A sample Akka Cluster application that uses ZooKeeper for service discovery
This is a demonstration of how to use ZooKeeper with the help of ConstructR
to bootstrap an Akka Cluster.
The process of discovering seed nodes and joining an existing cluster is a perilous journey especially in cloud
environments due to the ephemeral nature of instances. You can get into situations where you form two or more clusters
if you incorrectly detect that no seed nodes are up causing massive problems for your data. Thankfully, service discovery
mechanisms like Consul, etcd and ZooKeeper are there to help find out if existing seed nodes exist and ensure that a
single cluster forms correctly.
This project uses ConstructR backed by ConstructR-ZooKeeper
in order to bootstrap the cluster i.e. discover existing seed nodes if they exist and ensure that new nodes join the
existing cluster. Once the application is ready, Greeter Actors on all nodes will begin messaging Cluster Sharded
Member Actors so you can see some activity.
In order to run the cluster, we make use of Docker Compose and the
SBT Native Packager. First, create a Docker image in your local Docker
registry using sbt docker:publishLocal or using ./scripts/docker-image.sh. Once this has been complete, spin up the
set of services (ZooKeeper and 3 clustered application nodes) that represent this application via docker-compose up.
When you are satisfied, use docker-compose down -v to bring down the application.
Note: If you want to run multiple clustered applications directly on your host machine, you will need to change the default
port Akka Cluster uses for gossip
since all applications use the default port.
Using Docker Compose means that a Docker network is created and containers are automatically placed into this network.
This means that the containers are free to communicate with each other. We make the Akka Cluster applications talk to
ZooKeeper by using the Container DNS mechanism offered by Docker. The Akka Cluster applications will use ZooKeeper
(via ConstructR) to discover the seed nodes and join the same cluster and will communicate with each other.
Note: This does not take care of Split Brain Resolution, this takes care of bootstrapping a cluster. If you are
looking for an open-source Split Brain Resolver then see akka-batteries
for an excellent example and also refer to Niko Will’s talk.