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tendermint/spec/abci++/README.md
2022-02-17 13:02:48 +01:00

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---
order: 1
parent:
title: ABCI++
order: 3
---
# 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 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](https://github.com/tendermint/tendermint/blob/master/proto/spec/abci/types.proto).
This allows Tendermint to run with applications written in many programming languages.
This specification is split as follows:
- [Basic concepts and definitions](./abci++_basic_concepts_002_draft.md) - definitions and descriptions
of concepts that are needed to understand other parts of this sepcification.
- [Methods](./abci++_methods_002_draft.md) - complete details on all ABCI++ methods
and message types.
- [Requirements for the Application](./abci++_app_requirements_002_draft.md) - formal requirements
on the Application's logic to ensure liveness of Tendermint. These requirements define what
Tendermint expects from the Application.
- [Tendermint's expected behavior](./abci++_tmint_expected_behavior_002_draft.md) - specification of
how the different ABCI++ methods may be called by Tendermint. This explains what the Application
is to expect from Tendermint.
>**TODO** Re-read these and remove redundant info
- [Applications](../abci/apps.md) - how to manage ABCI application state and other
details about building ABCI applications
- [Client and Server](../abci/client-server.md) - for those looking to implement their
own ABCI application servers