Access 95 lets you drag an entire database to the Windows 95 Briefcase, creating a replica that can be moved to a floppy or a laptop for use at a remote site. This makes it necessary to ensure that changes made by remote users of replica databases are reflected in the original database. Access 95 solves this problem with data replication: the ability to coordinate changes in two or more copies of a database.
Access replication is deferred and asynchronous, which means a database copy may be changed at any time without coordination. To avoid database copies getting out of sync between replication cycles, higher-end products like Microsoft SQL Server 6 and Oracle use synchronous replication
, which requires constant communication between database copies, restrictions on which copies of a database may be changed, or both. Access's lack of such restrictions makes conflicts possible among its co-equal databases.
Access
replication happens at the row level
; Oracle and the upcoming Lotus Notes 4 are more granular, with field-level replication. When changes are made to a single field, Access copies the entire row containing the field to the other replica's equivalent row, erasing any data that was there. Replicating only the changed records is more economical than managing multiple copies of the entire database.
Managing replication adds substantial overhead. Each table that is a replicated object has at least three new fields added to it (Memo and OLE object fields require still more fields to track their update status). In addition, the database gets at least three new tables and a fourth category of tables to track update conflicts, which occur if two replicas chan
ge the same record after it has been updated.
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Access 95 uses a row-level replication scheme to coordinate changes in copies of a database. Changes may occur in both directions.
Replicating Access Databases
