queriers.md

Queriers

Start by creating the ./x/nameservice/querier.go file. This is the place to define which queries against application state users will be able to make. In our nameservice we want to expose two:

• resolve: This takes a name and returns the value that is stored by the nameservice. This is similar to a DNS query.
• whois: This takes a name and returns the price, value, and owner of the name. Used for figuring out how much names cost when you want to buy them.

Start by defining the NewQuerier function which acts a sub-router for queries to this module (similar to our NewHandler function). Note that because we don't have an interface similar to Msg for queries we need to manually define our switch statement cases (they can't be pulled off of the query .Name() function):

package nameservice

import (
	"github.com/cosmos/cosmos-sdk/codec"

	sdk "github.com/cosmos/cosmos-sdk/types"
	abci "github.com/tendermint/tendermint/abci/types"
)

// query endpoints supported by the governance Querier
const (
	QueryResolve = "resolve"
	QueryWhois   = "whois"
)

// NewQuerier is the module level router for state queries
func NewQuerier(keeper Keeper) sdk.Querier {
	return func(ctx sdk.Context, path []string, req abci.RequestQuery) (res []byte, err sdk.Error) {
		switch path[0] {
		case QueryResolve:
			return queryResolve(ctx, path[1:], req, keeper)
		case QueryWhois:
			return queryWhois(ctx, path[1:], req, keeper)
		default:
			return nil, sdk.ErrUnknownRequest("unknown nameservice query endpoint")
		}
	}
}

Now that the router is defined, define the inputs and responses for each query:

// nolint: unparam
func queryResolve(ctx sdk.Context, path []string, req abci.RequestQuery, keeper Keeper) (res []byte, err sdk.Error) {
	name := path[0]

	value := keeper.ResolveName(ctx, name)

	if value == "" {
		return []byte{}, sdk.ErrUnknownRequest("could not resolve name")
	}

	return []byte(value), nil
}

// nolint: unparam
func queryWhois(ctx sdk.Context, path []string, req abci.RequestQuery, keeper Keeper) (res []byte, err sdk.Error) {
	name := path[0]

	whois := Whois{}

	whois.Value = keeper.ResolveName(ctx, name)
	whois.Owner = keeper.GetOwner(ctx, name)
	whois.Price = keeper.GetPrice(ctx, name)

	bz, err2 := codec.MarshalJSONIndent(keeper.cdc, whois)
	if err2 != nil {
		panic("could not marshal result to JSON")
	}

	return bz, nil
}

// Whois represents a name -> value lookup
type Whois struct {
	Value string         `json:"value"`
	Owner sdk.AccAddress `json:"owner"`
	Price sdk.Coins      `json:"price"`
}

Notes on the above code:

• Here our Keeper's' getters and setters come into heavy use. When building any other applications that use this module you may need to go back and define more getters/setters to access the pieces of state you need.
• If your application needs some custom response types (Whois here), define them in this file.

Now that you have ways to mutate and view your module state its time to put the finishing touches on it! Register your types in the Amino encoding format next!