Defining Default Values via Genesis

In the last section, we saw how to initialize the default values of our precompiled contracts via the Configure function. While straightforward, it would be ideal for us (and developers who don't write in Go) to initialize default values via the genesis JSON. Here's how to do just that.

Modifying config.go

The first step is to extract the value from the JSON file. Go to config.go and look at the Config struct:

// Config implements the precompileconfig.Config interface and
// adds specific configuration for StringStore.
type Config struct {
    precompileconfig.Upgrade
    // CUSTOM CODE STARTS HERE
    // Add your own custom fields for Config here
}

Within the Config struct, we can find another struct: the Upgrade struct. It is not necessary to look at the definition of this struct. However, it is useful to know that this struct allows for the blockTimestamp key to be parsed in our genesis.json file.

"stringStoreConfig" : {
    "blockTimestamp": 0,
}

To pass in default values via the genesis JSON, we must complete the following:

• Define a key-value in the genesis JSON
• Update our Config struct so it is able to parse our new key-value

In the case of StringStore, after defining blockTimestamp, we will add another key-pair for our default string:

"stringStoreConfig" : {
    "blockTimestamp": 0,
    "defaultString": "EGS"
}

Next, we will want to update our Config struct:

type Config struct {
    precompileconfig.Upgrade
    // CUSTOM CODE STARTS HERE
    // Add your own custom fields for Config here
    DefaultString string `json:"defaultString,omitempty"`
}

In the above snippet, we are defining a new field of type string named DefaultString. With respect to its initialization, the value of DefaultString is derived from the JSON key defaultString from our genesis JSON (the value json:"defaultString,omitempty" tells Go to ignore the JSON field if we do not define it in our genesis JSON).

At this point, we have defined our new key-value pair in the genesis JSON and incorporated the logic so that precompile-evm can parse the new key-value.

However, one step remains. Although precompile-evm can parse the new key-value, we are not actually utilizing it anywhere. Our last-step is to update Configure method in module.go so it can read our new key-value pair.

func (*configurator) Configure(chainConfig contract.ChainConfig, cfg precompileconfig.Config, state contract.StateDB, _ contract.BlockContext) error {
    config, ok := cfg.(*Config)
    if !ok {
        return fmt.Errorf("incorrect config %T: %v", config, config)
    }
    // CUSTOM CODE STARTS HERE
    StoreString(state, config.DefaultString)
 
    return nil
}