Spring Boot Tomcat Session Replication on Kubernetes Using Hazelcast

April 14, 2020

Spring Boot is a framework that helps to build microservices easily. In the Java ecosystem, it’s one of the most preferred ways of building microservices as of now. In a microservice architecture, there are different options to share data among the running services: one of them is caching, which Spring Boot leverages. It also provides session scope data access using built-in Tomcat HTTP sessions. But when running multiple microservices, it quickly becomes a hassle to replicate sessions and share their data across microservices.

Imagine that you have a microservice which uses Tomcat HTTP sessions, and you would like to scale it horizontally. You can create multiple microservice instances and use a load balancer to access them. However, each microservice instance will not have the same session data and the session won’t be consistent through microservices when the requests reach different instances. One option is to use sticky sessions but even if you use sticky sessions, you will lose some session data when a microservice instance is crashed/shut down. To prevent any data loss and provide consistency, session data should be replicated through microservice instances.

There are multiple solutions for different environments and setups, but in this blog post we will find out how we can replicate sessions through Spring Boot microservices using Hazelcast with minimal configuration settings. We will also show how it can run in Kubernetes environments through a simple demo application.


  • Apache Maven to build and run the project.
  • A containerization software for building containers. We will use Docker in this guide.
  • A Kubernetes environment. We will use local minikube environment as k8s for demonstration.

Session Replication Sample

This post contains a basic Spring Boot microservice code sample using Hazelcast Tomcat Session Manager. You can see the whole project here and start building your app in the final directory.

However, we will start from scratch and build the application step-by-step.

Getting Started

First, clone the Git repository below. It contains two directories, /initial contains the starting project that we will build upon, and /final contains the finished result.

$ git clone
$ cd springboot-tomcat-session-replication-on-kubernetes/initial/

Running the Spring Application

The application in the initial directory is a simple Spring Boot application.
It has three different endpoints:

  1. / is the homepage returning “Homepage” string only
  2. /put is the page where key and value are saved to the current session as an attribute
  3. /get is the page where the values in the current session can be obtained by keys

The application can be ran using the commands below:

$ mvn clean package
$ java -jar target/springboot-tomcat-session-replication-on-kubernetes-0.1.0.jar

Now the app is running at http://localhost:8080. One can test it by using the following commands in another console prompt:

$ curl "localhost:8080"
$ curl --cookie cookies.txt --cookie-jar cookies.txt -s -L "localhost:8080/put?key=myKey&value=hazelcast"
$ curl --cookie cookies.txt --cookie-jar cookies.txt -s -L "localhost:8080/get?key=myKey"

Notice, we have to use cookies when testing the application since the data is saved in HTTP sessions. Thus, we need to access the same session. The output should be similar to the following:


value is set to hazelcast since it was put in the second command. podName is set to null because we are not running the application in a Kubernetes environment… yet. After testing, you can stop the running application.

Running the Containerized Application

In order to create the Docker image of the application, we will use the Jib tool and its related Maven plugin. It allows you to build containers from Java applications without a Docker file or even changing the pom.xml file. To build the image, run the command below:

$ mvn clean compile

This command:

  1. Compiles the application
  2. Creates a Docker image
  3. And registers it in the local container registry

Now, let’s run the container using the following command:

$ docker run -p 5000:8080 springboot-tomcat-session-replication-on-kubernetes:0.1.0

This command runs the application and binds the local 5000 port to the 8080 port of the container. Now, we are able to access the application using the following commands:

$ curl "localhost:5000"
$ curl --cookie cookies.txt --cookie-jar cookies.txt -s -L "localhost:5000/put?key=myKey&value=hazelcast"
$ curl --cookie cookies.txt --cookie-jar cookies.txt -s -L "localhost:5000/get?key=myKey"

The results will be the same as before. Kill the running application after testing. Now, we have a container image to deploy on Kubernetes.

Running the Application on Kubernetes

To run the app on Kubernetes, we need a running environment. As stated before, we will be using minikube for that. After this point, we assume that your Kubernetes environment is running.

We will use a deployment configuration that builds a service with two pods. Each of these pods will run one container which is built with our application image. Please see our example configuration file (named as kubernetes.yaml) in the repository. Click here to download this file. Note, that the MY_POD_NAME environment variable is set in order to reach the pod name in the application.

After downloading the configuration file, the containers can be deployed like that:

$ kubectl apply -f kubernetes.yaml

At this point, we should have a running deployment. Let’s check if everything is alright by getting the pod list:

$ kubectl get pods

It is time to test the running application. But first, the IP address of the running Kubernetes cluster is required to access it. By using minikube, we can get the cluster IP using the command below:

$ minikube ip

We have the cluster IP address, thus we can test our application:

$ curl --cookie cookies.txt --cookie-jar cookies.txt -s -L "http://[CLUSTER-IP]:31000/put?key=myKey&value=hazelcast"
$ while true; do curl --cookie cookies.txt --cookie-jar cookies.txt -s -L [CLUSTER-IP]:31000/get?key=myKey;echo; sleep 2; done

The second command makes a request in a loop in order to see the responses from both pods. At some point, a result similar to the one below should be displayed:


This means the pod named hazelcast-tomcatsessionreplication-statefulset-1 got the put request and stored the value in its local session storage. However, the other pod couldn’t get this data and displayed null since there is no session replication between pods. To replicate the sessions, we will use Hazelcast in the next step. Now, delete the deployment using the following command:

$ kubectl delete -f kubernetes.yaml

Session Replication using Hazelcast Tomcat Session Manager

To configure session replication, let’s first add some dependencies to the pom.xml file:


The first dependency is for Hazelcast Tomcat Session Manager, the second one is for Hazelcast IMDG itself, and the last one is for Hazelcast’s Kubernetes Discovery Plugin. The latter helps Hazelcast members to discover each other on a Kubernetes environment.

At this point, only some configuration beans are necessary to enable session replication:

public Config hazelcastConfig() {
    Config config = new Config();
    config.setProperty( "hazelcast.logging.type", "slf4j" );
    JoinConfig joinConfig = config.getNetworkConfig().getJoin();
    return config;

public HazelcastInstance hazelcastInstance(Config hazelcastConfig) {
    return Hazelcast.getOrCreateHazelcastInstance(hazelcastConfig);

public WebServerFactoryCustomizer<TomcatServletWebServerFactory> customizeTomcat(HazelcastInstance hazelcastInstance) {
    return (factory) -> {
        factory.addContextCustomizers(context -> {
            HazelcastSessionManager manager = new HazelcastSessionManager();
  1. The first bean creates a Hazelcast Config object to configure Hazelcast members. We enable the Kubernetes configuration for discovery.
  2. The second bean creates the Hazelcast member using the hazelcastConfig bean.
  3. The third one customizes Tomcat instance in Spring Boot to use Hazelcast Tomcat Session Manager.

Please note that the Hazelcast instance name for HazelcastSessionManager object and the instance name for Hazelcast config bean should be the same. Otherwise, HazelcastSessionManager wouldn’t access the running Hazelcast instance.

Our application with session replication is now ready to go. We do not need to change anything else because we are already using HTTP sessions to store data.

Running the Application with Tomcat Session Replication in Kubernetes Environment

Before deploying our updated application on Kubernetes, one should create a rbac.yaml file which can be found here. This is the role-based access control (RBAC) configuration which is used to give access to the Kubernetes Master API from pods. Hazelcast requires read access to auto-discover other Hazelcast members and form Hazelcast cluster. After creating/downloading the file, apply it using the command below:

$ kubectl apply -f rbac.yaml

Now, we can build the container image again and deploy it on Kubernetes:

$ mvn clean compile
$ kubectl apply -f kubernetes.yaml

The application is running and it is time to test it again:

$ curl --cookie cookies.txt --cookie-jar cookies.txt -s -L "http://[CLUSTER-IP]:31000/put?key=myKey&value=hazelcast"
$ while true; do curl --cookie cookies.txt --cookie-jar cookies.txt -s -L [CLUSTER-IP]:31000/get?key=myKey;echo; sleep 2; done

Since session replication is now configured, the output is similar to the following:


Both of the pods have the same value and session data is replicated across applications!


In this post, we first developed a simple microservices application which uses HTTP sessions to store data. The usage is very simple, but the session data is not accessible from all microservices on a Kubernetes environment if the sessions are not replicated. In order to replicate the sessions, we used Hazelcast Tomcat Session Manager. The configuration is just a matter of configuring three beans. Finally, we succeeded to replicate the sessions among pods, which helped us to access the same data from all microservices.

The complete source code for this post can be found here.

For additional resources, please see:

About the Author

About the Author

Alparslan Avci

Alparslan Avci

Software Developer

Alparslan works for Hazelcast as a Software Developer. He is a passionate Java developer and loves to think in a distributed and object-oriented way. Prior to joining Hazelcast, Alparslan worked in several Java projects including a web search engine and enterprise financial anti-fraud solutions. He also contributes to open-source projects like Apache Nutch and currently a PMC member in Apache Gora. Alparslan holds an MS degree in Software Engineering of Distributed Systems from Kungliga tekniska hogskolan in Stockholm, Sweden.

Latest Blogs

Hazelcast Command Line is released!

Hazelcast on Azure: Best Practices for Deployment

View all blogs by the author

Subscribe to the blog

Follow us on: