Frequently Asked Questions


When are validations defined in the design enforced?

There is a trade-off between performance and robustness wrt enforcing the validations defined in the service design. Goa trusts that the user code does the right thing and only validates external data (“input”). This means goa validates incoming requests server side and responses client side. This way your code is always guaranteed to get valid data but doesn’t have to pay the price of validation for each response being written server side or request being sent client side.

When is a generated struct field a pointer?

There are a few considerations taken into account by the code generation algorithms to decide whether a generated struct field should be a pointer or not. The goal is to avoid using pointers when not necessary as they tend to complicate code and provide opportunity for errors. This discussion only affects attributes using one of the primitive types. Fields that correspond to attributes that are objects always use pointers. Fields that correspond to attributes that are arrays or maps never use pointers.

The general idea is that if a type design specifies that a certain attribute is required or has a default value then the corresponding field should never be nil and therefore does not need to be a pointer. However the generated code that decodes incoming HTTP requests or responses must account for the fact that these fields may be missing (the request or response is malformed) and thus have to use data structures that use pointers for all fields to test for nil values in the unmarshaled data.

The table below lists whether fields generated for user type attributes that are primitives use pointers (*) or direct values (-).

  • (s) : data structure generated for the server
  • © : data structure generated for the client
Properties / Data Structure Payload / Result Req. Body (s) Resp. Body (s) Req. Body © Resp. Body ©
Required OR Default - * - - *
Not Required, No Default * * * * *

How are default values used?

The DSL allows for specifying default values for attributes. The default values are used in two places by the code generators.

When generating marshaling code (server side to marshal the response or client side to marshal the request) the default value is used to initialize the data structure field if it is nil. As discussed in the previous section this cannot happen if the attribute is defined with a primitive type since in this case the field is not a pointer. However this can happen for attributes that are arrays or maps.

When generating unmarshaling code (server side to unmarshal an incoming request or client side to unmarshal a response) the default value is used to set the value of missing fields. Note that if the attribute is required then the generated code returns an error if the corresponding field is missing. So this only applies for non required attributes with default values. For gRPC endpoints, the default value behavior when unmarshaling varies because protocol buffer v3 sets a default value (i.e. type’s zero value) for a missing field internally when it encodes the message. See https://developers.google.com/protocol-buffers/docs/proto3#default. It is impossible to know whether these zero values are set by the application itself or by protocol buffer. Therefore, when unmarshaling a protocol buffer message, goa sets the default values only for the optional attributes only if the message contains zero values for such fields. Optional attributes of boolean type is an exception, i.e., if attributes of boolean type are zero values (false), unmarshalling code does not set default values to the corresponding target attributes. Attributes that are required and have default values are not set with the default values if they are zero values.

How are optional attributes unmarshalled in gRPC?

Protocol buffer v3 does not differentiate between optional and required attributes (see https://developers.google.com/protocol-buffers/docs/proto3#default). It is impossible to know whether the zero values are set by the application or by protocol buffer. When generating unmarshaling code (server side to unmarshal incoming protocol buffer request message or client side to unmarshal incoming protocol buffer response message), the optional attributes are set to non-nil value only if the corresponding protocol buffer message fields are non-zero values. Optional attributes of boolean type is an exception, i.e., if attributes of boolean type are zero values (false), unmarshalling code sets the corresponding target attributes to the zero values.

How are views for a result type computed?

Views can be defined on a result type. If a method returns a result type * the service method returns an extra view along with the result and error if the result type has more than one view. The generated endpoint function uses this view to create a viewed result type. * a views package is generated at the service level which defines a viewed result type for each method result. This viewed result type has identical field names and types but uses pointers for all fields so that view specific validation logic may be generated. Constructors are generated in the service package to convert a result type to a viewed result type and vice versa.

The server side response marshaling code marshals the viewed result type returned by the endpoint into a server type omitting any nil attributes. The view used to render the result type is passed to the client in “Goa-View” header.

The client side response unmarshaling code unmarshals the response into the client type which is then transformed to the viewed result type and sets the view attribute of the viewed result type from the Goa-View header. It validates the attributes in the viewed result type as defined by the view and converts the viewed result type into the service result type using the appropriate constructor.

NOTE: If a result type is defined without any views, a “default” view is added to the result type by goa. If you don’t care about views, you can define a method result using the Type DSL which will bypass the view-specific logic.