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ADR 054: Semver Compatible SDK Modules

ADR 054: Semver Compatible SDK Modules

Changelog

  • 2022-04-27: First draft

Status

DRAFT

Abstract

In order to move the Cosmos SDK to a system of decoupled semantically versioned modules which can be composed in different combinations (ex. staking v3 with bank v1 and distribution v2), we need to reassess how we organize the API surface of modules to avoid problems with go semantic import versioning and circular dependencies. This ADR explores various approaches we can take to addressing these issues.

Context

There has been a fair amount of desire in the community for semantic versioning in the SDK and there has been significant movement to splitting SDK modules into standalone go modules. Both of these will ideally allow the ecosystem to move faster because we won't be waiting for all dependencies to update synchronously. For instance, we could have 3 versions of the core SDK compatible with the latest 2 releases of CosmWasm as well as 4 different versions of staking . This sort of setup would allow early adopters to aggressively integrate new versions, while allowing more conservative users to be selective about which versions they're ready for.

In order to achieve this, we need to solve the following problems:

  1. because of the way go semantic import versioning (SIV) works, moving to SIV naively will actually make it harder to achieve these goals
  2. circular dependencies between modules need to be broken to actually release many modules in the SDK independently
  3. pernicious minor version incompatibilities introduced through correctly evolving protobuf schemas without correct unknown field filtering

Note that all the following discussion assumes that the proto file versioning and state machine versioning of a module are distinct in that:

  • proto files are maintained in a non-breaking way (using something like buf breaking to ensure all changes are backwards compatible)
  • proto file versions get bumped much less frequently, i.e. we might maintain cosmos.bank.v1 through many versions of the bank module state machine
  • state machine breaking changes are more common and ideally this is what we'd want to semantically version with go modules, ex. x/bank/v2, x/bank/v3, etc.

Problem 1: Semantic Import Versioning Compatibility

Consider we have a module foo which defines the following MsgDoSomething and that we've released its state machine in go module example.com/foo:

package foo.v1;

message MsgDoSomething {
  string sender = 1;
  uint64 amount = 2;
}

service Msg {
  DoSomething(MsgDoSomething) returns (MsgDoSomethingResponse);
}

Now consider that we make a revision to this module and add a new condition field to MsgDoSomething and also add a new validation rule on amount requiring it to be non-zero, and that following go semantic versioning we release the next state machine version of foo as example.com/foo/v2.

// Revision 1
package foo.v1;

message MsgDoSomething {
  string sender = 1;
  
  // amount must be a non-zero integer.
  uint64 amount = 2;
  
  // condition is an optional condition on doing the thing.
  //
  // Since: Revision 1
  Condition condition = 3;
}

Approaching this naively, we would generate the protobuf types for the initial version of foo in example.com/foo/types and we would generate the protobuf types for the second version in example.com/foo/v2/types.

Now let's say we have a module bar which talks to foo using this keeper interface which foo provides:

type FooKeeper interface {
	DoSomething(MsgDoSomething) error
}

Scenario A: Backward Compatibility: Newer Foo, Older Bar

Imagine we have a chain which uses both foo and bar and wants to upgrade to foo/v2, but the bar module has not upgraded to foo/v2.

In this case, the chain will not be able to upgrade to foo/v2 until bar has upgraded its references to example.com/foo/types.MsgDoSomething to example.com/foo/v2/types.MsgDoSomething.

Even if bar's usage of MsgDoSomething has not changed at all, the upgrade will be impossible without this change because example.com/foo/types.MsgDoSomething and example.com/foo/v2/types.MsgDoSomething are fundamentally different incompatible structs in the go type system.

Scenario B: Forward Compatibility: Older Foo, Newer Bar

Now let's consider the reverse scenario, where bar upgrades to foo/v2 by changing the MsgDoSomething reference to example.com/foo/v2/types.MsgDoSomething and releases that as bar/v2 with some other changes that a chain wants. The chain, however, has decided that it thinks the changes in foo/v2 are too risky and that it'd prefer to stay on the initial version of foo.

In this scenario, it is impossible to upgrade to bar/v2 without upgrading to foo/v2 even if bar/v2 would have worked 100% fine with foo other than changing the import path to MsgDoSomething (meaning that bar/v2 doesn't actually use any new features of foo/v2).

Now because of the way go semantic import versioning works, we are locked into either using foo and bar OR foo/v2 and bar/v2. We cannot have foo + bar/v2 OR foo/v2 + bar. The go type system doesn't allow this even if both versions of these modules are otherwise compatible with each other.

Naive Mitigation

A naive approach to fixing this would be to not regenerate the protobuf types in example.com/foo/v2/types but instead just update example.com/foo/types to reflect the changes needed for v2 (adding condition and requiring amount to be non-zero). Then we could release a patch of example.com/foo/types with this update and use that for foo/v2. But this change is state machine breaking for v1. It requires changing the ValidateBasic method to reject the case where amount is zero, and it adds the condition field which should be rejected based on ADR 020 unknown field filtering. So adding these changes as a patch on v1 is actually incorrect based on semantic versioning. Chains that want to stay on v1 of foo should not be importing these changes because they are incorrect for v1.

Problem 2: Circular dependencies

None of the above approaches allow foo and bar to be separate modules if for some reason foo and bar depend on each other in different ways. For instance, we can't have foo import bar/types while bar imports foo/types.

We have several cases of circular module dependencies in the SDK (ex. staking, distribution and slashing) that are legitimate from a state machine perspective. Without separating the API types out somehow, there would be no way to independently semantically version these modules without some other mitigation.

Problem 3: Handling Minor Version Incompatibilities

Imagine that we solve the first two problems but now have a scenario where bar/v2 wants the option to use MsgDoSomething.condition which only foo/v2 supports. If bar/v2 works with foo v1 and sets condition to some non-nil value, then foo will silently ignore this field resulting in a silent logic possibly dangerous logic error. If bar/v2 were able to check whether foo was on v1 or v2 and dynamically, it could choose to only use condition when foo/v2 is available. Even if bar/v2 were able to perform this check, however, how do we know that it is always performing the check properly. Without some sort of framework-level unknown field filtering, it is hard to know whether these pernicious hard to detect bugs are getting into our app and a client-server layer such as ADR 033: Inter-Module Communication may be needed to do this.

Solutions

Approach A) Separate API and State Machine Modules

One solution (first proposed in https://github.com/cosmos/cosmos-sdk/discussions/10582) is to isolate all protobuf generated code into a separate module from the state machine module. This would mean that we could have state machine go modules foo and foo/v2 which could use a types or API go module say foo/api. This foo/api go module would be perpetually on v1.x and only accept non-breaking changes. This would then allow other modules to be compatible with either foo or foo/v2 as long as the inter-module API only depends on the types in foo/api. It would also allow modules foo and bar to depend on each other in that both of them could depend on foo/api and bar/api without foo directly depending on bar and vice versa.

This is similar to the naive mitigation described above except that it separates the types into separate go modules which in and of itself could be used to break circular module dependencies. It has the same problems as the naive solution, otherwise, which we could rectify by:

  1. removing all state machine breaking code from the API module (ex. ValidateBasic and any other interface methods)
  2. embedding the correct file descriptors for unknown field filtering in the binary

Migrate all interface methods on API types to handlers

To solve 1), we need to remove all interface implementations from generated types and instead use a handler approach which essentially means that given a type X, we have some sort of resolver which allows us to resolve interface implementations for that type (ex. sdk.Msg or authz.Authorization). For example:

func (k Keeper) DoSomething(msg MsgDoSomething) error {
	var validateBasicHandler ValidateBasicHandler
	err := k.resolver.Resolve(&validateBasic, msg)
	if err != nil {
		return err
	}   
	
	err = validateBasicHandler.ValidateBasic()
	...
}

In the case of some methods on sdk.Msg, we could replace them with declarative annotations. For instance, GetSigners can already be replaced by the protobuf annotation cosmos.msg.v1.signer. In the future, we may consider some sort of protobuf validation framework (like https://github.com/bufbuild/protoc-gen-validate but more Cosmos-specific) to replace ValidateBasic.

Pinned FileDescriptor's

To solve 2), state machine modules must be able to specify what the version of the protobuf files was that they were built against. For instance if the API module for foo upgrades to foo/v2, the original foo module still needs a copy of the original protobuf files it was built with so that ADR 020 unknown field filtering will reject MsgDoSomething when condition is set.

The simplest way to do this may be to embed the protobuf FileDescriptors into the module itself so that these FileDescriptors are used at runtime rather than the ones that are built into the foo/api which may be different. Using buf build, go embed, and a build script we can probably come up with a solution for embedding FileDescriptors into modules that is fairly straightforward.

Potential limitations to generated code

One challenge with this approach is that it places heavy restrictions on what can go in API modules and requires that most of this is state machine breaking. All or most of the code in the API module would be generated from protobuf files, so we can probably control this with how code generation is done, but it is a risk to be aware of.

For instance, we do code generation for the ORM that in the future could contain optimizations that are state machine breaking. We would either need to ensure very carefully that the optimizations aren't actually state machine breaking in generated code or separate this generated code out from the API module into the state machine module. Both of these mitigations are potentially viable but the API module approach does require an extra level of care to avoid these sorts of issues.

Minor Version Incompatibilities

This approach in and of itself does little to address any potential minor version incompatibilities and the requisite unknown field filtering. Likely some sort of client-server routing layer which does this check such as ADR 033: Inter-Module communication is required to make sure that this is done properly. We could then allow modules to perform a runtime check given a MsgClient, ex:

func (k Keeper) CallFoo() error {
	if k.interModuleClient.MinorRevision(k.fooMsgClient) >= 2 {
		k.fooMsgClient.DoSomething(&MsgDoSomething{Condition: ...})
    } else {
        ...
    }
}

To do the unknown field filtering itself, the ADR 033 router would need to use the protoreflect API to ensure that no fields unknown to the receiving module are set. This could result in an undesirable performance hit depending on how complex this logic is.

Approach B) Changes to Generated Code

An alternate approach to solving the versioning problem is to change how protobuf code is generated and move modules mostly or completely in the direction of inter-module communication as described in ADR 033. In this paradigm, a module could generate all the types it needs internally - including the API types of other modules - and talk to other modules via a client-server boundary. For instance, if bar needs to talk to foo, it could generate its own version of MsgDoSomething as bar/internal/foo/v1.MsgDoSomething and just pass this to the inter-module router which would somehow convert it to the version which foo needs (ex. foo/internal.MsgDoSomething).

Currently, two generated structs for the same protobuf type cannot exist in the same go binary without special build flags (see https://developers.google.com/protocol-buffers/docs/reference/go/faq#fix-namespace-conflict). A relatively simple mitigation to this issue would be to set up the protobuf code to not register protobuf types globally if they are generated in an internal/ package. This will require modules to register their types manually with the app-level protobuf registry, this is similar to what modules already do with the InterfaceRegistry and amino codec.

If modules only do ADR 033 message passing then a naive and non-performant solution for converting bar/internal/foo/v1.MsgDoSomething to foo/internal.MsgDoSomething would be marshaling and unmarshaling in the ADR 033 router. This would break down if we needed to expose protobuf types in Keeper interfaces because the whole point is to try to keep these types internal/ so that we don't end up with all the import version incompatibilities we've described above. However, because of the issue with minor version incompatibilities and the need for unknown field filtering, sticking with the Keeper paradigm instead of ADR 033 may be unviable to begin with.

A more performant solution (that could maybe be adapted to work with Keeper interfaces) would be to only expose getters and setters for generated types and internally store data in memory buffers which could be passed from one implementation to another in a zero-copy way.

For example, imagine this protobuf API with only getters and setters is exposed for MsgSend:

type MsgSend interface {
	proto.Message
	GetFromAddress() string
	GetToAddress() string
	GetAmount() []v1beta1.Coin
    SetFromAddress(string)
    SetToAddress(string)
    SetAmount([]v1beta1.Coin)
}

func NewMsgSend() MsgSend { return &msgSendImpl{memoryBuffers: ...} }

Under the hood, MsgSend could be implemented based on some raw memory buffer in the same way that Cap'n Proto and FlatBuffers so that we could convert between one version of MsgSend and another without serialization (i.e. zero-copy). This approach would have the added benefits of allowing zero-copy message passing to modules written in other languages such as Rust and accessed through a VM or FFI. It could also make unknown field filtering in inter-module communication simpler if we require that all new fields are added in sequential order, ex. just checking that no field > 5 is set.

Also, we wouldn't have any issues with state machine breaking code on generated types because all the generated code used in the state machine would actually live in the state machine module itself. Depending on how interface types and protobuf Anys are used in other languages, however, it may still be desirable to take the handler approach described in approach A. Either way, types implementing interfaces would still need to be registered with an InterfaceRegistry as they are now because there would be no way to retrieve them via the global registry.

In order to simplify access to other modules using ADR 033, a public API module (maybe even one remotely generated by Buf) could be used by client modules instead of requiring to generate all client types internally.

The big downsides of this approach are that it requires big changes to how people use protobuf types and would be a substantial rewrite of the protobuf code generator. This new generated code, however, could still be made compatible with the google.golang.org/protobuf/reflect/protoreflect API in order to work with all standard golang protobuf tooling.

It is possible that the naive approach of marshaling/unmarshaling in the ADR 033 router is an acceptable intermediate solution if the changes to the code generator are seen as too complex. However, since all modules would likely need to migrate to ADR 033 anyway with this approach, it might be better to do this all at once.

Approach C) Don't address these issues

If the above solutions are seen as too complex, we can also decide not to do anything explicit to enable better module version compatibility, and break circular dependencies.

In this case, when developers are confronted with the issues described above they can require dependencies to update in sync (what we do now) or attempt some ad-hoc potentially hacky solution.

One approach is to ditch go semantic import versioning (SIV) altogether. Some people have commented that go's SIV (i.e. changing the import path to foo/v2, foo/v3, etc.) is too restrictive and that it should be optional. The golang maintainers disagree and only officially support semantic import versioning. We could, however, take the contrarian perspective and get more flexibility by using 0.x-based versioning basically forever.

Module version compatibility could then be achieved using go.mod replace directives to pin dependencies to specific compatible 0.x versions. For instance if we knew foo 0.2 and 0.3 were both compatible with bar 0.3 and 0.4, we could use replace directives in our go.mod to stick to the versions of foo and bar we want. This would work as long as the authors of foo and bar avoid incompatible breaking changes between these modules.

Or, if developers choose to use semantic import versioning, they can attempt the naive solution described above and would also need to use special tags and replace directives to make sure that modules are pinned to the correct versions.

Note, however, that all of these ad-hoc approaches, would be vulnerable to the minor version compatibility issues described

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