# Encoding

The codec is used everywhere in the Cosmos SDK to encode and decode structs and interfaces. The specific codec used in the Cosmos SDK is called go-amino.

# Pre-requisite Readings

# Encoding

The Cosmos SDK utilizes two binary wire encoding protocols, Amino and Protocol Buffers, where Amino is an object encoding specification. It is 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 three 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. Finally, HybridCodec, a codec that utilizes Protobuf for binary serialization and Amino for JSON serialization. The HybridCodec is typically the codec that used in majority in situations as it's easier to use for client and state serialization.

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 RegisterCodec 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:

Copy 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)

Another important use of the Amino 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.

Copy // TxDecoder unmarshals transaction bytes type TxDecoder func(txBytes []byte) (Tx, Error) // TxEncoder marshals transaction to bytes type TxEncoder func(tx Tx) ([]byte, error)

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

Copy // DefaultTxDecoder logic for standard transaction decoding func DefaultTxDecoder(cdc *codec.Codec) sdk.TxDecoder { return func(txBytes []byte) (sdk.Tx, sdk.Error) { var tx = StdTx{} if len(txBytes) == 0 { return nil, sdk.ErrTxDecode("txBytes are empty") } // StdTx.Msg is an interface. The concrete types // are registered by MakeTxCodec err := cdc.UnmarshalBinaryLengthPrefixed(txBytes, &tx) if err != nil { return nil, sdk.ErrTxDecode("error decoding transaction").TraceSDK(err.Error()) } return tx, nil } } // DefaultTxEncoder logic for standard transaction encoding func DefaultTxEncoder(cdc *codec.Codec) sdk.TxEncoder { return func(tx sdk.Tx) ([]byte, error) { return cdc.MarshalBinaryLengthPrefixed(tx) } }

# Gogoproto

Modules are encouraged to utilize Protobuf encoding for their respective types. If modules do not contain any interfaces (e.g. Account or Content), then they may simply accept a Marshaler as the codec which is implemented via the HybridCodec without any further customization.

However, if modules are to handle type interfaces, they should seek to extend the Marshaler interface contract for these types (e.g. MarshalAccount). Note, they should still use a HybridCodec internally. These extended contracts will typically use concrete types with unique oneof messages.

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