> ## Documentation Index
> Fetch the complete documentation index at: https://cosmos-docs-sync-security-docs.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Overview

# ABCI++

## Introduction

ABCI++ is a major evolution of ABCI (**A**pplication **B**lock**c**hain **I**nterface).
Like its predecessor, ABCI++ is the interface between CometBFT (a state-machine
replication engine) and the actual state machine being replicated (i.e., the Application).
The API consists of a set of *methods*, each with a corresponding `Request` and `Response`
message type.

The methods are always initiated by CometBFT. The Application implements its logic
for handling all ABCI++ methods.
Thus, CometBFT always sends the `Request*` messages and receives the `Response*` messages
in return.

All ABCI++ messages and methods are defined in [protocol buffers](https://github.com/cometbft/cometbft/blob/v0.38.x/proto/tendermint/abci/types.proto).
This allows CometBFT to run with applications written in many programming languages.

This specification is split as follows:

* [Overview and basic concepts](/cometbft/latest/spec/abci/Outline) - interface's overview and concepts
  needed to understand other parts of this specification.
* [Methods](/cometbft/latest/spec/abci/Methods) - complete details on all ABCI++ methods
  and message types.
* [Requirements for the Application](/cometbft/latest/spec/abci/Requirements-for-the-Application) - formal requirements
  on the Application's logic to ensure CometBFT properties such as liveness. These requirements define what
  CometBFT expects from the Application; second part on managing ABCI application state and related topics.
* [CometBFT's expected behavior](/cometbft/latest/spec/abci/CometBFTs-expected-behavior) - specification of
  how the different ABCI++ methods may be called by CometBFT. This explains what the Application
  is to expect from CometBFT.
* [Example scenarios](/cometbft/latest/spec/abci/Introduction) - specific scenarios showing why the Application needs to account
  for any CometBFT's behaviour prescribed by the specification.
* [Client and Server](/cometbft/latest/spec/abci/Client-and-server) - for those looking to implement their
  own ABCI application servers.