Monday, September 29, 2014

Replication from Oracle to MariaDB the simple way - Part 2

The theme for this series of posts is, and indicated in the previous post, "Try and try, again", and there will be more of this now when I start to make this work by playing with Oracle, with PL/SQL and with the restrictions of Oracle Express (which is the version I have available).

So, what we have right now is a way of "sending" SQL statements from Oracle to MariaDB, the question is when and how to send them from Oracle. The idea for this was then to use triggers on the Oracle tables to send the data to MariaDB, like this, assuming we are trying to replicate the orders table from Oracle to MariaDB:
In Oracle, and assuming that the extproc I have that created to send UDP messages is called oraudp, then I would do something like this:
CREATE OR REPLACE TRIGGER orders_insert
AFTER INSERT ON orders
FOR EACH ROW
BEGIN
   CALL oraudp('INSERT INTO orders VALUES '||
     :new.order_id||', '||:new.customer_id||', '||
     :new.amount||')');
END;
/

CREATE OR REPLACE TRIGGER orders_update
AFTER UPDATE ON orders
FOR EACH ROW
BEGIN
   CALL oraudp('UPDATE orders SET order_id = '||:new.order_id||
     ', customer_id = '||:new.customer_id||', amount = '||:new.amount||
     ' WHERE order_id = '||:old.order_id);
END;
/

CREATE OR REPLACE TRIGGER orders_delete
AFTER DELETE ON orders
FOR EACH ROW
BEGIN
    CALL oraudp('DELETE FROM orders WHERE order_id = '||:old.order_id);
END;
/
 As I noted in my previous post though, DML TRIGGERs execute when the DML statement they are defined for executes, not when that DML is committed, which means that if I replicate to MariaDB in a trigger and then roll back, the data will still be replicated to MariaDB. So the above design wasn't such a good one after all. But the theme is, as I said, try and try again, so off to think and some googling.

Google helped me find a workaround, which is by using Oracle MATERIALIZED VIEWs. And I know what you think, you think "What has dear old Karlsson been smoking this time, I bet it's not legal", how is all this related to MATERIALIZED VIEWs? Not at all, actually, but this is a bit of a kludge, it does actually work, and this is the nature of a kludge (I've done my fair share of these over the years). There are two attributes of Oracle MATERIALIZED VIEWs that makes these work for us:
  • A MATERIALIZED VIEW can be updated ON COMMIT, which is one of the two ways a MATERIALIZED VIEW is updated (the other is ON DEMAND). So when there is a COMMIT that affects the tables underlying the MATERIALIZED VIEW, we have something happening.
  • And the above would be useless if you couldn't attach a TRIGGER to a MATERIALIZED VIEW, but you can!
So, to make this work, I can define a MATERIALIZED VIEW, one that I don't really use for any other purpose, on the table I want to replicate. So then the design gets something like this, again using the orders table as an example:
In practice, if we continue to use theorders table, this is what I would do in Oracle:
CREATE MATERIALIZED VIEW orders_mvREFRESH ON COMMIT
AS SELECT order_id, customer_id, amount
FROM orders;

CREATE OR REPLACE TRIGGER orders_mv_insert
AFTER INSERT ON orders_mv
FOR EACH ROW
BEGIN
   CALL oraudp('INSERT INTO orders VALUES '||
     :new.order_id||', '||:new.customer_id||', '||
     :new.amount||')');
END;
/

CREATE OR REPLACE TRIGGER orders_mv_update
AFTER UPDATE ON orders_mv
FOR EACH ROW
BEGIN
   CALL oraudp('UPDATE orders SET order_id = '||:new.order_id||
     ', customer_id = '||:new.customer_id||', amount = '||:new.amount||
     ' WHERE order_id = '||:old.order_id);
END;
/

CREATE OR REPLACE TRIGGER orders_mv_delete
AFTER DELETE ON orders_mv
FOR EACH ROW
BEGIN
    CALL oraudp('DELETE FROM orders WHERE order_id = '||:old.order_id);
END;


Now we have a solution that actually works, and it also works with Oracle transactions, nothing will be sent to MariaDB if a transaction is rolled back. There is a slight issue with this though, you might think it's no big deal, but fact is that performance sucks when we start adding data to the orders table, and the reason for this is that the default way for Oracle to refresh a materialized view is to delete all rows and then add the new ones. In short, Oracle doesn't keep track of the changes to the base table, except writing them to the base table just as usual, so when the MATERIALIZED VIEW is to be refreshed, all rows in the materialized view will first be deleted, and then all the rows in the base table inserted. So although this works, it is painfully slow.

So it's back to the drawing board, but we are getting closer, now it works at least, we just have a small performance problem to fix. I'll show you how I did that in the next post in this series, so see you all soon. And don't you dare skip a class...

/Karlsson

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