ADR0006: Where to implement model serialization

Add decision record about the design of de/serialization between
TUF metadata class model and wire line metadata formats.

Chosen option: Serialization and class model are decoupled, but the
class model provides conversion helper methods.

Signed-off-by: Lukas Puehringer <[email protected]>

docs/adr/0006-where-to-implemenent-model-serialization.md

@@ -0,0 +1,116 @@
+# Separate metadata serialization from metadata class model but keep helpers
+
+Technical Story: https://github.com/theupdateframework/tuf/pull/1279
+
+## Context and Problem Statement
+In the course of implementing a class-based role metadata model we have also
+reviewed options on how to design serialization infrastructure between wire
+formats and the class model. In an initial attempt we have implemented
+serialization on the metadata class (see option 1), but issues with inheritance
+and calls for more flexibility have caused us to rethink this approach.
+
+## Decision Drivers
+* A class-based role metadata model (see ADR4) requires serialization routines
+  from and to wire formats
+* TUF integrators may require custom serialization implementations for custom
+  wire formats
+* Readability and simplicity of implementation for users and maintainers
+* Recognizability of specification
+
+## Considered Options
+1. Serialization in metadata classes
+2. Serialization in metadata subclasses
+3. Serialization separated from metadata classes
+4. Compromise 1: Default serialization methods in metadata classes /
+   non-default serialization separated
+5. Compromise 2: Serialization separated / dict conversion helper methods for
+   default serialization in metadata classes
+
+## Decision Outcome
+Chosen option: "Compromise 2", because implementing dict conversion as methods
+on a corresponding class is idiomatic and allows for well-structured code.
+Together with a separated serialization interface, it provides both ease of use
+and maintenance, and full flexibility with regards to custom serialization
+implementations and wire formats.
+
+## Pros and Cons of the Options
+
+### Option 1: Serialization in metadata classes
+
+Serialization is implemented on metadata classes, e.g.
+`Metadata.serialize_as_json()`, etc.
+
+* Good, because serialization for any object is encapsulated within the
+  corresponding class and thus structured in small code chunks, using the
+  already existing hierarchical class model structure.
+* Good, because the TUF specification is heavily based on json, even if only
+  for illustrative purposes, thus this option facilitates recognizability.
+* Bad, because it might suggest that TUF is limited to json alone.
+* Bad, because it does not facilitate custom serialization implementations.
+* Bad, because it can get complicated with inheritance in the class model.
+  *NOTE: a workaround exists in #1279.*
+
+### Option 2: Serialization in metadata subclasses
+Serialization is implemented on metadata subclasses, e.g.
+`JsonMetadata.serialize()`, etc.
+
+* Good, because the wire format is decoupled from the base classes, not giving
+  the impression that TUF is limited to json, and facilitating custom
+  implementations.
+* Bad, because a user needs to decide on serialization ahead of time, when
+  instantiating the metadata objects.
+* Bad, because the metadata model has many classes, which would all need to be
+  subclassed accordingly.
+
+### Option 3: Serialization separated from metadata classes
+Serialization is implemented independently of the metadata class, e.g. by
+defining an abstract `Serializer` interface, which must be implemented in
+subclasses, e.g. `JsonSerializer`, etc.
+
+* Good, because the wire format is completely decoupled from the class model,
+  not giving the impression that TUF is limited to json, and facilitating
+  custom implementations.
+* Good, because it can serve as exact blueprint for custom implementations.
+* Bad, because a decoupled serialization implementation needs to "re-implement"
+  the entire class hierarchy, likely in a procedural manner.
+
+### Option 4: Compromise 1
+Default json serialization is implemented on the metadata class as described in
+(1), but can be overridden using an independent `Serializer` interface as
+described in (3).
+
+* Good, for the reasons outlined in options (1) and (3), i.e. encapsulation
+  within classes but decoupled class model and wire format.
+* Bad, because it creates two different code paths for default and non-default
+  wire formats making the code more complex and prone to deteriorate,
+  especially on the non-default path.
+* Bad, because the on-the-class default implementation can not be used as
+  blueprint for custom implementations.
+
+### Option 5: Compromise 2
+Serialization is implemented independently of the metadata class as described
+in (3). However, the *meat* of the default `JsonSerializer`, i.e. conversion
+between metadata objects and dicts, is implemented on the metadata class, e.g.
+as `Metadata.to_dict()`, etc.
+
+* Good, for the reasons outlined in options (1) and (3), i.e. encapsulation
+  within classes but decoupled class model and wire format, without the
+  disadvantage in (4) of having two completely different code paths.
+* Good, because it makes the separate default serializer a minimal wrapper
+  around the dict conversion methods.
+* Good, because other serialization implementations might also make use of dict
+  conversion methods.
+* Good, because conversion between class objects and dicts is akin to type
+  casting, which is idiomatic to implement on the class.
+* Bad, because the on-the-class default implementation can not be used as
+  blueprint for custom implementations.
+
+## Links
+* [ADR4: Add classes for complex metadata attributes (decision driver)](/Users/lukp/tuf/tuf/docs/adr/0004-extent-of-OOP-in-metadata-model.md)
+* [PR: Add simple TUF role metadata model (implements option 1)](https://github.com/theupdateframework/tuf/pull/1112)
+  - [details about separation of serialization and instantiation](https://github.com/theupdateframework/tuf/commit/f63dce6dddb9cfbf8986141340c6fac00a36d46e)
+  - [code comment about issues with inheritance](https://github.com/theupdateframework/tuf/blob/9401059101b08a18abc5e3be4d60e18670693f62/tuf/api/metadata.py#L297-L306)
+* [PR: New metadata API: add MetadataInfo and TargetFile classes (recent ADR discussion impetus)](https://github.com/theupdateframework/tuf/pull/1223)
+  - [more discussion about issues with inheritance](https://github.com/theupdateframework/tuf/pull/1223#issuecomment-737188686)
+* [SSLIB/Issue: Add metadata container classes (comparison of options 1 and 2)](https://github.com/secure-systems-lab/securesystemslib/issues/272)
+* [tuf-on-a-plane parser (implements option 3)](https://github.com/trishankatdatadog/tuf-on-a-plane/blob/master/src/tuf_on_a_plane/parsers/)

docs/adr/index.md

@@ -10,6 +10,7 @@ This log lists the architectural decisions for tuf.
 - [ADR-0003](0003-where-to-develop-TUF-1-0-0.md) - Develop TUF 1.0.0 in a subdirectory of the current TUF implementation
 - [ADR-0004](0004-extent-of-OOP-in-metadata-model.md) - Add classes for complex metadata attributes
 - [ADR-0005](0005-use-google-python-style-guide.md) - Use Google Python style guide with minimal refinements
+- [ADR-0006](0006-where-to-implemenent-model-serialization.md) - Separate metadata serialization from metadata class model but keep helpers
 
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