Query

Tutorial.Nameservice.Query

module Tutorial.Nameservice.Query where

import Nameservice.Modules.Nameservice.Types (Whois)
import Nameservice.Modules.Nameservice.Store (Name, whoisMap)
import Polysemy (Members)
import Tendermint.SDK.BaseApp (QueryEffs, StoreLeaf, RouteQ, storeQueryHandler)
import qualified Tendermint.SDK.BaseApp.Store.Map as M
import Servant.API ((:>))

The way to query application state is via the query message which uses a url like format. The SDK tries to abstract as much of this away as possible. For example, if you want to only serve state that you have registered with the store via the IsKey class, then things are very easy. If you need to make joins to serve requests, we support this as well and it's not hard, but we will skip this for now.

In the case we just want to serve data we have registered with the IsKey class, we simply need to declare some types:

type Api = "whois" :> StoreLeaf (M.Map Name Whois)

Api is the list of valid urls for this module. In this case, when unpacked it will create a single endpoint <URL_ROOT>/whois that expects a value of type Name as the data field in the ABCI query object. Technically speaking it is a Name prefixed by some other storage related prefixes dictated by the module, but we will hold off on this for now. If you use the automatically generated client libraries, you don't need to worry about this.

To serve all the data, we can use the storeQueryHandler function by supplying the appropriate store we want to serve.

querier
  :: Members QueryEffs r
  => RouteQ Api r
querier = storeQueryHandler whoisMap

Here RouteQ is a type family that can build a server from the Api type to handle incoming requests. It is similar to how servant works, and is largely vendored from that codebase.

Note that more advanced queries are possible other than just serving what is in storage. For example you might want to use joins to fulfill requests or use query parameters in the url. These are all possible, but we won't go into details here as they are not used in the app.