There's a lot of talk about blockchain, but how does the technology behind it actually work? For developers, getting some hands-on experience is the fastest way to get familiair with new technologies. So let's build a blockchain, then! In this session, we're going to build one in plain old Java, and have it working in 40 minutes. We'll cover key concepts of a blockchain: transactions, blocks, mining, proof-of-work, and reaching consensus in the blockchain network. After this session, you'll have a better understanding of core aspects of blockchain technology.

1. Let’s build a blockchain... in 40 minutes!
@MichelSchudel
michel@craftsmen.nl
Amsterdam | April 2-3, 2019

6. transactiontransaction
Central
authority
Ledger
transactiontransaction
transaction
transation
Distributed
ledger
Distributed
ledger
Distributed
ledger
Distributed
ledger
@MichelSchudel

7. 1. A Blockchain is an implementation of a distributed ledger
t1t2t3
t1t2t3
t1t2t3
t1t2t3
t1t2t3
t1t2t4
@MichelSchudel

8. 2. A Blockchain is an sequential, immutable chain of records
called Blocks
Block
transactiontransactiontransaction
Block
transactiontransactiontransaction
Block
transactiontransactiontransaction
@MichelSchudel

9. index : 2
previous hash
payload
transaction
transaction
transaction
3. Immutability is implemented by hashing
index : 1
previous hash: 0
Genesis block
transaction
index : 3
previous hash
payload
transaction
transaction
transaction
@MichelSchudel

10. Let’s build a Blockchain, then!
@MichelSchudel

11. Let’s build a Blockchain, then!
BlockchainRestController
Blockchain TransactionPool
BlockBlockBlockBlock
TransactionTransactionTransactionTransaction
Manages Manages
Has Has
API
GET /api/blockchain
POST /api/createtransaction
GET /api/pendingtransactions
POST /api/mine
Network
Interacts
with
@MichelSchudel

12. Step 1: Create the initial blockchain
index : 1
previous hash: 0
Genesis block
@MichelSchudel

13. Kudo’s (from, to, kudos)
Pool of transactions
Out of scope:
• Transaction integrity (signing)
• Transaction inputs and outputs
Transaction pool
transaction
transaction
transaction
Step 2: Create something to store in
the blocks
@MichelSchudel

14. • New block should contain hash of
previous block (immutability, trust)
• Creating a block requires proof-of-
work / stake (supports consensus)
• Creating a block should give a reward
(incentive, generation transaction)
index : 2
previous hash
payload
transaction
transaction
transaction
index : 1
previous hash: 0
Genesis block
Step 3: making (mining) a new block
@MichelSchudel

15. Proof-of-work
• It should be hard to create a new
block
• Makes cheating unattractive
• It should be easy to verify a new
block
@MichelSchudel

16. Remember this Nonce thingy?
“(new block)” “00005c3d2d...”SHA256 hash
“(new block with nonce x)” “00005c3df8...”SHA256 hash
X = 100
Creating a proof-of-work (hard)
Verifying proof-of-work (easy)
@MichelSchudel

17. • The longest blockchain that is valid “wins”
• A blockchain is valid when
• Previous hash field of each block matches hash of previous block
• Proof-of-work (nonce) is verified and correct for each block
Step 4: consensus with other nodes
@MichelSchudel

18. Step 5: Implementing decentralization
• Transactions are propagated
• Everybody should have the chance to mine
• Blocks are propagated
• Validation is performed before accepting
• This could prevent downloading all chains
@MichelSchudel

19. Summary
We created a blockchain in 40 minutes with:
• Blocks
• Transactions
• Mining
• Reaching consensus
• Propagation
Now go and build one yourself!
https://gitlab.com/craftsmen/workshop-blockchain
@MichelSchudel

20. https://gitlab.com/craftsmen/workshop-blockchain
@MichelSchudel
michel@craftsmen.nl

21. Code slides
(in case you didn’t see the demo)

22. Create the block structure
public class Block {
private long index;
private Set<Transaction> transactions = new HashSet<>();
private String previousHash;
private long nonce;
…getters, setters, toString etc.
}

23. Making the initial chain
public class Blockchain {
private LinkedList<Block> blocks = new LinkedList<>();
static Blockchain create() {
Blockchain blockchain = new Blockchain();
Block block = new Block();
block.setIndex(1);
block.setPreviousHash("0");
block.setNonce(0);
blockchain.blocks.add(block);
return blockchain;
}
}

24. Creating a transaction
public class Transaction {
private String id;
private String from;
private String to;
private int kudos;
..getters, setters…
… equals and hashcode based on id only!
}

25. Creating a transaction pool
public class TransactionPool {
private Set<Transaction> transactions = new HashSet<>();
public void addTransaction(Transaction transaction) {
transactions.add(transaction);
}
public void clearTransactions() {
transactions.clear();
}
public Set<Transaction> getAllTransactions() {
return new HashSet<>(transactions);
}
}

26. Creating a transaction
@PostMapping("/api/createtransaction")
public long createTransaction(@RequestBody Transaction transaction) {
//give the transaction an id
transaction.setId(UUID.randomUUID().toString());
//add the transaction to the pool
transactionPool.addTransaction(transaction);
//return the height of the next block
return blockchain.height() + 1;
}

27. Mining a new block
@PostMapping("/api/mine")
public Block mine() {
Block block = getBlockchain().mine(transactionPool.getAllTransactions());
transactionPool.clearTransactions();
return block;
}

28. Mining a new block
public class Blockchain {
...
public Block mine(Set<Transaction> allTransactions) {
Block block = new Block();
block.setIndex(this.blocks.size() + 1);
block.setPreviousHash(DigestUtils.sha256Hex(blocks.getLast().toString()));
block.setTransactions(allTransactions);
//create reward
Transaction reward = new Transaction();
reward.setId(UUID.randomUUID().toString());
reward.setFrom("");
reward.setTo(“Michel for creating this block");
reward.setKudos(3);
allTransactions.add(reward);
block.calculateProofOfWork();
blocks.add(block);
return block;
}
}

29. Proof of work
public class Block {
…
public void calculateProofOfWork() {
this.nonce = 0;
while (!DigestUtils.sha256Hex(this.toString()).startsWith("0000")) {
this.nonce++;
}
}
}

30. Comparing with other blockchains
In your service…
public void init() {
this.blockchain = Blockchain.create();
this.blockchain = network.retrieveBlockchainsFromPeers()
.stream()
.filter(b -> b.isValid())
.filter(b -> b.height() > this.blockchain.height())
.max(Comparator.comparing(Blockchain::height))
.orElse(this.blockchain);
}

31. Comparing with other blockchains
public boolean isValid() {
for (int i = blocks.size() - 1; i > 0; i--) {
Block currentBlock = blocks.get(i);
Block previousBlock = blocks.get(i - 1);
if (!previousHashMatches(previousBlock, currentBlock)) {
System.out.println("previous hash doesn't match!");
return false;
}
if (!currentBlock.isValid()) {
System.out.println("proof of work is invalid");
return false;
}
}
return true;
}
private boolean previousHashMatches(Block previousBlock, Block currentBlock) {
return currentBlock.getPreviousHash()
.equals(DigestUtils.sha256Hex(previousBlock.toString()));
}

32. Comparing with other blockchains
public class Block {
…
boolean isValid() {
return DigestUtils.sha256Hex(this.toString()).startsWith("0000");
}
…
}

33. Propagating transactions
@PostMapping("/api/createtransaction")
public long createTransaction(@RequestBody Transaction transaction) {
…create transaction…
network.notifyPeersOfNewTransaction(transaction);
}

34. Receiving transactions from the network
@PostMapping("/api/addtransaction")
public void newTransactionReceived(@RequestBody Transaction transaction) {
if (!transactionPool.getAllTransactions().contains(transaction)) {
transactionPool.addTransaction(transaction);
network.notifyPeersOfNewTransaction(transaction);
}
}

35. Propagating blocks
@PostMapping("/api/mine")
public Block mine() {
Block block = getBlockchain().mine(transactionPool.getAllTransactions());
transactionPool.clearTransactions();
//propgate transaction
network.notifyPeersOfNewBlock(block);
return block;
}

36. Receiving blocks from the network
@PostMapping("/api/addblock")
public void newBlockReceived(@RequestBody Block block) {
//only add block if it is valid
if (blockchain.isValid(block)) {
blockchain.addBlock(block);
//clear all transactions that are already in the block
transactionPool.clearTransactions(block.getTransactions());
//propagate block through the network
network.notifyPeersOfNewBlock(block);
}
}

37. Receiving blocks from the network
public class Blockchain {
...
private boolean previousHashMatches(Block previousBlock, Block currentBlock) {
return currentBlock.getPreviousHash()
.equals(DigestUtils.sha256Hex(previousBlock.toString()));
}
public boolean isValid(Block block) {
return block.isValid() && previousHashMatches(blocks.getLast(), block);
}
public void addBlock(Block block) {
blocks.add(block);
}
}