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.
