cosmos-sdk/docs/architecture/adr-020-protobuf-transaction-encoding.md

ADR 020: Protocol Buffer Transaction Encoding

Changelog

• 2020 March 06: Initial Draft
• 2020 March 12: API Updates

Status

Proposed

Context

This ADR is a continuation of the motivation, design, and context established in ADR 019, namely, we aim to design the Protocol Buffer migration path for the client-side of the Cosmos SDK.

Specifically, the client-side migration path primarily includes tx generation and signing, message construction and routing, in addition to CLI & REST handlers and business logic (i.e. queriers).

With this in mind, we will tackle the migration path via two main areas, txs and querying. However, this ADR solely focuses on transactions. Querying should be addressed in a future ADR, but it should build off of these proposals.

Decision

Transactions

Since the messages that an application is known and allowed to handle are specific to the application itself, so must the transactions be specific to the application itself. Similar to how we described in ADR 019, the concrete types will be defined at the application level via Protobuf oneof.

The application will define a single canonical Message Protobuf message with a single oneof that implements the SDK's Msg interface.

Example:

// app/codec/codec.proto

message Message {
  option (cosmos_proto.interface_type) = "github.com/cosmos/cosmos-sdk/types.Msg";

  oneof sum {
    cosmos_sdk.x.bank.v1.MsgSend              msg_send             = 1;
    cosmos_sdk.x.bank.v1.MsgMultiSend         msg_multi_send       = 2;
    cosmos_sdk.x.crisis.v1.MsgVerifyInvariant msg_verify_invariant = 3;
    // ...
  }
}

Because an application needs to define it's unique Message Protobuf message, it will by proxy have to define a Transaction Protobuf message that encapsulates this Message type. The Transaction message type must implement the SDK's Tx interface.

Example:

// app/codec/codec.proto

message Transaction {
  cosmos_sdk.x.auth.v1.StdTxBase base = 1;
  repeated Message               msgs = 2;
}

Note, the Transaction type includes StdTxBase which will be defined by the SDK and includes all the core field members that are common across all transaction types. Developers do not have to include StdTxBase if they wish, so it is meant to be used as an auxiliary type.

Signing

Signing of a Transaction must be canonical across clients and binaries. In order to provide canonical representation of a Transaction to sign over, clients must obey the following rules:

• Encode SignDoc (see below) via Protobuf's canonical JSON encoding.
  • Default must be stripped from the output!
  • JSON keys adhere to their Proto-defined field names.
• Generate canonical JSON to sign via the JSON Canonical Form Spec.
  • This spec should be trivial to interpret and implement in any language.

// app/codec/codec.proto

message SignDoc {
  StdSignDocBase base = 1;
  repeated Message msgs = 2;
}

CLI & REST

Currently, the REST and CLI handlers encode and decode types and txs via Amino JSON encoding using a concrete Amino codec. Being that some of the types dealt with in the client can be interfaces, similar to how we described in ADR 019, the client logic will now need to take a codec interface that knows not only how to handle all the types, but also knows how to generate transactions, signatures, and messages.

type Generator interface {
  NewTx() ClientTx
}

type ClientTx interface {
  sdk.Tx
  codec.ProtoMarshaler

  SetMsgs(...sdk.Msg) error
  GetSignatures() []sdk.Signature
  SetSignatures(...sdk.Signature)
  GetFee() sdk.Fee
  SetFee(sdk.Fee)
  GetMemo() string
  SetMemo(string)

  CanonicalSignBytes(cid string, num, seq uint64) ([]byte, error)
}

We then update CLIContext to have a new field: Marshler.

Then, each module's client handler will at the minimum accept a Marshaler instead of a concrete Amino codec and a Generator along with an AccountRetriever so that account fields can be retrieved for signing. If the module needs to work with any interface types, it will use the Codec interface defined by the module which also extends Marshaler.

Future Improvements

Requiring application developers to have to redefine their Message Protobuf types can be extremely tedious and may increase the surface area of bugs by potentially missing one or more messages in the oneof.

To circumvent this, an optional strategy can be taken that has each module define it's own oneof and then the application-level Message simply imports each module's oneof. However, this requires additional tooling and the use of reflection.

Example:

// app/codec/codec.proto

message Message {
  option (cosmos_proto.interface_type) = "github.com/cosmos/cosmos-sdk/types.Msg";

  oneof sum {
    bank.Msg = 1;
    staking.Msg = 2;
    // ...
  }
}

Consequences

Positive

• Significant performance gains.
• Supports backward and forward type compatibility.
• Better support for cross-language clients.

Negative

• Learning curve required to understand and implement Protobuf messages.
• Less flexibility in cross-module type registration. We now need to define types at the application-level.
• Client business logic and tx generation become a bit more complex as developers have to define more types and implement more interfaces.

Neutral

References