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Smart Contracts

Shamwari supports WebAssembly (WASM) smart contracts for extensible blockchain logic.

Contract Model​

Smart contracts in Shamwari run in a deterministic WASM runtime with:

  • Sandboxed execution: Isolated memory and resources
  • Gas metering: Computation cost limits
  • Deterministic: Same input always produces same output
  • Stateful: Persistent contract storage on-chain

Writing Contracts​

Basic Structure​

export function init(context) {
    context.set("owner", context.sender());
    context.set("balance", 0);
}

export function transfer(context, amount, to) {
    let sender = context.sender();
    let balance = context.get("balance");
    
    require(balance >= amount, "Insufficient balance");
    require(sender !== to, "Cannot transfer to self");
    
    context.sub("balance", amount);
    context.set(to, context.get(to) + amount);
}

Available APIs​

FunctionDescription
context.sender()Get transaction sender account
context.get(key)Read contract storage
context.set(key, value)Write contract storage
context.add(key, amount)Add to numeric storage
context.sub(key, amount)Subtract from numeric storage
context.require(cond, msg)Conditional abort
context.emit(event, data)Emit contract event

Deployment​

# Compile contract
shamwari contract compile my_contract.js

# Deploy to chain
curl -X POST http://localhost:6876/nxt \
  -d "requestType=uploadContractFile" \
  -d "contractName=MyContract" \
  -d "contract=@my_contract.wasm" \
  -d "secretPhrase=your_secret_phrase"

Contract References​

# Set contract reference (link account to contract)
curl -X POST http://localhost:6876/nxt \
  -d "requestType=setContractReference" \
  -d "account=NXT-account" \
  -d "contract=1234567890123456789" \
  -d "secretPhrase=your_secret_phrase"

Verification​

# Verify contract integrity
curl "http://localhost:6876/nxt?requestType=verifyContract&contract=1234567890123456789"

Contract Runner Status​

# Check contract execution state
curl "http://localhost:6876/nxt?requestType=getContractRunnerStatus"

Best Practices​

  1. Minimize Gas: Optimize loops and storage access
  2. Validate Inputs: Always check parameters before use
  3. Handle Errors: Use try/catch for external calls
  4. Event Logging: Emit events for important state changes
  5. Upgrade Path: Design contracts for future upgrades

Limitations​

  • Contract size: Maximum 128 KB
  • Execution time: 100 ms timeout
  • Storage: 1 MB per contract
  • Gas per block: Limited by block size
  • uploadContractFile - Deploy contract binary
  • downloadContractFile - Retrieve contract
  • verifyContract - Verify contract integrity
  • setContractReference - Link account to contract
  • getContractReferences - List account contracts
  • getContractRunnerStatus - Check runner state