Validate JSON against a protobuf-like schema

Problem Statement

Given a schema definition and a JSON object, write a function is_valid(schema, data) that returns True if the data conforms to the schema and False otherwise.

Schema Definition

The schema is defined as a dictionary where keys are field names. Each value is a dictionary with:

• type: Either "string", "int", or a dict (for nested messages).
• required: A boolean indicating if the field must be present.

Input Rules

• If type is a nested dict, the data for that field must also be a dictionary and must be recursively validated against that nested schema.
• If a field is required and missing from data, return False.
• If a field exists in data but its type does not match the schema, return False.
• Extra fields present in data that are not defined in the schema should be ignored (or considered valid).

Examples

schema = {
    "id": {"type": "int", "required": True},
    "metadata": {
        "type": {
            "source": {"type": "string", "required": True}
        },
        "required": False
    }
}

# Returns True
is_valid(schema, {"id": 1, "metadata": {"source": "sensor_a"}})

# Returns False (missing required 'source')
is_valid(schema, {"id": 1, "metadata": {}})

Constraints

• data will always be a dictionary.
• The schema structure is a directed acyclic graph (no self-referencing schemas).
• You do not need to validate deep type-checking for types other than int, str, and dict.