tendermint/docs/architecture/adr-009-ABCI-design.md

ADR 009: ABCI UX Improvements

Changelog

23-06-2018: Some minor fixes from review 07-06-2018: Some updates based on discussion with Jae 07-06-2018: Initial draft to match what was released in ABCI v0.11

Context

The ABCI was first introduced in late 2015. It's purpose is to be:

• a generic interface between state machines and their replication engines
• agnostic to the language the state machine is written in
• agnostic to the replication engine that drives it

This means ABCI should provide an interface for both pluggable applications and pluggable consensus engines.

To achieve this, it uses Protocol Buffers (proto3) for message types. The dominant implementation is in Go.

After some recent discussions with the community on github, the following were identified as pain points:

• Amino encoded types
• Managing validator sets
• Imports in the protobuf file

See the references for more.

Imports

The native proto library in Go generates inflexible and verbose code. Many in the Go community have adopted a fork called gogoproto that provides a variety of features aimed to improve the developer experience. While gogoproto is nice, it creates an additional dependency, and compiling the protobuf types for other languages has been reported to fail when gogoproto is used.

Amino

Amino is an encoding protocol designed to improve over insufficiencies of protobuf. It's goal is to be proto4.

Many people are frustrated by incompatibility with protobuf, and with the requirement for Amino to be used at all within ABCI.

We intend to make Amino successful enough that we can eventually use it for ABCI message types directly. By then it should be called proto4. In the meantime, we want it to be easy to use.

PubKey

PubKeys are encoded using Amino (and before that, go-wire). Ideally, PubKeys are an interface type where we don't know all the implementation types, so its unfitting to use oneof or enum.

Addresses

The address for ED25519 pubkey is the RIPEMD160 of the Amino encoded pubkey. This introduces an Amino dependency in the address generation, a functionality that is widely required and should be easy to compute as possible.

Validators

To change the validator set, applications can return a list of validator updates with ResponseEndBlock. In these updates, the public key must be included, because Tendermint requires the public key to verify validator signatures. This means ABCI developers have to work with PubKeys. That said, it would also be convenient to work with address information, and for it to be simple to do so.

AbsentValidators

Tendermint also provides a list of validators in BeginBlock who did not sign the last block. This allows applications to reflect availability behaviour in the application, for instance by punishing validators for not having votes included in commits.

InitChain

Tendermint passes in a list of validators here, and nothing else. It would benefit the application to be able to control the initial validator set. For instance the genesis file could include application-based information about the initial validator set that the application could process to determine the initial validator set. Additionally, InitChain would benefit from getting all the genesis information.

Header

ABCI provides the Header in RequestBeginBlock so the application can have important information about the latest state of the blockchain.

Decision

Imports

Move away from gogoproto. In the short term, we will just maintain a second protobuf file without the gogoproto annotations. In the medium term, we will make copies of all the structs in Golang and shuttle back and forth. In the long term, we will use Amino.

Amino

To simplify ABCI application development in the short term, Amino will be completely removed from the ABCI:

• It will not be required for PubKey encoding
• It will not be required for computing PubKey addresses

That said, we are working to make Amino a huge success, and to become proto4. To facilitate adoption and cross-language compatibility in the near-term, Amino v1 will:

• be fully compatible with the subset of proto3 that excludes oneof
• use the Amino prefix system to provide interface types, as opposed to oneof style union types.

That said, an Amino v2 will be worked on to improve the performance of the format and its useability in cryptographic applications.

PubKey

Encoding schemes infect software. As a generic middleware, ABCI aims to have some cross scheme compatibility. For this it has no choice but to include opaque bytes from time to time. While we will not enforce Amino encoding for these bytes yet, we need to provide a type system. The simplest way to do this is to use a type string.

PubKey will now look like:

message PubKey {
    string type
    bytes data
}

where type can be:

• "ed225519", with data = <raw 32-byte pubkey>
• "secp256k1", with data = <33-byte OpenSSL compressed pubkey>

As we want to retain flexibility here, and since ideally, PubKey would be an interface type, we do not use enum or oneof.

Addresses

To simplify and improve computing addresses, we change it to the first 20-bytes of the SHA256 of the raw 32-byte public key.

We continue to use the Bitcoin address scheme for secp256k1 keys.

Validators

Add a bytes address field:

message Validator {
    bytes address
    PubKey pub_key
    int64 power
}

RequestBeginBlock and AbsentValidators

To simplify this, RequestBeginBlock will include the complete validator set, including the address, and voting power of each validator, along with a boolean for whether or not they voted:

message RequestBeginBlock {
  bytes hash
  Header header
  LastCommitInfo last_commit_info
  repeated Evidence byzantine_validators
}

message LastCommitInfo {
  int32 CommitRound
  repeated SigningValidator validators
}

message SigningValidator {
    Validator validator
    bool signed_last_block
}

Note that in Validators in RequestBeginBlock, we DO NOT include public keys. Public keys are larger than addresses and in the future, with quantum computers, will be much larger. The overhead of passing them, especially during fast-sync, is significant.

Additional, addresses are changing to be simpler to compute, further removing the need to include pubkeys here.

In short, ABCI developers must be aware of both addresses and public keys.

ResponseEndBlock

Since ResponseEndBlock includes Validator, it must now include their address.

InitChain

Change RequestInitChain to give the app all the information from the genesis file:

message RequestInitChain {
    int64 time
    string chain_id
    ConsensusParams consensus_params
    repeated Validator validators
    bytes app_state_bytes
}

Change ResponseInitChain to allow the app to specify the initial validator set and consensus parameters.

message ResponseInitChain {
    ConsensusParams consensus_params
    repeated Validator validators
}

Header

Now that Tendermint Amino will be compatible with proto3, the Header in ABCI should exactly match the Tendermint header - they will then be encoded identically in ABCI and in Tendermint Core.

Status

Implemented

Consequences

Positive

• Easier for developers to build on the ABCI
• ABCI and Tendermint headers are identically serialized

Negative

• Maintenance overhead of alternative type encoding scheme
• Performance overhead of passing all validator info every block (at least its only addresses, and not also pubkeys)
• Maintenance overhead of duplicate types

Neutral

• ABCI developers must know about validator addresses

References

• ABCI v0.10.3 Specification (before this proposal)
• ABCI v0.11.0 Specification (implementing first draft of this proposal)
• Ed25519 addresses
• InitChain contains the Genesis
• PubKeys
• Notes on Header
• Gogoproto issues
• Absent Validators