alaa_wmde added a comment.
Reading the documentation of the constants in core Defines.php they don't
match the previously explained process to me.
/**@{
* Schema change migration flags.
*
* Used as values of a feature flag for an orderly transition from an old
* schema to a new schema. The numeric values of these constants are
compatible with the
* SCHEMA_COMPAT_XXX bitfield semantics. High bits are used to ensure that
the numeric
* ordering follows the order in which the migration stages should be used.
*
* - MIGRATION_OLD: Only read and write the old schema. The new schema need
not
* even exist. This is used from when the patch is merged until the schema
* change is actually applied to the database.
* - MIGRATION_WRITE_BOTH: Write both the old and new schema. Read the new
* schema preferentially, falling back to the old. This is used while the
* change is being tested, allowing easy roll-back to the old schema.
* - MIGRATION_WRITE_NEW: Write only the new schema. Read the new schema
* preferentially, falling back to the old. This is used while running the
* maintenance script to migrate existing entries in the old schema to the
* new schema.
* - MIGRATION_NEW: Only read and write the new schema. The old schema (and
the
* feature flag) may now be removed.
*/
define( 'MIGRATION_OLD', 0x00000000 | SCHEMA_COMPAT_OLD );
define( 'MIGRATION_WRITE_BOTH', 0x10000000 | SCHEMA_COMPAT_READ_BOTH |
SCHEMA_COMPAT_WRITE_BOTH );
define( 'MIGRATION_WRITE_NEW', 0x20000000 | SCHEMA_COMPAT_READ_BOTH |
SCHEMA_COMPAT_WRITE_NEW );
define( 'MIGRATION_NEW', 0x30000000 | SCHEMA_COMPAT_NEW );
As far as I have learned, we follow this process:
write/read old -> write both, read old -> write both, read new -> write/read
new
So either we have different migraiton processes, and we are using a different
one, that is:
write/read old => write both, read old and fallback to new, write both, read
new and fallback to old, write/read new.
Or I learned completely wrong.
So currently the implementation we have does the following:
on MIGRATION_OLD' => write/read old
on MIGRATION_WRITE_BOTH => write both, read old
on MIGRATION_WRITE_NEW' => write both, read new
on MIGRATION_NEW => write/read new
TASK DETAIL
https://phabricator.wikimedia.org/T225053
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