Response message: Composite message or optional fields

We are working with a set of web services and we're looking for the best option to return errors to the web service's consumer. This is the current response:

Response

• Some data about the server
• Some data about the user
• Some resulting data of executing the transaction

So, we need to return errors too. these are our options:

Composite message

we'll return two kinds of responses depending if the transaction was approved or had an error:

First:

• type identifier (this message is serialized. so I need to know which kind of message I'm dealing with, to deserialize the last part)
• Some data about the server
• Some data about the user
• Some resulting data of executing the transaction

Second:

• type identifier (this message is serialized. so I need to know which kind of message I'm dealing with, to deserialize the last part)
• Some data about the server
• Some data about the user
• The errors

Optional fields

the transaction data and error fields will be optional. if there's no errors I will know it was approved.

• Some data about the server
• Some data about the user
• Some resu开发者_运维百科lting data of executing the transaction
• The errors

Which option is more appropriate?

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This is discutable and more of a personal opinion than a best practice.

My personal favor is to use the Optional fields, because the error code is possible outcome of an operation. I would expect the client to always first check the (optional) error properties of the returned result before parsing the results. This allows to also return non-fatal errors and partial results together. Exclusive makes it so ... exclusive. Optional is more flexible.