Re: [Wikitech-l] Abstract Schema and Schema changes, request for help

Hello, In case you haven't done any changes on database schema of mediawiki

let me explain the process to you (if you know this, feel free to skip

paragraph): * Mediawiki core supports three types of RDBMS: MySQL, Sqlite, Postgres.

used to be five (plus Oracle and MSSQL) * For each one of these types, you need to do three parts: 1- Change the tables.sql file so new installations get the new schema 2- Make .sql

change file, like an "ALTER TABLE" for current installations so they can upgrade. 3- Wire that schema change file into *Updater.php file. * For example, this is a patch to drop a column: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/473601 This file

14 different files, adds 94 lines and removes 30. This is bad for several reasons: * It is extremely complicated to do a even a simple schema change.

something as simple as adding an column takes a whole day for me. There

lots of complicating factors, like Sqlite doesn't have ALTER TABLE, so

you want to make a patch for adding a column, you need to make a

table with the new column, copy the old table data to it, drop the old table and then rename the old table. ** Imagine the pain and sorrow when you want to normalize a table meaning you need to do several schema changes: 1- Add a table, 2- Add a column on the old table, 3- make the column not-nullable when it's filled and make the old column nullable instead 4- drop the old column. * It's almost impossible to test all DBMS types, I don't have MSSQL or Oracle installed and I don't even know their differences with MySQL. I assume most other developers are good in one type, not all. * Writing raw sqls, specially duplicated ones, and doubly specially when

don't have CI to test (because we won't install propriety software in our infra) is pretty much prone to error. My favourite one was that a new column on a table was actually added to the wrong table in MSSQL and it went unnoticed for two years (four releases, including one LTS). * It's impossible to support more DBMS types through extensions or other third party systems. Because the maintainer needs to keep up with all patches we add to core and write their equivalents. * For lots of reasons, these schemas are diverging, there have been

work to just reduce this to a minimum. There was a RFC to introduce abstract schema and schema changes and it

accepted and I have been working to implement this: https://phabricator.wikimedia.org/T191231 This is not a small task, and like any big work, it's important to cut it to small pieces and gradually improve things. So my plan is first, I abstract the schema (tables.sql files), then slowly I abstract schema changes. For now, the plan is to make these .sql files automatically generated through maintenance scripts. So we will have a file called tables.json and when running something like: php maintenance/generateSchemaSql.php --json maintenance/tables.json

maintenance/tables-generated.sql --type=mysql It would produce tables-generated.sql file. The code that produces it is Doctrine DBAL and this is already installed as a dev dependency of core because you would need Doctrine if you want to make a schema change, if

maintain an instance, you should not need anything. Most of the work for automatically generating schema is already merged and the last part that wires it (and migrates two tables) is up for review: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/595240 My request is that I need to make lots of patches and since I'm doing

in my volunteer capacity, I need developers to review (and potentially

with the work if you're excited about this like me). Let me know if

willing to be added in future patches and the current patch also welcomes any feedback: https://gerrit.wikimedia.org/r/c/mediawiki/core/+/595240 I have added the documentation in https://www.mediawiki.org/wiki/Manual:Schema_changes for the plan and future changes. The ideal goal is that when you want to do a schema

you just change tables.json and create a json file that is snapshot of before and after table (remember, sqlite doesn't have alter table,

it has to know the whole table). Also, once we are in a good shape in migrating mediawiki core, we can start cleaning up extensions. Any feedback is also welcome. Best -- Amir (he/him) _______________________________________________ Wikitech-l mailing list Wikitech-l(a)lists.wikimedia.org https://lists.wikimedia.org/mailman/listinfo/wikitech-l