Sybase SQL Remote DC38133-01-0902-01 User Manual
Who gets what?
Each time a row in a table is inserted, deleted, or updated, a message has to
be sent to those subscribed to the row. In addition, an update may cause the
subscription expression to change, so that the statement is sent to some
subscribers as a delete, some as an update, and some as an insert.
be sent to those subscribed to the row. In addition, an update may cause the
subscription expression to change, so that the statement is sent to some
subscribers as a delete, some as an update, and some as an insert.
☞
For details of what statements get sent to which subscribers, see
. For details on subscriptions, see the
following two chapters.
This section describes how SQL Remote sends the right operations to the
right recipients.
right recipients.
The task of determining who gets what is divided between the database
server and the Message Agent. The engine handles those aspects that are to
do with publications, while the Message Agent handles aspects to do with
subscriptions.
server and the Message Agent. The engine handles those aspects that are to
do with publications, while the Message Agent handles aspects to do with
subscriptions.
Adaptive Server
Anywhere actions
Anywhere actions
Adaptive Server Anywhere evaluates the subscription expression for each
update made to a table that is part of a publication. It adds the value of the
expression to the log, both before and after the update.
update made to a table that is part of a publication. It adds the value of the
expression to the log, both before and after the update.
Not the subscriber list
Adaptive Server Enterprise does not evaluate or enter into the log a list of
subscribers. The subscription expression (a property of the publication) is
evaluated and entered. All handling of subscribers is left to the Message
Agent.
subscribers. The subscription expression (a property of the publication) is
evaluated and entered. All handling of subscribers is left to the Message
Agent.
For a table that is part of more than one publication, the subscription
expression is evaluated before and after the update for each publication.
expression is evaluated before and after the update for each publication.
The addition of information to the log can affect performance in the
following cases:
following cases:
♦
Expensive expressions
When a subscription expression is expensive
to evaluate, it can affect performance.
♦
Many publications
When a table belongs to many publications, many
expressions must be evaluated. In contrast, the number of
subscriptions is
irrelevant.
♦
Many-valued expressions
Some expressions are many-valued. This
can lead to much additional in formation in the transaction log, with a
corresponding effect on performance.
corresponding effect on performance.
Adaptive Server
Enterprise actions
Enterprise actions
In a SQL Remote for Adaptive Server Enterprise publication, the
subscription expression must be a column. The subscription column
subscription expression must be a column. The subscription column
86