Encoding

Synopsis

While encoding in the Cosmos SDK used to be mainly handled by go-amino codec, the Cosmos SDK is moving towards using gogoprotobuf for both state and client-side encoding.

Pre-requisite Readings

• Anatomy of a Cosmos SDK application

Encoding

The Cosmos SDK supports two wire encoding protocols. Binary encoding is fulfilled by Protocol Buffers, specifically the gogoprotobuf implementation, which is a subset of Proto3 with an extension for interface support. Text encoding is fulfilled by Amino.

Due to Amino having significant performance drawbacks, being reflection-based, and not having any meaningful cross-language/client support, Amino is only used to generate JSON (Amino JSON) in order to support the Amino JSON sign mode, and for JSON RPC endpoints.

Binary wire encoding of types in the Cosmos SDK can be broken down into two main categories, client encoding and store encoding. Client encoding mainly revolves around transaction processing and signing, whereas store encoding revolves around types used in state-machine transitions and what is ultimately stored in the Merkle tree.

For storage encoding, module developers are encouraged to use Protobuf encoding for their types but may choose any encoding schema they like. The collections package automatically handles encoding and decoding of state for you.

In the codec package, there exist two core interfaces, BinaryCodec and JSONCodec, where the former encapsulates the current Amino interface except it operates on types implementing the latter instead of generic interface{} types.

The ProtoCodec, where both binary and JSON serialization are handled via Protobuf. This means that modules may use Protobuf encoding, but the types must implement ProtoMarshaler. If modules wish to avoid implementing this interface for their types, this is autogenerated via buf

Gogoproto

Modules are encouraged to utilize Protobuf encoding for their respective types. In the Cosmos SDK, we use the Gogoproto specific implementation of the Protobuf spec that offers speed and DX improvements compared to the official Google protobuf implementation.

Guidelines for protobuf message definitions

In addition to following official Protocol Buffer guidelines, we recommend using these annotations in .proto files when dealing with interfaces:

• use cosmos_proto.accepts_interface to annotate Any fields that accept interfaces
  • pass the same fully qualified name as protoName to InterfaceRegistry.RegisterInterface
  • example: (cosmos_proto.accepts_interface) = "cosmos.gov.v1beta1.Content" (and not just Content)
• annotate interface implementations with cosmos_proto.implements_interface
  • pass the same fully qualified name as protoName to InterfaceRegistry.RegisterInterface
  • example: (cosmos_proto.implements_interface) = "cosmos.authz.v1beta1.Authorization" (and not just Authorization)

Code generators can then match the accepts_interface and implements_interface annotations to know whether some Protobuf messages are allowed to be packed in a given Any field or not.

Transaction Encoding

Another important use of Protobuf is the encoding and decoding of transactions. Transactions are defined by the application or the Cosmos SDK but are then passed to the underlying consensus engine to be relayed to other peers. Since the underlying consensus engine is agnostic to the application, the consensus engine accepts only transactions in the form of raw bytes.

• The TxEncoder object performs the encoding.
• The TxDecoder object performs the decoding.

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.1/types/tx_msg.go#L117-L121

A standard implementation of both these objects can be found in the auth/tx module:

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.1/x/auth/tx/decoder.go

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.1/x/auth/tx/encoder.go

See ADR-020 for details of how a transaction is encoded.

Interface Encoding and Usage of Any

The Protobuf DSL is strongly typed, which can make inserting variable-typed fields difficult. Imagine we want to create a Profile protobuf message that serves as a wrapper over an account:

message Profile {
  // account is the account associated to a profile.
  cosmos.auth.v1beta1.BaseAccount account = 1;
  // bio is a short description of the account.
  string bio = 4;
}

In this Profile example, we hardcoded account as a BaseAccount. However, there are several other types of user accounts related to vesting, such as BaseVestingAccount or ContinuousVestingAccount. All of these accounts are different, but they all implement the AccountI interface. How would you create a Profile that allows all these types of accounts with an account field that accepts an AccountI interface?

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/types/account.go#L15-L32

In ADR-019, it has been decided to use Anys to encode interfaces in protobuf. An Any contains an arbitrary serialized message as bytes, along with a URL that acts as a globally unique identifier for and resolves to that message's type. This strategy allows us to pack arbitrary Go types inside protobuf messages. Our new Profile then looks like:

message Profile {
  // account is the account associated to a profile.
  google.protobuf.Any account = 1 [
    (cosmos_proto.accepts_interface) = "cosmos.auth.v1beta1.AccountI"; // Asserts that this field only accepts Go types implementing `AccountI`. It is purely informational for now.
  ];
  // bio is a short description of the account.
  string bio = 4;
}

To add an account inside a profile, we need to "pack" it inside an Any first, using codectypes.NewAnyWithValue:

var myAccount AccountI
myAccount = ... // Can be a BaseAccount, a ContinuousVestingAccount or any struct implementing `AccountI`

// Pack the account into an Any
accAny, err := codectypes.NewAnyWithValue(myAccount)
if err != nil {
  return nil, err
}

// Create a new Profile with the any.
profile := Profile {
  Account: accAny,
  Bio: "some bio",
}

// We can then marshal the profile as usual.
bz, err := cdc.Marshal(profile)
jsonBz, err := cdc.MarshalJSON(profile)

To summarize, to encode an interface, you must 1/ pack the interface into an Any and 2/ marshal the Any. For convenience, the Cosmos SDK provides a MarshalInterface method to bundle these two steps. Have a look at a real-life example in the x/auth module.

The reverse operation of retrieving the concrete Go type from inside an Any, called "unpacking", is done with the GetCachedValue() on Any.

profileBz := ... // The proto-encoded bytes of a Profile, e.g. retrieved through gRPC.
var myProfile Profile
// Unmarshal the bytes into the myProfile struct.
err := cdc.Unmarshal(profilebz, &myProfile)

// Let's see the types of the Account field.
fmt.Printf("%T\n", myProfile.Account)                  // Prints "Any"
fmt.Printf("%T\n", myProfile.Account.GetCachedValue()) // Prints "BaseAccount", "ContinuousVestingAccount" or whatever was initially packed in the Any.

// Get the address of the account.
accAddr := myProfile.Account.GetCachedValue().(AccountI).GetAddress()

It is important to note that for GetCachedValue() to work, Profile (and any other structs embedding Profile) must implement the UnpackInterfaces method:

func (p *Profile) UnpackInterfaces(unpacker codectypes.AnyUnpacker) error {
  if p.Account != nil {
    var account AccountI
    return unpacker.UnpackAny(p.Account, &account)
  }

  return nil
}

The UnpackInterfaces gets called recursively on all structs implementing this method, to allow all Anys to have their GetCachedValue() correctly populated.

For more information about interface encoding, and especially on UnpackInterfaces and how the Any's type_url gets resolved using the InterfaceRegistry, please refer to ADR-019.

Any Encoding in the Cosmos SDK

The above Profile example is a fictive example used for educational purposes. In the Cosmos SDK, we use Any encoding in several places (non-exhaustive list):

• the cryptotypes.PubKey interface for encoding different types of public keys,
• the sdk.Msg interface for encoding different Msgs in a transaction,
• the AccountI interface for encoding different types of accounts (similar to the above example) in the x/auth query responses,
• the EvidenceI interface for encoding different types of evidences in the x/evidence module,
• the AuthorizationI interface for encoding different types of x/authz authorizations,
• the Validator struct that contains information about a validator.

A real-life example of encoding the pubkey as Any inside the Validator struct in x/staking is shown in the following example:

https://github.com/cosmos/cosmos-sdk/blob/v0.52.0-beta.2/x/staking/types/validator.go#L43-L63

Any's TypeURL

When packing a protobuf message inside an Any, the message's type is uniquely defined by its type URL, which is the message's fully qualified name prefixed by a / (slash) character. In some implementations of Any, like the gogoproto one, there's generally a resolvable prefix, e.g. type.googleapis.com. However, in the Cosmos SDK, we made the decision to not include such prefix, to have shorter type URLs. The Cosmos SDK's own Any implementation can be found in github.com/cosmos/cosmos-sdk/codec/types.

The Cosmos SDK is also switching away from gogoproto to the official google.golang.org/protobuf (known as the Protobuf API v2). Its default Any implementation also contains the type.googleapis.com prefix. To maintain compatibility with the SDK, the following methods from "google.golang.org/protobuf/types/known/anypb" should not be used:

• anypb.New
• anypb.MarshalFrom
• anypb.Any#MarshalFrom

Instead, the Cosmos SDK provides helper functions in "github.com/cosmos/cosmos-proto/anyutil", which create an official anypb.Any without inserting the prefixes:

• anyutil.New
• anyutil.MarshalFrom

For example, to pack a sdk.Msg called internalMsg, use:

import (
-   "google.golang.org/protobuf/types/known/anypb"
+   "github.com/cosmos/cosmos-proto/anyutil"
)

- anyMsg, err := anypb.New(internalMsg.Message().Interface())
+ anyMsg, err := anyutil.New(internalMsg.Message().Interface())

- fmt.Println(anyMsg.TypeURL) // type.googleapis.com/cosmos.bank.v1beta1.MsgSend
+ fmt.Println(anyMsg.TypeURL) // /cosmos.bank.v1beta1.MsgSend

FAQ

How to create modules using protobuf encoding

Defining module types

Protobuf types can be defined to encode:

• state
• Msgs
• Query services
• genesis

Naming and conventions

We encourage developers to follow industry guidelines: Protocol Buffers style guide and Buf, see more details in ADR 023