cosmos-sdk/docs/core/encoding.md

Encoding

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

Pre-requisite Readings

• Anatomy of an SDK application {prereq}

Encoding

The Cosmos SDK utilizes two binary wire encoding protocols, Amino which is an object encoding specification and Protocol Buffers, a subset of Proto3 with an extension for interface support. See the Proto3 spec for more information on Proto3, which Amino is largely compatible with (but not with Proto2).

Due to Amino having significant performance drawbacks, being reflection-based, and not having any meaningful cross-language/client support, Protocol Buffers, specifically gogoprotobuf, is being used in place of Amino. Note, this process of using Protocol Buffers over Amino is still an ongoing process.

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 store encoding, protobuf definitions can exist for any type and will typically have an Amino-based "intermediary" type. Specifically, the protobuf-based type definition is used for serialization and persistence, whereas the Amino-based type is used for business logic in the state-machine where they may converted back-n-forth. Note, the Amino-based types may slowly be phased-out in the future so developers should take note to use the protobuf message definitions where possible.

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

In addition, there exists two implementations of Marshaler. The first being AminoCodec, where both binary and JSON serialization is handled via Amino. The second being ProtoCodec, where both binary and JSON serialization is handled via Protobuf.

This means that modules may use Amino or Protobuf encoding but the types must implement ProtoMarshaler. If modules wish to avoid implementing this interface for their types, they may use an Amino codec directly.

Amino

Every module uses an Amino codec to serialize types and interfaces. This codec typically has types and interfaces registered in that module's domain only (e.g. messages), but there are exceptions like x/gov. Each module exposes a RegisterLegacyAminoCodec function that allows a user to provide a codec and have all the types registered. An application will call this method for each necessary module.

Where there is no protobuf-based type definition for a module (see below), Amino is used to encode and decode raw wire bytes to the concrete type or interface:

bz := keeper.cdc.MustMarshalBinaryBare(typeOrInterface)
keeper.cdc.MustUnmarshalBinaryBare(bz, &typeOrInterface)

Note, there are length-prefixed variants of the above functionality and this is typically used for when the data needs to be streamed or grouped together (e.g. ResponseDeliverTx.Data)

Gogoproto

Modules are encouraged to utilize Protobuf encoding for their respective types.

FAQ

1. 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

2. How to update modules to protobuf encoding?

If modules do not contain any interfaces (e.g. Account or Content), then they may simply migrate any existing types that are encoded and persisted via their concrete Amino codec to Protobuf (see 1. for further guidelines) and accept a Marshaler as the codec which is implemented via the ProtoCodec without any further customization.

However, if a module type composes an interface, it must wrap it in the skd.Any (from /types package) type. To do that, a module-level .proto file must use google.protobuf.Any for respective message type interface types.

For example, in the x/evidence module defines an Evidence interface, which is used by the MsgSubmitEvidence. The structure definition must use sdk.Any to wrap the evidence file. In the proto file we define it as follows:

// proto/cosmos/evidence/v1beta1/tx.proto

message MsgSubmitEvidence {
  string              submitter = 1;
  google.protobuf.Any evidence  = 2 [(cosmos_proto.accepts_interface) = "Evidence"];
}

The SDK codec.Marshaler interface provides support methods MarshalInterface and UnmarshalInterface to easy encoding of state to Any.

Module should register interfaces using InterfaceRegistry which provides a mechanism for registering interfaces: RegisterInterface(protoName string, iface interface{}) and implementations: RegisterImplementations(iface interface{}, impls ...proto.Message) that can be safely unpacked from Any, similarly to type registration with Amino:

+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc4/codec/types/interface_registry.go#L25-L66

In addition, an UnpackInterfaces phase should be introduced to deserialization to unpack interfaces before they're needed. Protobuf types that contain a protobuf Any either directly or via one of their members should implement the UnpackInterfacesMessage interface:

type UnpackInterfacesMessage interface {
  UnpackInterfaces(InterfaceUnpacker) error
}

Guidelines for protobuf message definitions

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

• fields which accept interfaces should be annotated with cosmos_proto.accepts_interface using the same full-qualified name passed as protoName to InterfaceRegistry.RegisterInterface
• interface implementations should be annotated with cosmos_proto.implements_interface using the same full-qualified name passed as protoName to InterfaceRegistry.RegisterInterface

Transaction Encoding

Another important use of Protobuf is the encoding and decoding of transactions. Transactions are defined by the application or the SDK, but passed to the underlying consensus engine in order to be relayed to other peers. Since the underlying consensus engine is agnostic to the application, it only accepts transactions in the form of raw bytes. The encoding is done by an object called TxEncoder and the decoding by an object called TxDecoder.

+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc4/types/tx_msg.go#L83-L87

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

+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc4/x/auth/tx/decoder.go

+++ https://github.com/cosmos/cosmos-sdk/blob/v0.40.0-rc4/x/auth/tx/encoder.go

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