DMDB

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

1. Copies need to be synchronized

With an update everywhere approach, changes can be initiated at any of the copies. That is, any of the sites which owns a copy can update the value of the data item.

1. There is always one site which has all the updates
2. No inter-site synchronization is necessary (it takes place at the primary copy)

1. The load at the primary copy can be quite large
2. Reading the local copy may not yield the most up to date value

1. AC1 = All processors that reach a decision reach the same one (agreement, consensus).
2. AC2 = A processor cannot reverse its decision. 
3. AC3 = Commit can only be decided if all processors vote YES (no imposed decisions).
4. AC4 = If there are no failures and all processors voted YES, the decision will be to commit (non triviality).
5. AC5 = Consider an execution with normal failures. If all failures are repaired and no more failures occur for sufficiently long, then all processors will eventually reach a decision (liveness).

1. ATOMICITY: a transaction is executed in its entirety or not at all
2. ISOLATION: a transaction executes as if it were alone in the system
3. CONSISTENCY: a transaction executed in its entirety over a consistent DB produces a consistent DB
4. DURABILITY: committed changes of a transaction are never lost - can be recovered