refactor: replace snapshot design with SQLite-inspired reader-writer lock approach

IDEA.md

@@ -23,13 +23,14 @@ Build a clean, composable, and educational storage engine in Go that follows Log
 - Support reproducible startup states across restarts
 
 ### 4. Composable Primitives
-- Design clean interfaces for fundamental operations (reads, writes, snapshots, iteration)
+- Design clean interfaces for fundamental operations (reads, writes, iteration, transactions)
 - Enable building of higher-level abstractions (SQL, Gremlin, custom query languages)
 - Support both transactional and analytical workloads
 - Provide simple atomic write primitives that can be built upon:
-  - Leverage read snapshots from immutable LSM structure
+  - Leverage reader-writer lock for concurrency control (SQLite-inspired approach)
   - Support basic atomic batch operations
   - Ensure crash recovery through proper WAL handling
+  - Enable read operations during writes via WAL (following SQLite's concurrency model)
 
 ## Target Use Cases
 

PLAN.md

@@ -37,8 +37,7 @@ go-storage/
 │   │   └── footer/          # File footer and metadata
 │   ├── compaction/          # Compaction strategies
 │   ├── iterator/            # Merged iterator implementation
-│   ├── transaction/         # Transaction management with Snapshot + WAL
-│   │   ├── snapshot/        # Read snapshot implementation
+│   ├── transaction/         # Transaction management with WAL and locks
 │   │   └── txbuffer/        # Transaction write buffer
 │   └── engine/              # Main engine implementation with single-writer architecture
 │
@@ -98,9 +97,10 @@ go-storage/
 
 ### Phase F: Basic Atomicity and Advanced Features (2-3 weeks)
 1. Implement merged iterator across all levels
-2. Add snapshot capability for reads:
-   - Point-in-time view of the database
-   - Consistent reads across MemTable and SSTables
+2. Implement SQLite-inspired reader-writer concurrency:
+   - Reader-writer lock for basic isolation
+   - WAL-based reads for active transactions
+   - Support for read operations during writes
 3. Implement simple atomic batch operations:
    - Support atomic multi-key writes
    - Ensure proper crash recovery for batch operations
@@ -137,15 +137,17 @@ go-storage/
 
 ### Concurrency
 - Implement single-writer architecture for the main write path
-- Allow concurrent readers (snapshots) to proceed without blocking
-- Use appropriate synchronization for reader-writer coordination
-- Ensure proper isolation between transactions
+- Allow concurrent readers through reader-writer lock mechanism
+- Use SQLite-inspired WAL approach for reader-writer coordination:
+  - Writers write to WAL, readers can read from either main database or WAL
+  - Use appropriate synchronization with reader-writer lock
+- Ensure proper isolation between transactions through lock-based exclusion
 
 ### Batch Operation Management
 - Use WAL for atomic batch operation durability
-- Leverage LSM's natural versioning for snapshots
+- Leverage reader-writer lock for transactional isolation
 - Provide simple interfaces that can be built upon for transactions
-- Ensure proper crash recovery for batch operations
+- Ensure proper crash recovery for batch operations through WAL replay
 
 ### Go Idioms
 - Follow standard Go project layout

TODO.md

@@ -43,12 +43,12 @@ This document outlines the implementation tasks for the Go Storage Engine, organ
 
 - [x] Connect WAL replay to MemTable
   - [x] Create recovery logic to rebuild MemTable from WAL
-  - [x] Implement consistent snapshot reads during recovery
+  - [x] Implement consistent state reconstruction during recovery
   - [x] Handle errors during replay with appropriate fallbacks
 
 - [x] Test concurrent read/write scenarios
   - [x] Verify reader isolation during writes
-  - [x] Test snapshot consistency guarantees
+  - [x] Test consistency guarantees with concurrent operations
   - [x] Benchmark read/write performance under load
 
 ## Phase C: Persistent Storage
@@ -121,11 +121,11 @@ This document outlines the implementation tasks for the Go Storage Engine, organ
   - [x] Add efficient seeking capabilities
   - [x] Implement proper cleanup for resources
 
-- [ ] Add snapshot capability
-  - [ ] Create point-in-time view mechanism
-  - [ ] Implement consistent reads across all data sources
-  - [ ] Add resource tracking and cleanup
-  - [ ] Test isolation guarantees
+- [ ] Implement SQLite-inspired reader-writer concurrency
+  - [ ] Add reader-writer lock for basic isolation
+  - [ ] Implement WAL-based reads during active write transactions
+  - [ ] Design clean API for transaction handling
+  - [ ] Test concurrent read/write operations
 
 - [ ] Implement atomic batch operations
   - [ ] Create batch data structure for multiple operations
@@ -186,7 +186,7 @@ This document outlines the implementation tasks for the Go Storage Engine, organ
   - [ ] `Delete(ctx context.Context, key []byte, opts ...WriteOption) error`
   - [ ] `Batch(ctx context.Context, ops []Operation, opts ...WriteOption) error`
   - [ ] `NewIterator(opts IteratorOptions) Iterator`
-  - [ ] `Snapshot() Snapshot`
+  - [ ] `BeginTransaction(readOnly bool) (Transaction, error)`
   - [ ] `Close() error`
 
 - [ ] Implement error types