External Client

The external client is used to simulate an entity that wishes to get state from a Fabric network, along with proof that that state is part of the immutable ledger. This proof comes in the form of a membership witness of the state in an RSA accumulator. The RSA accumulator is a commitment to the entire state of the Fabric ledger. It is updated continuously as blocks are added to the ledger and is published periodically to a public bulletin board, in this case, Ethereum. To verify the proof, the external client queries both the smart contract on the Ethereum mainnet for the latest accumulator, and the Fabric commitment agent for the state and proof of state in the accumulator.

Prerequisites

Publish the RSA Accumulator library to MavenLocal

The external client uses the rsa-accumulator-kotlin library to verify membership proofs provided by the Fabric agent. This repository needs to be cloned, built, and published to a local Maven repository. Follow instructions in the repo to do this. Change line 30 in the build.gradle to point to your local Maven repository directory.

Get the Solidity contracts

This project copies the solidity smart contracts defined in the bulletin board repo and generates Java wrapper files from them. Ensure the bulletin board project is present at the same level as the directory structure as the commitment-agent project and run:

./scripts/get-solidity-contracts.sh

Start the Fabric network

The recommended network to use is Fabric network. Start the network and deploy and invoke the chaincode with:

make start
make deploy-cc
make invoke-cc

The invoke-cc make target starts a Fabric node.js application that submits CreateAsset transactions every 10 seconds. This can be cancelled with ctrl-c. The make invoke-cc can be used repeatedly without needing to restart the network.

Start the Fabric Agent

The Fabric commitment agent is used to maintain an accumulator for the ledger state. It returns state and proof of membership to the state on request. Clone the repo, update the build.gradle to point to the local Maven repo, and start the agent with:

make ethereum-client ORG="org1"
make fabric-client ORG="org1" INIT="true"

When the Ethereum client starts, make note of the address for the ledger state contract. This is needed by the external client to fetch the latest commitment.

Building and Running

The external client is a command line application. Build the binary with:

./gradlew installDist

The only available command is get-proof <state-key> <ledger-contract-address> <fabric-org-id>. Use the ledger contract address noted down from when starting the agent in the previous section. An example of running the get-proof command is:

./build/install/external-client/bin/external-client get-proof key1 0xe78a0f7e598cc8b0bb87894b0f60dd2a88d6a8ab org1

Coding principles

This codebase uses functional programming principles as much as possible. A functional library for Kotlin, called Arrow is used, primarily for error handling with the Either type.

Conventions

• Use immutable state.
• Catch exceptions as close as possible to their source and convert to Arrow's Either type.
• Implement functions as expressions. Flows that produce errors can be composed using map, flatMap and fold. Avoid statements with side effects in functions.
• Use recursion over loops (when tail recursion is possible to avoid stack overflow).

Example Gists:

• How to catch exceptions and convert to and Either type.
• Using flatMap to compose functions that return Eithers.
• Folding over an Either Error to reduce to a single type.

TODO

• Config file for the build.gradle