cosmos-sdk/docs/building-modules/simulator.md

Module Simulation

Prerequisites

• Cosmos Blockchain Simulator

Synopsis

This document details how to define each module simulation functions to be integrated with the application SimulationManager.

• Simulation package
  • Store decoders
  • Randomized genesis
  • Randomized parameters
  • Random weighted operations
  • Random proposal contents
• Registering the module simulation functions
• App simulator manager
• Simulation tests

Simulation package

Every module that implements the SDK simulator needs to have a x/<module>/simulation package which contains the primary functions required by the fuzz tests: store decoders, randomized genesis state and parameters, weighted operations and proposal contents.

Store decoders

Registering the store decoders is required for the AppImportExport. This allows for the key-value pairs from the stores to be decoded (i.e unmarshalled) to their corresponding types. In particular, it matches the key to a concrete type and then unmarshals the value from the KVPair to the type provided.

You can use the example here from the distribution module to implement your store decoders.

Randomized genesis

The simulator tests different scenarios and values for genesis parameters in order to fully test the edge cases of specific modules. The simulator package from each module must expose a RandomizedGenState function to generate the initial random GenesisState from a given seed.

Once the module genesis parameter are generated randomly (or with the key and values defined in a params file), they are marshaled to JSON format and added to the app genesis JSON to use it on the simulations.

You can check an example on how to create the randomized genesis here.

Randomized parameter changes

The simulator is able to test parameter changes at random. The simulator package from each module must contain a RandomizedParams func that will simulate parameter changes of the module throughout the simulations lifespan.

You can see how an example of what is needed to fully test parameter changes here

Random weighted operations

Operations are one of the crucial parts of the SDK simulation. They are the transactions (Msg) that are simulated with random field values. The sender of the operation is also assigned randomly.

Operations on the simulation are simulated using the full transaction cycle of a ABCI application that exposes the BaseApp.

Shown below is how weights are set:

+++ https://github.com/cosmos/cosmos-sdk/blob/master/x/staking/simulation/operations.go#L18

As you can see the weights are predefined in this case but there are options on how to override this behavior with different weights. One is allowing *rand.Rand to define a random weight for the operation, or you can inject your own predefined weights.

Here is how one can override the above package simappparams.

+++ https://github.com/cosmos/gaia/blob/master/sims.mk#L9-L22

For the last test a tool called runsim is used, this is used to parallelize go test instances, provide info to Github and slack integrations to provide information to your team on how the simulations are running.

Random proposal contents

Randomized governance proposals are also supported on the SDK simulator. Each module must define the governance proposal Contents that they expose and register them to be used on the parameters.

Registering simulation functions

Now that all the required functions are defined, we need to integrate them into the module pattern within the module.go:

+++ https://github.com/cosmos/cosmos-sdk/blob/v0.42.0/x/distribution/module.go

App Simulator manager

The following step is setting up the SimulatorManager at the app level. This is required for the simulation test files on the next step.

type CustomApp struct {
  ...
  sm *module.SimulationManager
}

Then at the instantiation of the application, we create the SimulationManager instance in the same way we create the ModuleManager but this time we only pass the modules that implement the simulation functions from the AppModuleSimulation interface described above.

func NewCustomApp(...) {
  // create the simulation manager and define the order of the modules for deterministic simulations
  app.sm = module.NewSimulationManager(
    auth.NewAppModule(app.accountKeeper),
    bank.NewAppModule(app.bankKeeper, app.accountKeeper),
    supply.NewAppModule(app.supplyKeeper, app.accountKeeper),
    ov.NewAppModule(app.govKeeper, app.accountKeeper, app.supplyKeeper),
    mint.NewAppModule(app.mintKeeper),
    distr.NewAppModule(app.distrKeeper, app.accountKeeper, app.supplyKeeper, app.stakingKeeper),
    staking.NewAppModule(app.stakingKeeper, app.accountKeeper, app.supplyKeeper),
    slashing.NewAppModule(app.slashingKeeper, app.accountKeeper, app.stakingKeeper),
  )

  // register the store decoders for simulation tests
  app.sm.RegisterStoreDecoders()
  ...
}