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Implement Chain Replication with tests
- Add ChainReplication implementation with Head, Middle, and Tail node roles - Implement chain replication message types (read/write requests and responses) - Add ChainReplicationTest with basic configuration and write tests - Fix socket timeouts and message handling in network layer - Update common components (Config, Message, Replica) to support chain replication - Include documentation in wal.md and quorum.md
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‎TODO.md‎

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# Chain Replication Implementation TODO
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## Phase 1: Basic Infrastructure
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### 1. Core Classes Setup
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- [ ] Create `ChainReplication.java` extending `Replica`
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- [ ] Define `NodeRole` enum (HEAD, MIDDLE, TAIL, JOINING)
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- [ ] Create basic state variables (role, successor, predecessor, store)
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- [ ] Implement basic constructor and initialization
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### 2. Message Types
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- [ ] Create `ChainWriteRequest.java`
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- [ ] Create `ChainWriteAck.java`
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- [ ] Create `ChainReadRequest.java`
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- [ ] Create `ChainReadResponse.java`
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- [ ] Add message IDs to `MessageId` enum
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### 3. Basic Test Infrastructure
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- [ ] Create `ChainReplicationTest.java` extending `ClusterTest`
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- [ ] Implement `setUp()` with 3-node chain configuration
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- [ ] Create helper assertion methods
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- [ ] Create `KVClient` extensions for chain operations
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## Phase 2: Basic Operations
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### 4. Chain Configuration
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- [ ] Implement `updateChainConfig()` method
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- [ ] Add test: `chainConfigurationTest()`
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- [ ] Add test: `roleAssignmentTest()`
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- [ ] Add test: `successorPredecessorTest()`
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### 5. Write Path - Basic
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- [ ] Implement `handleClientWrite()` for HEAD
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- [ ] Implement `handleChainWrite()` for forwarding
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- [ ] Implement `handleChainWriteAck()` for responses
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- [ ] Add test: `basicWriteTest()`
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- [ ] Add test: `writeToNonHeadFailsTest()`
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### 6. Read Path - Basic
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- [ ] Implement `handleClientRead()` for TAIL
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- [ ] Add test: `basicReadTest()`
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- [ ] Add test: `readFromNonTailFailsTest()`
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- [ ] Add test: `readAfterWriteTest()`
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## Phase 3: Consistency & Ordering
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### 7. Write Ordering
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- [ ] Implement version tracking in writes
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- [ ] Add test: `writeOrderingTest()`
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- [ ] Add test: `concurrentWritesTest()`
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- [ ] Add test: `writeVersioningTest()`
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### 8. Read Consistency
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- [ ] Ensure reads reflect latest committed writes
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- [ ] Add test: `readConsistencyTest()`
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- [ ] Add test: `readYourWritesTest()`
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- [ ] Add test: `multipleClientReadWriteTest()`
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## Phase 4: Failure Handling
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### 9. Basic Failure Detection
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- [ ] Implement heartbeat mechanism
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- [ ] Add failure detection logic
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- [ ] Add test: `nodeFailureDetectionTest()`
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- [ ] Add test: `heartbeatTimeoutTest()`
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### 10. Chain Reconfiguration
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- [ ] Implement chain reconfiguration protocol
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- [ ] Handle successor/predecessor updates
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- [ ] Add test: `basicChainReconfigurationTest()`
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- [ ] Add test: `headFailureReconfigurationTest()`
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- [ ] Add test: `tailFailureReconfigurationTest()`
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- [ ] Add test: `middleNodeFailureReconfigurationTest()`
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### 11. State Transfer
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- [ ] Implement state transfer protocol
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- [ ] Create state snapshot mechanism
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- [ ] Implement catch-up logic
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- [ ] Add test: `stateTransferTest()`
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- [ ] Add test: `nodeRecoveryTest()`
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- [ ] Add test: `catchUpAfterFailureTest()`
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## Phase 5: Performance & Durability
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### 12. Write Pipeline
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- [ ] Implement pipelined writes
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- [ ] Add performance metrics
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- [ ] Add test: `writeThroughputTest()`
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- [ ] Add test: `pipelinedWriteTest()`
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### 13. Durability
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- [ ] Implement `DurableKVStore` integration
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- [ ] Add persistence for chain configuration
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- [ ] Add persistence for node state
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- [ ] Add test: `persistenceAfterRestartTest()`
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- [ ] Add test: `recoveryFromDiskTest()`
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### 14. Performance Tests
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- [ ] Add test: `writeLatencyTest()`
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- [ ] Add test: `readThroughputTest()`
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- [ ] Add test: `concurrentOperationsTest()`
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- [ ] Add benchmarking suite
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## Phase 6: Edge Cases & Robustness
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### 15. Network Partitions
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- [ ] Handle network partition scenarios
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- [ ] Add test: `networkPartitionTest()`
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- [ ] Add test: `partitionHealingTest()`
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- [ ] Add test: `splitBrainPreventionTest()`
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### 16. Message Loss & Delays
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- [ ] Implement message retry mechanism
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- [ ] Handle delayed messages
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- [ ] Add test: `messageLossTest()`
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- [ ] Add test: `delayedMessageTest()`
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- [ ] Add test: `messageReorderingTest()`
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### 17. Configuration Changes
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- [ ] Implement dynamic chain expansion
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- [ ] Implement chain shrinking
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- [ ] Add test: `chainExpansionTest()`
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- [ ] Add test: `chainShrinkingTest()`
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- [ ] Add test: `reconfigurationDuringOperationsTest()`
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## Phase 7: Monitoring & Operations
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### 18. Metrics & Monitoring
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- [ ] Add operation latency tracking
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- [ ] Add throughput metrics
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- [ ] Add chain health metrics
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- [ ] Add test: `metricsTrackingTest()`
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### 19. Administrative Operations
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- [ ] Add chain status API
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- [ ] Add manual failover command
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- [ ] Add node replacement API
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- [ ] Add test: `administrativeOperationsTest()`
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### 20. Documentation
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- [ ] Write API documentation
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- [ ] Write operational guide
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- [ ] Write failure handling guide
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- [ ] Add example configurations
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- [ ] Document test scenarios
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## Completion Criteria
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- All tests passing
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- Performance benchmarks met
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- Documentation complete
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- Code review completed
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- Integration tests with other system components passing
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## Notes
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- Each task should be implemented following TDD:
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1. Write failing test
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2. Implement minimum code to pass
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3. Refactor
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4. Verify all tests still pass
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- Tasks within each phase can be parallelized if needed
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- Each task should include appropriate logging and error handling
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- Consider adding metrics for each operation type

‎agenda.md‎

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# 2-Day Distributed Systems Workshop Agenda
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> **Workshop Format**: Each teaching block is 40 minutes (≈ 30 min explanation + 10 min guided coding)
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> **Breaks**: 10–15 minutes between sessions
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> **Daily Structure**: ~4 hours teaching + ~45 minutes of breaks per day
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---
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## 📅 Day 1: Foundations & Basic Patterns
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### **Session 1** (40 min) 🎯 **Why Distribute?**
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- **Learning Goals:**
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- Resource ceilings and physical limits
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- Little's Law and performance modeling
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- Motivation for distributed patterns
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- **🛠️ Hands-on Lab:** Run provided disk-perf test; capture own numbers
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- **Break:** 10 minutes
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### **Session 2** (40 min) 🎯 **Partial Failure Mindset**
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- **Learning Goals:**
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- Probability of failure at scale
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- Network partitions and split-brain scenarios
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- Process pauses and their impact
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- **🛠️ Hands-on Lab:** Walkthrough of the 'replicate' framework with an example test to inject faults.
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- **📁 Reference:** `src/main/java/replicate/common/` and `src/test/java/replicate/common/`
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- **Break:** 10 minutes
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### **Session 3** (40 min) 🎯 **Write-Ahead Log Pattern**
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- **Learning Goals:**
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- Append-only discipline for durability
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- Recovery mechanisms and replay
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- WAL as foundation for other patterns
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- **🛠️ Hands-on Lab:** Execute and walkthrough `DurableKVStoreTest` for persistent key-value store.
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- **📁 Reference:** `src/test/java/replicate/wal/DurableKVStoreTest.java`
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- **Break:** 15 minutes
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### **Session 4** (40 min) 🎯 **Replication & Majority Quorum**
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- **Learning Goals:**
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- Write vs read quorums trade-offs
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- Quorum intersection properties
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- Universal Scalability Law curve analysis
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- **🛠️ Hands-on Lab:** Modify `QuorumKVStoreTest`: pass for 5-node/3-node clusters
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- **📁 Reference:** `src/test/java/replicate/quorum/QuorumKVStoreTest.java`
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- **End of Day 1**
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### 🍽️ **Lunch Break / Self-Paced Time**
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**Offline Activities:**
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- Review morning labs and concepts
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- Push completed work to GitHub
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- Optional: Explore additional resources
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---
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## 📅 Day 2: Consensus Algorithms & Advanced Patterns
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### **Session 5** (40 min) 🎯 **Why Simple Replication Fails**
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- **Learning Goals:**
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- Two-phase commit pitfalls
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- Recovery ambiguity problems
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- The need for consensus algorithms
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- **🛠️ Hands-on Lab:** Step through `DeferredCommitmentTest` and `RecoverableDeferredCommitmentTest`; explain why they hang
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- **📁 Reference:** `src/test/java/replicate/twophaseexecution/DeferredCommitmentTest.java`
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- **Break:** 10 minutes
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### **Session 6** (40 min) 🎯 **Single-Value Paxos**
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- **Learning Goals:**
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- Prepare/Accept phases explained
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- Recovery with generation numbers
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- Safety and liveness properties
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- **🛠️ Hands-on Lab:** Work with generation voting mechanism using existing Paxos tests
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- **📁 Reference:** `src/test/java/replicate/paxos/` and `src/test/java/replicate/generationvoting/`
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- **Break:** 10 minutes
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### **Session 7** (40 min) 🎯 **From Paxos to Multi-Paxos**
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- **Learning Goals:**
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- Replicated log concept and implementation
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- High-water mark for safe execution
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- Heartbeats and failure detection
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- **🛠️ Hands-on Lab:** Extend log to multi-slot using Multi-Paxos and Paxos Log implementations
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- **📁 Reference:** `src/test/java/replicate/multipaxos/` and `src/test/java/replicate/paxoslog/`
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- **Break:** 15 minutes
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### **Session 8** (40 min) 🎯 **RAFT vs Multi-Paxos in Practice**
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- **Learning Goals:**
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- Implementation optimizations comparison
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- Idempotent receiver pattern
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- Production considerations and future directions
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- **🛠️ Hands-on Lab:** Compare RAFT & Multi-Paxos implementations; annotate pros/cons
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- **📁 Reference:** `src/main/java/replicate/raft/` and `src/main/java/replicate/multipaxos/`
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- **End of Day 2**
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---
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## 📊 Workshop Summary
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### 🎯 **Learning Outcomes**
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- **8 teaching blocks** × 40 minutes each
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- **Hands-on labs** tied directly to core lecture concepts
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- **Built-in breaks** for focus & recovery
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- **Progressive assignments** that reinforce distributed systems primitives step-by-step
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### 🛠️ **Technical Skills Gained**
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- Understanding distributed systems fundamentals
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- Implementing Write-Ahead Log pattern
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- Working with quorum-based replication
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- Exploring consensus algorithms (Paxos, RAFT)
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- Hands-on experience with fault tolerance patterns
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### 🗂️ **Available Implementations**
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- **Consensus Algorithms:** Paxos, Multi-Paxos, RAFT, ViewStamped Replication
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- **Replication Patterns:** Chain Replication, Quorum-based KV Store
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- **Foundational Patterns:** WAL, Two-Phase Commit, Heartbeat Detection
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- **Network Layer:** Socket-based messaging, Request-waiting lists
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### 📁 **Key Files Reference**
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- **Core Framework:** `src/main/java/replicate/common/`
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- **WAL Implementation:** `src/main/java/replicate/wal/DurableKVStore.java`
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- **Quorum KV Store:** `src/main/java/replicate/quorum/QuorumKVStore.java`
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- **Chain Replication:** `src/main/java/replicate/chain/ChainReplication.java`
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- **Paxos Implementation:** `src/main/java/replicate/paxos/`
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- **RAFT Implementation:** `src/main/java/replicate/raft/`
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- **Tests Directory:** `src/test/java/replicate/`
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### 📚 **Resources & Next Steps**
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- All code examples and labs available on GitHub
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- Additional reading materials provided
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- Follow-up Q&A session for complex topics

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