identity value of the table on publisher Dbcc checkident (tbl_pub , noreseed)
Checking identity information: current identity value '4602'
The primary range is 4601-4701 & the secondary is 4701-4801 so adding 400 rows should override this Insert into pub.dbo.tbl_pub (col2) select * from #tmp Go Msg 548, Level 16, State 2, Line 1 The insert failed. It conflicted with an identity range check constraint in database 'Pub', replicated table 'dbo.tbl_pub', column 'col1'. If the identity column is automatically managed by replication, update the range as follows: for the Publisher, execute sp_adjustpublisheridentityrange; for the Subscriber, run the Distribution Agent or the Merge Agent. Dbcc checkident (tbl_pub , noreseed)
Checking identity information: current identity value '4802'
The current identity pushed to the limit but no rows were inserted. Here is what happened:
  1. SQL Server began inserting the data and assigning new Identity values; since the trigger fires per statement, it will not fire until the batch completes.
  2. Because the batch has more rows than the available free Identity slots (in this case, fewer than 200) it fails because the constraint is violated. However, the next identity value is already greater than the max value defined by the constraint.
  3. Because the batch failed, the trigger never fired and never allocated a new range.
  4. Any subsequent processes, even if ONE single row, will fail because the next available Identity slot, 4802, is greater than the limit of the secondary range (4801). This is when the applications fail and return error.
With single row operations , the trigger will fire and assign new ranges if both primary and secondary ranges were exhausted and the new ranges will be ready for future operations. Temporary resolution was following the advice of the error message and run sp_adjustpublisheridentityrange. The disadvantage however, is that it creates some gaps in the identity column because the insert rolled back and never used the Identity slots. Kindly note that rolling back a transaction does NOT set identity Seed to the value before the transaction start. If we want to reduce the gap then we can reseed the column to a value close to the most recent inserted value within the range.
[sourcecode language="sql"] select max(col1) +5 [reseed] from pub.dbo.tbl_pub where col1 between (select max(range_begin) from Distribution..MSmerge_identity_range_allocations where publication='pub1' and article= 'tbl_pub' and subscriber_db='Pub' and subscriber = 'MYINSTANCE') AND (select max(next_range_end) from Distribution..MSmerge_identity_range_allocations where publication='pub1' and article= 'tbl_pub' and subscriber_db='Pub' and subscriber = 'MYINSTANCE'); GO [/sourcecode] Both are manual workarounds, and this was not desired. Workarounds Few workarounds can work here, they all evolve around ensuring that the triggers fire to allocate new ranges including
  1. Increase the range assuming that a larger range can accommodate larger batches. Well,not quite true since the batches have varied sizes and user processes can reduce available identity slots to a number less than the batch rows count.
  2. Always assign new ranges when batches start. This creates gaps.
  3. Add some logic in the code that will check for the available Identity range VS batch row counts and insert if there is enough slots or assign new ranges if the rows count are more. We tried this but sometimes failed because some user processes added rows, exhausting range mid-way. It also created big gaps sometimes>If Current ID in the primary range then check the available ID count >>>> If the Available empty IDs > batch count then proceed >>>>>>>>>> If Available empty IDs If Current ID in the Secondary range then check the available ID count >>>> If the Available empty IDs > batch count then proceed >>>>>>>>>> If Available empty IDs < batch count then run sp_adjustpublisheridentityrange to assign new ranges
  4. Insert everything as single rows. This works, but it's very slow sometimes.
We ended up having a “Hybrid” approach :
  • Batch insert if the range permits, much faster
  • Row by row if the range does NOT accommodate all the rows. We could have stretched it a bit more by batch inserting till the end of the range and insert the rest using row-by-row but that's complicating it more.
[sourcecode language="sql"] Create table #temp ( SNo INT IDENTITY(1,1) , CARTONID nvarchar(40), STOREID int, STORECODE nvarchar(20), SUBSIDIARYID int ) -- insert the rows required into the temp table. SELECT DISTINCT rows INTO #temp from staging_instance.staging_database.dbo.table select @count= COUNT(*) +500 from #temp --five hundered rows in case user processes add rows in the meantime declare @range_begin bigint,@range_end bigint,@next_range_begin bigint,@next_range_end bigint , @current bigint select top 1 @range_begin =range_begin,@range_end =range_end,@next_range_begin =next_range_begin, @next_range_end=next_range_end from distribution..MSmerge_identity_range_allocations where article = 'table1' and subscriber = 'Publisher' order by range_begin desc select @current=IDENT_CURRENT('dbo.table1') set @i=1 BEGIN TRAN T1 If (@current< @range_end and @count < (@range_end - @current) ) or (@current > @next_range_begin AND @count < (@next_range_end - @current) ) Begin --Batch Insert INSERT INTO Table1 (col1, col2, col3, . . ,ColZ) * FROM #temp End If (@current< @range_end and @count > (@range_end - @current) ) or (@current > @next_range_begin AND @count > (@next_range_end - @current) ) Begin WHILE (@i <= @count) BEGIN INSERT INTO Table1 (col1, col2, col3, . . ,ColZ) * FROM #temp where ID = @i SELECT @i = @i + 1 end End COMMIT TRAN T1 [/sourcecode] Recommendations Besides the recommendations in Part 1 of this series, the lesson we learned from that engagement was basic : You need to understand your environment! If you have transactions or batches that run against tables with identity columns engaged in Merge replication, then you want to ensure that the batch rows can fit within the available ranges.
Topics: Microsoft SQL Server Technical Track Replication Sql Server Administration Merge Replication

No Comments Yet

Let us know what you think

See all

See all

Subscribe by email
Follow us
Love Your Data
Pythian Logo

© 2024 All rights reserved.