This exists to interact with the RDS Postgres database (which has an RDS Proxy in front).
It is configured as a 'data source' in AppSync, and chooses what SQL to run base on which GraphQL query/mutation is requested (present in the info.fieldName parameter of the input payload).
This provides a nice single place where DB interactions are defined. We use GraphQL Code Generator to generate type definitions from the GraphQL schema (which we use across the whole project) but combined with tsafe in /shared/graphql/operations.ts and @typescript-eslint/switch-exhaustiveness-check we guarantee at compile time that we have a handler for every database related GraphQL query/mutation. We also iterate this list in the /cdk/stacks.ts to generate the AppSync resolvers for each GraphQL query/mutation.