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110 lines
3.8 KiB
Markdown
110 lines
3.8 KiB
Markdown
## Database migrations
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We use [goose](https://github.com/pressly/goose) to handle database migrations. This allows us to execute
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both SQL deltas (e.g `ALTER TABLE ...`) as well as manipulate data in the database in Go using Go functions.
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To run a migration, the `goose` binary in this directory needs to be built:
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```
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$ go build ./cmd/goose
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```
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This binary allows Dendrite databases to be upgraded and downgraded. Sample usage for upgrading the roomserver database:
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```
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# for sqlite
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$ ./goose -dir roomserver/storage/sqlite3/deltas sqlite3 ./roomserver.db up
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# for postgres
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$ ./goose -dir roomserver/storage/postgres/deltas postgres "user=dendrite dbname=dendrite sslmode=disable" up
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```
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For a full list of options, including rollbacks, see https://github.com/pressly/goose or use `goose` with no args.
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### Rationale
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Dendrite creates tables on startup using `CREATE TABLE IF NOT EXISTS`, so you might think that we should also
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apply version upgrades on startup as well. This is convenient and doesn't involve an additional binary to run
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which complicates upgrades. However, combining the upgrade mechanism and the server binary makes it difficult
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to handle rollbacks. Firstly, how do you specify you wish to rollback? We would have to add additional flags
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to the main server binary to say "rollback to version X". Secondly, if you roll back the server binary from
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version 5 to version 4, the version 4 binary doesn't know how to rollback the database from version 5 to
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version 4! For these reasons, we prefer to have a separate "upgrade" binary which is run for database upgrades.
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Rather than roll-our-own migration tool, we decided to use [goose](https://github.com/pressly/goose) as it supports
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complex migrations in Go code in addition to just executing SQL deltas. Other alternatives like
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`github.com/golang-migrate/migrate` [do not support](https://github.com/golang-migrate/migrate/issues/15) these
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kinds of complex migrations.
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### Adding new deltas
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You can add `.sql` or `.go` files manually or you can use goose to create them for you.
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If you only want to add a SQL delta then run:
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```
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$ ./goose -dir serverkeyapi/storage/sqlite3/deltas sqlite3 ./foo.db create new_col sql
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2020/09/09 14:37:43 Created new file: serverkeyapi/storage/sqlite3/deltas/20200909143743_new_col.sql
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```
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In this case, the version number is `20200909143743`. The important thing is that it is always increasing.
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Then add up/downgrade SQL commands to the created file which looks like:
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```sql
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-- +goose Up
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-- +goose StatementBegin
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SELECT 'up SQL query';
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-- +goose StatementEnd
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-- +goose Down
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-- +goose StatementBegin
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SELECT 'down SQL query';
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-- +goose StatementEnd
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```
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You __must__ keep the `+goose` annotations. You'll need to repeat this process for Postgres.
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For complex Go migrations:
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```
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$ ./goose -dir serverkeyapi/storage/sqlite3/deltas sqlite3 ./foo.db create complex_update go
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2020/09/09 14:40:38 Created new file: serverkeyapi/storage/sqlite3/deltas/20200909144038_complex_update.go
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```
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Then modify the created `.go` file which looks like:
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```go
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package migrations
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import (
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"database/sql"
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"fmt"
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"github.com/pressly/goose"
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)
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func init() {
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goose.AddMigration(upComplexUpdate, downComplexUpdate)
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}
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func upComplexUpdate(tx *sql.Tx) error {
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// This code is executed when the migration is applied.
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return nil
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}
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func downComplexUpdate(tx *sql.Tx) error {
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// This code is executed when the migration is rolled back.
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return nil
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}
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```
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You __must__ import the package in `/cmd/goose/main.go` so `func init()` gets called.
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#### Database limitations
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- SQLite3 does NOT support `ALTER TABLE table_name DROP COLUMN` - you would have to rename the column or drop the table
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entirely and recreate it. ([example](https://github.com/matrix-org/dendrite/blob/master/userapi/storage/accounts/sqlite3/deltas/20200929203058_is_active.sql))
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More information: [sqlite.org](https://www.sqlite.org/lang_altertable.html)
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