Saturday, September 27, 2014

Replication from Oracle to MariaDB the simple way - Part 1

Yes, there is a simple way to do this. Although it might not be so simple unless you know how to do it, so let me show you how this can be done. It's actually pretty cool. But I'll do this over a number of blog posts, and this is just an introductory blog, covering some of the core concepts and components.

But getting this to work wasn't easy, I had to try several things before I got it right, and it's not really obvious how you make it work at first, so this is a story along the lines of "If at first you don't succeed mr Kidd" "Try and try again, mr Wint" from my favorite villains in the Bond movie "Diamonds are forever":
So, I had an idea of how to achieve replication from Oracle to MySQL and I had an idea on how to implement it, and it was rather simple, so why not try it.

So, part 1 then. Oracle has the ability to let you add a UDF (User Defined Procedure) just like MariaDB (and MySQL), but Oracle calls then extproc. So my first idea was an extproc that would send a message over UDP, any generic message. In this specific case though, I was to have a server that received that UDP message and then sent it to MariaDB as a query. So this server was connected to MariaDB and a message came in though UDP, and this message would of course be some DML statement. Simple and efficient, right? Of course UDP is not a reliable protocol, but for this usage it should be OK, right?

All that was needed after this was a set of triggers on the Oracle tables that would send a message to the server which would then send these to MySQL, and this message would be based on the changed data, as provided by the trigger, like a corresponding INSERT, UPDATE or DELETE statement. This should work. OK, let's get to work.

My first attempt at the server meant that a lot of UDP messages were lost. The reason for this was that there were just to many messages coming in bursts (this is how extprocs works in Oracle, nothing wrong with Oracle here, it's just the nature of the beast). The solution was to have the server be multithreaded, with one tight thread receiving messages on the UDP port, and then having a queue and another thread processing the queue and send the messages to MariaDB?

Well, this didn't work out either. The reason was that the means I did synchronization between the threads was using Mutexes, and this also slowed things down. It worked OK for a while, but some burst could easily mean that UDP messages were lost, and really not much was necessary in terms of load for this to happen.

So, what do we do now? I had two choices, either I skip UDP and go for a connected TCP solution instead. The issue with this was that partly the extra overhead this would cause on the Oracle end of things, and partly that this would increase the complexity of my extproc and server, and above all the API to all this. The other option was to go for a lock-less queue. Some googling and some testing allowed me to figure out how to do this with gcc and I could move on and test this. Fact is, it worked fine. And as UDP messages can't be routed, using UDP messages in and of themselves made things a bit more secure.

But then there was the issue with the TRIGGERs calling the extproc. A trigger is called on a DML operation, but that operation might later be rolled back, and in that case I would break the consistency of data. And for some reason, for all the enhancement to standard SQL that Oracle provides, (including DDL triggers), there are no ON COMMIT triggers. But there just had to be a way around that one, right?

So Try and try again, mr Wint. Yes, I will solve all this, but the story is to be continued in another blog post. Also, I will eventually provide with the code for all this. GPL code that you can use out of the box to replicate from Oracle to MariaDB. But not just yet.

So don't touch that dial!



Daniƫl van Eeden said...

I do think UDP is routable. Using a non-routable protocol is a extremely weak security measure. If security is important you should use DTLS. And besides TCP and UDP you could also consider SCTP, but thats also connection oriented.

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