feat: implement snapshot-based persistence (closes #39)

CLAUDE.md

@@ -0,0 +1,103 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Project Overview
+
+FKVS (Fast Key Value Store) is a high-performance key-value store written in C (C23 standard) with a single-threaded, non-blocking I/O multiplexed event loop. Default port: 5995.
+
+## Build Commands
+
+```bash
+# Full setup (submodules) and build
+make -f Makefile.fkvs setup-and-build
+
+# Build only (after initial setup)
+make -f Makefile.fkvs build
+
+# Run tests
+make -f Makefile.fkvs tests
+
+# Run a single test directly after building
+./test_counter
+./test_string_utils
+
+# macOS code signing (needed for network entitlements)
+make -f Makefile.fkvs codesign-server
+
+# Run the server
+./fkvs-server -c
+
+# Run the CLI client
+./fkvs-cli
+```
+
+Build uses CMake (minimum 3.31.2). The build output (executables) goes to the project root directory.
+
+## Architecture
+
+### Platform-Specific Event Dispatchers
+
+The server uses a compile-time selected event loop based on the target platform:
+- **macOS**: `src/io/event_dispatcher_kqueue.c` (kqueue)
+- **Linux**: `src/io/event_dispatcher_epoll.c` (epoll) or `src/io/event_dispatcher_io_uring.c` (io_uring if liburing is available)
+
+This is controlled in `CMakeLists.txt` via `APPLE`/`LINUX` conditionals and the `IO_URING_ENABLED` compile definition.
+
+### Compile Definitions
+
+Source files are compiled with either `SERVER` or `CLI` definition to conditionally include server-side or client-side code. This means shared headers (like `client.h`, `networking.h`) have `#ifdef SERVER`/`#ifdef CLI` blocks.
+
+### Command System
+
+Commands follow a split handler pattern:
+- `src/commands/server/server_command_handlers.c` — server-side execution of commands (SET, GET, INCR, etc.)
+- `src/commands/client/client_command_handlers.c` — client-side response parsing
+- `src/commands/common/command_registry.c` — command lookup table
+- `src/commands/common/command_parser.c` — client-side command parsing
+
+### Core Data Structures
+
+- **Hashtable** (`src/core/hashtable.c`) — hash table with collision chaining; stores two encoding types: `VALUE_ENTRY_TYPE_INT` (1) and `VALUE_ENTRY_TYPE_RAW` (2). The `value_entry_t` struct uses an `encoding` field (4-bit bitfield)
+- **Linked List** (`src/core/list.c`) — doubly-linked list used for client connection management
+
+### Key State Structures
+
+- `server_t` in `src/server.h` — central server state: database store (`db_t *` containing `hashtable_t *store`), expire_cursor (active expiration scan position), client list, counters, config
+- `client_t` in `src/client.h` — per-client state: 64KB read/write buffers, socket info, connection mode
+
+### Networking
+
+`src/networking/networking.c` handles socket setup, frame-based protocol processing, and supports both TCP and Unix domain sockets. Frame format is a custom binary protocol.
+
+### Configuration
+
+`server.conf` and `client.conf` are parsed by `src/config.c`. Config keys are simple `key value` pairs, one per line.
+
+## Code Style
+
+LLVM-based style with 4-space indentation, Linux brace style, no tabs. Formatting config is in `.clang-format`.
+
+## Testing
+
+Tests use C `assert.h` directly (no external test framework). Test sources are in `tests/`. Tests are registered with CMake's `ctest` in `CMakeLists.txt`.
+
+## Three Executables
+
+The project builds three separate executables from different `main()` entry points:
+- `fkvs-server` — `src/server.c`
+- `fkvs-cli` — `src/fkvs-cli.c` (uses linenoise for line editing)
+- `fkvs-benchmark` — `src/fkvs-benchmark.c` (multi-threaded benchmark client)
+
+## Development Guides
+
+Detailed technical guides for implementing features, fixing bugs, and understanding the architecture are in `docs/guides/`:
+
+- `docs/guides/adding-commands.md` — Step-by-step: 8 files to touch when adding a new command
+- `docs/guides/wire-protocol.md` — Binary frame format, command layouts, response types
+- `docs/guides/memory-management.md` — Pointer ownership: get_value() copies vs find_entry() direct access
+- `docs/guides/testing-guide.md` — Test patterns, CMake registration, now_monotonic_ms() in tests
+- `docs/guides/data-structures.md` — Hashtable, linked list, server_t (db_t contains hashtable_t *store)
+- `docs/guides/event-dispatchers-deep.md` — kqueue, epoll, io_uring internals and guards
+- `docs/guides/debugging-checklist.md` — Systematic checklist for code review and bug analysis
+- `docs/guides/build-and-config.md` — CMake variants, source file placement, server/client config

CMakeLists.txt

@@ -11,7 +11,7 @@ set(THREADS_PREFER_PTHREAD_FLAG ON)
 find_package(Threads REQUIRED)
 
 if(APPLE)
-  add_executable(fkvs-server src/memory.c src/counter.c src/client.c src/core/list.c src/config.c src/networking/networking.c src/server.c src/core/hashtable.c src/commands/common/command_registry.c src/commands/server/server_command_handlers.c src/io/event_dispatcher_kqueue.c src/ttl.c)
+  add_executable(fkvs-server src/memory.c src/counter.c src/client.c src/core/list.c src/config.c src/networking/networking.c src/server.c src/core/hashtable.c src/commands/common/command_registry.c src/commands/server/server_command_handlers.c src/io/event_dispatcher_kqueue.c src/ttl.c src/persistence/persistence.c)
   target_compile_definitions(fkvs-server PRIVATE SERVER)
 
   add_executable(fkvs-benchmark src/string_utils.c src/fkvs-benchmark.c src/client.c src/core/list.c src/networking/networking.c src/commands/common/command_parser.c src/commands/client/client_command_handlers.c)
@@ -20,11 +20,11 @@ elseif(LINUX)
   find_library(LIBURING liburing)
 
   if(LIBURING)
-    add_executable(fkvs-server src/memory.c src/counter.c src/client.c src/core/list.c src/config.c src/networking/networking.c src/server.c src/core/hashtable.c src/commands/common/command_registry.c src/commands/server/server_command_handlers.c src/io/event_dispatcher_io_uring.c src/ttl.c)
+    add_executable(fkvs-server src/memory.c src/counter.c src/client.c src/core/list.c src/config.c src/networking/networking.c src/server.c src/core/hashtable.c src/commands/common/command_registry.c src/commands/server/server_command_handlers.c src/io/event_dispatcher_io_uring.c src/ttl.c src/persistence/persistence.c)
     target_link_libraries(fkvs-server PRIVATE LIBURING)
     target_compile_definitions(fkvs-server PRIVATE SERVER IO_URING_ENABLED)
   else()
-    add_executable(fkvs-server src/memory.c src/counter.c src/client.c src/core/list.c src/config.c src/networking/networking.c src/server.c src/core/hashtable.c src/commands/common/command_registry.c src/commands/server/server_command_handlers.c src/io/event_dispatcher_epoll.c src/ttl.c)
+    add_executable(fkvs-server src/memory.c src/counter.c src/client.c src/core/list.c src/config.c src/networking/networking.c src/server.c src/core/hashtable.c src/commands/common/command_registry.c src/commands/server/server_command_handlers.c src/io/event_dispatcher_epoll.c src/ttl.c src/persistence/persistence.c)
     target_compile_definitions(fkvs-server PRIVATE SERVER)
   endif()
 
@@ -37,15 +37,19 @@ add_executable(test_counter tests/test_counter.c src/counter.c)
 add_executable(test_string_utils tests/test_string_utils.c src/string_utils.c)
 add_executable(test_integration tests/test_integration.c src/core/hashtable.c src/commands/common/command_registry.c src/commands/common/command_parser.c src/commands/server/server_command_handlers.c src/counter.c src/ttl.c)
 target_compile_definitions(test_integration PRIVATE SERVER)
+add_executable(test_persistence tests/test_persistence.c src/core/hashtable.c src/persistence/persistence.c src/ttl.c src/counter.c)
+target_compile_definitions(test_persistence PRIVATE SERVER)
 target_link_libraries(test_counter)
 target_link_libraries(test_string_utils)
 target_link_libraries(test_integration)
+target_link_libraries(test_persistence)
 
 # Enable testing
 enable_testing()
 add_test(NAME CounterTest COMMAND test_counter)
 add_test(NAME StringUtilsTest COMMAND test_string_utils)
 add_test(NAME IntegrationTest COMMAND test_integration)
+add_test(NAME PersistenceTest COMMAND test_persistence)
 add_executable(fkvs-cli src/string_utils.c src/fkvs-cli.c src/config.c src/commands/common/command_parser.c src/commands/client/client_command_handlers.c)
   target_link_libraries(fkvs-benchmark PRIVATE Threads::Threads)
   target_link_libraries(fkvs-cli PUBLIC linenoise)

server.conf

@@ -8,5 +8,9 @@ show-logo true
 # can be processed at one time during an iteration of the event loop
 event-loop-max-events 100000
 # unixsocket /tmp/fkvs/fkvs.sock
+# save-interval <seconds> <min-changes>
+# Default: save-interval 300 1 (every 5 min if at least 1 change)
+# Set to: save-interval 0 0 to disable persistence
+# snapshot-path fkvs-dump.rdb
 # Enable io_uring for pro-reactive I/O handling on Linux
 use-io-uring false

src/config.c

@@ -21,6 +21,12 @@ server_t load_server_config(const char *path)
     server.num_clients = 0;
     server.uds_socket_path = NULL;
     server.socket_domain = TCP_IP;
+    server.save_interval = 300;
+    server.save_changes_threshold = 1;
+    server.save_tick_count = 0;
+    server.dirty = 0;
+    server.shutdown_requested = 0;
+    server.snapshot_path = "fkvs-dump.rdb";
     if (path) {
         server.config_file_path = path;
     } else {
@@ -93,6 +99,18 @@ server_t load_server_config(const char *path)
             }
         }
 
+        if (strcmp(key, "save-interval") == 0) {
+            server.save_interval = atoi(value);
+            char changes[512];
+            if (sscanf(line, "%*s %*s %s", changes) == 1) {
+                server.save_changes_threshold = atoi(changes);
+            }
+        }
+
+        if (strcmp(key, "snapshot-path") == 0) {
+            server.snapshot_path = strdup(value);
+        }
+
         if (strcmp(key, "use-io-uring") == 0) {
             if (strcmp(value, "true") == 0) {
                 server.use_io_uring = true;

src/fkvs_time.h

@@ -0,0 +1,14 @@
+#ifndef FKVS_TIME_H
+#define FKVS_TIME_H
+
+#include <stdint.h>
+#include <time.h>
+
+static inline int64_t fkvs_now_ms(void)
+{
+    struct timespec ts;
+    clock_gettime(CLOCK_REALTIME, &ts);
+    return (int64_t)ts.tv_sec * 1000 + ts.tv_nsec / 1000000;
+}
+
+#endif // FKVS_TIME_H

src/io/event_dispatcher_epoll.c

@@ -4,6 +4,7 @@
 #include "../networking/modes.h"
 #include "../networking/networking.h"
 #include "../server.h"
+#include "../persistence/persistence.h"
 #include "../ttl.h"
 #include "../utils.h"
 #include "event_dispatcher.h"
@@ -98,6 +99,9 @@ int run_event_loop()
             break;
         }
 
+        if (server.shutdown_requested)
+            shutdown_server();
+
         for (int i = 0; i < n; i++) {
             const uint32_t evt = events[i].events;
 
@@ -107,6 +111,15 @@ int run_event_loop()
                 read(tfd, &expirations, sizeof(expirations));
                 expire_sweep(server.database->store,
                              server.database->expires, 20);
+                if (server.save_interval > 0 &&
+                    server.dirty >= (uint64_t)server.save_changes_threshold) {
+                    server.save_tick_count++;
+                    if (server.save_tick_count >= server.save_interval * 10) {
+                        save_snapshot(server.database, server.snapshot_path);
+                        server.dirty = 0;
+                        server.save_tick_count = 0;
+                    }
+                }
                 continue;
             }
 

src/io/event_dispatcher_io_uring.c

@@ -4,6 +4,7 @@
 #include "../networking/modes.h"
 #include "../networking/networking.h"
 #include "../server.h"
+#include "../persistence/persistence.h"
 #include "../ttl.h"
 #include "../utils.h"
 #include "event_dispatcher.h"
@@ -80,13 +81,27 @@ int run_event_loop()
             break;
         }
 
+        if (server.shutdown_requested)
+            shutdown_server();
+
         client_t *c = io_uring_cqe_get_data(cqe);
 
         // Handle timer expiration for active sweep
         if (c == &timer_sentinel) {
             if (cqe->res > 0) {
                 expire_sweep(server.database->store,
                              server.database->expires, 20);
+                if (server.save_interval > 0 &&
+                    server.dirty >=
+                        (uint64_t)server.save_changes_threshold) {
+                    server.save_tick_count++;
+                    if (server.save_tick_count >=
+                        server.save_interval * 10) {
+                        save_snapshot(server.database, server.snapshot_path);
+                        server.dirty = 0;
+                        server.save_tick_count = 0;
+                    }
+                }
             }
             // Re-arm the timer read
             struct io_uring_sqe *sqe = io_uring_get_sqe(&ring);

src/io/event_dispatcher_kqueue.c

@@ -4,6 +4,7 @@
 #include "../core/list.h"
 #include "../networking/networking.h"
 #include "../server.h"
+#include "../persistence/persistence.h"
 #include "../ttl.h"
 #include "../utils.h"
 #include "event_dispatcher.h"
@@ -88,6 +89,9 @@ int run_event_loop()
             break;
         }
 
+        if (server.shutdown_requested)
+            shutdown_server();
+
         // We have new events
         for (int i = 0; i < n; i++) {
             const int ident_fd = (int)evs[i].ident;
@@ -96,6 +100,15 @@ int run_event_loop()
             if (evs[i].filter == EVFILT_TIMER) {
                 expire_sweep(server.database->store, server.database->expires,
                              20);
+                if (server.save_interval > 0 &&
+                    server.dirty >= (uint64_t)server.save_changes_threshold) {
+                    server.save_tick_count++;
+                    if (server.save_tick_count >= server.save_interval * 10) {
+                        save_snapshot(server.database, server.snapshot_path);
+                        server.dirty = 0;
+                        server.save_tick_count = 0;
+                    }
+                }
                 continue;
             }
 

src/networking/networking.c

@@ -15,6 +15,7 @@
 
 #ifdef SERVER
 
+#include "../commands/common/command_defs.h"
 #include "../commands/common/command_registry.h"
 #include "../counter.h"
 #include <errno.h>
@@ -171,9 +172,17 @@ void try_process_frames(client_t *c)
         }
 
         // Dispatch exactly one frame.
+        uint8_t cmd_id = c->buffer[2];
         dispatch_command(c, c->buffer, frame_len);
         increment_command_count(&server.metrics);
 
+        if (cmd_id == CMD_SET || cmd_id == CMD_DEL ||
+            cmd_id == CMD_INCR || cmd_id == CMD_DECR ||
+            cmd_id == CMD_INCR_BY || cmd_id == CMD_DECR_BY ||
+            cmd_id == CMD_EXPIRE || cmd_id == CMD_PERSIST) {
+            server.dirty++;
+        }
+
         // Shift any remaining bytes (back-to-back frames).
         size_t remain = c->buf_used - frame_len;
         if (remain)

src/persistence/crc32.h

@@ -0,0 +1,63 @@
+#ifndef CRC32_H
+#define CRC32_H
+
+#include <stddef.h>
+#include <stdint.h>
+
+static const uint32_t crc32_table[256] = {
+    0x00000000, 0x77073096, 0xEE0E612C, 0x990951BA, 0x076DC419, 0x706AF48F,
+    0xE963A535, 0x9E6495A3, 0x0EDB8832, 0x79DCB8A4, 0xE0D5E91E, 0x97D2D988,
+    0x09B64C2B, 0x7EB17CBD, 0xE7B82D07, 0x90BF1D91, 0x1DB71064, 0x6AB020F2,
+    0xF3B97148, 0x84BE41DE, 0x1ADAD47D, 0x6DDDE4EB, 0xF4D4B551, 0x83D385C7,
+    0x136C9856, 0x646BA8C0, 0xFD62F97A, 0x8A65C9EC, 0x14015C4F, 0x63066CD9,
+    0xFA0F3D63, 0x8D080DF5, 0x3B6E20C8, 0x4C69105E, 0xD56041E4, 0xA2677172,
+    0x3C03E4D1, 0x4B04D447, 0xD20D85FD, 0xA50AB56B, 0x35B5A8FA, 0x42B2986C,
+    0xDBBBC9D6, 0xACBCF940, 0x32D86CE3, 0x45DF5C75, 0xDCD60DCF, 0xABD13D59,
+    0x26D930AC, 0x51DE003A, 0xC8D75180, 0xBFD06116, 0x21B4F4B5, 0x56B3C423,
+    0xCFBA9599, 0xB8BDA50F, 0x2802B89E, 0x5F058808, 0xC60CD9B2, 0xB10BE924,
+    0x2F6F7C87, 0x58684C11, 0xC1611DAB, 0xB6662D3D, 0x76DC4190, 0x01DB7106,
+    0x98D220BC, 0xEFD5102A, 0x71B18589, 0x06B6B51F, 0x9FBFE4A5, 0xE8B8D433,
+    0x7807C9A2, 0x0F00F934, 0x9609A88E, 0xE10E9818, 0x7F6A0DBB, 0x086D3D2D,
+    0x91646C97, 0xE6635C01, 0x6B6B51F4, 0x1C6C6162, 0x856530D8, 0xF262004E,
+    0x6C0695ED, 0x1B01A57B, 0x8208F4C1, 0xF50FC457, 0x65B0D9C6, 0x12B7E950,
+    0x8BBEB8EA, 0xFCB9887C, 0x62DD1DDF, 0x15DA2D49, 0x8CD37CF3, 0xFBD44C65,
+    0x4DB26158, 0x3AB551CE, 0xA3BC0074, 0xD4BB30E2, 0x4ADFA541, 0x3DD895D7,
+    0xA4D1C46D, 0xD3D6F4FB, 0x4369E96A, 0x346ED9FC, 0xAD678846, 0xDA60B8D0,
+    0x44042D73, 0x33031DE5, 0xAA0A4C5F, 0xDD0D7CC9, 0x5005713C, 0x270241AA,
+    0xBE0B1010, 0xC90C2086, 0x5768B525, 0x206F85B3, 0xB966D409, 0xCE61E49F,
+    0x5EDEF90E, 0x29D9C998, 0xB0D09822, 0xC7D7A8B4, 0x59B33D17, 0x2EB40D81,
+    0xB7BD5C3B, 0xC0BA6CAD, 0xEDB88320, 0x9ABFB3B6, 0x03B6E20C, 0x74B1D29A,
+    0xEAD54739, 0x9DD277AF, 0x04DB2615, 0x73DC1683, 0xE3630B12, 0x94643B84,
+    0x0D6D6A3E, 0x7A6A5AA8, 0xE40ECF0B, 0x9309FF9D, 0x0A00AE27, 0x7D079EB1,
+    0xF00F9344, 0x8708A3D2, 0x1E01F268, 0x6906C2FE, 0xF762575D, 0x806567CB,
+    0x196C3671, 0x6E6B06E7, 0xFED41B76, 0x89D32BE0, 0x10DA7A5A, 0x67DD4ACC,
+    0xF9B9DF6F, 0x8EBEEFF9, 0x17B7BE43, 0x60B08ED5, 0xD6D6A3E8, 0xA1D1937E,
+    0x38D8C2C4, 0x4FDFF252, 0xD1BB67F1, 0xA6BC5767, 0x3FB506DD, 0x48B2364B,
+    0xD80D2BDA, 0xAF0A1B4C, 0x36034AF6, 0x41047A60, 0xDF60EFC3, 0xA867DF55,
+    0x316E8EEF, 0x4669BE79, 0xCB61B38C, 0xBC66831A, 0x256FD2A0, 0x5268E236,
+    0xCC0C7795, 0xBB0B4703, 0x220216B9, 0x5505262F, 0xC5BA3BBE, 0xB2BD0B28,
+    0x2BB45A92, 0x5CB36A04, 0xC2D7FFA7, 0xB5D0CF31, 0x2CD99E8B, 0x5BDEAE1D,
+    0x9B64C2B0, 0xEC63F226, 0x756AA39C, 0x026D930A, 0x9C0906A9, 0xEB0E363F,
+    0x72076785, 0x05005713, 0x95BF4A82, 0xE2B87A14, 0x7BB12BAE, 0x0CB61B38,
+    0x92D28E9B, 0xE5D5BE0D, 0x7CDCEFB7, 0x0BDBDF21, 0x86D3D2D4, 0xF1D4E242,
+    0x68DDB3F8, 0x1FDA836E, 0x81BE16CD, 0xF6B9265B, 0x6FB077E1, 0x18B74777,
+    0x88085AE6, 0xFF0F6A70, 0x66063BCA, 0x11010B5C, 0x8F659EFF, 0xF862AE69,
+    0x616BFFD3, 0x166CCF45, 0xA00AE278, 0xD70DD2EE, 0x4E048354, 0x3903B3C2,
+    0xA7672661, 0xD06016F7, 0x4969474D, 0x3E6E77DB, 0xAED16A4A, 0xD9D65ADC,
+    0x40DF0B66, 0x37D83BF0, 0xA9BCAE53, 0xDEBB9EC5, 0x47B2CF7F, 0x30B5FFE9,
+    0xBDBDF21C, 0xCABAC28A, 0x53B39330, 0x24B4A3A6, 0xBAD03605, 0xCDD70693,
+    0x54DE5729, 0x23D967BF, 0xB3667A2E, 0xC4614AB8, 0x5D681B02, 0x2A6F2B94,
+    0xB40BBE37, 0xC30C8EA1, 0x5A05DF1B, 0x2D02EF8D,
+};
+
+static inline uint32_t crc32_update(uint32_t crc, const void *buf, size_t len)
+{
+    const uint8_t *p = (const uint8_t *)buf;
+    crc = ~crc;
+    for (size_t i = 0; i < len; i++) {
+        crc = crc32_table[(crc ^ p[i]) & 0xFF] ^ (crc >> 8);
+    }
+    return ~crc;
+}
+
+#endif // CRC32_H