Oracle7 Server Distributed Systems Volume II: Replicated Data
Selecting a Conflict Resolution Method
The symmetric replication facility's declarative conflict resolution mechanism provides system-defined routines for resolving update and uniqueness conflicts.
These system-defined routines do not support the following situations:
- changes to primary key columns
- NULLs in the columns that you designate to resolve the conflict
- referential integrity constraint violations
For these situations, either provide your own conflict resolution routine or determine a method of resolving these errors after they are logged in the DefError view.
System-Defined Resolution Routines for Update Conflicts
The table below specifies the system-defined resolution methods for update conflicts that guarantee convergence in three types of replication environments.
| Any Number of Master Sites (4 methods)
| One or Two Master Sites (9 methods)
| One Master Site and Multiple Snapshots Sites (12 methods)
|
| latest timestamp
|
|
|
| additive
|
|
|
| minimum value (always decreasing)
|
|
|
| maximum value (always increasing)
|
|
|
|
| earliest timestamp
|
|
|
| minimum value
|
|
|
| maximum value
|
|
|
| highest priority site
|
|
|
| highest priority value
|
|
|
|
| average
|
|
|
| discard from snapshot sites
|
|
|
| overwrite master site
|
Table 6 - 1. System Defined Methods to Resolve Update Conflicts
Note: The conflict resolution methods you assign need to ensure data convergence and provide results that are appropriate for how your business uses the data.
System-Defined Resolution Routines for Unique Constraint Conflicts
The symmetric replication facility provides three methods for resolving uniqueness conflicts:
- append the global name of the originating site to the column value from the originating site
- append a generated sequence number to the column value from the originating site
- discard the row value from the originating site
Avoiding Ordering Conflicts
If you have more than one master site, none of these routines result in convergence, and these routines should only be used in conjunction with a notification facility. See the Conflict Notification section ![[*]](jump.gif)
.
If your replicated environment has more than two masters:
- the following conflict resolution methods cannot guarantee convergence: earliest timestamp, minimum value, maximum value, highest priority site, highest priority value
- network failures and infrequent pushing of the deferred remote procedure call (RPC) queue increase the likelihood of non-convergence for these methods
Table 6 - 2 shows how having three master sites can lead to ordering conflicts. Master Site A has priority 30; Master Site B has priority 25; and Master Site C has priority 10; x is a column of a particular row in a column group that is assigned the site-priority conflict resolution method.
Note: The highest priority is given to the site with the highest priority value. Priority values can be any Oracle number and do not have to be consecutive integers.
| Time
| Action
| Site A
| Site B
| Site C
|
| 1
| All sites are up and agree that x = 2.
| 2
| 2
| 2
|
| 2
| Site A updates x = 5.
| 5
| 2
| 2
|
| 3
| Site C becomes unavailable.
| 5
| 2
| down
|
| 4
| Site A pushes update to Site B.
Site A and Site B agree that x = 5.
Site C is still unavailable.
The update transaction remains in the queue at Site A.
| 5
| 5
| down
|
| 5
| Site C becomes available with x = 2.
Sites A and B agree that x = 5.
| 5
| 5
| 2
|
| 6
| Site B updates x = 5 to x = 7.
| 5
| 7
| 2
|
| 7
| Site B pushes the transaction to Site A.
Sites A and B agree that x = 7.
Site C still says x = 2.
| 7
| 7
| 2
|
| 8
| Site B pushes the transaction to Site C.
Site C says the old value of x = 2;
Site B says the old value of x = 5.
Oracle detects a conflict and resolves it by applying the update from Site B, which has a higher priority level (25) than Site C (10).
All site agree that x = 7.
| 7
| 7
| 7
|
| 9
| Site A successfully pushes its transaction (x = 5) to Site C.
Oracle detects a conflict because the current value at
Site C (x = 7) does not match the old value at Site A (x = 2).
Site A has a higher priority (30) than Site C (10).
Oracle resolves the conflict by applying the outdated update from Site A (x = 5).
Because of this ordering conflict, the sites no longer converge.
| 7
| 7
| 5
|
Table 6 - 2. Ordering Conflicts With Site Priority - More Than Two Masters
You can guarantee convergence when using priority groups if you require that the flow of ownership be ordered. For example, the workflow model dictates that information flow one-way through a three-step sequence:
1. From the ORDERING site
If the billing site receives a change to a row from the ordering site after the billing site received a change to that row from the shipping site, the billing site ignores the out-of-order change because the change from shipping has a higher priority.
Suggestion: To help determine which conflict resolution method to use, make a diagram or time-action table (such as Table 6 - 2) to help uncover any potential loopholes in your conflict resolution methodology.
Why Use Multiple Resolution Methods
The "Understanding Column Groups" section of this chapter explained that multiple column groups provide multiple conflict resolution methods for a single row. You can also use multiple conflict resolution methods for each column group. Use multiple resolutions methods
- to have one or more backup methods
- to receive notification of conflicts
Multiple resolution methods are applied in the sequence you set.
Your preferred conflict resolution method might not always succeed. You can specify a backup method to have a greater chance of conflict resolution without manual intervention.
Some system-defined resolution methods, such as latest timestamp, occasionally require a backup method to successfully resolve conflicts. (Site priority is a possible backup method.) The latest timestamp method uses a special timestamp column to determine and apply the most recent change. In the unlikely event that the row at the originating site and the row at another site change at precisely the same second, you must provide a backup method.
Note: Oracle stores time to a granularity of one second.
You can also provide a user-defined method that records conflict information or notifies the DBA if the conflict cannot be resolved. You can arrange to receive notification for all conflicts, or for only those conflicts that cannot be resolved. You can mix any number of user-defined and system-defined conflict resolution routines.
Summary of Standard Conflict Resolution Methods
Convergence means that all sites ultimately agree on the same value. Table 6 - 3 summarizes the system-defined conflict resolution methods, and whether they guarantee convergence between multiple master sites and their associated snapshot sites. Following sections describe each of these methods in greater detail.
| Conflict Type
| Resolution Methods
| Converge?
| Converge with >2 Masters?
|
| UPDATE
| minimum
| yes
| no, unless always decreasing
|
|
| maximum
| yes
| no, unless always increasing
|
|
| earliest timestamp
| yes
| no
|
|
| latest timestamp
| yes
| yes, with backup method
|
|
| priority group
| yes
| no, unless always increasing
|
|
| site priority
| yes
| no
|
|
| overwrite
| yes, 1 master only
| no
|
|
| discard
| yes, 1 master only
| no
|
|
| average (numeric only)
| yes, 1 master only
| no
|
|
| additive (numeric only, additive updates only)
| yes
| yes
|
| Uniqueness Constraint (UPDATE or INSERT)
| append site name
| no
| no
|
|
| append sequence
| no
| no
|
|
| ignore discard
| no
| no
|
| DELETE
| none
|
|
|
Table 6 - 3. Standard Conflict Resolution Methods