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Michel Schudel - Let's build a blockchain... in 40 minutes! - Codemotion Amsterdam 2019

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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.

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Michel Schudel - Let's build a blockchain... in 40 minutes! - Codemotion Amsterdam 2019

  1. 1. Let’s build a blockchain... in 40 minutes! @MichelSchudel michel@craftsmen.nl Amsterdam | April 2-3, 2019
  2. 2. transactiontransaction Central authority Ledger transactiontransaction transaction transation Distributed ledger Distributed ledger Distributed ledger Distributed ledger @MichelSchudel
  3. 3. 1. A Blockchain is an implementation of a distributed ledger t1t2t3 t1t2t3 t1t2t3 t1t2t3 t1t2t3 t1t2t4 @MichelSchudel
  4. 4. 2. A Blockchain is an sequential, immutable chain of records called Blocks Block transactiontransactiontransaction Block transactiontransactiontransaction Block transactiontransactiontransaction @MichelSchudel
  5. 5. 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
  6. 6. Let’s build a Blockchain, then! @MichelSchudel
  7. 7. 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
  8. 8. Step 1: Create the initial blockchain index : 1 previous hash: 0 Genesis block @MichelSchudel
  9. 9. 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
  10. 10. • 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
  11. 11. 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
  12. 12. 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
  13. 13. • 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
  14. 14. 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
  15. 15. 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
  16. 16. https://gitlab.com/craftsmen/workshop-blockchain @MichelSchudel michel@craftsmen.nl
  17. 17. Code slides (in case you didn’t see the demo)
  18. 18. 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. }
  19. 19. 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; } }
  20. 20. 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! }
  21. 21. 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); } }
  22. 22. 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; }
  23. 23. Mining a new block @PostMapping("/api/mine") public Block mine() { Block block = getBlockchain().mine(transactionPool.getAllTransactions()); transactionPool.clearTransactions(); return block; }
  24. 24. 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; } }
  25. 25. Proof of work public class Block { … public void calculateProofOfWork() { this.nonce = 0; while (!DigestUtils.sha256Hex(this.toString()).startsWith("0000")) { this.nonce++; } } }
  26. 26. 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); }
  27. 27. 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())); }
  28. 28. Comparing with other blockchains public class Block { … boolean isValid() { return DigestUtils.sha256Hex(this.toString()).startsWith("0000"); } … }
  29. 29. Propagating transactions @PostMapping("/api/createtransaction") public long createTransaction(@RequestBody Transaction transaction) { …create transaction… network.notifyPeersOfNewTransaction(transaction); }
  30. 30. 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); } }
  31. 31. Propagating blocks @PostMapping("/api/mine") public Block mine() { Block block = getBlockchain().mine(transactionPool.getAllTransactions()); transactionPool.clearTransactions(); //propgate transaction network.notifyPeersOfNewBlock(block); return block; }
  32. 32. 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); } }
  33. 33. 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); } }

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