ABCI++: Evolving Tendermint’s Modularity - Sergio Mena

Application BlockChain Interface (ABCI) is the boundary between consensus and state machine replication (SMR) for Tendermint-based blockchains in Cosmos. ABCI was designed with the main goal of hiding the complexities of consensus to the SMR application developers. This led to a fool-proof interface where all the application is required to do is ensure deterministic execution in order to run on top of consensus.

Over the years, the tradeoffs of such a simple, highly modular interface became evident. If ABCI could expose more elements of consensus execution, this would allow for a finer grained control of SMR applications, such as transaction reordering or improved validity checks. To overcome these limitations, a major evolution of ABCI, called ABCI++, has been introduced.

ABCI++ increases the control an application developer has over executions of SMR. This increased control comes at the cost of an increased risk for the applications to accidentally affect core properties of consensus. In ABCI, the only risk was non-determinism in the application that could halt the chain. In ABCI++, buggy applications can lead to additional problems for consensus.

In this talk we walk through the main innovations in ABCI++; we discuss the new use cases they enable, as well as the risks they introduce and how these risks are captured in the specification.