graphql-engine

This note is in Hasura.RQL.DDL.Warnings.

Warnings in metadata API

The metadata API handlers return EncJSON, which is just a Bytestring Builder. Now, in order to add warnings to the API response, we cannot use the runMetadataWarnings at the top level (i.e. in runMetadataQueryM) as appending something to the EncJSON will require us to parse the JSON and then re-serialize it. This is wasteful and we should avoid it.

As a result, we are using the MonadWarnings class to add warnings at the API handler level, i.e., the API handler will use the runMetadataWarnings function to run the handler and get the warnings. Then, the API handler will use the warnings to construct the response.

We can however avoid this by changing the return type of the metadata API handlers to something like:

data MetadataAPIOutput = RawOutput EncJSON | SuccessWithWarnings MetadataWarnings | InconsistentMetadataWithWarnings MetadataWarnings

This will allow us to cater to the metadata APIs:

• That contacts some external service and passes the raw response (like the export_metadata API).
• That returns a success message with warnings (like the replace_metadata v1 API).
• That returns inconsistent metadata with warnings (like the replace_metadata v2 API).

Also, we can expand the scope of MetadataAPIOutput to include other types of responses as well in the future.

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