libhotstuff
-----------

libhotstuff is a general-purpose BFT state machine replication library with
modularity and simplicity, suitable for building hybrid consensus
cryptocurrencies.

Paper
=====

This repo includes the prototype implementation evaluated in our *HotStuff: BFT
Consensus in the Lens of Blockchain* paper. The consensus protocol is also used
by Facebook in Libra_ project.

Feel free to contact us if you'd like to reproduce the results in the paper, or
tweak the code for your own project/product.

- Full paper: https://arxiv.org/abs/1803.05069
- PODC 2019 paper: https://dl.acm.org/citation.cfm?id=3331591

.. _Libra: https://github.com/libra

Features
========

- Simplicity. The protocol core logic implementation is as simple as the one
  specified in our paper. See ``consensus.h`` and ``consensus.cpp``.

- Modular design. You can use abstraction from the lowest level (the core
  protocol logic) to the highest level (the state machine replication service
  with network implementation) in your application, or override/redefine the
  detailed behavior to customize your own consensus.

- Liveness decoupled from safety. The liveness logic is entirely decoupled from
  safety. By defining your own liveness gadget ("PaceMaker"), you can implement
  your own liveness heuristics/algorithm.  The actual performance varies
  depending on your liveness implementation, but the safety is always intact.

- Friendly to blockchain systems. A PaceMaker could potentially be PoW-based and
  it makes it easier to build a hybrid consensus system, or use it at the core of
  some cryptocurrencies.

- Minimal. The project strives to keep code base small and implement just the
  basic functionality of state machine replication: to deliver a consistent
  command sequence to the library user. Application-specific parts are not
  included, but demonstrated in the demo program.

Try the Current Version
=======================

NOTICE: the project is still in-progress. Try at your own risk, and this
section may be incomplete and subject to changes.

::

    # install from the repo
    git clone https://github.com/hot-stuff/libhotstuff.git
    cd libhotstuff/
    git submodule update --init --recursive

    # ensure openssl and libevent are installed on your machine
    cmake -DCMAKE_BUILD_TYPE=Release -DBUILD_SHARED=ON -DHOTSTUFF_PROTO_LOG=ON
    make

    # start 4 demo replicas with scripts/run_demo.sh
    # then, start the demo client with scripts/run_demo_client.sh


    # Fault tolerance:
    # Try to run run_demo.sh first and then run_demo_client.sh, then use Ctrl-C
    # to terminate the proposing replica (e.g. replica 0). Leader rotation will
    # be scheduled. Try to kill and run run_demo_client.sh again, new commands
    # should still get through (be replicated) once the new leader becomes stable.

TODO
====

- Add a PoW-based Pacemaker
- Branch pruning & swapping (the current implementation stores the entire chain in memory)
- Limit the async events (improve robustness)
- Persistent protocol state (recovery?)