Sybase DC38133-01-0902-01 manual How statements are replicated, Replication of inserts and deletes

How statements are replicated

	SQL Remote replication is based on the transaction log, enabling it to
	replicate only changes to data, rather than all data, in each update. When we
	say that SQL Remote replicates data, we really mean that SQL Remote
	replicates SQL statements that modify data .
Only committed	SQL Remote replicates only statements in committed transactions, to ensure
transactions are	proper transaction atomicity throughout the replication setup and maintain a
replicated	consistency among the databases involved in the replication, albeit with
	some time lag while the data is replicated.
Primary keys	When an UPDATE or a DELETE is replicated, SQL Remote uses the
	primary key columns to uniquely identify the row being updated or deleted.
	All tables being replicated must have a declared primary key or uniqueness
	constraint. A unique index is not sufficient. The columns of the primary key
	are used in the WHERE clause of replicated updates and deletes. If a table
	has no primary key, the WHERE clause refers to all columns in the table.
An UPDATE is not	When a simple INSERT statement is entered at one database, it is sent to
always an UPDATE	other databases in the SQL Remote setup as an INSERT statement.
	However, not all statements are replicated exactly as they are entered by the
	client application. This section describes how SQL Remote replicates SQL
	statements. It is important to understand this material if you are to design a
	robust SQL Remote installation.

The Message Agent is the component that carries out the replication of statements.

Replication of inserts and deletes

INSERT and DELETE statements are the simplest replication case.

SQL Remote takes each INSERT or DELETE operation from the transaction log, and sends it to all sites that subscribe to the row being inserted or deleted.

If only a subset of the columns in the table is subscribed to, the INSERT statements sent to subscribers contains only those columns.

The Message Agent ensures that statements are not replicated to the user that initially entered them.

Replication of updates

UPDATE statements are not replicated exactly as the client application enters them. This section describes two ways in which the replicated

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