Several databases

For read replicas of a single database, see read replicas. This page is about databases that hold different data.

One context per database

Declare a context for each. Each has its own models, migrations, and cmd/migrate.

type Invoice struct {
	ID int64
}

type MainDB struct {
	ant.Context
	Users ant.Table[User]
}

type BillingDB struct {
	ant.Context
	Invoices ant.Table[Invoice]
}

A relation can't cross databases. Keep the key, and load the other side in a second query. Mark such a relation .NoForeignKey().

Routing by tenant

To give each large tenant a database of its own, keep a map of open contexts and pick one per request. Open each one once: an ant context owns a connection pool.

type Router struct {
	mu  sync.RWMutex
	dbs map[string]*AppDB
}

func (r *Router) For(ctx context.Context, tenant string) (*AppDB, error) {
	r.mu.RLock()
	db := r.dbs[tenant]
	r.mu.RUnlock()

	if db != nil {
		return db, nil
	}

	r.mu.Lock()
	defer r.mu.Unlock()

	if db := r.dbs[tenant]; db != nil {
		return db, nil
	}

	db, err := ant.Open[AppDB](ctx, postgres.Dialect, dsnFor(tenant))
	if err != nil {
		return nil, err
	}

	r.dbs[tenant] = db

	return db, nil
}

func dsnFor(tenant string) string {
	return "postgres://localhost/" + tenant
}

Apply migrations to each with migrate.Upfunc Up[DB any](ctx context.Context, db *DB, config ...Config) errorUp applies pending migrations, then repeatable ones that are new or changed., which is safe to call from several processes at once.

No cross-database transactions

A transaction can't span databases, and ant has no two-phase commit. Use an outbox to carry a change from one to the other.