Remove presence tracking from default values

This commit removes the recommendation for using presence tracking
default values, and instead advocates for serializing default values.
This ensures that whatever an implementation thinks the default value
is currently is unambiguoulsy honored by anything that deserializers
the message later at any point in time, including if the default
value for that member has since changed in the model. This also removes
the large amount of complexity that presence brings with it. While
this does make it harder for a default value to change, changing
defaults is meant to be very rare anyways. So the tradeoff here
seems worth it given changing defaults was and continues to be so
strongly discouraged.

designs/defaults-and-model-evolution.md

@@ -328,12 +328,10 @@ The default value of a member that targets a shape with a default value
 MUST NOT be removed (by changing the value to `null`) since that would
 transition the member from non-optional to optional in generated code.
 
-The default value of a member that targets a shape without a default value
-SHOULD NOT be changed. However, it MAY be necessary in extreme cases to change
-a default value if changing the default value addresses a customer-impacting
-issue or availability issue for a service. Changing default values can result
-in parties disagreeing on the default value of a member because they are using
-different versions of the same model.
+The default value of a member SHOULD NOT be changed. However, it MAY be
+necessary in rare cases to change a default value. Changing default values can
+result in parties disagreeing on the default value of a member because they are
+using different versions of the same model.
 
 
 ### Readers MUST NOT differentiate from omitted or defaulted
@@ -368,21 +366,12 @@ an appropriate default value for the member:
 
 ### Default value serialization
 
-Authoritative model consumers like servers SHOULD always serialize default
-values to remove any ambiguity about the value of the most up to date default
-value. However, to avoid information disclosure, servers SHOULD NOT serialize
-default values if the member is marked with the `@internal` trait.
-
-To allow servers to change default values if necessary, clients SHOULD NOT
-serialize default values unless the member is explicitly set to the default
-value. This implies that clients SHOULD implement a kind of "presence tracking"
-of defaulted members.
-
-A member that is both `@default` and `@required` SHOULD always be serialized,
-and implementations SHOULD NOT use any form of presence tracking to omit a
-member if the member is not explicitly set to the default value. It is a
-protocol-specific decision whether this is enforced in serialized messages;
-some protocols follow this strictly, whereas others may not.
+1. All default values SHOULD be serialized. This ensures that messages are
+   unambiguous, and ensures that messages do not change during deserialization
+   if the default value for a member changes after the message was serialized.
+2. To avoid information disclosure, implementations MAY choose to not serialize
+   a default values if the member is marked with the `@internal` trait.
+3. A member that is both `@default` and `@required` MUST be serialized.
 
 
 ### Impact on API design
@@ -671,8 +660,7 @@ then it would be possible to omit far more default values on the wire, because
 any time a required member is missing, a deserializer can safely assume the
 member transitioned from `@required` to `@default` and fill in the default
 zero value. This helps to avoid unintended information disclosure and can
-reduce payload sizes. The other major benefit is that presence tracking isn't
-needed because default values can never change.
+reduce payload sizes.
 
 However, only supporting default zero values means that service teams will need
 to add confusing enum values like `""` and `0` to enums and intEnums. It also
@@ -684,19 +672,35 @@ processes to ensure that a service team understands the implications of
 changing a default value.
 
 
-### Always send default values
-
-Always sending default values would be a big simplification to clients because
-it removes presence tracking. Always sending defaults would make it explicit
-as to what each party thinks the default value is. However, this would remove
-the ability for a service to change a default value if ever needed. While the
-service can change the default value and wait their consumers upgrade their
-clients the change will take effect, there is usually an extremely long-tail of
-users that will not upgrade to the latest version. If a service needs to change
-a default value due to an operational or customer impacting issue, then that
-change needs to take effect across all currently deployed clients as soon as
-possible, hence the need for presence tracking in clients to only send default
-values if they are explicitly set.
+### Omit default values or implement presence tracking
+
+If clients omit default values, then the default value for a member becomes
+the complete discretion of servers. This allows services to change default
+values for members and see the change take effect immediately with all
+previously generated clients. This can be achieved by always omitting the
+serialization of default values or through presence tracking.
+
+If clients simply omit the serialization of default values, then it could be
+problematic if a client actually wants to use whatever it thinks the current
+default value for a member is instead of an updated default value on the
+service. For example, if the client wants to use the default value of `10`,
+but the service has since updated the default value to `11`, the client will
+omit the serialization of `10` because it thinks that is the default, and the
+server will use the newly updated default value of `11`. Until the client
+updates to the latest version of the client, it is impossible for the client
+to use the previous default value with the service.
+
+Presence tracking is the act of tracking which structure members were set
+explicitly to a value vs which structure members were set by a default value.
+If a member was set by a default value, the client can omit the serialization
+of the member and delegate the default value to the server. If the member was
+explicitly set to a value, including the default value, the client will send
+the default value to the server, ensuring that even if the default is changed,
+the server will honor the client's choice. The major issue with presence
+tracking is that it has steep complexity and size tradeoffs because it
+requires tracking internal state changes on what could otherwise be stateless
+types. Implementations are free to use presence tracking, though it isn't a
+requirement of integrating with Smithy's default values.
 
 
 ## FAQ

docs/source-2.0/spec/type-refinement-traits.rst

@@ -142,27 +142,12 @@ of a member because they are using different versions of the same model.
 Default value serialization
 ---------------------------
 
-Authoritative model consumers like servers SHOULD always serialize default
-values to remove any ambiguity about the value of the most up to default
-value. However, to avoid information disclosure, servers SHOULD NOT serialize
-default values if the member is marked with the :ref:`internal-trait`.
-
-To allow servers to change default values if necessary, clients SHOULD NOT
-serialize default values unless the member is explicitly set to the default
-value or marked with the :ref:`default-trait`. This implies that clients
-SHOULD implement a kind of "presence tracking" of defaulted members so that
-the member is only serialized if it is explicitly set to the default value.
-
-
-Default and required
---------------------
-
-A member that is both ``@default`` and ``@required`` SHOULD always be
-serialized, and implementations SHOULD NOT use any form of presence tracking
-to omit a member if the member is not explicitly set to the default value.
-It is a protocol-specific decision as to whether this is enforced in
-serialized messages; some protocols follow this strictly whereas others may
-not.
+1. All default values SHOULD be serialized. This ensures that messages are
+   unambiguous so that messages do not change during deserialization if the
+   default value for a member changes after the message was serialized.
+2. To avoid information disclosure, implementations MAY choose to not serialize
+   a default values if the member is marked with the :ref:`internal-trait`.
+3. A member that is both ``@default`` and ``@required`` MUST be serialized.
 
 
 .. smithy-trait:: smithy.api#addedDefault