diff --git a/data-prepper-api/src/main/java/org/opensearch/dataprepper/model/event/DefaultEventHandle.java b/data-prepper-api/src/main/java/org/opensearch/dataprepper/model/event/DefaultEventHandle.java
index cb7c12da84..743309bf75 100644
--- a/data-prepper-api/src/main/java/org/opensearch/dataprepper/model/event/DefaultEventHandle.java
+++ b/data-prepper-api/src/main/java/org/opensearch/dataprepper/model/event/DefaultEventHandle.java
@@ -56,8 +56,10 @@ public Instant getExternalOriginationTime() {
 
     @Override
     public void release(boolean result) {
-        for (final BiConsumer<EventHandle, Boolean> consumer: releaseConsumers) {
-            consumer.accept(this, result);
+        synchronized (releaseConsumers) {
+            for (final BiConsumer<EventHandle, Boolean> consumer: releaseConsumers) {
+                consumer.accept(this, result);
+            }
         }
         AcknowledgementSet acknowledgementSet = getAcknowledgementSet();
         if (acknowledgementSet != null) {
@@ -67,6 +69,8 @@ public void release(boolean result) {
 
     @Override
     public void onRelease(BiConsumer<EventHandle, Boolean> releaseConsumer) {
-        releaseConsumers.add(releaseConsumer);
+        synchronized (releaseConsumers) {
+            releaseConsumers.add(releaseConsumer);
+        }
     }
 }