mukan-ignite/docs/versioned_docs/version-v0.25/kb/11-simapp.md

---
sidebar_position: 11
description: Test different scenarios for your chain. 

---

# Chain simulation

The Ignite CLI chain simulator can help you to run your chain based in randomized inputs for you can make fuzz testing and also benchmark test for your chain, simulating the messages, blocks, and accounts. You can scaffold a template to perform simulation testing in each module along with a boilerplate simulation methods for each scaffolded message.

## Module simulation

Every new module that is scaffolded with Ignite CLI implements the Cosmos SDK [Module Simulation](https://docs.cosmos.network/main/building-modules/simulator). 

- Each new message creates a file with the simulation methods required for the tests. 
- Scaffolding a `CRUD` type like a `list` or `map` creates a simulation file with `create`, `update`, and `delete` simulation methods in the `x/<module>/simulation` folder and registers these methods in `x/<module>/module_simulation.go`. 
- Scaffolding a single message creates an empty simulation method to be implemented by the user.

We recommend that you maintain the simulation methods for each new modification into the message keeper methods.

Every simulation is weighted because the sender of the operation is assigned randomly. The weight defines how much the simulation calls the message. 

For better randomizations, you can define a random seed. The simulation with the same random seed is deterministic with the same output.

## Scaffold a simulation

To create a new chain:

```bash
ignite scaffold chain mars
```

Review the empty `x/mars/simulation` folder and the `x/mars/module_simulation.go` file to see that a simulation is not registered. 

Now, scaffold a new message:

```bash
ignite scaffold list user address balance:uint state
```

A new file `x/mars/simulation/user.go` is created and is registered with the weight in the `x/mars/module_simulation.go` file. 

Be sure to define the proper simulation weight with a minimum weight of 0 and a maximum weight of 100. 

For this example, change the `defaultWeightMsgDeleteUser` to 30 and the `defaultWeightMsgUpdateUser` to 50. 

Run the `BenchmarkSimulation` method into `app/simulation_test.go` to run simulation tests for all modules:

```bash
ignite chain simulate
```

You can also define flags that are provided by the simulation. Flags are defined by the method `simapp.GetSimulatorFlags()`:

```bash
ignite chain simulate -v --numBlocks 200 --blockSize 50 --seed 33
```

Wait for the entire simulation to finish and check the result of the messages.

The default `go test` command works to run the simulation:

```bash
go test -v -benchmem -run=^$ -bench ^BenchmarkSimulation -cpuprofile cpu.out ./app -Commit=true
```

### Skip message

Use logic to avoid sending a message without returning an error. Return only `simtypes.NoOpMsg(...)` into the simulation message handler.

## Params

Scaffolding a module with params automatically adds the module in the `module_simulaton.go` file:

```bash
ignite s module earth --params channel:string,minLaunch:uint,maxLaunch:int
```

After the parameters are scaffolded, change the `x/<module>/module_simulation.go` file to set the random parameters into the `RandomizedParams` method. The simulation will change the params randomly according to call the function. 

## Invariants

Simulating a chain can help you prevent [chain invariants errors](https://docs.cosmos.network/main/building-modules/invariants). An invariant is a function called by the chain to check if something broke, invalidating the chain data.
To create a new invariant and check the chain integrity, you must create a method to validate the invariants and register all invariants.

For example, in `x/earth/keeper/invariants.go`:

```go
package keeper

import (
	"fmt"

	sdk "github.com/cosmos/cosmos-sdk/types"
	"github.com/tendermint/spn/x/launch/types"
)

const zeroLaunchTimestampRoute = "zero-launch-timestamp"

// RegisterInvariants registers all module invariants
func RegisterInvariants(ir sdk.InvariantRegistry, k Keeper) {
	ir.RegisterRoute(types.ModuleName, zeroLaunchTimestampRoute,
		ZeroLaunchTimestampInvariant(k))
}

// ZeroLaunchTimestampInvariant invariant that checks if the
// `LaunchTimestamp is zero
func ZeroLaunchTimestampInvariant(k Keeper) sdk.Invariant {
	return func(ctx sdk.Context) (string, bool) {
		all := k.GetAllChain(ctx)
		for _, chain := range all {
			if chain.LaunchTimestamp == 0 {
				return sdk.FormatInvariant(
					types.ModuleName, zeroLaunchTimestampRoute,
					"LaunchTimestamp is not set while LaunchTriggered is set",
				), true
			}
		}
		return "", false
	}
}
```

Now, register the keeper invariants into the `x/earth/module.go` file:

```go
// RegisterInvariants registers the capability module's invariants.
func (am AppModule) RegisterInvariants(ir sdk.InvariantRegistry) {
	keeper.RegisterInvariants(ir, am.keeper)
}
```