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

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# 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:
```go
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:
```go
rootCmd.AddCommand(
// ..
preUpgradeCommand(),
// ..
)
```