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ABCI++ is a major evolution of ABCI (Application Blockchain Interface).
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. This allows CometBFT 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 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 - specification of how the different ABCI++ methods may be called by CometBFT. This explains what the Application is to expect from CometBFT.
- Example scenarios - specific scenarios showing why the Application needs to account for any CometBFT's behaviour prescribed by the specification.
- Client and Server - for those looking to implement their own ABCI application servers.