Information Technology Reference
In-Depth Information
F
IG
. 5. Data replication synchronization.
the case of DB2 Everyplace, the server transactionally performs the appropriate
sequence of SQL DELETE, INSERT and UPDATE operations. In the case
of Lotus Notes, the client's version of the NSF (Notes Storage Facility File)
records are copied over the master copy.
If data replication is used to implement the
order entry
example, as orders are placed
on the disconnected device, the stock levels are correspondingly modified. During
synchronization, those stock table rows that were modified (by reducing the stock
level) are transmitted to the server.
Figure 5
sketches what happens during a suc-
cessful data replication synchronization. The synchronization middleware detects
(through the “dirty” bit value) that the client decremented the stock of staplers, and
that only nine staplers are currently in stock. Because the corresponding column
value was not concurrently modified in the server-side database, the middleware
changes the server-side value to match the client's value, resulting in a successful
synchronization.
3 . 2 M e t h o d R e p l a y
The method replay synchronization technique represents a change set as a log
of the method invocations performed on the disconnected device. Each log entry
constrains the information needed to replay a single method: e.g., the method name,
the method's signature, and the method's parameter values. In can thus be seen as the
“dual” of the data replication approach which logs the data modifications. A version
of log replay is used in the “Field Calls” in IMS Fast Path
[19]
, earlier work on long-
running transactions
[30]
, and IceCube
[17]
(see also
[18]
). The synchronization
process begins by transmitting the method log to the server.
Conflict detection
: the method invocations are replayed, in sequence, against
the server's current state. This simultaneously propagates the device's updates
to the server (if the method replay is successful) and detects a conflict (if the
