How Hasura migrations work
This is an explanation on how the Hasura migration system works. To understand how to use the system, refer to Migrations & 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, 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.
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 migration versions can be automatically generated by the Hasura
console or can be written by hand. They are stored as SQL files in a
For more details on the format of these files, refer to Migration file format reference.
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 using the catalog state API provided
by 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
apply command is used, each migration that are to be applied are
send via a
run_sql API request.
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
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 (CI/CD).