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

# ADR 009: ABCI UX Improvements

## 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 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 following relevent github issues:
- TODO

### Imports

The native Protobuf library in Go generates code that is inellegant and difficult to work with.
The solution in the Go community is to use a fork of it called `gogoproto`.
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 Protobuf4.

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 Protobuf4. In the meantime,
we want it to be easy to use.

### PubKey

PubKeys were previously encoded using Amino (and before that, go-wire).

### Addresses

The address for an ED25519 pubkey is currently the RIPEMD160 of the Amino
encoded pubkey.

### 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.


## 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 Protobuf4.
To facilitate adoption and cross-language compatibility in the near-term, Amino
v1 will:

- be fully compatible with the subset of Protobuf3 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>`

and generated types. At the least we should use a reflection-based protobuf so
we can just encode our own types, rather than using protobuf generated ones.

### 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

Change the following:

- Validator includes an optional `bytes address` field
- If the field is provided, it *MUST* correspond to the `pubkey.Address()`


```
message Validator {
    bytes address
    PubKey pub_key
    int64 power
}
```

### AbsentValidators

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

```
message SigningValidator {
    Validator validator
    bool signed_last_block
}
```

### InitChain

Change RequestInitChain and ResponseInitChain to

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

message ResponseInitChain {
    ConsensusParams consensus_params
    repeated Validator validators
}
```

## Status

Accepted.

## Consequences

### Positive

- Easier for developers to build on the ABCI

### Negative

- Maintenance overhead of alternative type encoding scheme
- Performance overhead of passing all the validator info every block
- Maintenance overhead of duplicate types

### Neutral