Tuesday, September 27, 2016

Database Schema Migration

Some thoughts: http://workingwithdevs.com/delivering-databases-migrations-vs-state/

This covers a lot of ground on the Declarative vs. Procedural question. It explains a lot of the considerations that lead to choosing a procedural schema evolution vs. a declarative schema with an implied change sequence to migrate to each new declared state.

The article calls the declarative "state-based" and procedural approach "migration-based".

My 2¢ are focused on this point: 
When using a state-based solution you will most often be using a diff tool like those provided by Redgate or Visual Studio to examine the differences and generate an upgrade script. While this is a very efficient solution for most changes, with table renames and a few other types of table refactoring they can do bad things, ...
This point about table refactoring is, for me, the show-stopper. Relational theory tells me that I can map any schema to any other schema using selection, projection, and join. I can denormalize data and I can normalize again via group-by clauses. I can reduce the original schema to a sequence of object-attribute-value triples, and restructure this into any desired new schema. 

Given enough time, a change tracking tool should be able to find a minimal-cost transformation from schema to schema. This might involve a complex search over a large state space, and it certainly involves creating costs for each alternative query plan. 

Pragmatically, I'm not sold on this being a good idea. And I'm rarely sure I even want to get involved in a fully automated solution. While a tool might be able to detect and automate a variety of simple changes, I think that developers must always vet those change scripts.

In particular, the search space is emphatically not limited to select, project, and join. There are also database unload-reload, index create and drop. There are even more complex operations like creating intermediate results which aren't part of the final database structure. With proper indices, these might actually be beneficial.

In some cases, the continuous operation requirements are such that we might have two copies of a database: one being used and the other being transformed. A logger tracks transactions in the older copy and a synchronizer replicates those transactions in the new copy. After the data is moved, the customer access is moved via a feature toggle from the old database to the new database.

Semantic Drift

Also important is the issue of semantic drift. When we're making structural changes where the "before" column names match the "after" column names, then there's little chance for semantic drift. There's still some possibility, though. We can (and sometimes do) repurpose columns, preserving the original name. In some cases, we might change a database constraint without renaming the column.

In the larger case, of course, it doesn't require "‘hot-fix’ changes to QA or even production databases" to create profound semantic changes. All it takes is an app developer deciding that a column should be repurposed. There's may be no structural change on the schema overall. 

A non-structural change in some past release could have implications for structural change in a future release. Imagine three columns in three tables with the same names. Two started out life as simple foreign keys to the third. But one became optional, and now the semantics don't match but the names do. Automated tools are unlikely to discern the intent here. 


It's all procedural migration. I'm not declarative ("state") tools can be trusted beyond discerning the changes and suggesting a possible migration.

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