feat: add transaction support (BEGIN/COMMIT/ROLLBACK) with WAL recovery (#137)

* feat: implement transaction support with WAL recovery

Signed-off-by: unknown <010samarjeet@gamil.com>

* fix: address transaction review feedback from CodeRabbit

Signed-off-by: unknown <010samarjeet@gamil.com>

---------

Signed-off-by: unknown <010samarjeet@gamil.com>
Co-authored-by: unknown <010samarjeet@gamil.com>

Author: samarjeet818

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to NexumDB will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Notes
+
+* Transaction WAL currently uses a full `scan_all()` snapshot on `BEGIN` as an MVP implementation. A follow-up is planned to replace this with an incremental per-mutation undo/redo log for better scalability.
+* Transaction isolation is currently a single-active-transaction model per `Executor` and does not implement MVCC across multiple sessions yet.
+
 ## [0.6.0](https://github.com/aviralgarg05/NexumDB/compare/v0.5.0...v0.6.0) (2026-02-06)
 
 

nexum_cli/src/main.rs

@@ -40,6 +40,7 @@ SQL COMMANDS:
 SPECIAL COMMANDS:
   SHOW TABLES            List all tables
   DESCRIBE <table>       Show table schema
+  BEGIN / COMMIT / ROLLBACK  Transaction control
   ASK <question>         Natural language query
   EXPLAIN <query>        Show query execution plan
 "#)]
@@ -245,6 +246,11 @@ fn print_help_summary() {
         "•".cyan(),
         "DESCRIBE <table>".yellow()
     );
+    println!(
+        "  {} {} - Transaction control",
+        "•".cyan(),
+        "BEGIN / COMMIT / ROLLBACK".yellow()
+    );
     println!(
         "  {} {} - Show query plan",
         "•".cyan(),
@@ -290,6 +296,9 @@ fn print_full_help() {
     println!("{}", "Special Commands:".yellow().bold());
     println!("  {} - List all tables", "SHOW TABLES".cyan());
     println!("  {} - Show table schema", "DESCRIBE <table>".cyan());
+    println!("  {} - Start transaction", "BEGIN".cyan());
+    println!("  {} - Commit transaction", "COMMIT".cyan());
+    println!("  {} - Roll back transaction", "ROLLBACK".cyan());
     println!("  {} - Natural language query", "ASK <question>".cyan());
     println!("  {} - Query execution plan", "EXPLAIN <query>".cyan());
     println!();
@@ -357,6 +366,28 @@ fn print_result(result: &ExecutionResult, json_output: bool) {
 
 fn print_result_json(result: &ExecutionResult) {
     let json = match result {
+        ExecutionResult::TransactionBegan { tx_id } => {
+            serde_json::json!({
+                "type": "transaction_began",
+                "transaction_id": tx_id,
+                "message": format!("Transaction {} started", tx_id)
+            })
+        }
+        ExecutionResult::TransactionCommitted { tx_id, writes } => {
+            serde_json::json!({
+                "type": "transaction_committed",
+                "transaction_id": tx_id,
+                "writes": writes,
+                "message": format!("Transaction {} committed ({} row mutation(s))", tx_id, writes)
+            })
+        }
+        ExecutionResult::TransactionRolledBack { tx_id } => {
+            serde_json::json!({
+                "type": "transaction_rolled_back",
+                "transaction_id": tx_id,
+                "message": format!("Transaction {} rolled back", tx_id)
+            })
+        }
         ExecutionResult::Created { table } => {
             serde_json::json!({
                 "type": "created",
@@ -446,6 +477,18 @@ fn print_result_json(result: &ExecutionResult) {
 
 fn print_result_formatted(result: &ExecutionResult) {
     match result {
+        ExecutionResult::TransactionBegan { tx_id } => {
+            print_success(&format!("Transaction {} started", tx_id));
+        }
+        ExecutionResult::TransactionCommitted { tx_id, writes } => {
+            print_success(&format!(
+                "Transaction {} committed ({} row mutation(s))",
+                tx_id, writes
+            ));
+        }
+        ExecutionResult::TransactionRolledBack { tx_id } => {
+            print_success(&format!("Transaction {} rolled back", tx_id));
+        }
         ExecutionResult::Created { table } => {
             print_success(&format!("Table '{}' created successfully", table));
         }