cosmos-sdk/docs/migrations/pre-upgrade.md

Pre-Upgrade Handling

Cosmovisor supports custom pre-upgrade handling. Use pre-upgrade handling when you need to implement application config changes that are required in the newer version before you perform the upgrade.

Using Cosmovisor pre-upgrade handling is optional. If pre-upgrade handling is not implemented, the upgrade continues.

For example, make the required new-version changes to app.toml settings during the pre-upgrade handling. The pre-upgrade handling process means that the file does not have to be manually updated after the upgrade.

Before the application binary is upgraded, Cosmovisor calls a pre-upgrade command that can be implemented by the application.

The pre-upgrade command does not take in any command-line arguments and is expected to terminate with the following exit codes:

Exit status code	How it is handled in Cosmosvisor
0	Assumes pre-upgrade command executed successfully and continues the upgrade.
1	Default exit code when pre-upgrade command has not been implemented.
30	pre-upgrade command was executed but failed. This fails the entire upgrade.
31	pre-upgrade command was executed but failed. But the command is retried until exit code 1 or 30 are returned.

Sample

Here is a sample structure of the pre-upgrade command:

func preUpgradeCommand() *cobra.Command {
	cmd := &cobra.Command{
		Use:   "pre-upgrade",
		Short: "Pre-upgrade command",
        Long: "Pre-upgrade command to implement custom pre-upgrade handling",
		Run: func(cmd *cobra.Command, args []string) {

			err := HandlePreUpgrade()

			if err != nil {
				os.Exit(30)
			}

			os.Exit(0)

		},
	}

	return cmd
}

Ensure that the pre-upgrade command has been registered in the application:

rootCmd.AddCommand(
		// ..
		preUpgradeCommand(),
		// ..
	)