diff --git a/lib/dat/reader.ts b/lib/dat/reader.ts
index 9440bd2..f91e889 100644
--- a/lib/dat/reader.ts
+++ b/lib/dat/reader.ts
@@ -3,7 +3,7 @@ import * as fs from 'node:fs';
 import { DATA_HEADER_SIZE } from './constants.js';
 import { DataProtocol, DataHeader } from './protocol.js';
 import { IndexReader } from '../idx/index.js';
-import type { Serializer, DataEntry } from './types.js';
+import type { Serializer, DataEntry, DataFileReaderOptions } from './types.js';
 
 export class DataReader<T> {
     private fd: number | null = null;
@@ -12,17 +12,18 @@ export class DataReader<T> {
     private indexReader: IndexReader;
     private serializer: Serializer<T>;
 
-    readonly dataPath: string;
-    readonly indexPath: string;
+    private dataPath: string | null = null;
+    private indexPath: string | null = null;
 
-    constructor(basePath: string, serializer: Serializer<T>) {
-        this.dataPath = `${basePath}.dat`;
-        this.indexPath = `${basePath}.idx`;
-        this.serializer = serializer;
-        this.indexReader = new IndexReader(this.indexPath);
+    constructor(options: DataFileReaderOptions<T>) {
+        this.serializer = options.serializer;
+        this.indexReader = new IndexReader();
     }
 
-    open(): void {
+    open(basePath: string): void {
+        this.dataPath = `${basePath}.dat`;
+        this.indexPath = `${basePath}.idx`;
+
         this.fd = fs.openSync(this.dataPath, 'r');
 
         // Read header only
@@ -30,7 +31,7 @@ export class DataReader<T> {
         fs.readSync(this.fd, headerBuf, 0, DATA_HEADER_SIZE, 0);
         this.header = DataProtocol.readHeader(headerBuf);
 
-        this.indexReader.open();
+        this.indexReader.open(this.indexPath);
     }
 
     private readRecord(offset: bigint, length: number): Buffer {
@@ -204,6 +205,10 @@ export class DataReader<T> {
         return this.indexReader.getHeader().latestSequence;
     }
 
+    getFlags(): number {
+        return this.indexReader.getFlags();
+    }
+
     close(): void {
         if (this.fd !== null) {
             fs.closeSync(this.fd);
diff --git a/lib/dat/types.ts b/lib/dat/types.ts
index acc00d8..7041b99 100644
--- a/lib/dat/types.ts
+++ b/lib/dat/types.ts
@@ -17,3 +17,7 @@ export interface DataFileOptions<T> {
     forceTruncate?: boolean;
     indexFileOpt: IndexFileOptions;
 }
+
+export interface DataFileReaderOptions<T> {
+    serializer: Serializer<T>;
+}
diff --git a/lib/dat/writer.ts b/lib/dat/writer.ts
index 9dbe1fd..4062e14 100644
--- a/lib/dat/writer.ts
+++ b/lib/dat/writer.ts
@@ -117,6 +117,7 @@ export class DataWriter<T> {
         this.currentOffset += BigInt(buf.length);
         ++this.recordCount;
 
+        this.writeHeader();
         return this.latestSequence;
     }
 
@@ -149,8 +150,11 @@ export class DataWriter<T> {
         return this.latestSequence;
     }
 
-    getNextSequence(): number {
-        return this.latestSequence + 1;
+    writeHeader(): void {
+        if (this.fd === null || !this.headerBuf) return;
+
+        DataProtocol.updateHeader(this.headerBuf, this.currentOffset, this.recordCount);
+        fs.writeSync(this.fd, this.headerBuf, 0, DATA_HEADER_SIZE, 0);
     }
 
     sync(): void {
@@ -159,8 +163,6 @@ export class DataWriter<T> {
         DataProtocol.updateHeader(this.headerBuf, this.currentOffset, this.recordCount);
         fs.writeSync(this.fd, this.headerBuf, 0, DATA_HEADER_SIZE, 0);
         fs.fsyncSync(this.fd);
-
-        this.indexWriter.syncAll();
     }
 
     close(): void {
@@ -175,6 +177,10 @@ export class DataWriter<T> {
         this.headerBuf = null;
     }
 
+    writeFlags(flags: number): void {
+        this.indexWriter.writeFlags(flags);
+    }
+
     getStats() {
         return {
             dataPath: this.dataPath,
diff --git a/lib/idx/protocol.ts b/lib/idx/protocol.ts
index 33d36ad..4031c5d 100644
--- a/lib/idx/protocol.ts
+++ b/lib/idx/protocol.ts
@@ -38,6 +38,7 @@ export class IndexProtocol {
         buf.writeBigUInt64LE(0n, 28);  // dataFileSize
         buf.writeUInt32LE(0, 36);  // latestSequence
         buf.writeUInt8(autoIncrementSequence ? 1 : 0, 40);  // autoIncrementSequence
+        buf.writeUInt32LE(0, 41);  // flags
         return buf;
     }
 
@@ -52,10 +53,15 @@ export class IndexProtocol {
             dataFileSize: buf.readBigUInt64LE(28),
             latestSequence: buf.readUInt32LE(36),
             autoIncrementSequence: buf.readUInt8(40) === 1,
-            reserved: buf.subarray(41, 64),
+            flags: buf.readUInt32LE(41),
+            reserved: buf.subarray(45, 64),
         };
     }
 
+    static writeFlags(buf: Buffer, flags: number): void {
+        buf.writeUInt32LE(flags, 41);
+    }
+
     static updateHeaderCounts(
         buf: Buffer,
         writtenCnt: number,
diff --git a/lib/idx/reader.ts b/lib/idx/reader.ts
index 3498565..653348c 100644
--- a/lib/idx/reader.ts
+++ b/lib/idx/reader.ts
@@ -7,14 +7,14 @@ export class IndexReader {
     private buffer: Buffer | null = null;
     private header: IndexHeader | null = null;
 
-    readonly path: string;
+    private path: string | null = null;
 
-    constructor(path: string) {
-        this.path = path;
+    constructor() {
     }
 
-    open(): void {
+    open(idxFilePath: string): void {
         // Read entire file into buffer (simpler than mmap for read-only access)
+        this.path = idxFilePath;
         this.buffer = fs.readFileSync(this.path);
         this.header = IndexProtocol.readHeader(this.buffer);
     }
@@ -24,6 +24,11 @@ export class IndexReader {
         return this.header;
     }
 
+    getFlags(): number {
+        if (!this.header) throw new Error('Index file not opened');
+        return this.header.flags;
+    }
+
     getEntry(index: number): IndexEntry | null {
         if (!this.buffer || !this.header) throw new Error('Index file not opened');
         if (index < 0 || index >= this.header.entryCount) return null;
diff --git a/lib/idx/types.ts b/lib/idx/types.ts
index 19191fa..fd38dfc 100644
--- a/lib/idx/types.ts
+++ b/lib/idx/types.ts
@@ -9,6 +9,7 @@ export interface IndexHeader {
     dataFileSize: bigint;
     latestSequence: number;
     autoIncrementSequence: boolean;
+    flags: number;
     reserved: Buffer;
 }
 
diff --git a/lib/idx/writer.ts b/lib/idx/writer.ts
index e9ffb13..5fa3d73 100644
--- a/lib/idx/writer.ts
+++ b/lib/idx/writer.ts
@@ -151,6 +151,9 @@ export class IndexWriter {
             this.dataFileSize = newDataEnd;
         }
 
+        // Sync immediately after write
+        this.writeHeader();
+
         return true;
     }
 
@@ -158,7 +161,7 @@ export class IndexWriter {
         return this.latestSequence;
     }
 
-    syncHeader(): void {
+    writeHeader(): void {
         if (!this.headerBuf || this.fd === null) return;
 
         // Update header counts
@@ -177,7 +180,7 @@ export class IndexWriter {
         if (this.fd === null) return;
 
         // Sync header first
-        this.syncHeader();
+        this.writeHeader();
 
         // Sync all file changes to disk
         fs.fsyncSync(this.fd);
@@ -198,6 +201,16 @@ export class IndexWriter {
         this.entryBuf = null;
     }
 
+    writeFlags(flags: number): void {
+        if (!this.headerBuf || this.fd === null) {
+            throw new Error('Index file not opened');
+        }
+
+        IndexProtocol.writeFlags(this.headerBuf, flags);
+        fs.writeSync(this.fd, this.headerBuf, 41, 4, 41);
+        fs.fsyncSync(this.fd);
+    }
+
     getStats() {
         return {
             path: this.path,