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Version: v0.52

Query Services

Synopsis

A Protobuf Query service processes queries. Query services are specific to the module in which they are defined, and only process queries defined within said module.

Pre-requisite Readings

Implementation of a module query service​

gRPC Service​

When defining a Protobuf Query service, a QueryServer interface is generated for each module with all the service methods:

type QueryServer interface {
    QueryBalance(context.Context, *QueryBalanceParams) (*types.Coin, error)
    QueryAllBalances(context.Context, *QueryAllBalancesParams) (*QueryAllBalancesResponse, error)
}

These custom queries methods should be implemented by a module's keeper, typically in ./keeper/grpc_query.go.

Here's an example implementation for the bank module:

x/bank/keeper/grpc_query.go
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Calling queries from the State Machine​

The cosmos.query.v1.module_query_safe protobuf annotation is used to state that a query that is safe to be called from within the state machine, for example:

  • a Keeper's query function can be called from another module's Keeper,
  • ADR-033 intermodule query calls,
  • CosmWasm contracts can also directly interact with these queries.

If the module_query_safe annotation set to true, it means:

  • The query is deterministic: given a block height it will return the same response upon multiple calls, and doesn't introduce any state-machine breaking changes across SDK patch versions.
  • Gas consumption never fluctuates across calls and across patch versions.

If you are a module developer and want to use module_query_safe annotation for your own query, you have to ensure the following things:

  • the query is deterministic and won't introduce state-machine-breaking changes without coordinated upgrades
  • it has its gas tracked, to avoid the attack vector where no gas is accounted for on potentially high-computation queries.