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}
The Cosmos SDK utilizes two binary wire encoding protocols, [Amino](https://github.com/tendermint/go-amino/) which is an object encoding specification and [Protocol Buffers](https://developers.google.com/protocol-buffers), a subset of Proto3 with an extension for
Modules are encouraged to utilize Protobuf encoding for their respective types. In the SDK, we use the [Gogoproto](https://github.com/gogo/protobuf) specific implementation of the Protobuf spec that offers speed and DX improvements compared to the official [Google protobuf implementation](https://github.com/protocolbuffers/protobuf).
In addition to [following official Protocol Buffer guidelines](https://developers.google.com/protocol-buffers/docs/proto3#simple), we recommend using these annotations in .proto files when dealing with interfaces:
- use `cosmos_proto.accepts_interface` to annote fields that accept interfaces
- pass the same fully qualified name as `protoName` to `InterfaceRegistry.RegisterInterface`
- annotate interface implementations with `cosmos_proto.implements_interface`
- pass the same fully qualified name as `protoName` to `InterfaceRegistry.RegisterInterface`
### Transaction Encoding
Another important use of Protobuf is the encoding and decoding of
[transactions](./transactions.md). Transactions are defined by the application or
the 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.
See [ADR-020](../architecture/adr-020-protobuf-transaction-encoding.md) 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](../basics/accounts.md):
```proto
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](../../x/auth/spec/05_vesting.md), 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?
In [ADR-019](../architecture/adr-019-protobuf-state-encoding.md), it has been decided to use [`Any`](https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/any.proto)s 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:
```protobuf
message Profile {
// account is the account associated to a profile.
google.protobuf.Any account = 1 [
(cosmos_proto.accepts_interface) = "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`:
```go
var myAccount AccountI
myAccount = ... // Can be a BaseAccount, a ContinuousVestingAccount or any struct implementing `AccountI`
To summarize, to encode an interface, you must 1/ pack the interface into an `Any` and 2/ marshal the `Any`. For convenience, the SDK provides a `MarshalInterface` method to bundle these two steps. Have a look at [a real-life example in the x/auth module](https://github.com/cosmos/cosmos-sdk/blob/v0.42.1/x/auth/keeper/keeper.go#L218-L221).
The reverse operation of retrieving the concrete Go type from inside an `Any`, called "unpacking", is done with the `GetCachedValue()` on `Any`.
```go
profileBz := ... // The proto-encoded bytes of a Profile, e.g. retrieved through gRPC.
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.
It is important to note that for `GetCachedValue()` to work, `Profile` (and any other structs embedding `Profile`) must implement the `UnpackInterfaces` method:
The `UnpackInterfaces` gets called recursively on all structs implementing this method, to allow all `Any`s 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](../architecture/adr-019-protobuf-state-encoding.md).
#### `Any` Encoding in the SDK
The above `Profile` example is a fictive example used for educational purposes. In the 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 `Msg`s in a transaction,
- the `AccountI` interface for encodinig 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.
A real-life example of encoding the pubkey as `Any` inside the Validator struct in x/staking is shown in the following example:
We encourage developers to follow industry guidelines: [Protocol Buffers style guide](https://developers.google.com/protocol-buffers/docs/style)
and [Buf](https://buf.build/docs/style-guide), see more details in [ADR 023](../architecture/adr-023-protobuf-naming.md)
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`
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`](https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/any.proto) 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:
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:
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: