Introduction to Replication Server

 

1.1    Introduction
1.2    Replication Server Components
1.3    What is the Difference Between SQL Remote and Replication Server?

Thanks go to Manish I Shah for major help with this introduction. 

1.1    Introduction

What is Replication Server

Replication Server moves transactions (insert, updates and deletes) at the table level from a source dataserver to one or more destination dataservers.  The dataserver could be ASE or other major DBMS flavour (including DB2, Informix, Oracle). The source and destinations need not be of the same type.

• Move data from one source to another.
• Move only a subset of data from source to destination. So, you can ‘subscribe’ to a subset of data, or a subset of the columns, in the source table, e.g. select * from clients where state = ‘NY’
• Manipulation/transformation of data when moving from source to destination. E.g. it can map data from a data-type in DB2 to an equivalent in Sybase.*
• Provide a warm-standby system.  Can be incorporated with Open Switch to provide a fairly seamless fail-over environment.
• Merge data from several source databases into one destination database (could be for a warehouse type environment for example).
• Move data through a complicated network down to branch offices, say, only sending the relevant data to each branch.

(* This is one of Sybase replication's real strengths, the ability to define function string classes which allow the conversion of statements from one SQL dialect to match the dialect of the destination machine.  Ed)

The data moves asynchronously. The time it takes to reach the destination depends on the size of your transaction, level of activity in that particular database (a database as in Sybase systems), the length of the chain (one or more replication servers that the transaction has to pass through to reach the destination), the thickness of pipe (network), how busy your replication server is etc. Usually, on a LAN, for small transactions, this is about a second.

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The source of data where client applications enter/delete and modify data.  As mentioned before, this need not be ASE, it can be Microsoft SQL Server, Oracle, DB2, Informix.  (I know that I should get a complete list.)

Log Transfer Manager (LTM) is a separate program/process which reads transaction log from the source server and transfers them to the replication server for further processing. With ASE 11.5, this has become part of ASE and is now called the Replication Agent.  However, you still need to use an LTM for non-ASE sources.  I imagine there is a version of LTM for each kind of source (DB2, Informix, Oracle etc).  When replication is active, you see one connection per each replicated database in the source dataserver (sp_who).

The replication server is an Open Server/Open Client application.  The server part receives transactions being sent by either the source ASE or the source LTM.  The client part sends these transactions to the target server which could be another replication server or the final dataserver.  As far as I know, the server does not include the client component of any of the other DBMSes out of the box.

Server in which the final replication server (in the queue) will repeat the transaction done on the primary. You will see a connection, one for each target database, in the target dataserver when the replication server is actively transferring data (when idle, the replication server disconnects or fades out in replication terminology).

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1.3    What is the Difference Between Replication Server and SQL Remote?

Both SQL Remote and Replication Server perform replication.  SQL Remote was originally part of the Adaptive Server Anywhere tool kit and is intended for intermittent replication.  (The classic example is that of a salesman connecting on a daily basis to upload sales and download new prices and inventory.)  Replication Server is intended for near real-time replication scenarios.

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