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CHAPTER 8 TRANSACTIONS
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typically the one the client is logged into initially will be the coordinator for the distributed transaction. This one site will ask the other sites if they are ready to commit. In effect, this site will go to the other sites and ask them to be prepared to commit. Each of the other sites reports back its prepared state as YES or NO. If any one of the sites votes NO, the entire transaction is rolled back. If all sites vote YES, the site coordinator broadcasts a message to make the commit permanent on each of the sites. This limits the window in which a serious error could occur. Prior to the voting on the 2PC, any distributed error would result in all of the sites rolling back. There would be no doubt as to the outcome of the transaction. After the order to commit or rollback, there again is no doubt as to the outcome of the distributed transaction. It is only during the very short window when the coordinator is collecting the votes that the outcome might be in doubt, after a failure. Assume, for example, we have three sites participating in the transaction with Site 1 being the coordinator. Site 1 has asked Site 2 to prepare to commit, and Site 2 has done so. Site 1 then asks Site 3 to prepare to commit, and it does so. At this point, Site 1 is the only site that knows the outcome of the transaction, and it is now responsible for broadcasting the outcome to the other sites. If an error occurs right now the network fails, Site 1 loses power, whatever Sites 2 and 3 will be left hanging. They will have what is known as an in-doubt distributed transaction. The 2PC protocol attempts to close the window of error as much as possible, but it can t close it entirely. Sites 2 and 3 must keep that transaction open, awaiting notification of the outcome from Site 1. If you recall from the architecture discussion in 5, Oracle Processes, it is the function of the RECO process to resolve this issue. This is also where COMMIT and ROLLBACK with the FORCE option come into play. If the cause of the problem was a network failure between Sites 1, 2, and 3, then the DBAs at Sites 2 and 3 could actually call the DBA at Site 1, ask him for the outcome, and apply the commit or rollback manually, as appropriate. There are some, but not many, limitations to what you can do in a distributed transaction, and they are reasonable (to me, anyway, they seem reasonable). The big ones are as follows: You can t issue a COMMIT over a database link. That is, you can t issue a COMMIT@remote_site. You may commit only from the site that initiated the transaction. You can t do DDL over a database link. This is a direct result of the preceding issue. DDL commits. You can t commit from any site other then the initiating site, hence you can t do DDL over a database link. You can t issue a SAVEPOINT over a database link. In short, you can t issue any transaction control statements over a database link. All transaction control is inherited from the session that opened the database link in the first place; you can t have different transaction controls in place in the distributed instances in your transaction.
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The lack of transaction control over a database link is reasonable, since the initiating site is the only one that has a list of everyone involved in the transaction. If in our three-site configuration, Site 2 attempted to commit, it would have no way of knowing that Site 3 was involved. In Oracle, only Site 1 can issue the commit command. At that point, it is permissible for Site 1 to delegate responsibility for distributed transaction control to another site. We can influence which site will be the actual commit site by setting the COMMIT_POINT_STRENGTH (a parameter) of the site. A COMMIT_POINT_STRENGTH associates a relative level of importance to a server in a distributed transaction. The more important the server (the more available the data needs to be), the more probable that it will coordinate the distributed transaction. You might want to do this if you need to perform a distributed transaction between your production machine and a test machine. Since the transaction coordinator is never in doubt as to the outcome of a transaction, it s best if the production machine coordinated the distributed transaction. You don t care so much if your test machine has some open transactions and locked resources. You certainly do care if your production machine does. The inability to do DDL over a database link is actually not so bad at all. First, DDL is rare. You do it once at installation or during an upgrade. Production systems don t do DDL (well, they shouldn t do
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