diff --git a/src/main/java/collector/H2/H2Client.java b/src/main/java/collector/H2/H2Client.java
index f3e4827..55e3a44 100644
--- a/src/main/java/collector/H2/H2Client.java
+++ b/src/main/java/collector/H2/H2Client.java
@@ -44,6 +44,7 @@ public void execSession(Session<Long, Long> session, IsolationLevel isolationLev
                         }
                     }
                     transaction.setSuccess(true);
+                    break;
                 } catch (SQLException e) {
                     connection.rollback();
                 }
diff --git a/src/main/java/collector/mysql/MySQLClient.java b/src/main/java/collector/mysql/MySQLClient.java
index 7691221..8531970 100644
--- a/src/main/java/collector/mysql/MySQLClient.java
+++ b/src/main/java/collector/mysql/MySQLClient.java
@@ -44,6 +44,7 @@ public void execSession(Session<Long, Long> session, IsolationLevel isolationLev
                         }
                     }
                     transaction.setSuccess(true);
+                    break;
                 } catch (SQLException e) {
                     connection.rollback();
                 }
diff --git a/src/main/java/collector/postgresql/PostgreSQLClient.java b/src/main/java/collector/postgresql/PostgreSQLClient.java
index aa53183..cff2b24 100644
--- a/src/main/java/collector/postgresql/PostgreSQLClient.java
+++ b/src/main/java/collector/postgresql/PostgreSQLClient.java
@@ -43,6 +43,7 @@ public void execSession(Session<Long, Long> session, IsolationLevel isolationLev
                         }
                     }
                     transaction.setSuccess(true);
+                    break;
                 } catch (SQLException e) {
                     connection.rollback();
                 }
diff --git a/src/main/java/generator/general/GeneralGenerator.java b/src/main/java/generator/general/GeneralGenerator.java
index 7f6035b..55543f2 100644
--- a/src/main/java/generator/general/GeneralGenerator.java
+++ b/src/main/java/generator/general/GeneralGenerator.java
@@ -10,9 +10,9 @@
 import org.apache.commons.math3.distribution.ZipfDistribution;
 
 import java.util.ArrayList;
+import java.util.HashMap;
 import java.util.Properties;
 import java.util.concurrent.Callable;
-import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
 
@@ -49,7 +49,7 @@ public GeneralGenerator(Properties config) {
     @Override
     public History<Long, Long> generate() {
         var history = new History<Long, Long>();
-        var counts = new ConcurrentHashMap<Long, Long>();
+        var counts = new HashMap<Long, Long>();
         ExecutorService executor = Executors.newFixedThreadPool((int) this.session);
         var todo = new ArrayList<Callable<Void>>();
         for (var iSession = 0; iSession < this.session; iSession++) {
@@ -71,8 +71,10 @@ public History<Long, Long> generate() {
                         long var = keyDistribution.sample();
                         long val = 0L;
                         if (type == Operation.Type.WRITE) {
-                            val = counts.getOrDefault(var, 1L);
-                            counts.put(var, val + 1);
+                            synchronized (counts) {
+                                val = counts.getOrDefault(var, 1L);
+                                counts.put(var, val + 1);
+                            }
                         }
                         synchronized (history) {
                             history.addOperation(history.getTransaction(txnId), type, var, val);