Decentralized Applications
Saravanan Vijayakumaran
sarva@ee.iitb.ac.in
Department of Electrical Engineering
Indian Institute of Technology Bombay
August 22, 2019
1 / 22
DApps
• Applications that let users own their data and run without a single
centralized operator (Source: https://app.co/faq)
• Decentralized vs Distributed
• Distributed does not mean decentralized
• A single entity could be controlling a distributed system
• Pros
• Uncensorable
• Transparency, Privacy (DApps are typically open source)
• Cons
• Usability (slow, error-prone)
• Difficult to build and/or maintain
P2P systems are hard. The only thing harder than a distributed system is a
distributed system you don’t control. A system that will attack you, and that is
running stuff you don’t want it to run.
Juan Benet
• Pre-Bitcoin examples
• Email (if everyone doesn’t use GMail)
• BitTorrent
2 / 22
DApp Frameworks
• Permissionless
• Ethereum
• Blockstack
• IPFS (libp2p)
• . . . and many more
• Permissioned
• Quorum
• Corda
• Hyperledger Fabric
• . . . and many more
• DApp directories
• https://app.co/
• https://www.stateofthedapps.com/
3 / 22
Ethereum
Ethereum
• A blockchain platform for building decentralized applications
• Application code and state is stored on a blockchain
• Two types of transactions
• Contract creation
• Message calls
• Contract creation transactions create new contracts on the
blockchain
• Message call transactions call methods in an existing contract
• Input data to contract methods is specified
5 / 22
Storage Contract
1 pragma solidity ^0.4.0;
2
3 contract SimpleStorage {
4 uint storedData;
5
6 function set(uint x) public {
7 storedData = x;
8 }
9
10 function get() public view returns (uint) {
11 return storedData;
12 }
13 }
https://solidity.readthedocs.io/en/v0.4.24/
introduction-to-smart-contracts.html#storage
6 / 22
Currency Example
1 pragma solidity ^0.4.7;
2
3 contract Coin {
4 address public minter;
5 mapping (address => uint) public balances;
6
7 event Sent(address from, address to, uint amount);
8
9 constructor() public {
10 minter = msg.sender;
11 }
12
13 function mint(address receiver, uint amount) public {
14 if (msg.sender != minter) return;
15 balances[receiver] += amount;
16 }
17
18 function send(address receiver, uint amount) public {
19 if (balances[msg.sender] < amount) return;
20 balances[msg.sender] -= amount;
21 balances[receiver] += amount;
22 emit Sent(msg.sender, receiver, amount);
23 }
24 }
7 / 22
Initial Coin Offerings
• Also called token sales
• Ethereum is the most popular platform for ICOs
• Each ICO implements a ERC-20 token contract (link)
• Investments in ICOs was about $7 billion in 2017
• Some notable ICOs
• Basic Attention Token, May 2017, $35 million in 30 seconds
• Kik, Sep 2017, $100 million
• Filecoin, Jan 2018, $257 million
• Many of the ICO-funded projects have failed
• Used to execute “pump-and-dump” schemes
8 / 22
Ethereum DApp Examples
• CryptoKitties
• Allows players to purchase, breed, and sell virtual cats
• Each CryptoKitty is a non-fungible token using the ERC-721
standard
• Game popularity caused network congestion in Dec 2017
• The highest selling cat cost 246 ETH in Dec 2017 (≈ $117,000)
• Fomo3D (https://fomo3d.hostedwiki.co/)
• Decentralized exchanges (https://idex.market)
9 / 22
Other DApp Examples
• Graphite Docs (https://www.graphitedocs.com/about)
• Decentralized version of Google Docs
• Why? Privacy, Censor resistance
• Built using Blockstack
• Textile (https://www.textile.photos/)
• Decentralized photo sharing built on IPFS
• Peerpad (https://peerpad.net/)
• A P2P realtime collaborative editing tool built using IPFS
• Radicle (http://radicle.xyz/)
• IPFS-based replacement for GitHub
10 / 22
Bitmessage
• Decentralized, encrypted, P2P communications protocol
• Released by Jonathan Warren in Nov 2012
• Downloads increased fivefold in June 2013 after news of NSA
email surveillance
• Inspired by the Bitcoin protocol
• Identities are hashes of public keys
• Messages are broadcast over a network instead of blocks
• Each message needs PoW attached (to prevent spam)
• Messages live only for two days (by default)
• Source https://github.com/Bitmessage/PyBitmessage
11 / 22
Permissioned Blockchains
Permissioned Blockchains
• Private network of nodes which create and maintain a blockchain
• Proof-of-authority consensus is used instead of PoW
• A valid block is one with a certain number of approvers
• Motivation: A shared ledger of facts about assets
• Popular frameworks
• Hyperledger Fabric
• Corda
• Quorum
13 / 22
Hyperledger Fabric
• Hyperledger
• Collaborative blockchain effort hosted by Linux Foundation
• Mission: Create enterprise grade, open source distributed ledger
frameworks
• Launched in 2016
• Fabric
• Permissioned distributed ledger framework with smart contracts
• Originated in IBM in mid-2015 as Open Blockchain (OBC) project
• Initial implementation completed in Dec 2015
• IBM joined Hyperledger in Feb 2016 and donated OBC code
14 / 22
Ledger
Image credit: https://hyperledger-fabric.readthedocs.io/en/
release-1.3/ledger/ledger.html
15 / 22
World State
Image credit: https://hyperledger-fabric.readthedocs.io/en/
release-1.3/ledger/ledger.html
16 / 22
Blockchain
Image credit: https://hyperledger-fabric.readthedocs.io/en/
release-1.3/ledger/ledger.html
17 / 22
Blocks
Image credit: https://hyperledger-fabric.readthedocs.io/en/
release-1.3/ledger/ledger.html
18 / 22
Ledger Updates
Phase 1: Proposal
Image credit: https://hyperledger-fabric.readthedocs.io/en/
release-1.3/peers/peers.html
• Application sends transaction proposal to some peers for
endorsement
• Peers execute the transaction and append signatures endorsing
the proposal
• Phase 1 ends when application receives sufficient responses
19 / 22
Ledger Updates
Phase 2: Packaging
Image credit: https://hyperledger-fabric.readthedocs.io/en/
release-1.3/peers/peers.html
• Endorsed transaction proposals are packaged into a block by the
orderer
20 / 22
Ledger Updates
Phase 3: Validation
Image credit: https://hyperledger-fabric.readthedocs.io/en/
release-1.3/peers/peers.html
• Orderer distributes blocks to all peers
• Each peer checks that a block satisfies the organizational
endorsement policy and applies to ledger
21 / 22
References
• Chapter 1 of Decentralized Applications by Siraj Raval,
https://www.oreilly.com/library/view/
decentralized-applications/9781491924532/ch01.html
• Ethereum Wikipedia Article https://en.wikipedia.org/wiki/Ethereum
• ICO Wikipedia Article
https://en.wikipedia.org/wiki/Initial_coin_offering
• CryptoKitties Wikipedia Article
https://en.wikipedia.org/wiki/CryptoKitties
• Solidity Documentation https://solidity.readthedocs.io
• Graphite Docs https://www.graphitedocs.com/about
• Textile https://www.textile.photos/
• Peerpad https://peerpad.net/
• Radicle http://radicle.xyz/
• Fabric Documentation https://hyperledger-fabric.readthedocs.io/
22 / 22
