diff --git a/04/src/PersistenceManager.java b/04/src/PersistenceManager.java
index 6164cd1..a7a0e0f 100644
--- a/04/src/PersistenceManager.java
+++ b/04/src/PersistenceManager.java
@@ -1,63 +1,147 @@
 import org.jetbrains.annotations.Contract;
 
+import java.util.*;
+
 /**
  * Persistence manager using the Singleton pattern.
  */
-public class PersistenceManager
-{
-    private final static PersistenceManager instance;
+public class PersistenceManager {
+    private final static PersistenceManager               instance;
+    private              Hashtable<Integer, String>       _pageBuffer;
+    private              Hashtable<Integer, Set<Integer>> _transactionPageBuffer;
+    private              Hashtable<Integer, Boolean>      _transactions;
+    private              Hashtable<Integer, Integer>      _pageLSN;
+    private              int                              _nextTransactionNumber;
+    private              int                              _nextLogSequenceNumber;
     
     static {
         try {
             instance = new PersistenceManager();
-        }
-        catch (Throwable e) {
+        } catch (Throwable e) {
             throw new RuntimeException(e.getMessage());
         }
     }
     
     /**
      * Returns an instance of the persistence manager.
+     *
      * @return instance
      */
     @Contract(pure = true)
-    public static PersistenceManager getInstance()
-    {
+    public static PersistenceManager getInstance() {
         return instance;
     }
     
     /**
      * private constructor
      */
-    private PersistenceManager()
-    {}
+    private PersistenceManager() {
+        _pageBuffer = new Hashtable<>();
+        _transactionPageBuffer = new Hashtable<>();
+        _transactions = new Hashtable<>();
+        _pageLSN = new Hashtable<>();
+        _nextTransactionNumber = 1;
+        _nextLogSequenceNumber = 1;
+    }
     
     /**
      * Begins the transaction and returns the transaction ID.
+     *
      * @return transaction ID
      */
-    public int beginTransaction()
-    {
-        return 0;
+    public int beginTransaction() {
+        _transactions.put(_nextTransactionNumber, false);
+        // return the next transaction number and increase it by one afterwards
+        return _nextTransactionNumber++;
     }
     
     /**
      * Commits the transaction specified by given ID.
-     * @param taid transaction ID
+     *
+     * @param taid
+     *         transaction ID
      */
-    public void commit(int taid)
-    {
-    
+    public void commit(int taid) {
+        if (!_transactions.containsKey(taid)) {
+            throw new IllegalArgumentException("No transaction with given ID exists.");
+        }
+        // only perform commit actions if transaction isn't commited yet
+        if (!_transactions.get(taid)) {
+            _transactions.replace(taid, true);
+        }
     }
     
     /**
      * Writes given data into given page in given transaction.
-     * @param taid transaction ID
-     * @param pageid page ID
-     * @param data data
+     *
+     * @param taid
+     *         transaction ID
+     * @param pageid
+     *         page ID
+     * @param data
+     *         data
      */
-    public void write(int taid, int pageid, String data)
-    {
+    public void write(int taid, int pageid, String data) {
+        if (!_transactions.containsKey(taid)) {
+            throw new IllegalArgumentException("No transaction with given ID exists.");
+        }
+        if (_transactions.get(taid)) {
+            throw new IllegalArgumentException("Write operations cannot be performed for committed transactions.");
+        }
+        Set<Integer> pageIDs = _transactionPageBuffer.getOrDefault(taid, new HashSet<>());
+        pageIDs.add(pageid);
+        _transactionPageBuffer.put(taid, pageIDs);
+        _pageBuffer.put(pageid, data);
+        // log changes
+        int lsn = log(taid, pageid, data);
+        _pageLSN.put(pageid, lsn);
+        // call persist
+        persist();
+    }
     
+    /**
+     * Logs the necessary information to the log file.
+     *
+     * @param taid
+     *         affected transaction
+     * @param pageid
+     *         modified page
+     * @param data
+     *         written data
+     * @return log sequence number
+     */
+    private int log(int taid, int pageid, String data) {
+        return _nextLogSequenceNumber++;
+    }
+    
+    /**
+     * Checks for full buffer and persists data of committed transactions to storage if buffer is full.
+     */
+    private void persist() {
+        if (_pageBuffer.size() <= 5) {
+            return;
+        }
+        
+        // TODO persist the data of committed transactions
+        Enumeration<Integer> transactionIDs       = _transactions.keys();
+        List<Integer>        commitedTransactions = new ArrayList<>();
+        while (transactionIDs.hasMoreElements()) {
+            Integer taid      = transactionIDs.nextElement();
+            boolean committed = _transactions.get(taid);
+            if (committed) {
+                commitedTransactions.add(taid);
+            }
+        }
+        
+        for (int taid : commitedTransactions) {
+            // TODO persist data of committed transaction
+            Set<Integer> pageIDs = _transactionPageBuffer.get(taid);
+            for (int pageid : pageIDs) {
+                // TODO persist data of pageID
+                _pageBuffer.remove(pageid);
+            }
+            _transactionPageBuffer.remove(taid);
+            _transactions.remove(taid);
+        }
     }
 }