diff --git a/src/main/java/com/phonepe/sdk/pg/common/configs/CredentialConfig.java b/src/main/java/com/phonepe/sdk/pg/common/configs/CredentialConfig.java
index 4e3d7f7..9eb2e03 100644
--- a/src/main/java/com/phonepe/sdk/pg/common/configs/CredentialConfig.java
+++ b/src/main/java/com/phonepe/sdk/pg/common/configs/CredentialConfig.java
@@ -31,5 +31,5 @@ public class CredentialConfig {
 
     @NotBlank private String clientSecret;
 
-    @NotBlank private Integer clientVersion;
+    private int clientVersion;
 }
diff --git a/src/main/java/com/phonepe/sdk/pg/common/constants/BaseUrl.java b/src/main/java/com/phonepe/sdk/pg/common/constants/BaseUrl.java
index 80d2c07..9507341 100644
--- a/src/main/java/com/phonepe/sdk/pg/common/constants/BaseUrl.java
+++ b/src/main/java/com/phonepe/sdk/pg/common/constants/BaseUrl.java
@@ -31,7 +31,7 @@ public class BaseUrl {
 
     public static final String PRODUCTION_EVENTS_HOST_URL =
             "https://api.phonepe.com/apis/pg-ingestion";
-    public static final String SANDBOX_EVENTS_HOST_URL = "http://locahost";
+    public static final String SANDBOX_EVENTS_HOST_URL = "http://localhost";
 
     public static final String TESTING_URL = "http://localhost:30419";
 }
diff --git a/src/main/java/com/phonepe/sdk/pg/common/events/publisher/EventPublisherFactory.java b/src/main/java/com/phonepe/sdk/pg/common/events/publisher/EventPublisherFactory.java
index 905862e..1fb1800 100644
--- a/src/main/java/com/phonepe/sdk/pg/common/events/publisher/EventPublisherFactory.java
+++ b/src/main/java/com/phonepe/sdk/pg/common/events/publisher/EventPublisherFactory.java
@@ -28,8 +28,20 @@ public class EventPublisherFactory {
     private final ObjectMapper objectMapper;
     private final OkHttpClient okHttpClient;
     private final String hostUrl;
-    private static EventPublisher cachedEventPublisher;
+    private static volatile EventPublisher cachedEventPublisher;
+    private static final EventPublisher NO_OP_EVENT_PUBLISHER =
+            new EventPublisher() {
+                @Override
+                public void run() {}
+            };
 
+    /**
+     * Constructs an EventPublisherFactory with required dependencies.
+     *
+     * @param objectMapper the Jackson ObjectMapper
+     * @param okHttpClient the OkHttpClient
+     * @param hostUrl the event host URL
+     */
     public EventPublisherFactory(
             final ObjectMapper objectMapper,
             final OkHttpClient okHttpClient,
@@ -39,13 +51,12 @@ public EventPublisherFactory(
         this.hostUrl = hostUrl;
     }
 
-    public static void setCachedEventPublisher(EventPublisher eventPublisher) {
+    public static synchronized void setCachedEventPublisher(EventPublisher eventPublisher) {
         EventPublisherFactory.cachedEventPublisher = eventPublisher;
     }
 
-    public EventPublisher getEventPublisher(boolean shouldPublishEvents) {
+    public synchronized EventPublisher getEventPublisher(boolean shouldPublishEvents) {
         if (shouldPublishEvents) {
-            // For different clients, the eventSender should be same
             if (Objects.isNull(cachedEventPublisher)) {
                 EventPublisherFactory.setCachedEventPublisher(
                         QueuedEventPublisher.builder()
@@ -58,9 +69,6 @@ public EventPublisher getEventPublisher(boolean shouldPublishEvents) {
             }
             return EventPublisherFactory.cachedEventPublisher;
         }
-        return new EventPublisher() {
-            @Override
-            public void run() {}
-        };
+        return NO_OP_EVENT_PUBLISHER;
     }
 }
diff --git a/src/main/java/com/phonepe/sdk/pg/common/events/queue/EventQueue.java b/src/main/java/com/phonepe/sdk/pg/common/events/queue/EventQueue.java
index 1c2699f..9bb4d7a 100644
--- a/src/main/java/com/phonepe/sdk/pg/common/events/queue/EventQueue.java
+++ b/src/main/java/com/phonepe/sdk/pg/common/events/queue/EventQueue.java
@@ -19,22 +19,13 @@
 import java.util.concurrent.ConcurrentLinkedQueue;
 
 public interface EventQueue {
+    void add(BaseEvent data);
 
-    default void add(BaseEvent data) {}
+    boolean isEmpty();
 
-    default boolean isEmpty() {
-        return true;
-    }
+    int size();
 
-    default int size() {
-        return 0;
-    }
+    BaseEvent poll();
 
-    default BaseEvent poll() {
-        return null;
-    }
-
-    default ConcurrentLinkedQueue<BaseEvent> getQueue() {
-        return null;
-    }
+    ConcurrentLinkedQueue<BaseEvent> getQueue();
 }
diff --git a/src/main/java/com/phonepe/sdk/pg/common/exception/ExceptionMapper.java b/src/main/java/com/phonepe/sdk/pg/common/exception/ExceptionMapper.java
index 2e7cd68..59615e7 100644
--- a/src/main/java/com/phonepe/sdk/pg/common/exception/ExceptionMapper.java
+++ b/src/main/java/com/phonepe/sdk/pg/common/exception/ExceptionMapper.java
@@ -21,10 +21,12 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import lombok.SneakyThrows;
+import lombok.experimental.UtilityClass;
 
+@UtilityClass
 public class ExceptionMapper {
 
-    public static Map<Integer, Class<?>> codeToException =
+    public final Map<Integer, Class<?>> codeToException =
             Stream.of(
                             new AbstractMap.SimpleEntry<>(400, BadRequest.class),
                             new AbstractMap.SimpleEntry<>(401, UnauthorizedAccess.class),