diff --git a/packages/api/src/utility/JsonLinesDatastore.js b/packages/api/src/utility/JsonLinesDatastore.js
index eed749143..7ed08076b 100644
--- a/packages/api/src/utility/JsonLinesDatastore.js
+++ b/packages/api/src/utility/JsonLinesDatastore.js
@@ -3,38 +3,62 @@ const lineReader = require('line-reader');
 class JsonLinesDatastore {
   constructor(file) {
     this.file = file;
-    this.readerInfo = {
-      reader: null,
-      readedDataRowCount: 0,
-      readedSchemaRow: false,
-      isReading: false,
-      closeAfterRead: null,
-      closeAfterReadPromise: null,
-    };
+    this.reader = null;
+    this.readedDataRowCount = 0;
+
+    this.readedSchemaRow = false;
+    this.isReading = false;
+    this.closeAfterRead = null;
+    this.closeAfterReadCallback = null;
+    this.closeAfterReadPromise = null;
+
+    this.waitForReadyPromise = null;
+    this.waitForReadyResolve = null;
+
+    // this.readerInfo = {
+    //   reader: null,
+    //   readedDataRowCount: 0,
+    //   readedSchemaRow: false,
+    //   isReading: false,
+    //   closeAfterRead: null,
+    //   closeAfterReadPromise: null,
+    // };
   }
 
-  async closeReader() {
-    if (!this.readerInfo) return Promise.resolve();
-    return new Promise((resolve, reject) => {
-      this.readerInfo.reader.close((err) => {
-        if (err) reject(err);
-        resolve();
-      });
-    });
+  closeReader() {
+    if (!this.reader) return;
+    const reader = this.reader;
+    this.reader = null;
+    reader.close();
   }
 
-  async notifyChanged() {
-    if (this.readerInfo && this.readerInfo.isReading) {
-      if (this.readerInfo.closeAfterRead) {
-        return this.readerInfo.closeAfterReadPromise;
+  waitForReady() {
+    if (this.isReading) {
+      if (this.waitForReadyResolve) {
+        return this.waitForReadyPromise;
       }
       const promise = new Promise((resolve, reject) => {
-        this.readerInfo.closeAfterRead = resolve;
+        this.waitForReadyResolve = resolve;
       });
-      this.readerInfo.closeAfterReadPromise = promise;
+      this.waitForReadyPromise = promise;
+      return promise;
+    }
+    return Promise.resolve();
+  }
+
+  async notifyChanged(callback) {
+    if (this.isReading) {
+      this.closeAfterReadCallback = callback;
+      if (this.closeAfterRead) {
+        return this.closeAfterReadPromise;
+      }
+      const promise = new Promise((resolve, reject) => {
+        this.closeAfterRead = resolve;
+      });
+      this.closeAfterReadPromise = promise;
       return promise;
     } else {
-      await this.closeReader();
+      this.closeReader();
     }
   }
 
@@ -43,30 +67,21 @@ class JsonLinesDatastore {
       lineReader.open(this.file, (err, reader) => {
         if (err) reject(err);
 
-        const readerInfo = {
-          reader,
-          readedDataRowCount: 0,
-          readedSchemaRow: false,
-          isReading: true,
-          closeAfterRead: null,
-          closeAfterReadPromise: null,
-        };
-        this.readerInfo = readerInfo;
-        resolve(readerInfo);
+        resolve(reader);
       })
     );
   }
 
-  readLine(readerInfo) {
+  readLine() {
     return new Promise((resolve, reject) => {
-      const { reader } = readerInfo;
+      const reader = this.reader;
       if (!reader.hasNextLine()) {
         resolve(null);
         return;
       }
       reader.nextLine((err, line) => {
-        if (readerInfo.readedSchemaRow) readerInfo.readedDataRowCount += 1;
-        else readerInfo.readedSchemaRow = true;
+        if (this.readedSchemaRow) this.readedDataRowCount += 1;
+        else this.readedSchemaRow = true;
         if (err) reject(err);
         resolve(line);
       });
@@ -74,40 +89,58 @@ class JsonLinesDatastore {
   }
 
   async ensureReader(offset) {
-    if (this.readerInfo && this.readerInfo.readedDataRowCount > offset) {
-      await this.closeReader();
+    console.log('ENSURE', offset);
+    for (;;) {
+      await this.waitForReady();
+      if (this.readedDataRowCount > offset) {
+        this.closeReader();
+      }
+      if (!this.reader) {
+        const reader = await this.openReader();
+        if (this.isReading) {
+          reader.close(); // throw away this reader
+          continue; // reader is already used by other getRows, wait for free reader
+        }
+        this.reader = reader;
+        this.isReading = true;
+        break;
+      } else {
+        break;
+      }
     }
-    let readerInfo = this.readerInfo;
-    if (!readerInfo || !readerInfo.reader) {
-      readerInfo = await this.openReader();
+    if (!this.readedSchemaRow) {
+      await this.readLine(); // skip structure
     }
-    readerInfo.isReading = true;
-    if (!readerInfo.readedSchemaRow) {
-      await this.readLine(readerInfo); // skip structure
+    while (this.readedDataRowCount < offset) {
+      await this.readLine();
     }
-    while (readerInfo.readedDataRowCount < offset) {
-      await this.readLine(readerInfo);
-    }
-    return readerInfo;
   }
 
   async getRows(offset, limit) {
-    const readerInfo = await this.ensureReader(offset);
+    await this.ensureReader(offset);
     const res = [];
     for (let i = 0; i < limit; i += 1) {
-      const line = await this.readLine(readerInfo);
+      const line = await this.readLine();
       if (line == null) break;
       res.push(JSON.parse(line));
     }
-    readerInfo.isReading = false;
-    if (readerInfo.closeAfterRead) {
-      await this.closeReader();
-      // socket.emit(`jsldata-stats-${jslid}`, readerInfo.closeAfterReadAndSendStats);
-      const resolve = readerInfo.closeAfterRead;
-      readerInfo.closeAfterRead = null;
-      readerInfo.closeAfterReadPromise = null;
+    this.isReading = false;
+    if (this.closeAfterRead) {
+      if (this.closeAfterReadCallback) this.closeAfterReadCallback();
+      this.closeReader();
+      const resolve = this.closeAfterRead;
+      this.closeAfterRead = null;
+      this.closeAfterReadPromise = null;
+      this.closeAfterReadCallback = null;
       resolve();
     }
+    if (this.waitForReadyResolve) {
+      const resolve = this.waitForReadyResolve;
+      this.waitForReadyResolve = null;
+      this.waitForReadyPromise = null;
+      resolve();
+    }
+    console.log('RETURN', res.length);
     return res;
   }
 }