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Migrations at start-up: PostgreSQL and CockroachDB

Every file of the PostgreSQL and CockroachDB versions side by side, with the lines that differ marked.

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Configuration

sgl.yamldiffers
PostgreSQL
CockroachDB
1version: 1
1version: 1
2
2
3databases:
3databases:
4 - name: harbour
4 - name: harbour
5 dialect: postgres
5 dialect: cockroachdb
6 migrations: db/migrations
6 migrations: db/migrations
7 queries: db/queries
7 queries: db/queries
8 go:
8 go:
9 package: store
9 package: store
10 out: internal/store
10 out: internal/store
11 driver: database/sql
12 migrate:
13 dsn: ${env:DATABASE_URL}

Migrations

db/migrations/0001_boats.up.sqldiffers
PostgreSQL
CockroachDB
1CREATE TABLE boats (
1CREATE TABLE boats (
2 id BIGINT GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
2 id INT8 NOT NULL DEFAULT unique_rowid() PRIMARY KEY,
3 name TEXT NOT NULL
3 name STRING NOT NULL
4);
4);
db/migrations/0001_boats.down.sqlidentical
PostgreSQL
CockroachDB
1DROP TABLE boats;
1DROP TABLE boats;
db/migrations/0002_boat_home_port.up.sqldiffers
PostgreSQL
CockroachDB
1ALTER TABLE boats ADD COLUMN home_port TEXT NOT NULL DEFAULT 'Padstow';
1ALTER TABLE boats ADD COLUMN home_port STRING NOT NULL DEFAULT 'Padstow';
db/migrations/0002_boat_home_port.down.sqlidentical
PostgreSQL
CockroachDB
1ALTER TABLE boats DROP COLUMN home_port;
1ALTER TABLE boats DROP COLUMN home_port;

Queries

AddBoatidentical
PostgreSQL · db/queries/boats.sql
CockroachDB · db/queries/boats.sql
1-- sgl.query(AddBoat, exec)
1-- sgl.query(AddBoat, exec)
2INSERT INTO boats (name, home_port) VALUES ($1, $2);
2INSERT INTO boats (name, home_port) VALUES ($1, $2);
ListBoatsdiffers
PostgreSQL · db/queries/boats.sql
CockroachDB · db/queries/boats.sql
1-- sgl.query(ListBoats, many)
1-- sgl.query(ListBoats, many)
2SELECT id, name, home_port FROM boats ORDER BY id;
2SELECT name, home_port FROM boats ORDER BY name;

What sgl generates

AddBoatidentical
PostgreSQL
CockroachDB
1func (queries *Queries) AddBoat(ctx context.Context, params AddBoatParams) error
1func (queries *Queries) AddBoat(ctx context.Context, params AddBoatParams) error
2
2
3type AddBoatParams struct {
3type AddBoatParams struct {
4 Name string
4 Name string
5 HomePort string
5 HomePort string
6}
6}
ListBoatsdiffers
PostgreSQL
CockroachDB
1func (queries *Queries) ListBoats(ctx context.Context) ([]ListBoatsRow, error)
1func (queries *Queries) ListBoats(ctx context.Context) ([]ListBoatsRow, error)
2
2
3type ListBoatsRow struct {
3type ListBoatsRow struct {
4 ID int64 `json:"id"`
5 Name string `json:"name"`
4 Name string `json:"name"`
6 HomePort string `json:"home_port"`
5 HomePort string `json:"home_port"`
7}
6}

The program

example.godiffers
PostgreSQL
CockroachDB
1package main
1package main
2
2
3import (
3import (
4 "context"
4 "context"
5 "database/sql"
5 "database/sql"
6 "errors"
6 "fmt"
7 "fmt"
7 "io"
8 "io"
8 "io/fs"
9 "io/fs"
10 "sync"
9
11
10 "seagull.sh/seagull/examples/postgres/027_migrations_at_startup/internal/store"
12 "seagull.sh/seagull/examples/cockroachdb/027_migrations_at_startup/internal/store"
11 "seagull.sh/seagull/sdk/migrate"
13 "seagull.sh/seagull/sdk/migrate"
12 "seagull.sh/seagull/sdk/migrate/migrate_backend_postgres"
14 "seagull.sh/seagull/sdk/migrate/migrate_backend_cockroachdb"
13)
15)
14
16
15// run starts the program twice against the same database. The first start applies both
17// run deploys the program against an empty database the way an orchestrator does,
16// migrations before the program uses the schema; the second finds nothing to do.
18// starting several replicas at the same moment, each of which migrates before it works.
19// Then one replica restarts and finds nothing to migrate.
17//
20//
18// Takes database (*sql.DB) which is the database to migrate, empty or not.
21// Takes database (*sql.DB) which is the database to migrate, empty or not.
19// Takes migrations (fs.FS) which holds db/migrations, the embed.FS in main.go.
22// Takes migrations (fs.FS) which holds db/migrations, the embed.FS in main.go.
20// Takes output (io.Writer) which receives what each start did.
23// Takes output (io.Writer) which receives what each start did.
21//
24//
22// Returns error when migrating or a query fails.
25// Returns error when migrating or a query fails.
23func run(ctx context.Context, database *sql.DB, migrations fs.FS, output io.Writer) error {
26func run(ctx context.Context, database *sql.DB, migrations fs.FS, output io.Writer) error {
24 for _, boat := range []string{"Kittiwake", "Puffin"} {
27 replicas := []string{"Kittiwake", "Puffin", "Gannet"}
25 if err := start(ctx, database, migrations, boat, output); err != nil {
28 applied, err := deploy(ctx, database, migrations, replicas)
26 return err
29 if err != nil {
27 }
30 return err
28 }
31 }
32 fmt.Fprintf(output, "%d replicas started at once and applied %d migrations between them\n", len(replicas), applied)
33
34 restarted, err := start(ctx, database, migrations, "Cormorant")
35 if err != nil {
36 return err
37 }
38 fmt.Fprintf(output, "a replica restarted and applied %d migrations\n", restarted)
39
40 if err := printBoats(ctx, database, output); err != nil {
41 return err
42 }
43 if err := refuseAdvisoryLock(database, migrations, output); err != nil {
44 return err
45 }
29 return printHistory(ctx, database, migrations, output)
46 return printHistory(ctx, database, migrations, output)
30}
47}
31
48
49// deploy starts one replica per boat at the same moment and waits for them all.
50//
51// The replicas here share this process's connection pool, where real replicas are
52// separate processes with pools of their own. The lock lives in the database, so it
53// serialises them the same way.
54//
55// Takes database (*sql.DB) which is the database.
56// Takes migrations (fs.FS) which holds the migration files.
57// Takes boats ([]string) which names the boat each replica adds.
58//
59// Returns int which is how many migrations the replicas applied between them.
60// Returns error which joins every replica's failure.
61func deploy(ctx context.Context, database *sql.DB, migrations fs.FS, boats []string) (int, error) {
62 applied := make([]int, len(boats))
63 failures := make([]error, len(boats))
64 var replicas sync.WaitGroup
65 for index, boat := range boats {
66 replicas.Go(func() {
67 applied[index], failures[index] = start(ctx, database, migrations, boat)
68 })
69 }
70 replicas.Wait()
71
72 total := 0
73 for _, count := range applied {
74 total += count
75 }
76 return total, errors.Join(failures...)
77}
78
32// start is what the program does each time it starts: migrate first, then work. Here the
79// start is what the program does each time it starts: migrate first, then work. Here the
33// work is adding a boat and listing the harbour.
80// work is adding a boat.
34//
81//
35// Takes database (*sql.DB) which is the database.
82// Takes database (*sql.DB) which is the database.
36// Takes migrations (fs.FS) which holds the migration files.
83// Takes migrations (fs.FS) which holds the migration files.
37// Takes boat (string) which names the boat to add.
84// Takes boat (string) which names the boat to add.
38// Takes output (io.Writer) which receives what happened.
39//
85//
40// Returns error when migrating or a query fails.
86// Returns int which is how many migrations this start applied.
41func start(ctx context.Context, database *sql.DB, migrations fs.FS, boat string, output io.Writer) error {
87// Returns error when migrating or the query fails.
42 // migrate_backend_postgres.New() is the PostgreSQL backend: it knows how to create the
88func start(ctx context.Context, database *sql.DB, migrations fs.FS, boat string) (int, error) {
43 // history table and run each migration in a transaction, as PostgreSQL's DDL is
89 // migrate_backend_cockroachdb.New() is the PostgreSQL backend with a lock table in place
44 // transactional. It also holds an advisory lock while it works, so when several copies
90 // of an advisory lock, which CockroachDB does not have. Up creates sgl_migration_lock if
45 // of a service start at once, one migrates and the rest wait, then find nothing to do.
91 // it is missing and holds its one row with SELECT ... FOR UPDATE in an open transaction,
46 // NewMigrator reads the numbered *.up.sql and *.down.sql files from the directory inside
92 // so a second replica waits on that row until the first commits.
47 // migrations.
93 //
48 migrator, err := migrate.NewMigrator(database, migrate_backend_postgres.New(), migrations, "db/migrations")
94 // By default CockroachDB keeps a SERIALIZABLE transaction's FOR UPDATE locks in memory
95 // and drops them when a range splits or moves, which a schema change such as 0002's ADD
96 // COLUMN ... DEFAULT can cause. The backend turns on
97 // enable_durable_locking_for_serializable for its lock transaction, so the lock holds
98 // until the migrations are done; that needs CockroachDB 23.2 or later.
99 //
100 // The migrations run on a second connection, one statement at a time and not in a
101 // transaction: CockroachDB cannot roll a schema change back as part of a larger
102 // transaction, so the backend records each finished statement instead. A file that fails
103 // partway is marked dirty and resumes from the statement that failed once the cause is
104 // fixed. Keeping one schema change per file keeps each step its own record.
105 migrator, err := migrate.NewMigrator(database, migrate_backend_cockroachdb.New(), migrations, "db/migrations")
49 if err != nil {
106 if err != nil {
50 return err
107 return 0, err
51 }
108 }
52
109
53 // Up applies every migration not yet in the history, in version order, and returns how
110 // Up applies every migration not yet in the history and returns how many it applied. It
54 // many it applied. On a database that is up to date it applies none.
111 // takes the lock only when something is pending, and reads the history again once it
112 // holds the lock, so a replica that waited finds the work done.
55 applied, err := migrator.Up(ctx)
113 applied, err := migrator.Up(ctx)
56 if err != nil {
114 if err != nil {
57 return fmt.Errorf("migrating: %w", err)
115 return 0, fmt.Errorf("migrating: %w", err)
58 }
116 }
59 fmt.Fprintf(output, "start-up: applied %d migrations\n", applied)
60
117
61 queries := store.New(database)
118 queries := store.New(database)
62 if err := queries.AddBoat(ctx, store.AddBoatParams{Name: boat, HomePort: "Padstow"}); err != nil {
119 if err := queries.AddBoat(ctx, store.AddBoatParams{Name: boat, HomePort: "Padstow"}); err != nil {
63 return fmt.Errorf("adding %s: %w", boat, err)
120 return 0, fmt.Errorf("adding %s: %w", boat, err)
64 }
121 }
65 boats, err := queries.ListBoats(ctx)
122 return applied, nil
123}
124
125// printBoats lists the harbour, which every replica wrote to once the schema existed.
126//
127// Takes database (*sql.DB) which is the migrated database.
128// Takes output (io.Writer) which receives the list.
129//
130// Returns error when the query fails.
131func printBoats(ctx context.Context, database *sql.DB, output io.Writer) error {
132 boats, err := store.New(database).ListBoats(ctx)
66 if err != nil {
133 if err != nil {
67 return fmt.Errorf("listing boats: %w", err)
134 return fmt.Errorf("listing boats: %w", err)
68 }
135 }
69 for _, row := range boats {
136 for _, row := range boats {
70 fmt.Fprintf(output, " %d %-9s of %s\n", row.ID, row.Name, row.HomePort)
137 fmt.Fprintf(output, " %-9s of %s\n", row.Name, row.HomePort)
71 }
138 }
72 return nil
139 return nil
73}
140}
74
141
75// printHistory prints the migration history the migrator keeps in the database.
142// refuseAdvisoryLock shows that the CockroachDB backend rejects an advisory lock, as a
143// configuration copied from PostgreSQL might ask for, before it touches the database.
76//
144//
145// Takes database (*sql.DB) which is the database.
146// Takes migrations (fs.FS) which holds the migration files.
147// Takes output (io.Writer) which receives the refusal.
148//
149// Returns error when the backend accepts the advisory lock.
150func refuseAdvisoryLock(database *sql.DB, migrations fs.FS, output io.Writer) error {
151 backend := migrate_backend_cockroachdb.New(migrate.WithLock(migrate.LockAdvisory))
152 _, err := migrate.NewMigrator(database, backend, migrations, "db/migrations")
153 if err == nil {
154 return errors.New("an advisory lock on CockroachDB gave no error, want migrate.ErrUnsupportedLock")
155 }
156 if !errors.Is(err, migrate.ErrUnsupportedLock) {
157 return fmt.Errorf("an advisory lock on CockroachDB gave %w, want migrate.ErrUnsupportedLock", err)
158 }
159 fmt.Fprintf(output, "advisory lock refused: %v\n", err)
160 return nil
161}
162
163// printHistory prints the migration history the migrator keeps in the database, and the
164// table its lock uses.
165//
77// Takes database (*sql.DB) which is the migrated database.
166// Takes database (*sql.DB) which is the migrated database.
78// Takes migrations (fs.FS) which holds the migration files.
167// Takes migrations (fs.FS) which holds the migration files.
79// Takes output (io.Writer) which receives the history.
168// Takes output (io.Writer) which receives the history.
80//
169//
81// Returns error when the history cannot be read.
170// Returns error when the history cannot be read.
82func printHistory(ctx context.Context, database *sql.DB, migrations fs.FS, output io.Writer) error {
171func printHistory(ctx context.Context, database *sql.DB, migrations fs.FS, output io.Writer) error {
83 backend := migrate_backend_postgres.New()
172 backend := migrate_backend_cockroachdb.New()
84 migrator, err := migrate.NewMigrator(database, backend, migrations, "db/migrations")
173 migrator, err := migrate.NewMigrator(database, backend, migrations, "db/migrations")
85 if err != nil {
174 if err != nil {
86 return err
175 return err
87 }
176 }
88 statuses, err := migrator.Status(ctx)
177 statuses, err := migrator.Status(ctx)
89 if err != nil {
178 if err != nil {
90 return fmt.Errorf("reading migration status: %w", err)
179 return fmt.Errorf("reading migration status: %w", err)
91 }
180 }
92 fmt.Fprintf(output, "history in %s:\n", backend.Names().History)
181 names := backend.Names()
182 fmt.Fprintf(output, "history in %s, locked through %s:\n", names.History, names.MigrationLock)
93 for _, status := range statuses {
183 for _, status := range statuses {
94 fmt.Fprintf(output, " %04d %-15s applied=%t\n", status.Version, status.Name, status.Applied)
184 fmt.Fprintf(output, " %04d %-15s applied=%t\n", status.Version, status.Name, status.Applied)
95 }
185 }
96 return nil
186 return nil
97}
187}