# Encoding

# Pre-requisite Readings

# Encoding

Every Cosmos SDK application exposes a global codec to marshal/unmarshal structs and interfaces in order to store and/or transfer them. As of now, the codec used in the Cosmos SDK is go-amino, which possesses the following important properties:

  • Interface support.
  • Deterministic encoding of value (which is required considering that blockchains are deterministic replicated state-machines).
  • Upgradeable schemas.

The application's codec is typically initialized in the application's constructor function, where it is also passed to each of the application's modules via the basic manager.

Among other things, the codec is used by module's keepers to marshal objects into []byte before storing them in the module's KVStore, or to unmarshal them from []byte when retrieving them:

Copy // typical pattern to marshal an object to []byte before storing it bz := keeper.cdc.MustMarshalBinaryBare(object) //typical pattern to unmarshal an object from []byte when retrieving it keeper.cdc.MustUnmarshalBinaryBare(bz, &object)

Alternatively, it is possible to use MustMarshalBinaryLengthPrefixed/MustUnmarshalBinaryLengthPrefixed instead of MustMarshalBinaryBare/MustUnmarshalBinaryBare for the same encoding prefixed by a uvarint encoding of the object to encode.

Another important use of the codec is the encoding and decoding of transactions. Transactions are defined at the Cosmos SDK level, 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 []byte. 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) } }

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