Chapter 15: Achieving High Availability Through Replication

Introduction  Use replication to copy data to different locations in your enterprise  There are several reasons Move data closer to the users To reduce locking conflictsTo allow site autonomy To remove the impact of read- intensive operations © Wiley Inc All Rights Reserved.

Publisher/Subscriber Metaphor  There are several key terms used in replication Publisher  the source database where replication begins. It makes data available for replication Subscriber  the destination database where replication ends Distributor  the intermediary between the publisher and subscriber. It receives published transactions or snapshots and then stores and forwards these publications to the subscribers Publication  is the storage container for different articles. A subscriber can  subscribe to an individual article or an entire publication. Article  the data, transactions, or stored procedures that are stored within a publication. This is the actual information that is going to be replicated. © Wiley Inc All Rights Reserved.

Articles  An article is data in a table  It can be the whole table or just a subset Horizontal partitioning  Rows are filtered out Vertical partitioning  Columns are filtered out © Wiley Inc All Rights Reserved.

Publications  A publication is a logical collection of articles  Subscribers subscribe to a publication They do not need to read all of the articles though © Wiley Inc All Rights Reserved.

Replication Factors  Factors that affect replication are Autonomy  Level of subscriber independence Will the replicated data be considered read-only? How long will the data at a subscriber be valid? How often do you need to connect to the distributor to download more data? Latency  Frequency of data updates Transactional Consistency  Do all the transactions that are stored need to be applied at the same time and in order? What happens if there is a delay in the processing? © Wiley Inc All Rights Reserved.

Using Transactional Replication  Transactions from the publisher are stored on the distributor  Subscribers receive and apply transactions They should treat data as read-only  Some latency and autonomy can be introduced  Subscribers do not need to be in contact with publishers at all times © Wiley Inc All Rights Reserved.

Using Snapshot Replication  Moves an entire copy of the published items © Wiley Inc All Rights Reserved.

Snapshot With Updating Subscribers  Initial replication works just like snapshot replication  Subscribers use 2PC to update the publisher  Publishers must approve subscriber updates © Wiley Inc All Rights Reserved.

Using Merge Replication  This allows subscribers to make changes to their local data  All changes are merged with all other subscribers When all subscribers have the same data, they are in convergence © Wiley Inc All Rights Reserved.

Using Queued Updating  Used with transactional and snapshot replication  Allows subscribers to update publishers  Updates are queued, not immediate  There are other Restrictions © Wiley Inc All Rights Reserved.

Replication Internals  There are two types of subscriptions  Push Configured and maintained at the publisher  Pull Configured and maintained at the subscriber © Wiley Inc All Rights Reserved.

Publication Issues  Timestamp datatype This is replicated as binary data in transactional and snapshot Data is not replicated in merge  Identity values You can assign a range of values to replicate  User-defined datatypes These must be created on each subscriber  Not for replication Prevents objects from being replicated © Wiley Inc All Rights Reserved.

Publication Restrictions  Replicated tables must have a primary key. Except in snapshot replication.  Publications cannot span multiple databases.  Varchar(max), nvarchar(max), and varbinary(max) data is not replicated in transactional or merge replication. Because of their size, these objects must be refreshed by running a snapshot.  You cannot replicate from the master, model, MSDB, or tempdb databases. © Wiley Inc All Rights Reserved.

Distributor Issues  Ensure you have enough hard disk space for the Distribution working folder and the distribution database.  Do not let the distribution database’s transaction log fill up.  The distribution database will store all transactions from the publisher to the subscriber. It will also track when those transactions were applied.  Snapshots and merge data are stored in the Distribution working folder.  Be aware of the size and number of articles being published.  Text, ntext, and image datatypes are replicated only when you use a snapshot.  A higher degree of latency can significantly increase your storage space requirements.  Know how many transactions per synchronization cycle there are. © Wiley Inc All Rights Reserved.

Replication Backup  There are four possible strategies Publisher only Publisher and distributor Publisher and subscriber(s) Publisher, distributor and subscriber(s) © Wiley Inc All Rights Reserved.