How Hasura migrations work (config v2)

Introduction

This is an explanation on how the Hasura migration system works. To understand how to use the system, refer to Migrations & Metadata.

Metadata

Let's first talk about metadata. Whenever you do certain actions on the console or via the API, Hasura records it in the metadata catalogue which is a schema called hdb_catalog in your Postgres database. For example, if you track a table, a new entry is created in the hdb_catalog.hdb_table table in Postgres. Similarly, there are more tables in this schema to track relationships, event triggers, custom functions and remote schemas.

All information in this schema can be exported as files. Export options are available on the console, CLI and via the API. These files when imported to an existing or new Hasura instance, will clear out the hdb_catalog schema on that instance and populates it again with the imported data. One thing to note is that all the Postgres resources the metadata refers to should already exist when the import happens, otherwise Hasura will throw an error.

Migrations

While metadata can be exported as files as a representation of the state of Hasura, you might want more granular step-by-step checkpoints on the evolution of the state. You might also want to track the Postgres schema changes through Hasura's migration system.

Migrations are stored and applied as steps (or versions). A migration step (or version) contains changes to the Postgres schema. The migration version can also store the up migration (creating resources) and the down migration (deleting resources). For example, migration version 1 can include the SQL statements required to create a table called profile as the up migration and SQL statements to drop this table as the down migration.

The migration versions can be automatically generated by the Hasura console or can be written by hand. They are stored as SQL files in a directory called migrations.

For more details on the format of these files, refer to Migration file format reference (config v2).

When someone executes migrate apply using the Hasura CLI, the CLI will first read the migration files present in the designated directory. The CLI would then contact the Hasura Server and get the status of all migrations applied to the server by reading the hdb_catalog.schema_migrations table. Each row in this table denotes a migration version that is already applied on the server.

By comparing these two sets of versions, the CLI derives which versions are already applied and which are not. The CLI would then go ahead and apply the migrations on the server. This is done by executing the actions against the database through the Hasura metadata APIs. Whenever the apply command is used, all migrations that are to be applied are executed in a Postgres transaction (through a bulk API call). The advantage of doing this is that if there are any errors, all actions are rolled back and the user can properly debug the error without worrying about partial changes.

The default action of the migrate apply command is to execute all the up migrations. In order to roll back changes, you would need to execute down migrations using the --down flag on the CLI.

This guide provides an overall idea of how the system works. For more details on how to actually use the system, refer to Migrations & Metadata (config v2).