Analysis Phase - Learning Module

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Transaction Table: Tracking Active Transactions for Recovery

Knowing Who Was In Flight When Disaster Struck

When a database crashes, the recovery system faces two fundamental questions about every transaction that was running at failure time:

Did this transaction commit? If yes, its effects must be present in the recovered database (durability).
If not committed, what operations must be undone? Uncommitted work must be rolled back as if it never happened (atomicity).

The Transaction Table answers both questions. It maintains runtime state about every active transaction—its current status, the log records it generated, and the information needed to undo its effects. This page explores the Transaction Table in exhaustive technical detail, examining its structure, lifecycle management, checkpoint persistence, and indispensable role in the ARIES Analysis Phase.

What You Will Learn

By the end of this page, you will understand the complete architecture of the Transaction Table, how entries are created and updated throughout transaction lifecycles, how the table survives checkpoints, and how the Analysis Phase uses it to identify transactions requiring undo. You'll be able to trace transaction state transitions and explain how the table enables correct crash recovery.

The Purpose of the Transaction Table

The Transaction Table serves as the runtime registry of all active transactions in the database system. Its primary purposes are:

During Normal Operation:

Track the current state of each transaction (active, committing, aborting)
Maintain pointers to transaction log records for efficient rollback
Support transaction isolation and concurrency control
Enable administrative functions (viewing active transactions, killing runaway transactions)

During Recovery:

Identify transactions that were active at crash time
Determine which transactions committed versus which need undo
Provide the starting point for undo operations (the lastLSN of each uncommitted transaction)
Contribute to calculating the RedoLSN

Transaction Table vs Dirty Page Table: Complementary Roles
Aspect	Dirty Page Table	Transaction Table
Tracks	Modified database pages	Active transactions
Key Field	recLSN (first dirty LSN)	lastLSN (most recent operation)
Guides Recovery	Redo phase starting point	Undo phase targets
Entry Lifetime	From first dirty to disk write	From BEGIN to COMMIT/ABORT
Checkpoint Role	Snapshots dirty pages	Snapshots active transactions

Two Tables, Complete Information

The DPT tells recovery 'these pages might need redo.' The Transaction Table tells recovery 'these transactions need undo.' Together, they provide complete information for reconstructing a consistent database state from any crash point.

Transaction Table Architecture

The Transaction Table is an in-memory data structure mapping transaction identifiers to their runtime state. Each entry contains all information necessary for both normal operation and crash recovery.

Core Structure:

The table is typically implemented as a hash table for O(1) access, with each entry containing transaction metadata and log navigation information.

transaction_table.h
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// Transaction states in the lifecycle
enum class TransactionState {
    ACTIVE,      // Transaction is running, executing operations
    PREPARING,   // 2PC: prepare phase initiated
    COMMITTING,  // Commit initiated, log records being written
    COMMITTED,   // Commit complete (entry may be removed soon)
    ABORTING,    // Rollback in progress
    ABORTED      // Rollback complete (entry may be removed soon)
};
 
// Transaction Table entry structure
struct TransactionEntry {
    TransactionID txn_id;        // Unique transaction identifier
    TransactionState state;       // Current lifecycle state
    LSN           first_lsn;     // LSN of first log record for this txn
    LSN           last_lsn;      // LSN of most recent log record for this txn
    LSN           undo_next_lsn; // Next LSN to process during undo (follows prevLSN chain)
    Timestamp     start_time;     // When transaction began
    UserID        user_id;        // Owner of the transaction
    int           lock_count;     // Number of locks held (for monitoring)
    int           log_records;    // Count of log records generated
};
 
// The Transaction Table itself
class TransactionTable {
private:
    HashMap<TransactionID, TransactionEntry> entries;
    Mutex                                     table_lock;
    
public:
    // Create entry when transaction begins
    void beginTransaction(TransactionID tid);
    
    // Update lastLSN when transaction logs an operation
    void recordLogEntry(TransactionID tid, LSN log_lsn);
    
    // Update state during commit/abort
    void setCommitting(TransactionID tid);
    void setCommitted(TransactionID tid);
    void setAborting(TransactionID tid);
    void setAborted(TransactionID tid);
    
    // Remove entry when transaction completes
    void removeTransaction(TransactionID tid);
    
    // Recovery support
    vector<TransactionEntry> snapshot();
    TransactionEntry* getEntry(TransactionID tid);
    vector<TransactionID> getUncommittedTransactions();
};

Key Fields Explained:

Transaction ID (txn_id): A globally unique identifier assigned when the transaction begins. Typically a monotonically increasing 64-bit integer, possibly incorporating a timestamp or server ID for distributed systems.

State: The current position in the transaction lifecycle. This state machine governs what operations are legal and what recovery actions are needed.

First LSN (firstLSN): The Log Sequence Number of the first log record generated by this transaction. Used for:

Calculating RedoLSN (minimum firstLSN may influence where redo starts)
Debugging and auditing (when did this transaction start logging?)

Last LSN (lastLSN): The LSN of the most recent log record for this transaction. This is the starting point for undo—rollback follows the prevLSN chain backward from here.

Undo Next LSN (undoNextLSN): During rollback (either normal abort or recovery undo), this field tracks progress. It points to the next log record that must be undone, following the prevLSN chain.

The prevLSN Chain

Each log record contains a 'prevLSN' field pointing to the previous log record of the same transaction. This forms a backward-linked chain enabling efficient rollback without scanning the entire log. The lastLSN in the Transaction Table is the head of this chain.

Transaction Lifecycle and Table Updates

The Transaction Table is updated at every significant point in a transaction's lifecycle. Understanding these updates is essential for tracing how the table reaches its crash-time state.

Lifecycle State Machine:

Converting Mermaid diagram...

Event 1: Transaction Begin

When a transaction starts, a new entry is created with initial state:

txn_begin.cpp
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void TransactionTable::beginTransaction(TransactionID tid) {
    lock_guard<Mutex> lock(table_lock);
    
    // Create new entry
    TransactionEntry entry;
    entry.txn_id = tid;
    entry.state = TransactionState::ACTIVE;
    entry.first_lsn = INVALID_LSN;  // No log records yet
    entry.last_lsn = INVALID_LSN;
    entry.undo_next_lsn = INVALID_LSN;
    entry.start_time = getCurrentTime();
    entry.user_id = getCurrentUser();
    entry.lock_count = 0;
    entry.log_records = 0;
    
    entries[tid] = entry;
    
    log_debug("Transaction %d started", tid);
}
 
// Note: The BEGIN transaction itself may or may not be logged.
// ARIES does not require logging BEGIN, but some systems do for auditing.

Event 2: Recording Operations

Every data modification generates a log record. The Transaction Table must be updated to track the latest LSN:

record_operation.cpp
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void TransactionTable::recordLogEntry(TransactionID tid, LSN log_lsn) {
    lock_guard<Mutex> lock(table_lock);
    
    auto it = entries.find(tid);
    if (it == entries.end()) {
        throw InvalidTransactionError("Transaction not found");
    }
    
    TransactionEntry& entry = it->second;
    
    // Update firstLSN only if this is the first log record
    if (entry.first_lsn == INVALID_LSN) {
        entry.first_lsn = log_lsn;
    }
    
    // Always update lastLSN to the most recent record
    entry.last_lsn = log_lsn;
    entry.log_records++;
    
    log_debug("Transaction %d: logged LSN %d (total records: %d)",
              tid, log_lsn, entry.log_records);
}
 
// The log record itself contains:
// - LSN (assigned by log manager)
// - TransactionID
// - prevLSN (the PREVIOUS lastLSN of this transaction - forms undo chain)
// - Type (UPDATE, INSERT, DELETE, etc.)
// - PageID
// - Undo data (old value)
// - Redo data (new value)

Event 3: Commit Processing

Committing involves writing a COMMIT log record and transitioning through states:

commit_processing.cpp
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TransactionResult TransactionManager::commitTransaction(TransactionID tid) {
    // Step 1: Transition to COMMITTING state
    txn_table.setCommitting(tid);
    
    // Step 2: Write COMMIT log record
    // This record indicates the transaction's decision to commit
    LogRecord commit_record;
    commit_record.type = LogType::COMMIT;
    commit_record.txn_id = tid;
    commit_record.prev_lsn = txn_table.getEntry(tid)->last_lsn;
    
    LSN commit_lsn = log_manager.appendLog(commit_record);
    txn_table.recordLogEntry(tid, commit_lsn);
    
    // Step 3: Force log to disk (CRUCIAL for durability!)
    // All log records up to commit_lsn must be on stable storage
    log_manager.flushTo(commit_lsn);
    
    // Step 4: Transition to COMMITTED
    // At this point, the transaction is durably committed
    txn_table.setCommitted(tid);
    
    // Step 5: Release locks
    lock_manager.releaseAllLocks(tid);
    
    // Step 6: Write END log record (optional but useful)
    // Indicates that commit processing is complete
    LogRecord end_record;
    end_record.type = LogType::END;
    end_record.txn_id = tid;
    end_record.prev_lsn = commit_lsn;
    
    log_manager.appendLog(end_record);  // No need to force - just informational
    
    // Step 7: Remove from transaction table
    txn_table.removeTransaction(tid);
    
    return TransactionResult::SUCCESS;
}

The Critical Flush

The log flush in step 3 is the moment of truth for durability. Before this point, a crash would cause the transaction to be rolled back during recovery. After this point, the transaction is committed regardless of subsequent crashes. This is why write-ahead logging is fundamental—the COMMIT record must reach stable storage before we tell the application 'commit successful.'

Transaction Abort and Rollback

When a transaction must be aborted—whether due to user request, constraint violation, deadlock, or error—the system must undo all its effects. The Transaction Table provides the starting point for this rollback.

Rollback Process:

Write an ABORT log record
Follow the prevLSN chain from lastLSN backward
For each undoable log record, perform the compensating action
Write CLR (Compensation Log Record) for each undo action
Write END record when rollback is complete
Remove entry from Transaction Table

abort_processing.cpp
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TransactionResult TransactionManager::abortTransaction(TransactionID tid) {
    TransactionEntry* entry = txn_table.getEntry(tid);
    if (!entry) {
        throw InvalidTransactionError("Transaction not found");
    }
    
    // Step 1: Transition to ABORTING state
    txn_table.setAborting(tid);
    
    // Step 2: Write ABORT log record
    LogRecord abort_record;
    abort_record.type = LogType::ABORT;
    abort_record.txn_id = tid;
    abort_record.prev_lsn = entry->last_lsn;
    
    LSN abort_lsn = log_manager.appendLog(abort_record);
    txn_table.recordLogEntry(tid, abort_lsn);
    
    // Step 3: Perform rollback - follow prevLSN chain
    entry->undo_next_lsn = entry->last_lsn;  // Start from most recent
    
    while (entry->undo_next_lsn != INVALID_LSN) {
        LogRecord record = log_manager.readLog(entry->undo_next_lsn);
        
        if (record.isUndoable()) {
            // Perform the compensating action
            undoOperation(record);
            
            // Write CLR (Compensation Log Record)
            LogRecord clr;
            clr.type = LogType::CLR;
            clr.txn_id = tid;
            clr.prev_lsn = entry->last_lsn;
            clr.undo_next_lsn = record.prev_lsn;  // Points to next record to undo
            clr.page_id = record.page_id;
            clr.redo_data = record.undo_data;  // CLR's redo = original undo
            
            LSN clr_lsn = log_manager.appendLog(clr);
            txn_table.recordLogEntry(tid, clr_lsn);
        }
        
        // Move to previous log record in the chain
        entry->undo_next_lsn = record.prev_lsn;
    }
    
    // Step 4: Write END record
    LogRecord end_record;
    end_record.type = LogType::END;
    end_record.txn_id = tid;
    log_manager.appendLog(end_record);
    
    // Step 5: Release locks
    lock_manager.releaseAllLocks(tid);
    
    // Step 6: Transition to ABORTED and remove
    txn_table.setAborted(tid);
    txn_table.removeTransaction(tid);
    
    return TransactionResult::ABORTED;
}

CLRs Are Redo-Only

Compensation Log Records (CLRs) are crucial for crash recovery during rollback. If the system crashes mid-rollback, recovery can continue the rollback by following CLRs. CLRs are never undone—they represent completed undo work. Their undoNextLSN field enables recovery to skip already-undone records.

Checkpoint Integration

During checkpointing, the Transaction Table snapshot is written to the log alongside the Dirty Page Table. This enables recovery to reconstruct the transaction state at checkpoint time.

What Gets Persisted:

For each active transaction at checkpoint time, the checkpoint record stores:

Transaction ID
Current state
lastLSN (most recent log record)
undoNextLSN (if transaction was already in rollback)

The firstLSN is technically derivable from the log but is often included for convenience and correctness verification.

checkpoint_txn_table.cpp
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struct CheckpointTransactionEntry {
    TransactionID txn_id;
    TransactionState state;
    LSN last_lsn;
    LSN undo_next_lsn;
};
 
vector<CheckpointTransactionEntry> TransactionTable::snapshotForCheckpoint() {
    lock_guard<Mutex> lock(table_lock);
    
    vector<CheckpointTransactionEntry> snapshot;
    
    for (const auto& [tid, entry] : entries) {
        // Include all non-completed transactions
        if (entry.state != TransactionState::COMMITTED &&
            entry.state != TransactionState::ABORTED) {
            
            CheckpointTransactionEntry ckpt_entry;
            ckpt_entry.txn_id = entry.txn_id;
            ckpt_entry.state = entry.state;
            ckpt_entry.last_lsn = entry.last_lsn;
            ckpt_entry.undo_next_lsn = entry.undo_next_lsn;
            
            snapshot.push_back(ckpt_entry);
        }
    }
    
    log_info("Transaction table snapshot: %d active transactions", snapshot.size());
    return snapshot;
}
 
void RecoveryManager::performFuzzyCheckpoint() {
    // Step 1: Write BEGIN_CHECKPOINT
    LSN begin_ckpt_lsn = log_manager.appendLog(BEGIN_CHECKPOINT, {});
    
    // Step 2: Snapshot both tables (order doesn't matter for correctness)
    vector<DirtyPageEntry> dpt_snapshot = dirty_page_table.snapshot();
    vector<CheckpointTransactionEntry> txn_snapshot = txn_table.snapshotForCheckpoint();
    
    // Step 3: Write END_CHECKPOINT with both tables
    EndCheckpointRecord end_ckpt;
    end_ckpt.dirty_page_table = dpt_snapshot;
    end_ckpt.transaction_table = txn_snapshot;
    
    LSN end_ckpt_lsn = log_manager.appendLog(END_CHECKPOINT, end_ckpt);
    
    // Step 4: Force checkpoint records to disk
    log_manager.flushTo(end_ckpt_lsn);
    
    // Step 5: Update master record
    updateMasterRecord(begin_ckpt_lsn);
    
    log_info("Checkpoint complete: DPT=%d entries, TxnTable=%d entries",
             dpt_snapshot.size(), txn_snapshot.size());
}

Fuzzy Checkpoint Implications

Because fuzzy checkpoints don't stop transactions, the checkpoint snapshot represents state at a specific instant. Transactions may have progressed between BEGIN_CHECKPOINT and END_CHECKPOINT. The Analysis Phase accounts for this by scanning log records after the checkpoint to update both tables.

Transaction Table Reconstruction During Analysis

The Analysis Phase reconstructs the Transaction Table to reflect its crash-time state. This involves loading the checkpoint snapshot and then processing all log records after the checkpoint.

Analysis Phase Transaction Table Processing:

analysis_txn_table.cpp
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void AnalysisPhase::reconstructTransactionTable() {
    // Step 1: Load transaction table from checkpoint
    CheckpointRecord ckpt = readLastCheckpoint();
    
    for (const auto& entry : ckpt.transaction_table) {
        TransactionEntry txn;
        txn.txn_id = entry.txn_id;
        txn.state = entry.state;
        txn.last_lsn = entry.last_lsn;
        txn.undo_next_lsn = entry.undo_next_lsn;
        
        transaction_table.insert(entry.txn_id, txn);
    }
    
    log_info("Loaded %d transactions from checkpoint", 
             ckpt.transaction_table.size());
    
    // Step 2: Scan log from checkpoint to end
    LSN current_lsn = ckpt.checkpoint_lsn;
    
    while (current_lsn != END_OF_LOG) {
        LogRecord record = log_manager.readLog(current_lsn);
        TransactionID tid = record.txn_id;
        
        // Handle different log record types
        switch (record.type) {
            case LogType::UPDATE:
            case LogType::INSERT:
            case LogType::DELETE:
            case LogType::CLR:
                // Regular operations: update or create transaction entry
                if (!transaction_table.contains(tid)) {
                    // Transaction started after checkpoint
                    TransactionEntry entry;
                    entry.txn_id = tid;
                    entry.state = TransactionState::UNDO;  // Mark for undo
                    entry.last_lsn = current_lsn;
                    entry.undo_next_lsn = current_lsn;
                    transaction_table.insert(tid, entry);
                    
                    log_debug("Analysis: New transaction %d found at LSN %d", 
                             tid, current_lsn);
                } else {
                    // Update existing entry
                    TransactionEntry& entry = transaction_table.get(tid);
                    entry.last_lsn = current_lsn;
                    
                    // For CLR, update undoNextLSN
                    if (record.type == LogType::CLR) {
                        entry.undo_next_lsn = record.undo_next_lsn;
                    } else {
                        entry.undo_next_lsn = current_lsn;
                    }
                }
                break;
                
            case LogType::COMMIT:
                // Transaction committed - will be removed later
                if (transaction_table.contains(tid)) {
                    transaction_table.get(tid).state = TransactionState::COMMITTING;
                }
                break;
                
            case LogType::ABORT:
                // Transaction was aborting at crash
                if (transaction_table.contains(tid)) {
                    transaction_table.get(tid).state = TransactionState::UNDO;
                }
                break;
                
            case LogType::END:
                // Transaction fully completed - remove from table
                transaction_table.remove(tid);
                log_debug("Analysis: Transaction %d completed, removed", tid);
                break;
        }
        
        current_lsn = record.next_lsn;
    }
    
    // Step 3: Mark all remaining transactions as needing UNDO
    // Any transaction still in the table without an END record didn't complete
    for (auto& [tid, entry] : transaction_table) {
        if (entry.state != TransactionState::COMMITTING) {
            entry.state = TransactionState::UNDO;
        }
    }
    
    log_info("Analysis complete: %d transactions require undo",
             transaction_table.getUndoCount());
}

COMMIT vs END Records

A COMMIT record indicates the transaction decided to commit, but an END record indicates commit processing completed. If the system crashed between COMMIT and END, the transaction IS committed (its effects must survive), but it won't have an END record. The Analysis Phase must correctly identify this case—the transaction should NOT be in the undo list.

Handling the COMMIT-but-no-END Case:

This subtle scenario deserves special attention. Consider:

Transaction T1 writes COMMIT at LSN 1000
System crashes before END record
During recovery, Analysis sees COMMIT but no END

T1 should NOT be undone—it committed. The state should be COMMITTING, and it should be removed from the undo list during the redo/undo phases (after redo replays any effects and confirms durability).

Determining Undo Requirements

After the Analysis Phase completes, the Transaction Table contains all transactions that require undo processing. The Undo Phase uses this information to roll back uncommitted work.

Identifying Losers (Transactions to Undo):

A transaction is a 'loser' requiring undo if:

It was ACTIVE at crash time (no COMMIT record)
It was ABORTING at crash time (rollback was in progress)
It had an ABORT record but no END record

determine_undo_list.cpp
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struct UndoTask {
    TransactionID txn_id;
    LSN           undo_next_lsn;  // Where to start undoing
};
 
vector<UndoTask> AnalysisPhase::determineUndoList() {
    vector<UndoTask> undo_list;
    
    for (const auto& [tid, entry] : transaction_table) {
        bool needs_undo = false;
        
        switch (entry.state) {
            case TransactionState::ACTIVE:
                // Was running, never committed
                needs_undo = true;
                break;
                
            case TransactionState::UNDO:
                // Explicitly marked for undo (started after checkpoint, no commit)
                needs_undo = true;
                break;
                
            case TransactionState::ABORTING:
                // Was in the middle of rollback
                needs_undo = true;
                break;
                
            case TransactionState::COMMITTING:
                // Had COMMIT record - do NOT undo
                // Will write END during recovery to clean up
                needs_undo = false;
                log_debug("Transaction %d committed - will write END", tid);
                break;
                
            case TransactionState::COMMITTED:
            case TransactionState::ABORTED:
                // Fully completed - should have been removed
                // If still here, something is wrong
                log_error("Unexpected completed transaction %d in table", tid);
                break;
        }
        
        if (needs_undo) {
            UndoTask task;
            task.txn_id = tid;
            task.undo_next_lsn = entry.undo_next_lsn;
            undo_list.push_back(task);
            
            log_info("Transaction %d added to undo list, starting at LSN %d",
                    tid, entry.undo_next_lsn);
        }
    }
    
    // Sort by undoNextLSN descending for efficient undo
    // Start with most recent operations first
    sort(undo_list.begin(), undo_list.end(),
         [](const UndoTask& a, const UndoTask& b) {
             return a.undo_next_lsn > b.undo_next_lsn;
         });
    
    return undo_list;
}
 
/*
 * Example After Analysis:
 * 
 * Transaction Table:
 *   T1: state=UNDO, lastLSN=1050, undoNextLSN=1050
 *   T2: state=COMMITTING, lastLSN=1100
 *   T3: state=UNDO, lastLSN=1200, undoNextLSN=1150 (CLR advanced it)
 * 
 * Undo List (sorted by undoNextLSN descending):
 *   T3: undoNextLSN=1150
 *   T1: undoNextLSN=1050
 * 
 * Undo will process: LSN 1150 (T3), then 1050 (T1), then their prevLSN chains
 */

Why Sort Descending?

Processing undo tasks from highest to lowest LSN enables an efficient single backward pass through the log. As each transaction's prevLSN chain is followed, the log pointer naturally moves backward. This is more I/O efficient than jumping around the log randomly.

Summary: Transaction Table Mastery

The Transaction Table is the complement to the Dirty Page Table—together they provide complete information for crash recovery. Let's consolidate the essential concepts:

Key Takeaways

•Purpose: Tracks active transactions, their states, and log records for both normal operation and crash recovery.
•Key Fields: txn_id (identifier), state (lifecycle position), lastLSN (most recent log record), undoNextLSN (rollback progress).
•Lifecycle Management: Updated at BEGIN, each operation, COMMIT/ABORT, and removed at END.
•Rollback Support: The lastLSN initiates the prevLSN chain traversal for undoing transaction effects; CLRs track progress.
•Checkpoint Persistence: Snapshots of active transactions are written to the log, enabling recovery to start from checkpoint state.
•Analysis Reconstruction: Loads checkpoint snapshot, scans subsequent log, identifies 'loser' transactions needing undo.

What's Next:

With both the Dirty Page Table and Transaction Table understood, we'll examine how the Analysis Phase scans the log to update these structures. The log scanning process is where raw log records transform into actionable recovery information, determining exactly what redo and undo work must occur.

Page Complete

You now have a comprehensive understanding of the Transaction Table—its structure, lifecycle updates, checkpoint integration, and critical role in identifying transactions requiring undo. Combined with DPT knowledge, you understand the two core data structures that make ARIES recovery intelligent and efficient.