Project: Cortex (Observability and Analysis / Observability)
File: MIGRATIONS.md

A single logical migration is represented as two separate migration files, one to migrate "up" to the specified version from the previous version, and a second to migrate back "down" to the previous version. These migrations can be provided by any one of the supported . The ordering and direction of the migration files is determined by the filenames used for them. `migrate` expects the filenames of migrations to have the format: {version}_{title}.up.{extension} {version}_{title}.down.{extension} The `title` of each migration is unused, and is only for readability. Similarly, the `extension` of the migration files is not checked by the library, and should be an appropriate format for the database in use (`.sql` for SQL variants, for instance). Versions of migrations may be represented as any 64 bit unsigned integer. All migrations are applied upward in order of increasing version number, and downward by decreasing version number. Common versioning schemes include incrementing integers: 1initializeschema.down.sql 1initializeschema.up.sql 2addtable.down.sql 2addtable.up.sql ... Or timestamps at an appropriate resolution: 1500360784initializeschema.down.sql 1500360784initializeschema.up.sql 1500445949addtable.down.sql 1500445949addtable.up.sql ... But any scheme resulting in distinct, incrementing integers as versions is valid. It is suggested that the version number of corresponding `up` and `down` migration files be equivalent for clarity, but they are allowed to differ so long as the relative ordering of the migrations is preserved. The migration files are permitted to be "empty", in the event that a migration is a no-op or is irreversible. It is recommended to still include both migration files by making the whole migration file consist of a comment. If your database does not support comments, then deleting the migration file will also work. Note, an actual empty file (e.g. a 0 byte file) may cause issues with your database since migrate will attempt to run an empty query. In this case, deleting the migration file will also work. For the rational of this behavior see: The format of the migration files themselves varies between database systems. Different databases have different semantics around schema changes and when and how they are allowed to occur (for instance, ). As such, the `migrate` library has little to no checking around the format of migration sources. The migration files are generally processed directly by the drivers as raw operations. Best practice for writing schema migration is that all migrations should be reversible. It should in theory be possible for run migrations down and back up through any and all versions with the state being fully cleaned and recreated by doing so. By adhering to this recommended practice, development and deployment of new code is cleaner and easier (cleaning database state for a new feature should be as easy as migrating down to a prior version, and back up to the latest). As opposed to some other migration libraries, `migrate` represents up and down migrations as separate files. This prevents any non-standard file syntax from being introduced which may result in unintended behavior or errors, depending on what database is processing the file. While it is technically possible for an up or down migration to exist on its own without an equivalently versioned counterpart, it is strongly recommended to always include a down migration which cleans up the state of the corresponding up migration.