ADR 033: Protobuf-based Inter-Module Communication

Changelog

• 2020-10-05: Initial Draft

Status

Proposed

Abstract

This ADR introduces a system for permissioned inter-module communication leveraging the protobuf Query and Msg service definitions defined in ADR 021 and ADR 031 which provides:

• stable protobuf based module interfaces to potentially later replace the keeper paradigm
• stronger inter-module object capabilities (OCAPs) guarantees
• module accounts and sub-account authorization

Context

In the current Cosmos SDK documentation on the Object-Capability Model, it is stated that:

We assume that a thriving ecosystem of Cosmos SDK modules that are easy to compose into a blockchain application will contain faulty or malicious modules.

There is currently not a thriving ecosystem of Cosmos SDK modules. We hypothesize that this is in part due to:

1. lack of a stable v1.0 Cosmos SDK to build modules off of. Module interfaces are changing, sometimes dramatically, from point release to point release, often for good reasons, but this does not create a stable foundation to build on.
2. lack of a properly implemented object capability or even object-oriented encapsulation system which makes refactors of module keeper interfaces inevitable because the current interfaces are poorly constrained.

x/bank Case Study

Currently the x/bank keeper gives pretty much unrestricted access to any module which references it. For instance, the SetBalance method allows the caller to set the balance of any account to anything, bypassing even proper tracking of supply.

There appears to have been some later attempts to implement some semblance of OCAPs using module-level minting, staking and burning permissions. These permissions allow a module to mint, burn or delegate tokens with reference to the module’s own account. These permissions are actually stored as a []string array on the ModuleAccount type in state.

However, these permissions don’t really do much. They control what modules can be referenced in the MintCoins, BurnCoins and DelegateCoins*** methods, but for one there is no unique object capability token that controls access — just a simple string. So the x/upgrade module could mint tokens for the x/staking module simple by calling MintCoins(“staking”). Furthermore, all modules which have access to these keeper methods, also have access to SetBalance negating any other attempt at OCAPs and breaking even basic object-oriented encapsulation.

Decision

Based on ADR-021 and ADR-031, we introduce the Inter-Module Communication framework for secure module authorization and OCAPs. When implemented, this could also serve as an alternative to the existing paradigm of passing keepers between modules. The approach outlined here-in is intended to form the basis of a Cosmos SDK v1.0 that provides the necessary stability and encapsulation guarantees that allow a thriving module ecosystem to emerge.

Of particular note — the decision is to enable this functionality for modules to adopt at their own discretion. Proposals to migrate existing modules to this new paradigm will have to be a separate conversation, potentially addressed as amendments to this ADR.

New "Keeper" Paradigm

In ADR 021, a mechanism for using protobuf service definitions to define queriers was introduced and in ADR 31, a mechanism for using protobuf service to define Msgs was added. Protobuf service definitions generate two golang interfaces representing the client and server sides of a service plus some helper code. Here is a minimal example for the bank cosmos.bank.Msg/Send message type:

package bank

type MsgClient interface {
	Send(context.Context, *MsgSend, opts ...grpc.CallOption) (*MsgSendResponse, error)
}

type MsgServer interface {
	Send(context.Context, *MsgSend) (*MsgSendResponse, error)
}

ADR 021 and ADR 31 specifies how modules can implement the generated QueryServer and MsgServer interfaces as replacements for the legacy queriers and Msg handlers respectively.

In this ADR we explain how modules can make queries and send Msgs to other modules using the generated QueryClient and MsgClient interfaces and propose this mechanism as a replacement for the existing Keeper paradigm. To be clear, this ADR does not necessitate the creation of new protobuf definitions or services. Rather, it leverages the same proto based service interfaces already used by clients for inter-module communication.

Using this QueryClient/MsgClient approach has the following key benefits over exposing keepers to external modules:

1. Protobuf types are checked for breaking changes using buf and because of the way protobuf is designed this will give us strong backwards compatibility guarantees while allowing for forward evolution.
2. The separation between the client and server interfaces will allow us to insert permission checking code in between the two which checks if one module is authorized to send the specified Msg to the other module providing a proper object capability system (see below).
3. The router for inter-module communication gives us a convenient place to handle rollback of transactions, enabling atomicy of operations (currently a problem). Any failure within a module-to-module call would result in a failure of the entire transaction

This mechanism has the added benefits of:

• reducing boilerplate through code generation, and
• allowing for modules in other languages either via a VM like CosmWasm or sub-processes using gRPC

Inter-module Communication

To use the Client generated by the protobuf compiler we need a grpc.ClientConn interface implementation. For this we introduce a new type, ModuleKey, which implements the grpc.ClientConn interface. ModuleKey can be thought of as the "private key" corresponding to a module account, where authentication is provided through use of a special Invoker() function, described in more detail below.

Blockchain users (external clients) use their account's private key to sign transactions containing Msgs where they are listed as signers (each message specifies required signers with Msg.GetSigner). The authentication checks is performed by AnteHandler.

Here, we extend this process, by allowing modules to be identified in Msg.GetSigners. When a module wants to trigger the execution a Msg in another module, its ModuleKey acts as the sender (through the ClientConn interface we describe below) and is set as a sole "signer". It's worth to note that we don't use any cryptographic signature in this case. For example, module A could use its A.ModuleKey to create MsgSend object for /cosmos.bank.Msg/Send transaction. MsgSend validation will assure that the from account (A.ModuleKey in this case) is the signer.

Here's an example of a hypothetical module foo interacting with x/bank:

package foo


type FooMsgServer {
  // ...

  bankQuery bank.QueryClient
  bankMsg   bank.MsgClient
}

func NewFooMsgServer(moduleKey RootModuleKey, ...) FooMsgServer {
  // ...

  return FooMsgServer {
    // ...
    modouleKey: moduleKey,
    bankQuery: bank.NewQueryClient(moduleKey),
    bankMsg: bank.NewMsgClient(moduleKey),
  }
}

func (foo *FooMsgServer) Bar(ctx context.Context, req *MsgBarRequest) (*MsgBarResponse, error) {
  balance, err := foo.bankQuery.Balance(&bank.QueryBalanceRequest{Address: fooMsgServer.moduleKey.Address(), Denom: "foo"})

  ...

  res, err := foo.bankMsg.Send(ctx, &bank.MsgSendRequest{FromAddress: fooMsgServer.moduleKey.Address(), ...})

  ...
}

This design is also intended to be extensible to cover use cases of more fine grained permissioning like minting by denom prefix being restricted to certain modules (as discussed in #7459).

ModuleKeys and ModuleIDs

A ModuleKey can be thought of as a "private key" for a module account and a ModuleID can be thought of as the corresponding "public key". From the ADR 028, modules can have both a root module account and any number of sub-accounts or derived accounts that can be used for different pools (ex. staking pools) or managed accounts (ex. group accounts). We can also think of module sub-accounts as similar to derived keys - there is a root key and then some derivation path. ModuleID is a simple struct which contains the module name and optional "derivation" path, and forms its address based on the AddressHash method from the ADR-028:

type ModuleID struct {
  ModuleName string
  Path []byte
}

func (key ModuleID) Address() []byte {
  return AddressHash(key.ModuleName, key.Path)
}

In addition to being able to generate a ModuleID and address, a ModuleKey contains a special function called Invoker which is the key to safe inter-module access. The Invoker creates an InvokeFn closure which is used as an Invoke method in the grpc.ClientConn interface and under the hood is able to route messages to the appropriate Msg and Query handlers performing appropriate security checks on Msgs. This allows for even safer inter-module access than keeper's whose private member variables could be manipulated through reflection. Golang does not support reflection on a function closure's captured variables and direct manipulation of memory would be needed for a truly malicious module to bypass the ModuleKey security.

The two ModuleKey types are RootModuleKey and DerivedModuleKey:

type Invoker func(callInfo CallInfo) func(ctx context.Context, request, response interface{}, opts ...interface{}) error

type CallInfo {
  Method string
  Caller ModuleID
}

type RootModuleKey struct {
  moduleName string
  invoker Invoker
}

func (rm RootModuleKey) Derive(path []byte) DerivedModuleKey { /* ... */}

type DerivedModuleKey struct {
  moduleName string
  path []byte
  invoker Invoker
}

A module can get access to a DerivedModuleKey, using the Derive(path []byte) method on RootModuleKey and then would use this key to authenticate Msgs from a sub-account. Ex:

package foo

func (fooMsgServer *MsgServer) Bar(ctx context.Context, req *MsgBar) (*MsgBarResponse, error) {
  derivedKey := fooMsgServer.moduleKey.Derive(req.SomePath)
  bankMsgClient := bank.NewMsgClient(derivedKey)
  res, err := bankMsgClient.Balance(ctx, &bank.MsgSend{FromAddress: derivedKey.Address(), ...})
  ...
}

In this way, a module can gain permissioned access to a root account and any number of sub-accounts and send authenticated Msgs from these accounts. The Invoker callInfo.Caller parameter is used under the hood to distinguish between different module accounts, but either way the function returned by Invoker only allows Msgs from either the root or a derived module account to pass through.

Note that Invoker itself returns a function closure based on the CallInfo passed in. This will allow client implementations in the future that cache the invoke function for each method type avoiding the overhead of hash table lookup. This would reduce the performance overhead of this inter-module communication method to the bare minimum required for checking permissions.

To re-iterate, the closure only allows access to authorized calls. There is no access to anything else regardless of any name impersonation.

Below is a rough sketch of the implementation of grpc.ClientConn.Invoke for RootModuleKey:

func (key RootModuleKey) Invoke(ctx context.Context, method string, args, reply interface{}, opts ...grpc.CallOption) error {
  f := key.invoker(CallInfo {Method: method, Caller: ModuleID {ModuleName: key.moduleName}})
  return f(ctx, args, reply)
}

AppModule Wiring and Requirements

In ADR 031, the AppModule.RegisterService(Configurator) method was introduced. To support inter-module communication, we extend the Configurator interface to pass in the ModuleKey and to allow modules to specify their dependencies on other modules using RequireServer():

type Configurator interface {
   MsgServer() grpc.Server
   QueryServer() grpc.Server

   ModuleKey() ModuleKey
   RequireServer(msgServer interface{})
}

The ModuleKey is passed to modules in the RegisterService method itself so that RegisterServices serves as a single entry point for configuring module services. This is intended to also have the side-effect of greatly reducing boilerplate in app.go. For now, ModuleKeys will be created based on AppModule.Name(), but a more flexible system may be introduced in the future. The ModuleManager will handle creation of module accounts behind the scenes.

Because modules do not get direct access to each other anymore, modules may have unfulfilled dependencies. To make sure that module dependencies are resolved at startup, the Configurator.RequireServer method should be added. The ModuleManager will make sure that all dependencies declared with RequireServer can be resolved before the app starts. An example module foo could declare it's dependency on x/bank like this:

package foo

func (am AppModule) RegisterServices(cfg Configurator) {
  cfg.RequireServer((*bank.QueryServer)(nil))
  cfg.RequireServer((*bank.MsgServer)(nil))
}

Security Considerations

In addition to checking for ModuleKey permissions, a few additional security precautions will need to be taken by the underlying router infrastructure.

Recursion and Re-entry

Recursive or re-entrant method invocations pose a potential security threat. This can be a problem if Module A calls Module B and Module B calls module A again in the same call.

One basic way for the router system to deal with this is to maintain a call stack which prevents a module from being referenced more than once in the call stack so that there is no re-entry. A map[string]interface{} table in the router could be used to perform this security check.

Queries

Queries in Cosmos SDK are generally un-permissioned so allowing one module to query another module should not pose any major security threats assuming basic precautions are taken. The basic precaution that the router system will need to take is making sure that the sdk.Context passed to query methods does not allow writing to the store. This can be done for now with a CacheMultiStore as is currently done for BaseApp queries.

Internal Methods

In many cases, we may wish for modules to call methods on other modules which are not exposed to clients at all. For this purpose, we add the InternalServer method to Configurator:

type Configurator interface {
   MsgServer() grpc.Server
   QueryServer() grpc.Server
   InternalServer() grpc.Server
}

As an example, x/slashing's Slash must call x/staking's Slash, but we don't want to expose x/staking's Slash to end users and clients.

Internal protobuf services will be defined in a corresponding internal.proto file in the given module's proto package.

Services registered against InternalServer will be callable from other modules but not by external clients.

An alternative solution to internal-only methods could involve hooks / plugins as discussed here. A more detailed evaluation of a hooks / plugin system will be addressed later in follow-ups to this ADR or as a separate ADR.

Authorization

By default, the inter-module router requires that messages are sent by the first signer returned by GetSigners. The inter-module router should also accept authorization middleware such as that provided by ADR 030. This middleware will allow accounts to otherwise specific module accounts to perform actions on their behalf. Authorization middleware should take into account the need to grant certain modules effectively "admin" privileges to other modules. This will be addressed in separate ADRs or updates to this ADR.

Future Work

Other future improvements may include:

• custom code generation that:
  • simplifies interfaces (ex. generates code with sdk.Context instead of context.Context)
  • optimizes inter-module calls - for instance caching resolved methods after first invocation
• combining StoreKeys and ModuleKeys into a single interface so that modules have a single OCAPs handle
• code generation which makes inter-module communication more performant
• decoupling ModuleKey creation from AppModule.Name() so that app's can override root module account names
• inter-module hooks and plugins

Alternatives

MsgServices vs x/capability

The x/capability module does provide a proper object-capability implementation that can be used by any module in the Cosmos SDK and could even be used for inter-module OCAPs as described in #5931.

The advantages of the approach described in this ADR are mostly around how it integrates with other parts of the Cosmos SDK, specifically:

• protobuf so that:
  • code generation of interfaces can be leveraged for a better dev UX
  • module interfaces are versioned and checked for breakage using buf
• sub-module accounts as per ADR 028
• the general Msg passing paradigm and the way signers are specified by GetSigners

Also, this is a complete replacement for keepers and could be applied to all inter-module communication whereas the x/capability approach in #5931 would need to be applied method by method.

Consequences

Backwards Compatibility

This ADR is intended to provide a pathway to a scenario where there is greater long term compatibility between modules. In the short-term, this will likely result in breaking certain Keeper interfaces which are too permissive and/or replacing Keeper interfaces altogether.

Positive

• an alternative to keepers which can more easily lead to stable inter-module interfaces
• proper inter-module OCAPs
• improved module developer DevX, as commented on by several particpants on Architecture Review Call, Dec 3
• lays the groundwork for what can be a greatly simplified app.go
• router can be setup to enforce atomic transactions for module-to-module calls

Negative

• modules which adopt this will need significant refactoring

Neutral

Test Cases [optional]

References

• ADR 021
• ADR 031
• ADR 028
• ADR 030 draft
• Object-Capability Model