cosmos-sdk/docs/core/upgrade.md

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# In-Place Store Migrations

::: warning
Read and understand all of the in-place store migration documentation before you run a migration on a live chain.
:::

Upgrade your app modules smoothly with custom in-place migration logic. {synopsis}

The Cosmos SDK uses two methods to perform upgrades. 

- Exporting the entire application state to a JSON file using the `export` CLI command, making changes, and then starting a new binary with the changed JSON file as the genesis file. See the [Chain Upgrade Guide](../migrations/chain-upgrade-guide-040.md#upgrade-procedure). 

- Version v0.43 and later can perform upgrades in place to significantly decrease the upgrade time for chains with a larger state. Use the [Migration Upgrade Guide](../building-modules/upgrade.md) guide to set up your application modules to take advantage of in-place upgrades.
  

This document provides steps to use the In-Place Store Migrations upgrade method.

## Tracking Module Versions

Each module gets assigned a consensus version by the module developer. The consensus version serves as the breaking change version of the module. The SDK keeps track of all module consensus versions in the x/upgrade `VersionMap` store. During an upgrade, the difference between the old `VersionMap` stored in state and the new `VersionMap` is calculated by the Cosmos SDK. For each identified difference, the module-specific migrations are run and the respective consensus version of each upgraded module is incremented.


## Genesis State

When starting a new chain, the consensus version of each module must be saved to state during the application's genesis. To save the consensus version, add the following line to the `InitChainer` method in `app.go`:

```diff
func (app *MyApp) InitChainer(ctx sdk.Context, req abci.RequestInitChain) abci.ResponseInitChain {
  ...
+ app.UpgradeKeeper.SetModuleVersionMap(ctx, app.mm.GetVersionMap())
  ...
}
```

This information is used by the Cosmos SDK to detect when modules with newer versions are introduced to the app. 

### Consensus Version
The consensus version is defined on each app module by the module developer and serves as the breaking change version of the module. The consensus version informs the SDK on which modules need to be upgraded. For example, if the bank module was version 2 and an upgrade introduces bank module 3, the SDK upgrades the bank module and runs the "version 2 to 3" migration script.

### Version Map
The version map is a mapping of module names to consensus versions. The map is persisted to x/upgrade's state for use during in-place migrations. When migrations finish, the updated version map is persisted to state. 

## Upgrade Handlers

Upgrades use an `UpgradeHandler` to facilitate migrations. The `UpgradeHandler` functions implemented by the app developer must conform to the following function signature. These functions retrieve the `VersionMap` from x/upgrade's state and return the new `VersionMap` to be stored in x/upgrade after the upgrade. The diff between the two `VersionMap`s determines which modules need upgrading.

```golang
type UpgradeHandler func(ctx sdk.Context, plan Plan, fromVM VersionMap) (VersionMap, error)
```

Inside these functions, you must perform any upgrade logic to include in the provided `plan`. All upgrade handler functions must end with the following line of code:

```golang
  return app.mm.RunMigrations(ctx, cfg, fromVM)
```

## Running Migrations

Migrations are run inside of an `UpgradeHandler` using  `app.mm.RunMigrations(ctx, cfg, vm)`. The `UpgradeHandler` functions describe the functionality to occur during an upgrade. The `RunMigration` function loops through the `VersionMap` argument and runs the migration scripts for all versions that are less than the versions of the new binary app module. After the migrations are finished, a new `VersionMap` is returned to persist the upgraded module versions to state.

```golang
cfg := module.NewConfigurator(...)
app.UpgradeKeeper.SetUpgradeHandler("my-plan", func(ctx sdk.Context, plan upgradetypes.Plan, vm module.VersionMap) (module.VersionMap, error) {

    // ...
    // do upgrade logic
    // ...

    // RunMigrations returns the VersionMap
    // with the updated module ConsensusVersions
    return app.mm.RunMigrations(ctx, vm)
})
```

To learn more about configuring migration scripts for your modules, see the [Migration Upgrade Guide](../building-modules/upgrade.md).

## Adding New Modules During Upgrades

You can introduce entirely new modules to the application during an upgrade. New modules are recognized because they have not yet been registered in `x/upgrade`'s `VersionMap` store. In this case, `RunMigrations` calls the `InitGenesis` function from the corresponding module to set up its initial state.

## Overwriting Genesis Functions

The Cosmos SDK offers modules that the application developer can import in their app. These modules often have an `InitGenesis` function already defined.

You can write your own `InitGenesis` function for an imported module. To do this, manually trigger your custom genesis function in the upgrade handler.

::: warning
You MUST manually set the consensus version in the version map passed to the `UpgradeHandler` function. Without this, the SDK will run the Module's existing `InitGenesis` code even if you triggered your custom function in the `UpgradeHandler`. 
:::

```go
import foo "github.com/my/module/foo"

app.UpgradeKeeper.SetUpgradeHandler("my-plan", func(ctx sdk.Context, plan upgradetypes.Plan, vm module.VersionMap)  (module.VersionMap, error) {
  
    // Register the consensus version in the version map
    // to avoid the SDK from triggering the default 
    // InitGenesis function. 
    vm["foo"] = foo.AppModule{}.ConsensusVersion()

    // Run custom InitGenesis for foo
    app.mm["foo"].InitGenesis(ctx, app.appCodec, myCustomGenesisState)

    return app.mm.RunMigrations(ctx, cfg, vm)
})
```

If you do not have a custom genesis function and want to skip the module's default genesis function, you can simply register the module with the version map in the `UpgradeHandler` as shown in the example:

```go
import foo "github.com/my/module/foo"

app.UpgradeKeeper.SetUpgradeHandler("my-plan", func(ctx sdk.Context, plan upgradetypes.Plan, vm module.VersionMap)  (module.VersionMap, error) {
  
    // Set foo's version to the latest ConsensusVersion in the VersionMap.
    // This will skip running InitGenesis on Foo
    vm["foo"] = foo.AppModule{}.ConsensusVersion()

    return app.mm.RunMigrations(ctx, cfg, vm)
})
```

## Syncing a Full Node to an Upgraded Blockchain

You can sync a full node to an existing blockchain which has been upgraded using Cosmovisor 

In order to successfully sync, you must start with the initial binary that the blockchain started with at genesis. Cosmovisor will handle downloading and switching to the binaries associated with each sequential upgrade. 

To learn more about Cosmovisor, see the [Cosmovisor Quick Start](../run-node/cosmovisor.md).