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Building an application is complicated and requires a lot of testing. The Cosmos SDK provides a way to test your application in a real-world environment: a testnet. We allow developers to take the state from their mainnet and run tests against the state. This is useful for testing upgrade migrations, or for testing the application in a real-world environment.

​
Testnet Setup

We will be breaking down the steps to create a testnet from mainnet state. 
/ InitSimAppForTestnet is broken down into two sections:
  / Required Changes: Changes that, if not made, will cause the testnet to halt or panic
  / Optional Changes: Changes to customize the testnet to one's liking (lower vote times, fund accounts, etc)

func InitSimAppForTestnet(app *SimApp, newValAddr bytes.HexBytes, newValPubKey crypto.PubKey, newOperatorAddress, upgradeToTrigger string) *SimApp {
  ...
}

​
Required Changes

​
Staking

When creating a testnet the important part is migrate the validator set from many validators to one or a few. This allows developers to spin up the chain without needing to replace validator keys. 
ctx := app.BaseApp.NewUncachedContext(true, tmproto.Header{
})
    pubkey := &ed25519.PubKey{
    Key: newValPubKey.Bytes()
}

pubkeyAny, err := types.NewAnyWithValue(pubkey)
    if err != nil {
    tmos.Exit(err.Error())
}

	/ STAKING
	/

	/ Create Validator struct for our new validator.
	_, bz, err := bech32.DecodeAndConvert(newOperatorAddress)
    if err != nil {
    tmos.Exit(err.Error())
}

bech32Addr, err := bech32.ConvertAndEncode("simvaloper", bz)
    if err != nil {
    tmos.Exit(err.Error())
}
    newVal := stakingtypes.Validator{
    OperatorAddress: bech32Addr,
    ConsensusPubkey: pubkeyAny,
    Jailed:          false,
    Status:          stakingtypes.Bonded,
    Tokens:          sdk.NewInt(900000000000000),
    DelegatorShares: sdk.MustNewDecFromStr("10000000"),
    Description: stakingtypes.Description{
    Moniker: "Testnet Validator",
},
    Commission: stakingtypes.Commission{
    CommissionRates: stakingtypes.CommissionRates{
    Rate:          sdk.MustNewDecFromStr("0.05"),
    MaxRate:       sdk.MustNewDecFromStr("0.1"),
    MaxChangeRate: sdk.MustNewDecFromStr("0.05"),
},
},
    MinSelfDelegation: sdk.OneInt(),
}

	/ Remove all validators from power store
    stakingKey := app.GetKey(stakingtypes.ModuleName)
    stakingStore := ctx.KVStore(stakingKey)
    iterator := app.StakingKeeper.ValidatorsPowerStoreIterator(ctx)
    for ; iterator.Valid(); iterator.Next() {
    stakingStore.Delete(iterator.Key())
}

iterator.Close()

	/ Remove all validators from last validators store
	iterator = app.StakingKeeper.LastValidatorsIterator(ctx)
    for ; iterator.Valid(); iterator.Next() {
    app.StakingKeeper.LastValidatorPower.Delete(iterator.Key())
}

iterator.Close()

	/ Add our validator to power and last validators store
	app.StakingKeeper.SetValidator(ctx, newVal)

err = app.StakingKeeper.SetValidatorByConsAddr(ctx, newVal)
    if err != nil {
    panic(err)
}

app.StakingKeeper.SetValidatorByPowerIndex(ctx, newVal)

app.StakingKeeper.SetLastValidatorPower(ctx, newVal.GetOperator(), 0)
    if err := app.StakingKeeper.Hooks().AfterValidatorCreated(ctx, newVal.GetOperator()); err != nil {
    panic(err)
}

​
Distribution

Since the validator set has changed, we need to update the distribution records for the new validator. 
/ Initialize records for this validator across all distribution stores
	app.DistrKeeper.ValidatorHistoricalRewards.Set(ctx, newVal.GetOperator(), 0, distrtypes.NewValidatorHistoricalRewards(sdk.DecCoins{
}, 1))

app.DistrKeeper.ValidatorCurrentRewards.Set(ctx, newVal.GetOperator(), distrtypes.NewValidatorCurrentRewards(sdk.DecCoins{
}, 1))

app.DistrKeeper.ValidatorAccumulatedCommission.Set(ctx, newVal.GetOperator(), distrtypes.InitialValidatorAccumulatedCommission())

app.DistrKeeper.ValidatorOutstandingRewards.Set(ctx, newVal.GetOperator(), distrtypes.ValidatorOutstandingRewards{
    Rewards: sdk.DecCoins{
}})

​
Slashing

We also need to set the validator signing info for the new validator. 
/ SLASHING
	/

	/ Set validator signing info for our new validator.
    newConsAddr := sdk.ConsAddress(newValAddr.Bytes())
    newValidatorSigningInfo := slashingtypes.ValidatorSigningInfo{
    Address:     newConsAddr.String(),
    StartHeight: app.LastBlockHeight() - 1,
    Tombstoned:  false,
}

app.SlashingKeeper.ValidatorSigningInfo.Set(ctx, newConsAddr, newValidatorSigningInfo)

​
Bank

It is useful to create new accounts for your testing purposes. This avoids the need to have the same key as you may have on mainnet. 
/ BANK
	/
    defaultCoins := sdk.NewCoins(sdk.NewInt64Coin("ustake", 1000000000000))
    localSimAppAccounts := []sdk.AccAddress{
    sdk.MustAccAddressFromBech32("cosmos12smx2wdlyttvyzvzg54y2vnqwq2qjateuf7thj"),
		sdk.MustAccAddressFromBech32("cosmos1cyyzpxplxdzkeea7kwsydadg87357qnahakaks"),
		sdk.MustAccAddressFromBech32("cosmos18s5lynnmx37hq4wlrw9gdn68sg2uxp5rgk26vv"),
		sdk.MustAccAddressFromBech32("cosmos1qwexv7c6sm95lwhzn9027vyu2ccneaqad4w8ka"),
		sdk.MustAccAddressFromBech32("cosmos14hcxlnwlqtq75ttaxf674vk6mafspg8xwgnn53"),
		sdk.MustAccAddressFromBech32("cosmos12rr534cer5c0vj53eq4y32lcwguyy7nndt0u2t"),
		sdk.MustAccAddressFromBech32("cosmos1nt33cjd5auzh36syym6azgc8tve0jlvklnq7jq"),
		sdk.MustAccAddressFromBech32("cosmos10qfrpash5g2vk3hppvu45x0g860czur8ff5yx0"),
		sdk.MustAccAddressFromBech32("cosmos1f4tvsdukfwh6s9swrc24gkuz23tp8pd3e9r5fa"),
		sdk.MustAccAddressFromBech32("cosmos1myv43sqgnj5sm4zl98ftl45af9cfzk7nhjxjqh"),
		sdk.MustAccAddressFromBech32("cosmos14gs9zqh8m49yy9kscjqu9h72exyf295afg6kgk"),
		sdk.MustAccAddressFromBech32("cosmos1jllfytsz4dryxhz5tl7u73v29exsf80vz52ucc")
}

  / Fund localSimApp accounts
    for _, account := range localSimAppAccounts {
    err := app.BankKeeper.MintCoins(ctx, minttypes.ModuleName, defaultCoins)
    if err != nil {
    tmos.Exit(err.Error())
}

err = app.BankKeeper.SendCoinsFromModuleToAccount(ctx, minttypes.ModuleName, account, defaultCoins)
    if err != nil {
    tmos.Exit(err.Error())
}
	
}

​
Upgrade

If you would like to schedule an upgrade the below can be used. 
/ UPGRADE
	/
    if upgradeToTrigger != "" {
    upgradePlan := upgradetypes.Plan{
    Name:   upgradeToTrigger,
    Height: app.LastBlockHeight(),
}

err = app.UpgradeKeeper.ScheduleUpgrade(ctx, upgradePlan)
    if err != nil {
    panic(err)
}
	
}

​
Optional Changes

If you have custom modules that rely on specific state from the above modules and/or you would like to test your custom module, you will need to update the state of your custom module to reflect your needs

​
Running the Testnet

Before we can run the testnet we must plug everything together. in root.go, in the initRootCmd function we add: 
  server.AddCommands(rootCmd, simapp.DefaultNodeHome, newApp, createSimAppAndExport, addModuleInitFlags)
	++ server.AddTestnetCreatorCommand(rootCmd, simapp.DefaultNodeHome, newTestnetApp, addModuleInitFlags)
Next we will add a newTestnetApp helper function: 
/ newTestnetApp starts by running the normal newApp method. From there, the app interface returned is modified in order
/ for a testnet to be created from the provided app.
func newTestnetApp(logger log.Logger, db cometbftdb.DB, traceStore io.Writer, appOpts servertypes.AppOptions) servertypes.Application {
	/ Create an app and type cast to an SimApp
	app := newApp(logger, db, traceStore, appOpts)
	simApp, ok := app.(*simapp.SimApp)
	if !ok {
		panic("app created from newApp is not of type simApp")
	}

	newValAddr, ok := appOpts.Get(server.KeyNewValAddr).(bytes.HexBytes)
	if !ok {
		panic("newValAddr is not of type bytes.HexBytes")
	}
	newValPubKey, ok := appOpts.Get(server.KeyUserPubKey).(crypto.PubKey)
	if !ok {
		panic("newValPubKey is not of type crypto.PubKey")
	}
	newOperatorAddress, ok := appOpts.Get(server.KeyNewOpAddr).(string)
	if !ok {
		panic("newOperatorAddress is not of type string")
	}
	upgradeToTrigger, ok := appOpts.Get(server.KeyTriggerTestnetUpgrade).(string)
	if !ok {
		panic("upgradeToTrigger is not of type string")
	}

	/ Make modifications to the normal SimApp required to run the network locally
	return simapp.InitSimAppForTestnet(simApp, newValAddr, newValPubKey, newOperatorAddress, upgradeToTrigger)
}
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