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libschema - database schema migration for libraries

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Install:

go get github.com/muir/libschema

Libraries

Libschema provides a way for Go libraries to manage their own database migrations.

Tying migrations to libraries supports two things: the first is source code locality: the migrations can be next to the code that uses the tables that the migrations address.

The second is support for migrations in third-party libraries. This is a relatively unexplored and unsolved problem: how can an open source (or proprietary) library specify and maintain a database schema? Libschema hopes to start solving this problem.

Register and execute

Migrations are registered:

schema := libschema.New(ctx, libschema.Options{})

sqlDB, err := sql.Open("postgres", "....")

database, err := lspostgres.New(logger, "main-db", schema, sqlDB) 
// or
database, singlestoreHelper, err := lssinglestore.New(logger, "main-db", schema, sqlDB)
// or
database, mysqlHelper, err := lssinglestore.New(logger, "main-db", schema, sqlDB)

database.Migrations("MyLibrary",
	lspostgres.Script("createUserTable", `
		CREATE TABLE users (
			name	text,
			id	bigint
		)`
	}),
	lspostgres.Script("addLastLogin", `
		ALTER TABLE users
			ADD COLUMN last_login timestamp
		`
	}),
)

Migrations are then run run later in the order that they were registered.

err := schema.Migrate(context)

Computed Migrations

Migrations may be SQL strings or migrations can be done in Go:

database.Migrations("MyLibrary", 
	lspostgres.Computed("importUsers", func(_ context.Context, _ Migration, tx *sql.Tx) error {
		// code to import users here
	}),
)

Asynchronous migrations

The normal mode for migrations is to run the migrations synchronously when schema.Migrate() is called. Asynchronous migrations are started when schema.Migrate() is called but they're run in the background in a go-routine. If there are later migrations, after the asynchronous migration, they'll force the asynchronous migration to be synchronous unless they're also asynchronous.

Version blocking

Migrations can be tied to specific code versions so that they are not run until conditions are met. This is done with SkipRemainingIf. This be used to backfill data.

database.Migrations("MyLibrary",
	...
	lspostgres.Script("addColumn", `
			ALTER TABLE users
				ADD COLUMN rating`,
	libschema.SkipThisAndFollowingIf(func() bool {
		return semver.Compare(version(), "3.11.3") < 1
	})),
	lspostgres.Script("fillInRatings", `
			UPDATE	users
			SET	rating = ...
			WHERE	rating IS NULL;

			ALTER TABLE users
				MODIFY COLUMN rating SET NOT NULL;`,
	libschema.Asychronous),
)

Cross-library dependencies

Although it is best if the schema from one library is independent of the schema for another, sometimes that's not possible, especially if you want to enforce foriegn key constraints.

Use After() to specify a cross-library dependency.

database.Migrations("users",
	...
	lspostgres.Script("addOrg", `
			ALTER TABLE users
				ADD COLUMN org TEXT,
				ADD ADD CONSTRAINT orgfk FOREIGN KEY (org)
					REFERENCES org (name) `, 
		libschema.After("orgs", "createOrgTable")),
)

database.Migrations("orgs",
	...
	lspostgres.Script("createOrgTable", `
		...
	`),
)

Transactions

For databases that support transactions on metadata, all migrations will be wrapped with a BEGIN and COMMIT. For databases that do not support transactions on metadata, migrations will be split into individual commands and run one at a time. If only some of the commands succeed, the migration will be marked as partially complete. If the migration is revised, then the later parts can be re-tried as long as the earlier parts are not modified. This does not apply to Compute()ed migrations.

Command line

The OverrideOptions can be added as command line flags that change the behavior of calling schema.Migrate()

--migrate-only			Call os.Exit() after completing migrations
--migrate-database		Migrate only one logical database (must match NewDatabase)
--migrate-dsn			Override *sql.DB 
--no-migrate			Skip all migrations
--error-if-migrate-needed	Return error if there are outstanding synchronous migrations
--migrate-all-synchronously	Treat asychronous migrations as synchronous

Ordering and pull requests

Migrations are run the order that they're defined. If the set of migrations is updated so that there are new migrations that are earlier in the table than migrations that have already run, this is not considered an error and the new migrations will be run anyway. This allows multiple branches of code with migrations to be merged into a combined branch without hassle.

Migrations can have explicit dependencies and these dependencies can cross between libraries so that one library's migrations can depend on anothers.

Code structure

Registering the migrations before executing them is easier if using library singletons. Library singletons can be supported by using nserve or fx. With nserve, migrations can be given their own hook.

Driver inclusion and database support

Like database/sql, libschema requires database-specific drivers:

libschema currently supports: PostgreSQL, SingleStore, MySQL. It is relatively easy to add additional databases.

Forward only

Libschema does not support reverse migrations. If you need to fix a migration, fix forward. The history behind this is that reverse migrations are rarely the right answer for production systems and the extra work for maintaining reverse migrations is does not have enough of a payoff during development to be worth the effort.

One way to get the benefits of reverse migrations for development is to put enough enough reverse migrations to reverse to the last production schema at the end of the migration list but protected by a gateway:

libschema.SkipThisAndRemainingIf(func() bool {
	return os.Getenv("LIBMIGRATE_REVERSE_TO_PROD") != "true"
}),

This set of reverse migrations would always be small since it would just be enough to take you back to the current production release.

Patterns for applying migrations

When using a migration tool like libschema there are several reasonable patterns one can follow to apply migrations to produciton code.

Down-Up deploys

The simplist pattern is to deploy migrations synchronously when rolling out updates. If you take your service down to do deploys then your migrations do not have to be backwards compatible. This has the huge upside of allowing your schema to eveolve easily and avoid the build up of technical debt. For example, if you have a column whose name is sub-optimal, you can simply rename it and change the code that uses it at the same time.

To minimimize downtime so that the downtime doesn't matter in practice, run expensive migrations asynchronously. Asychronous migrations are harder to define because they should be broken up into a whole bunch of smallish transactions. The RepeatUntilNoOp() decorator may be useful.

Green-Blue deploys

When you decide to run without downtime, one consequence is that all migrations must be backwards compatible with the deployed code.

DDL operations that are backwards compatible include:

From a coding point-of-view, the simplest way to manage developing with these restrictions is to separate the migration into a separate pull request from any other code changes. Tests must still pass in the pull request that just has the migration. Local and CI testing should apply the migration and validate that the the existing code isn't broken by the change in database schema.

Only after the migration has been deployed can code that uses the migration be deployed. When using git, this can be done by having layered side branches:

graph LR;
 mob(migration-only branch)
 code(code branch)
 cleanup(cleanup migration branch)
 main --> mob --> code --> cleanup;

Kubernetes and slow migrations

One issue with using libschema to deploy changes is that servers can take a long time to come up if there are expensive migrations that need to be deployed first. A solution for this is to use OverrideOptions to separate the migrations into a separate step and run them in an init container.

To do this use the MigrateOnly / --migrate-only option on your main program when running it in the init container.

Then use the ErrorIfMigrateNeeded / --error-if-migrate-needed option on your main program when it starts up for normal use.

Code Stability

Libschema is still subject to changes. Anything that is not backwards compatible will be clearly documented and will fail in a way that does not cause hidden problems. For example, switching from using "flag" to using OverrideOptions will trigger an obvious breakage if you try to use a flag that no longer works.

Anticpated changes for the future: