DMDB

Synchronous replication propagates any changes to the data immediately to all ex- isting copies. Moreover, the changes are propagated within the scope of the trans- action making the changes. The ACID properties apply to all copy updates.

1. No inconsistencies (identical copies)
2. Reading the local copy yields the most up to date value
3. Changes are atomic

1. A transaction has to update all sites (longer execution time, worse response time)

Asynchronous replication first executes the updating transaction on the local copy. Then the changes are propagated to all other copies. While the propagation takes place, the copies are inconsistent (they have different values). The time the copies are inconsistent is an ad- justable parameter which is application de- pendent.

1. A transaction is always local (good response time)

1. Data inconsistencies
2. A local read does not always return the most up to date value
3. Changes to all copies are not guaranteed Replication is not transparent

With a primary copy approach, there is only one copy which can be updated (the master), all others (secondary copies) are updated re- flecting the changes to the master.

1. Load is evenly distributed
2. any site can run a transaction