A DataSource[R, A] requires an environment R and is capable of executing requests of type A.

Data sources must implement the method runAll which takes a collection of requests and returns an effect with a CompletedRequestMap containing a mapping from requests to results. The type of the collection of requests is a Chunk[Chunk[A]]. The outer Chunk represents batches of requests that must be performed sequentially. The inner Chunk represents a batch of requests that can be performed in parallel. This allows data sources to introspect on all the requests being executed and optimize the query.

Data sources will typically be parameterized on a subtype of Request[A], though that is not strictly necessarily as long as the data source can map the request type to a Request[A]. Data sources can then pattern match on the collection of requests to determine the information requested, execute the query, and place the results into the CompletedRequestsMap one of the constructors in CompletedRequestMap$. Data sources must provide results for all requests received. Failure to do so will cause a query to die with a QueryFailure when run.