# ABCI++

# Introduction

ABCI++ is a major evolution of ABCI (Application Blockchain Interface). Like its predecessor, ABCI++ is the interface between Tendermint (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 Tendermint. The Application implements its logic for handling all ABCI++ methods. Thus, Tendermint always sends the Request* messages and receives the Response* messages in return.

All ABCI++ messages and methods are defined in protocol buffers. This allows Tendermint to run with applications written in many programming languages.

This specification is split as follows:

  • Overview and basic concepts - interface's overview and concepts needed to understand other parts of this specification.
  • Methods - complete details on all ABCI++ methods and message types.
  • Requirements for the Application - formal requirements on the Application's logic to ensure Tendermint properties such as liveness. These requirements define what Tendermint expects from the Application; second part on managing ABCI application state and related topics.
  • Tendermint's expected behavior - specification of how the different ABCI++ methods may be called by Tendermint. This explains what the Application is to expect from Tendermint.
  • Client and Server - for those looking to implement their own ABCI application servers