N-Way Replication in Oracle 11g Streams: Part 1

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N-way replication refers to a Streams environment where there are multiple sources. In this article, we will still use the STRM1 and STRM2 databases but with a little twist; making both databases the source. By making both STRM1 and STRM2 sources, we need to first consider a couple of unique situations and do a little more pre-planning, specifically for N-Way replication.

The concepts and techniques used to configure a 2-way replication can then be used to scale to N-way replication. We all need to crawl before we run, the better you crawl (understand) this article, the easier it will be to scale up to N-way replication. Pay close attention and learn the technique so that you can implement it well.

We need to repeat this—Streams is not Failover.

We need to repeat this—Streams is not Failover.

No, that is not a typo. The authors are passionate about Streams and want to see you successfully implement it. To successfully implement Streams, you need to know not to step into the trap of using it for Failover.

Both authors have done some work where Failover was the requirement. Streams is not a Failover solution. Failover is handled by Oracle Data Guard, NOT Oracle Streams. Streams is about distributing the data to multiple locations. On more than one occasion, Streams was used as a Failover technology because it can distribute data to multiple locations. Do not fall into the trap of using the wrong tool for the wrong job. Streams distributes (replicates) data. As such, there will always be some difference between the databases in a Streams environment. All replication technology has this problem. The only time where all of the databases are in sync is, when there is no activity and all replication has been applied to all target locations.

If you need Failover, then use the proper tool. Oracle Data Guard is for Failover. It has the necessary processes to guarantee a different level of failover from a primary site to a secondary site, whereas Streams is a Replication tool that distributes data. Just remember the following, when there is a discussion of Replication and Failover that comes up:

  • Streams distributes data, it is built for replication
  • Data Guard is built for Failover

Pre-planning for N-way replication

When we set up N-way replication, we must consider the possibility of a collision of data. Since we have multiple sources of data, it is possible for the exact same data to be inputted on any or all of the sources at the exact same time. When this happens, it is a conflict. This example is just one type of conflict that can happen in N-way replication environments. The types of conflict that can occur are as follows:

  • Update conflict: When transactions from different databases try to update the same row at nearly the same time.
  • Delete conflict: When one transaction deletes a row and the next transaction tries to update or delete the row. Transactions originate from different databases.
  • Unique conflict: When transactions from different databases violate a primary or unique constraint, the first transaction is accepted. The second transaction obtains the conflict.
  • Foreign key conflict : This happens when a transaction from a Source tries to insert a child record before the parent record exists.

The good news is that Oracle has provided built-in conflict resolution in Streams that solves the most common situations. The built-in solutions are as follows:

  • OVERWRITE
  • DISCARD
  • MAXIMUM
  • MINIMUM

We will provide an example of conflict resolution after we build our N-way replication. In our case, we will use MAXIMUM. As part of the pre-planning for N-way replication, we highly suggest creating a simple table such as the Setup Table.

Avoiding Conflict

As conflict requires additional pre-planning and configuration, one begins to wonder, “Are there techniques so that we can configure N-way replication without the possibility of conflict?” The simple answer to the question is “Yes”. The not-so simple answer is that there is some configuration magic that needs to be done and the devil is in the details.

Limiting who and what can be updated is one method of avoiding conflict. Think of it this way— there is no conflict if we agree to who and what can update the specific data. User 1 can only update his specific data and no one else can do that. Similarly, user 2 can only update his specific data. So, user 1 and user 2 can never cause a conflict. Now this may be a little bit difficult depending on the application. This can be implemented with the use of offset sequences. One sequence produces only odd values, and another produces only even values. We could also use a combination of sequence and some unique characteristics of the database.

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