Clearly separate compatibility rules for input and output schemas.

chapters/compatibility.adoc

@@ -41,25 +41,63 @@ expected that code changes are necessary.
 == {SHOULD} prefer compatible extensions
 
 API designers should apply the following rules to evolve RESTful APIs for
-services in a backward-compatible way:
+services in a backward-compatible way.
+
+In general:
 
-* Add only optional, never mandatory fields.
 * Never change the semantic of fields (e.g. changing the semantic from
   customer-number to customer-id, as both are different unique customer keys)
+* Consider <<251>> in case a URL has to change.
+
+The compatibility of schema changes depends on whether the input and/or output objects are defined.
+
+For schemas used in input only:
+
+* Add optional fields, but never mandatory fields.
+* Make mandatory fields optional, but not vice-versa.
+* Don't remove fields(*).
 * Input fields may have (complex) constraints being validated via server-side
   business logic. Never change the validation logic to be more restrictive and
   make sure that all constraints are clearly defined in description.
-* `enum` ranges can be reduced when used as input parameters, only if the server
-  is ready to accept and handle old range values too. The range can be reduced
-  when used as output parameters.
-* `enum` ranges cannot be extended when used for output parameters — clients may
-  not be prepared to handle it. However, enum ranges can be extended when used
-  for input parameters.
+* `enum` ranges can be reduced, only if the server
+  continues to accept and handle old range values.
+* `enum` ranges can be extended.
+
+(*) Hint: Removing a field can be considered as a compatible change that does not break clients, in case the service still accepts and possibly ignores it if sent by the client. However, removed fields allow later adding a same-named field with different type or semantic (which is harder to catch). We therefore define field removal as non-compatible.  
+
+For schemas used in output only:
+
+* Add (mandatory or optional) fields.
+* Make optional fields mandatory, but not vice-versa.
+* Don't remove fields (*).
+* `enum` ranges can be reduced.
+* `enum` ranges *cannot* be extended — clients may
+  not be prepared to handle it.
+* You <<112>> that are used for output parameters and likely to
+  be extended with growing functionality. The API definition should be updated
+  first before returning new values.
+
+(*) Hint: Removing an optional field can be considered as a compatible change that does not break clients. However, removed fields allow later adding a same-named field with different type or semantic (which is harder to catch). We therefore define optional field removal as non-compatible. 
+
+For schemas used in both input and output (which is typical in
+many cases), both of these rule sets combine, i.e. you can only do changes which
+are allowed in both input and output.
+
+* Add only optional, never mandatory fields.
+* Don't remove any fields.
+* Don't make mandatory fields optional or make optional fields mandatory.
+* Input fields may have (complex) constraints being validated via server-side
+  business logic. Never change the validation logic to be more restrictive and
+  make sure that all constraints are clearly defined in description.
+* `enum` ranges can be reduced only if the server is ready to still accept and
+   handle old values. They **cannot** be extended.
 * You <<112>> that are used for output parameters and likely to
   be extended with growing functionality. The API definition should be updated
   first before returning new values.
-* Consider <<251>> in case a URL has to change.
 
+Input/Output here is from the perspective of a service implementing and
+owning the API. For the rare case of APIs implemented by other services
+(and consumed by the owning service), this turns around.
 
 [#109]
 == {SHOULD} design APIs conservatively