Now we have a basic idea about what a DApp, blockchain, and DLT is, let's have an overview of what platforms are available to build a permissioned blockchain applications and DApps. We will only go through the ones that are popular on the market, and for which there is a demand.
Ethereum is the most popular DApp after Bitcoin. Ethereum is a decentralized platform that allows us to build other blockchain-based DApps on top of it. In Ethereum, we build DApps using Ethereum smart contracts. Smart contracts are applications that run exactly as programmed without any possibility of downtime, censorship, fraud, or third-party interference. Ethereum can be thought of as a platform to deploy and run smart contracts. Ethereum supports two consensus protocols, PoW and PoA (Clique).
The main public Ethereum network uses PoW for consensus. If you want to deploy your own private Ethereum network, then you have to use PoA. PoW requires a lot of computation power to keep the blockchain secure, therefore it's good for public blockchain use, whereas PoA doesn't have any such computation power requirement; instead it requires a few authority nodes in the network to achieve consensus.
Quorum is a decentralized platform that allows us to build permissioned blockchain-based DApps on top of it. Actually, Quorum is a fork of Ethereum (actually Quorum is a fork of Go Ethereum, which is an implementation of Ethereum using Golang), therefore if you have ever worked on Ethereum then you will find it easy to learn and build permissioned blockchains using Quorum. Many enterprises select Quorum for building blockchains because of Ethereum's large community, which makes it easy to find Ethereum developers. What makes Quorum different from Ethereum is that it supports privacy (it lets parties do transactions privately); peer whitelisting, so you can mention a list of other nodes that are allowed to connect to your node (in Ethereum this needs to be done at network level); many different flavors of consensus protocols suitable for permissioned blockchain, and provides very high performance.
Quorum currently supports three consensus protocols, QuorumChain, IBFT, and Raft. We will skip QuorumChain in this book, as Raft and IBFT fulfil all our requirements.
Microsoft Azure provides BaaS to easily build your own Quorum network on the Cloud. But, in this book, we will learn how to install it manually, and we won't be using BaaS.
Popular node software for Ethereum include Go Ethereum, Ethereum C++, and Parity. Parity also supports two other consensus protocols, other than Ethereum's PoW, which are specifically designed for permissioned blockchains. These consensus protocols are Aura and Tendermint. Many Ethereum developers use parity compared to Quorum when they don't need the extra features provided by Quorum.
As parity doesn't provide any unique features compared to Quorum, we will be skipping parity in this book. But, once you finish this book, you will find it really easy to grasp parity's concepts and will be able to build something using it too.
MultiChain is a platform to build permissioned blockchain-based DApps. Unique features of MultiChain include permissions management, data streams, and assets. It doesn't support smart contracts. This is an example of a non-smart contract-based platform for building blockchain-based DApps. MultiChain uses round robin validation consensus.
Initially MultiChain was based on the idea of managing ownership and transfer of assets on blockchain. Operations on assets includes issuance, reissuance, transfer, atomic exchange, escrow, and destruction of assets. Later on, data streams were introduced to provide a different flavor of representing data in MultiChain. Any number of streams can be created in a MultiChain, and each stream acts as an independent append-only collection of items. Operations on streams include creating streams, writing, subscribing, indexing, and retrieving. So, basically, a blockchain use case on MultiChain can be built on a foundation of assets or streams. Finally, permission management is used to control who can connect, transact, create assets/streams, mine/validate, and administrate.
MultiChain provides maximal compatibility with the Bitcoin ecosystem, including the peer-to-peer protocol, transaction/block formats, the UTXO model, and Bitcoin Core APIs/runtime parameters. So, before you start learning MultiChain, it's better to learn how Bitcoin works at a high level at least.
Before we get into what Hyperledger Fabric 1.0 is, we need to understand what Hyperledger is specifically. Hyperledger is an umbrella project of open source blockchains and related tools, started in December 2015 by the Linux Foundation. At the time of writing this book, there are four projects under Hyperledger:Fabric, Sawtooth, Iroha, and Burrow.
Hyperledger Fabric is the most popular project under Hyperledger. IBM is the main contributer to the project. IBM's Bluemix also provides BaaS to build your own Fabric network on the Cloud easily.
Hyperledger Fabric 1.0 is a platform to build your own permissioned blockchain-based applications. Currently, at the time of writing this book, Hyperledger Fabric 1.0 supports only distributed architecture, and for the creation of blocks it depends on a central trusted node called the orderer. It supports smart contracts, network permissioning, privacy, and other features. In HLF 1.0, there is a special kind of node called as OSN, which is hosted by a trusted party. This OSN creates blocks and distributes to peers in networks. As you trust this node, there is no need for consensus. HLD 1.0 currently supports CouchDB and LevelDB to store the state of the blockchain. Peers in the network store the state of the blockchain in the LevelDB database, by default.
HLF 1.0 has a concept of channels to achieve privacy. A channel is a sub-blockchain in the network and allows certain parties to be part of a channel depending on configuration. Actually, every transaction has to belong to a channel and when the HLF 1.0 network is deployed, a default channel is created. OSN can see all the data in all the channels, therefore it should a trusted party. Technically, it's possible to configure the network to have multiple OSNs hosting different channels if you cannot trust a single party for all channels. Even if the traffic is going to be huge or OSN availability is critical, then you can plug Kafka into OSN for better performance and increased stability. We can even have multiple OSNs per channel connected via Kafka if high availability is required.
Fabric 1.0 has a feature called transaction endorsement, which provides a mechanism of taking approvals from certain parties before sending a transaction. When we say that a transaction has been endorsed by a member in the network, we mean that the member has verified the transaction. Every chaincode (smart contracts in HLF) has an endorsement policy defined to it at the time of deployment. The policy states which members has to endorse the transactions associated with this chaincode. The default policy states that any one member of the channel has to sign the transaction. But, we can define custom policies containing AND and OR operators.
Also, peers of the same channel broadcast blocks to each other regardless of the presence or absence of OSN, but in the absence of OSN new blocks cannot be created for the channel. Peers broadcast blocks using a special protocol called as gossip data dissemination protocol.
HLF 1.0 has very advanced membership features to control network membership, and that are also internal to a specific organization. In HLF 1.0, you can write chaincodes in Java or Go programming languages. In the future, Fabric 1.0 will come with the Simple Byzantine Fault Tolerance(SBFT) consensus protocol and some other features that will enable us to build DApps. Similarly, there are various new features that are under development and will be released in future as a sub-version of the product.
Note
The best way to get started with building your first HLF 1.0 application is the check out examples at https://github.com/hyperledger/fabric-samples and modify them according to your application needs. You can find HLF 1.0 detailed docs at http://hyperledger-fabric.readthedocs.io/en/latest/.
BigchainDB is a decentralized database that uses blockchain. BigchainDB is highly scalable and customizable. It uses the Blockchain data structure. It supports features such as rich permissioning, petabytes capacity, advanced querying, linear scaling, and so on. At the time of writing this book, BigchainDB is not production-ready but can be used for building Proof of Concepts (PoCs). We will learn how it works, and will create a basic PoC using it, in later chapters.
InterPlanetary File System (IPFS) is a decentralized filesystem. IPFS uses Distributed Hash Table (DHT) and Merkle Direct Acyclic Graph (DAG) data structures. It uses a protocol similar to Torrent to decide how to move the data around the network. One of the advanced features of IPFS is that it supports file versioning. To achieve file versioning, it uses data structures similar to Git.
Although it's called as a decentralized filesystem, it doesn't adhere to a major property of a filesystem, namely, when we store something in filesystem, it should be there until deleted. But, IPFS doesn't work this way. Each node doesn't store all files, instead it stores only those files it needs. Therefore, if a file is not popular, then many nodes will not have the file therefore there is a huge chance of the file disappearing in the network. Due to this, we can call IPFS a decentralised peer-to-peer file-sharing application. We will learn about about how it works in later chapters.
Corda is a platform on which to build your own permissioned DLT-based applications. Corda is a product of R3. R3 is an enterprise software firm working with over 100 banks, financial institutions, regulators, trade associations, professional services firms, and technology companies to develop Corda. The latest version of Corda is 1.0, which aims to replace legacy softwares used for financial transactions, and enables organisations to digitalize various business process that were cumbersome using legacy software systems:
The preceding diagram shows the high level architecture of a Corda network. Let's understand Corda's architecture at a high level. The idea of R3's Corda is to provide a shared trusted ledger for financial transactions. R3's Corda is not a blockchain platform, therefore there is no concept of blocks, global broadcasts, and so on. All the transactions are point to point. Corda applications are not decentralized. In Corda, smart contracts are called as CorDapps and they are written in either Java or Kotlin.
Infrastructure services form the nodes in the network that should be hosted by the trusted parties. Network Map publishes IP addresses of all the other nodes, so that nodes can reach out to other nodes. Permissioning service gives permission to nodes to join the network ; the node will receive a root-authority-signed TLS certificate from the network's permissioning service if permitted to join the network. Notaries provide transaction ordering and time stamping services (optionally, a notary also acts as the timestamping authority, verifying that a transaction occurred during a specific time-window before notarizing it). A notary service may be a single network node, a cluster of mutually-trusting nodes, or a cluster of mutually-distrusting nodes.
Notaries are expected to be hosted by enterprises that the network doesn't trust, therefore consensus is required between the notaries, due to which Corda provides various pluggable consensus protocols, such as Raft, BFT, and so on.
Sometimes, Corda applications need to depend on external application APIs. For example, a multi-currency bank-to-bank payment application built using Corda will need to fetch the exchange rate. In this scenario, the node initiating the transaction can fetch the exchange rate and put on the transaction, but how can you trust that node? Also, every node cannot simply re-fetch the exchange rate to verify if it's correct because by the time other nodes fetch it the rate might have changed, and also this is not a scalable solution. Therefore, Corda provides oracles to solve this issue. There can be one or more oracles in the network. An Oracle is a service that acts as a bridge for communication between two applications. In Corda, the transaction initiator can fetch the information from outside the Corda network and get the information signed from Oraclize to prove its validity. Optionally, Oraclize can also provide the information to the transaction initiator on request. Obviously, the Oraclize should be hosted by trusted parties with respect to what information they provide and sign.
Corda supports any pluggable RDBMS (currently, it is using the H2 database) to store smart contracts data. Data privacy is maintained as to which nodes can see the transactions. Multisignature support is also given by the framework, which enables multiple nodes to sign a transaction. One of the major downsides of Corda is that as there is no global broadcasting, each node has to maintain its own backup and failover redundancy in a traditional way as there is no redundancy built into the network. A node will store transactions and retry sending the messages to the recipient until the recipient has successfully received it. Once the messages are received, the sender has no more responsibility.
As all transactions are not broadcasted to all parties in the network, to prevent a double spend (a double spend is an attack on DLTs to spend the same money twice, transfer the same asset twice and so on), we use notaries. Notaries contain all the unconsumed UTXOs, and after notarization they mark them as consumed and add the new unconsumed ones to their state. The transaction purposer gets the transaction notarized by a notary before sending to the other parties for commit.
A notary will only be able to sign a transaction if it has earlier signed input states of the transaction. But, this may not always be the case, therefore Corda also lets us change the state's appointed notary. This situation can occur mostly due to the following reasons:
- A transaction consuming states that have different appointed notaries
- A node wishes to use a different notary for achieving privacy or efficiency
Before these transactions can be created, the states must first be repointed to all have the same notary. This is achieved using a special notary-change transaction.
CorDapps are not like smart contracts of other platforms. They don't have a state. Their purpose is to just validate if the outputs produced from the inputs are correct. Every UTXO points to a CorDapp. CorDapps define the format of UTXOs. In a transaction, we can have UTXOs of multiple CorDapps, and in these cases each CorDapp will run only once and validate all the inputs and outputs belonging to it. For a transaction to be valid, it must be contractually valid; the CorDapp should approve it.
Apart from inputs and outputs, transactions might consist of commands too, small data packets that the platform doesn't decipher itself but which help CorDapps to process the inputs and outputs. A command is a piece of data associated with some public keys. Commands are used to provide additional information to the CorDapps that it cannot get via the UTXOs. The platform assures that the transaction is signed by every key listed in the commands before the contracts start to execute. Thus, the CorDapp can trust that all listed keys have signed the transaction, but is responsible for verifying that the intended parties have signed it. Public keys may be random or identityless for privacy, or linked to a well-known legal identity.
Oracles provide signed information to the transaction purposer in the form of commands that encapsulate a specific fact, and list the oracle as a required signer.
Also, transactions can contain a hash of attachments. Attachments are ZIP/JAR files. Attachments are useful when there's a large fragment of data that can be reused acrossseveral different transactions.
It is possible that while verifying a proposed transaction, the node may not have all the transactions of the transaction chain that it needs to verify. Therefore, Corda lets the node request the missing transactions from the proposer(s). It's always true that the transaction proposer will have all the transactions of the required transaction chain, as they would have requested it when verifying the transaction and created the purposed transaction's input states.
Finally, once the transaction is committed, you can query the Vault (which keeps track of both unconsumed and consumed states).
Note
To learn more about Corda and build your first Corda app, visit https://docs.corda.net/, which contains detailed documentation. There are several example apps that you can download and experiment with.
Sawtooth is a decentralized platform to build your own permissioned DApps. The main contributer to Sawtooth is Intel. What makes Sawtooth special is that it uses a Trusted Execution Environment (TEE) (it currently supports Intel's SGX only) for consensus, which makes the network very safe and trustworthy and increases trust in the final result of the consensus.
Note
The TEE is a secure area of the main processor. It guarantees that the code and data loaded inside is protected with respect to confidentiality and integrity. The TEE as an isolated execution environment that provides security features such as isolated execution, integrity of Trusted Applications, along with confidentiality of their assets.
Proof of Elapsed Time (PoET) is the name of the consensus protocol that Sawtooth uses. In PoET, there are special types of nodes called as validators. Validators must run their node on an SGX-supported CPU. This is how PoET works.
Every validator requests a wait time from an enclave (a trusted function). The validator with the shortest wait time for a particular transaction block is elected the leader. One function, say CreateTimer, creates a timer for a transaction block that is guaranteed to have been created by the enclave. Another function, say CheckTimer, verifies that the timer was created by the enclave and, if it has expired, creates an attestation that can be used to verify that the validator did, in fact, wait the allotted time before claiming the leadership role. PoET randomly distributes leadership election across the entire population of validators. The probability of election is proportional to the resources contributed (in this case, resources are general-purpose processors with a TEE). An attestation of execution provides information for verifying that the certificate was created within the enclave (and that the validator waited the allotted time). Further, the low cost of participation increases the likelihood that the population of validators will be large, increasing the robustness of the consensus algorithm.
Sawtooth also supports smart contracts (specifically, Ethereum smart contracts can be executed on Sawtooth). Performance-wise, Sawtooth scales well in terms of large numbers of transactions and nodes.
