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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.

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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..

> fsiAddPrinter(fun (d: SystemDateTime) -> dToString());; val it : unit = () > query();; val it : seq<string * SystemDateTime> = seq [("Joe", 14/02/1965 00:00:00); ("Mary", 15/09/1985 00:00:00)] The definition of query uses sequence expressions that locally define new IDisposable objects such as conn, comm, and reader using declarations of the form use var = expr These ensure that the locally defined connection, command, and reader objects are disposed after exhausting the entire sequence See s 4, 8, and 9 for more details on sequence expressions of this kind F# sequences are on-demand (that is, lazy), and the definition of query does not itself open a connection to the database This is done when the sequence is first iterated, and indeed a connection is maintained until the sequence is exhausted.

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

DDL). Second, there is a method to do DDL over a database link, in a fashion, using the job queue facility, DBMS_JOB or, in Oracle 10g, the scheduler package, DBMS_SCHEDULER. Instead of trying to do DDL over the link, you use the link to schedule a remote job to be executed as soon as you commit. In that fashion, the job runs on the remote machine, is not a distributed transaction, and can do the DDL. In fact, this is the method by which the Oracle Replication Services perform distributed DDL to do schema replication.

Note that the command object s ExecuteReader method returns a DataReader instance that is used to extract the typed data returned from the query You can read from the resulting sequence in a straightforward manner using a sequence iterator For instance, we can use a simple anonymous function to print data on the screen: > query() |> Seqiter (fun (fn, bday) -> printfn "%s has birthday %O" fn bday);; Joe has birthday 14/02/1965 00:00:00 Mary has birthday 15/09/1985 00:00:00 val it : unit = () The query brings the data from the database in-memory, though still as a lazy sequence You can then use standard F# in-memory data transformations on the result: > query() |> Seqfilter (fun (nm, bday) -> bday < SystemDateTimeParse("01/01/1985")) |> Seq.