1. DAT 412:Transactional Replication Internals
Matt Hollingsworth
Program Manager
Microsoft Corporation

2. Agenda Review General Agent Overview Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

3. Transactional Replication
Snapshot
Agent
\\Snapshot Share
Publishing
database
Subscribing
database
Distribution
Agent DB
log
Distribution
database
Log Reader
Agent
Snapshot Agent writes initial data to Snapshot Share
Publisher changes tracked and extracted by Log Reader Agent
Distribution database (store) acts as reliable queue
Distribution Agent forwards changes to Subscribers

4. Transactional Replication Updating subscribers
Publisher
Distributor
Read-Only
Subscriber
Queued Updating
Subscriber
Queue
NETWORK
Queue Reader Agent
Immediate Updating
Subscriber
2PC, RPC

5. Agenda Review General Agent Overview Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

6. General Agent Overview
Agents implement the core functions of replication
Initiated via SQLAgent or from command line
Maintain connections to repl server instances
Connection info supplied on agent command line
Can use integrated or SQL Server security
SQL Server security: SecurityMode=0, login and password
Integrated security: Security Mode=1, no login or password
LoginTimeOut and QueryTimeOut configurable
Can be scheduled or run on demand
Continuous: sleeps for PollingInterval between loops
Logging and History
HistoryVerboseLevel [1|2|3]
OutputVerboseLevel [0|1|2]
Output
ErrorFile

7. Agenda Review General Agent Properties Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

8. Snapshot Agent Algorithm
Connects to the Publisher and Distributor
For each article in the publication
Reads publisher schema metadata
Generates schema scripts into snapshot directory according to user article options (.sch, .idx, .dri files)
Bulk copies table data into snapshot directory (.bcp files) according to user defined row and column filters
Updates status for subscription to active

9. Snapshot Agent Parameters
MaxBcpThreads
Improves parallelism during BCP out
Misc.
ReplicationType =1 (transactional)
RowDelimiter
FieldDelimiter

10. Snapshot Sync Properties
Controls how the snapshot is generated and applied
sp_addpublication
@sync_method
sets snapshot format (character or native)
sets snapshot concurrency behavior
@immediate_sync
true: subscriber can retrieve the initial data immediately after subscribing
false: subscriber must wait for next snapshot to synchronize
@pre/post_snapshot_script
executes user defined custom scripts

11. Snapshot Scripting Properties
How to configure
none, delete, drop or truncate
0x00 User specified creation script
0x01 Object creation
0x02 Custom SP’s
0x10 Clustered indexes
0x20 Convert UDT’s to base data types
0x40 Nonclustered indexes
0x80 PK Indexes
0x100 Triggers
0x200 FK Constraints
0x800 Defaults
0x Column-level collation
0x Extended properties
0x UK Constraints
0x PK Constraints
UI: configure via article properties page

12. Agenda Review General Agent Overview Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

13. Log Reader Agent Algorithm
Connects to the publisher and distributor
Scans the publisher log, building a list of committed tx which are marked for replication = sp_replcmds.
For each command in the committed transactions
For each publication receiving the command
inserts data into the distributor using the user specified article format and column filter = sp_Msadd_repl_cmds27 if the row matches the specified user row filter
Moves replication truncation mark in log = sp_repldone
Commits transaction batch at distributor
Logs history messages into the distribution database
Note: Log Reader must run before a concurrent snapshot is available for subscribers

14. demo
Log Reader Agent Profiler Trace

15. Log Reader Agent Parameters
Batching
ReadBatchSize – max number of transactions read from publisher log per processing loop
always honors a tx boundary
ReadBatchThreshold – max number of commands read from the publisher log per processing loop
MaxCmdsInTran – max number of commands written to distributor as a transaction
Warning: forces a commit when the limit is reached. This may result in transactional inconsistency at the subscriber in certain cases.
Other Parameters
PacketSize
MessageInterval

16. Command Formats
The Log Reader writes statements as “commands” into the distributor MSrepl_commands table
The command format is defined by the user as an article property
These commands are applied directly to the subscriber by the distribution agent
This flexibility is valuable for application specific processing and performance tuning

17. “None” Command Format
Log Reader skips the specified command type instead of inserting it into the distribution db
When to use
Great for auditing, use “None” for the delete command
How to configure
= ‘None’
UI: configure via article properties page

18. “SQL” Command Format
Commands stored in distribution DB as SQL inserts, updates, and deletes
When to use
Heterogeneous subscribers are present
Lower distribution performance is acceptable
Less SQL plan sharing occurs at the subscriber
How to configure
= ‘SQL’
To include column names in insert statements, = 8
ODBC parameterization improves performance. = 16 or 24
UI: configure via article properties page

19. Custom Command Formats
Commands stored in distribution db as stored procedure calls = sp_Msins/upd/del<article>
When to use
Highest performance distribution
Provides application flexibility since the stored procedure can be customized
No heterogeneous subscribers are present
How to configure
= 0x02 to generate subscriber side stored procedures
= 16 to improve performance via parameterization
UI: configure via article properties page

20. ‘CALL’ Command Format How SP parameters are specified When to use
Inserts
all column values
Updates
initial pk column values
all final column values for the updated row
Delete parameters generated
pk column values
When to use
for highest insert and delete performance
when custom subscriber logic doesn’t require the values for columns in deleted rows
How to configure
= ‘CALL’
UI: configure via article properties page

21. ‘MCALL’ Command Format
How SP parameters are specified
Inserts
not supported
Updates
initial pk column values
final column values for the specific columns updated
bitmask indicating which columns were updated
Deletes
When to use
for highest update performance
when custom subscriber logic doesn’t require the before values for columns
How to configure
= ‘MCALL’
UI: configure via article properties page

22. ‘XCALL’ Command Format
How SP parameters are specified
Inserts
not supported
Updates
all initial and final column values
Deletes
all initial column values
When to use
For the most flexible subscriber side custom logic and transforms
How to configure
= ‘XCALL’
UI: article properties page

23. demo
Browsing Distributor Commands with
sp_browsereplcmds

24. User Trigger Considerations
Some publisher statements are applied as different sets of statements at the subscriber
Concurrent snapshot processing applies compensating commands after snapshot is delivered to subscriber
By default, SQL Server logs updates to unique constraints in replicated tables as as deletes followed by inserts
SQL Server 2000 SP1 added new dbcc option that converts single row unique constraint updates from delete/insert pairs back into single update statements for the subscriber
This trace flag is not available for queued and immediate updating subscribers
How to configure
dbcc traceon(8207,-1)
Note: updates to columns in unique keys that affect multiple rows are always applied to the subscriber as deletes followed by inserts.

25. Stored Proc Considerations
Two SP publishing methods exist:
Replicate the rows affected by the SP
Replicate the execution of the SP
When to use
Publish execution for SP’s that affect large numbers of rows. Provides significant performance improvement (10x).
How to configure
value of ’proc exec’ replicates via RPC
value of ’serializable proc exec’ replicates via RPC if publisher call is within a serializable transaction
other values will replicate the individual rows affected by the stored procedure
UI: article properties page

26. View Considerations Standard Views Indexed Views
Replicated as scripted schema object definitions
Requires publishing the underlying table(s)
Indexed Views
Replicated as a base object like a table
only the changes to the indexed view data are replicated
underlying table(s) do not need to be published

27. Text/Image Considerations
Size
Server option ‘max text repl size’ sets the largest text/image value on the replicated table
Data beyond the max size is not allowed
Error: Length of text data 1000 to be replicated exceeds configured maximum: 500."
Text/Image Update Limitations with Repl
No updates during concurrent snapshot
Repeatable read tx isolation level or greater required when using text pointers
XCALL before image not supported
No updates at subscriber for text/image

28. Loopback Considerations
Includes originating server instance and database with each distributor command to prevent looping command distribution scenarios
When to use
Necessary for Bi-directional (reciprocal) topologies where each database publishes and subscribes to the other
How to Configure
= ‘true’ for both subscriptions

29. Backup Considerations
Publisher restored to previous point-in-time
Problem: distributor and subscriber may contain rows later than restored database!
Solution: set “sync with backup” option on publisher to prevent commands from inserting into distributor until the log entries are backed up
Side Effect: increases command distribution latency
Distributor restored to previous point-in-time
Problem: Distributor loses commands that have not been delivered to subscribers when they are already truncated from publisher log!
Solution: set “sync with backup” option on distributor to prevent publisher log truncation until the distributor has backed up entries
Side Effect: may increase size of publisher log
Subscriber restored to previous point-in-time
No issues if within the distributor retention interval since transaction timestamp on subscriber is also restored

30. Agenda Review General Agent Overview Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

31. Distribution Agent Algorithm
Connects to the distributor, subscriber, and snapshot share
Applies scripts and bcp’s snapshot data into subscriber. Depends on status and
Gets next batch of commands for the subscription from the distributor MSrepl_commands table = sp_msget_repl_commands
Determines last applied xact id for subscription by querying MSreplication_subscriptions transaction_timestamp
Applies the literal commands directly to the subscriber
Commits the batch

32. demo
Distribution Agent Profiler Trace

33. Distribution Agent Parameters
Performance related parameters
BcpBatchSize
CommitBatchSize
CommitBatchThreshold
MaxBcpThreads
MaxDeliveredTransactions
UseInprocLoader
Misc parameters
MessageInterval
TransactionsPerHistory
NoTextInitOnSync
ProfileName
DefinitionFile
Publication
QuotedIdentifier [0|1|2]
Note: FTP parameters are obsolete

34. Distribution Tuning Tips
Offload distribution agent load
Configure pull subscriptions
Tune databases
Minimize indexes on subscriber
Tune I/O for subscriber and distributor: RAID 0+1 for data, separate log
Increase Batching
Increase CommitBatchSize, CommitBatchThreshold, and MaxBcpBatchSize parameters
Increase agent parallelism
Use independent agent: =’true’
Configure multiple publications for each subscriber according to transactional requirements
Increase MaxBcpThreads parameters

35. Scripting Replication Stored Procs
To manually generate subscriber replication procs
exec sp_helparticle (to get article id)
exec sp_scriptinsproc
exec sp_scriptupdproc
exec sp_scriptdelproc
To manually generate queued conflict tables and triggers
sp_makeconflicttable
sp_addsynctriggers

36. Scripting Replication Stored Procs(2)
Useful for manually generating an update proc that is optimized for tables with large numbers of indexes
Exec Sp_scriptdynupdproc
Subscriber Procedure
Indexes on subscriber table
Throughput, cmds/sec
Default MCALL
1 clustered index
1,570
1 clustered index, 10 nonclustered indexes
110
Dynamic MCALL
1,180

37. Agenda Review General Agent Overview Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

38. Queued Subscriber Trigger Logic
SQL DML modification to subscription table occurs
Replication trigger fires and inserts data into local subscriber queue
Queue can be a SQL Server table (msreplication_queue) or MSMQ
Queue Reader Agent will later collect these changes from the queue and apply to the publisher

39. Queue Reader Agent Algorithm
Connects to the distributor, subscriber, and publisher
For each subscription serviced by the distributor
Gets next batch of commands from the subscriber queue = MSreplication_queue via stored procedure = sp_replsqlqgetrows
Applies commands to the publisher table sp_MSsync_ins/upd/del_<table>_#
Note: manages an internal thread pool to keep CPU’s busy and increase parallelization. Groups approximately 1000 commands per batch.
Commits the publisher rows
Conflicts detected via msrepl_tran_version uniqueidentifier column differences
Note: ranged updates (update foo set x=x+1) to unique key columns will fail at the publisher since they are applied as individual update statements.

40. demo
Queue Reader Agent Profiler Trace

41. Queue Reader Agent Parms
You can configure who wins in case of conflicting updates
ResolverState [1|2|3]
Publisher wins conflict
Publisher wins conflict with reinitialization of subscriber
Subscriber overwrites existing Publisher values

42. Administering the Queues
sp_getqueuedrows
Returns the transactions that are stored in the queue
sp_replqueuemonitor
Returns the commands that are stored in the queue
sp_setreplfailovermode
Switches between immediate updating and queued updating modes

43. Agenda Review General Agent Overview Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

44. Distributor Cleanup Agents
Distribution Cleanup
Deletes rows from the distribution Msrepl_cmds and Msrepl_transactions table which are outside the distribution database retention period
sp_MSdistribution_cleanup
History and Logging Cleanup
Deletes rows from each distributor agent history table which are outside the history retention period
Expired Subscription Cleanup
Removes subscriptions which have not synchronized within the publication retention period
sp_expired_subscription_cleanup

45. Other Replication Agents
Replication Checkup
Raises error for all agents that have not logged messages within the specified interval
Re-init Subs with Data Validation Failures
Marks all subscriptions for re-initialization that have failed a validation check
sp_MSreinit_failed_subscriptions

46. Agenda Review General Agent Overview Snapshot Agent Log Reader Agent
Distribution Agent
Queue reader Agent
Other Replication Agents
Summary

47. Replication Summary Fast Flexible Consistent
Can deliver 2500 commands per second
Latency is often a few seconds
Flexible
Modular agent approach maximizes application flexibility
Stored procedures and DMO permit fine-grained control
Consistent
Data is kept transactionally consistent

48. Other Resources
DAT411 Programming and Deploying Microsoft SQL Server 2000 Replication: Lessons Learned 
Thursday 15: :15, Room 3
Transactional Replication Performance Tuning whitepaper

49. Don’t forget to complete the on-line Session Feedback form on the Attendee Web site

50. © 2002 Microsoft Corporation. All rights reserved.
This presentation is for informational purposes only. Microsoft makes no warranties, express or implied, in this summary.