Saturday, August 06, 2005

The Database Patch Runner: Table Centric Views

On Wilfred van der Deijl's blog we got into a nice discussion about grabbing a table centric history of the patches applied to a database when using a functional patch centric upgrade organisation.

I think I've made it obvious in the past that I massively favour the functional grouping, since I think it makes the job of the developer and the person performing the upgrade that much clearer. However, I acknowledge the perceived need for a table history, and started thinking about how you might produce it.

Just as I was getting to some sort of conclusion, Wilfred put together a couple of great ideas... and I thought I'd give them a critical review.

Forgive me for this Wilfred... I really wanted to get this down, I think it's a great topic!

Group the patches by table, then add interpreted comments to the files in order to state which patch a given ALTER TABLE statement belongs with

I'm not sure about this solution for a simple reason. I don't like organising the source files by tables; I think this makes the developer's job that much harder.

In producing a patch the developer needs to update several files and cannot easily see the order in which these changes are applied.

I simple situation where this becomes very important is where a column needs to be moved from one table to another. In order to do so, I cannot see how this can be done without changing at least two files; one that adds the new column, and one that drops the old.. In addition, you need to find somewhere to put the data migration script. Does this go into to the DDL file for the table having the column added? Or do you could add a third file into the mix.

I have found in the past that the create column and data migration get put into the table scripts and there is a manual process added to ensure that the existing column gets dropped. The process is then forgotten in the heat of the production upgrade and the redundant column remains in the schema.

In addition, with the table centric grouping it is easy for a developer to miss a step in the DDL, a column isn't added to a table, for example. By having the functional grouping it's easier to spot this mistake since the developer has a single file that lists all the DDL applicable for that functional change.

Finally, there is a need to add that extra step... adding the comments to the table create scripts. I have an objection to this in the same way that I have an objection to JavaDoc comments. People simply don't add them, and nobody notices / cares until such time as those missing comments are crucial. By then it's too late. The comments could be policed, but whenever I see something that needs policing I see something that needs changing so the policing is no longer needed.

Another way would be to stick to your way with a SQL script per patch and just store all statements in the database... [snip] ... have system triggers that log all DDL statement against particular objects

I love this idea. I think this has got a lot going for it. The big advantage of this technique is that the person writing the patch does not need to change the way they work in order for it to be implemented.

As part of the patch runner installation it can add a system trigger to the current schema that will log the changes to the schema as they occur. This detail log can state which object that has changed, the change that was made, and the patch that issued the change.

The trigger can be notified of which patch is currently running (as stated by Wilfred) by the patch runner logging the name of the current patch in a package variable, a temporary table or some other session specific temporary store.

Once the patch runner has completed it can then produce a report stating which changes were applied by that upgrade, grouped by table. It can produce a report of all changes ever made to the database. If so required it could list the changes classified by some arbitrary grouping of objects (system functional area / table sizes).

As Wilfred states, there is a downside to this... the data is in the database, rather than version control. It is generated after the change has been applied. However, this data can be made available in version control fairly easily. The simplest solution is to have the report run regularly on a development or test server. Copy the report into version control and away you go. If you're running a nightly build then this can be done automatically each night from the test server in advance of the codeset being tagged.

Alternatively you can provide a simple user interface in order to query this information more interactively. If, for example, you're developing browser based applications this will be a fairly simple tool to produce within your normal toolset.

Additionally, such a system DDL trigger can be used to report on ANY changes made to the database, not just those applied through the official upgrade process. Having produced reports that will list object changes from the database it would be a trivial task to change those reports to list any objects that have changed without being ran through the patch runner. It may be that in a strict environment a change made outside of the patch runner (and therefore the standard application upgrade) could be blocked. Obviously, any developer or DBA that knows the structure will be able to work around it without too much work, but this then means that you are certain that the person doing so is consciously making a change outside of the accepted process, rather than because they are not aware that such a process exists.

As I've stated earlier, on Wilfred's blog, I'm not sure our organisation has a need for the table centric view of the changes made to a database. In fact I strongly urge those people who do so to ask why it is they do. I'd love to hear the reasons why... I feel as though I'm missing something!

That said, I think the second solution proposed by Wilfred is spot on, and if we were looking to produce a table centric view we would definitely follow that avenue first.

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Anonymous said...

Thanks for the insight, Rob.

The more I think about it, the more I feel for your functional centric aproach in stead of my initial feeling of table centric version history. I'll start implementing our version control and build automation in September and will definitely have a look at your approach with the additional DDL triggers.

Thanks for the great discussions. They were really helpfull in getting a clear picture

Andrew Beacock said...

I'm not convinced by the table-centric view either, it splits up the context of the change too much.

It reminds me of a previous job where we had Sybase database scripts split up by type, so a table script, a trigger script, a user scripts, a stored procedure script, etc. It meant than anything other than a very minor change required updating multiple files.

These files were under ClearCase (file locking SCM) with about 100 developers in the codebase, so it made for some very difficult changes, more floor walking and talking than coding!