Running database migrations


LiteFS is a single-writer system so only the primary node can write to the database. When you deploy a new version of your application, you will likely need to run a database migration but the first node you deploy to won’t necessarily be the primary node.

There are two options for performing migrations when a node starts up: auto-promotion & write forwarding.


The recommended way to run migrations when you deploy is to have candidate nodes automatically promote themselves after they’ve connected to the cluster and are sync’d up. This can be done by setting the lease.promote field to true in the configuration and then running your migrations as one of your exec commands.

  candidate: ${FLY_REGION == PRIMARY_REGION}
  promote: true

  - cmd: "rails db:migrate"
    if-candidate: true

  - cmd: "rails server"

In this example litefs.yml config, we are marking nodes whose region (FLY_REGION) is equal to the primary region of the app (PRIMARY_REGION). On startup, they will request a primary handoff from the current primary node and become the primary themselves.

Once they are promoted, LiteFS will execute the rails db:migrate command on the node if it is a candidate. Then it will run rails server on all nodes.

Your migrations must be idempotent as they will be run on each candidate node. Your application must also handle running against the next version of the database schema as replica nodes can receive database updates before a new deployment of the app has completed.

Using the run command

You can also perform migrations separately from your deploy by logging into a candidate node and using litefs run with the -promote command:

litefs run -promote -- /path/to/

This will perform a primary handoff before running the given script.

Write forwarding

All LiteFS nodes also have the ability to halt writes on the primary node and temporarily perform writes on the local node. These changes are then forwarded back to the primary node and propagated out to the rest of the cluster.

To perform a remote write, you can use the litefs run command and the -with-halt-lock-on flag to specify the database you wish to lock.

litefs run -with-halt-lock-on /litefs/db -- bin/rails db:migrate

This command is meant for small, infrequent transactions as it is slower than writing to the primary node. Please note that the HALT lock is limited to 30 seconds before it is automatically released to ensure that replicas do not permanently lock the primary.

You can find a reference implementation of a client library in the litefs-go repository.