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Concord-BFT: a Distributed Trust Infrastructure

License clang-tidy Build Status Build Status Build Status Build Status codecoverage


Concord-bft is a generic state machine replication library that can handle malicious (byzantine) replicas.

Its implementation is based on the algorithm described in the paper SBFT: a Scalable Decentralized Trust Infrastructure for Blockchains.

It is designed to be used as a core building block for replicated distributed data stores, and is especially suited to serve as the basis of permissioned Blockchain systems.

For a real-life integration example, please take a look at Project Concord, a highly scalable and energy-efficient distributed trust infrastructure for consensus and smart contract execution.


We abide by semantic versioning. Public APIs can change at any time until we reach version 1.0. We will, however, bump the minor version when a backwards incompatible change is made.


Install and Build (Ubuntu Linux 18.04)

Concord-BFT supports two kinds of builds: native and docker.

The docker build is strongly recommended.


cd concord-bft
make test

Run make help to see more commands.


  • The output binaries are stored in the host's concord-bft/build.
  • Makefile is configurable. For example, if you want to use another compiler you may pass it to the make:

Other build options, including passthrough options for CMake, are defined in the Makefile and prefixed with CONCORD_BFT_. Variables that are capable of being overridden on the commandline are set with the Make conditional operator ?= and are at the beginning of Makefile. Please check that file for options.

Select comm module

One option that is worth calling out explicitly is the communication (transport) library. Transport defaults to TLS and can be configured explicitly by setting the CONCORD_BFT_CMAKE_TRANSPORT flag. The flag defaults to TLS, but also supports UDP and TCP. These can be useful because the use of pinned certificates for TLS requires an out of band setup.

See for an example. This script is used in apollo tests. For production usage, an out of band deployment for each replica must be used to avoid revealing private keys to each replica.


git clone
cd concord-bft
sudo ./ # Installs all dependencies and 3rd parties
mkdir build
cd build
cmake ..
sudo make test

By default the build chooses the active compiler on the native build platform. In order to force compilation by clang you can use the following command.

CC=clang CXX=clang++ cmake ..

In order to turn on or off various options, you need to change your cmake configuration. This is done by passing arguments to cmake with a -D prefix: e.g. cmake -DBUILD_TESTING=OFF. Note that make must be run afterwards to build according to the configuration. Please see CMakeLists.txt for configurable options.

Select comm module

One option that is worth calling out explicitly is the communication (transport) library.

We support both UDP and TCP communication. UDP is the default. In order to enable TCP communication, build with -DBUILD_COMM_TCP_PLAIN=TRUE in the cmake instructions shown above. If set, the test client will run using TCP. If you wish to use TCP in your application, you need to build the TCP module as mentioned above and then create the communication object using CommFactory and passing PlainTcpConfig object to it.

We also support TCP over TLS communication. To enable it, change the BUILD_COMM_TCP_TLS flag to TRUE in the main CMakeLists.txt file. When running simpleTest using the - there is no need to create TLS certificates manually. The script will use the (located under the scripts/linux folder) to create certificates. The latter can be used to create TLS files for any number of replicas, e.g. when extending existing tests.

As we used pinned certificates for TLS, the user will have to manually provide these. THey can use the script as an example.

C++ Linter

The C++ code is statically checked by clang-tidy as part of the CI.
To check code before submitting PR, please run make tidy-check.

Detailed information about clang-tidy checks.

(Optional) Python client

The python client is required for running tests. If you do not want to install python, you can configure the build of concord-bft by running cmake -DBUILD_TESTING=OFF .. from the build directory for native builds, and CONCORD_BFT_CMAKE_BUILD_TESTING=TRUE make for docker builds.

The python client requires python3(>= 3.5) and trio, which is installed via pip.

python3 -m pip install --upgrade trio

Adding a new dependency or tool

The CI builds and runs tests in a docker container. To add a new dependency or tool, follow the steps below:

  • Rebase against master
  • In order to add/remove dependencies update the file
  • Build new release/debug images: make build-docker-images
  • Check images current version in Makefile and Makefile
  • Tag the new images (at least one):
    • For release: docker tag concord-bft:latest concordbft/concord-bft:<version>
    • For debug: docker tag concord-bft-debug:latest concordbft/concord-bft-debug:<version>
      where version is current version + 1.
  • Update the version in the Makefile
  • Make sure that Concord-BFT is built and tests pass with the new images: RUN_WITH_DEBUG_IMAGE=TRUE make clean build test
  • Ask one of the maintainers for a temporary write permission to Docker Hub repository(you need to have a Docker ID)
  • Push the images (at least one):
    • For release: docker push concordbft/concord-bft:<version>
    • For debug: docker push concordbft/concord-bft-debug:<version>
  • Create a PR for the update:
    • The PR must contain only changes related to the updates in the image
    • PR's summary has to be similar to Docker update to version release=<new version> debug=<new version>
    • PR's message has to list the changes made in the image content and preferably the reason
    • Submit the PR

Important notes:

  1. Adding dependencies or tools directly to the Dockerfile is strongly not recommended because it breaks the native build support.
  2. If any tools are installed during the build but not needed for the actual compilation/debugging/test execution(for example, git), please remove them(Dockerfile is an example). The reason is that the image is supposed to be as tiny as possible.

Apollo testing framework

The Apollo framework provides utilities and advanced testing scenarios for validating Concord BFT's correctness properties, regardless of the running application/execution engine. For the purposes of system testing, we have implemented a "Simple Key-Value Blockchain" (SKVBC) test application which runs on top of the Concord BFT consensus engine.

Apollo enables running all test suites (without modification) against any supported BFT network configuration (in terms of n, f, c and other parameters).

Various crash or byzantine failure scenarios are also covered (including faulty replicas and/or network partitioning).

Apollo test suites run regularly as part of Concord BFT's continuous integration pipeline.

Please find more details about the Apollo framework here

Run examples

Simple test application (4 replicas and 1 client on a single machine)

Tests are compiled into in the build directory and can be run from anywhere as long as they aren't moved.

Run the following from the top level concord-bft directory:


Using simple test application via Python script

You can use the script to run various configurations via a simple command line interface. Please find more information here

Example application

This example demo application shows some capabilities of the Concord-BFT consensus-based byzantine fault-tolerant state machine replication library. For Concord-BFT users who are interested in learning more about Concord-BFT and its uses, this application offers a demonstration and instruction. Overall, any blockchain application based on concord-bft consensus may be created using this example application.

Use the script to run the example application. This script is also used to perform this demo via the command line interface with different configurations.

Please see here for more information about the example/demo application.

Directory Structure

  • bftengine: concord-bft codebase
    • include: external interfaces of concord-bft (to be used by client applications)
    • src: internal implementation of concord-bft
      • tests: tests and usage examples
  • threshsign: crypto library that supports digital threshold signatures
    • include: external interfaces of threshsign (to be used by client applications)
    • src: internal implementation of threshsign
      • tests: tests and usage examples
  • scripts: build scripts
  • tests: BFT engine system tests


The concord-bft project team welcomes contributions from the community. If you wish to contribute code and you have not signed our contributor license agreement (CLA), our bot will update the issue when you open a Pull Request. For any questions about the CLA process, please refer to our FAQ. For more detailed information, refer to


The library calls std::terminate() when it cannot continue in a safe manner. In that way, users can install a handler that does something different than just calling std::abort().


Concord-BFT Slack.

Request a Slack invitation via


concord-bft is available under the Apache 2 license.