Clustered Akka Application Deployment

For our typical stateless services, switching to docker was relatively straightforward. The instances of these services do not need to know about each other, they simply accept requests behind a load balancer. The deployment of a clustered Akka application however, requires every instance (node in akka) to know about all other running instances in order to form a cluster.

Before we dive into the deployments let’s cover some basics of cluster formation in Akka:

Cluster Formation Primer

In order to form a cluster every node needs be configured with the address of at least one existing node in the cluster, a list of initial contact points into the cluster. Any node which serves as an initial contact point for other nodes to join the cluster is known as a seed node. Other than being designated as initial contact points, seed nodes function exactly the same as other nodes in the cluster.

When a node launches, it reaches out to all nodes in its configured seed list, sending a cluster join request to the first seed that responds. The node which receives the join request tells other nodes about the request via a gossip protocol. Once every node in the cluster sees the join request from the new node, it becomes a member in the cluster.

A simplified example of a node joining a cluster with two nodes. Some details are omitted for simplicity

But how can a node join a cluster if it is the first node to join? Isn’t this a classic chicken and egg problem? As it turns out, the first seed node in the seed list of all nodes will send a join request to itself when it launches, becoming the first member of the cluster. It is thus important that all seed lists have the same first node.

For a more detailed treatment of cluster formation, consult the Akka Cluster Docs.

The Challenge

Now that we have an understanding of cluster formation we can discuss some of the challenges present when deploying to ECS:

All nodes must run inside Docker containers

All nodes receive cluster level communication via their remoting port , hence they must be addressable on this port from other container instances in the ECS cluster

, hence they must be addressable on this port from other container instances in the ECS cluster One or more seed nodes must be configured to bootstrap the cluster

We will discuss strategies to overcome these challenges under the constraints of ECS

Prerequisites

JDK8+

Docker

SBT

An existing ECS Cluster

Access to an AWS Account

Working knowledge of Docker

Overview

Our first step towards deployment will be to package our clustered application as a Docker container and publish it to a Docker registry. We will use SBT to accomplish these steps.

Once our artifact is published we will tackle the problem of making nodes addressable across container instances. We will then run our nodes by creating a Task Definition and ECS Service.

In order to manage the seed list of every node we will utilize an Akka library called ConstructR which uses Apache Zookeeper to store a centralized seed list.

We have created a sample clustered application which addresses the steps and challenges above. The sample application can be found on Github at Theatre Booking Application

Dockerizing Applications with SBT Native Packager

Using the SBT Native Packager plugin it is trivial to package your applications as Docker containers. We can utilize any docker base image that has JRE 8 and bash installed.

Enable sbt native packager plugin to dockerize the application by adding the following to project/plugins.sbt :

addSbtPlugin("com.typesafe.sbt" % "sbt-native-packager" % "1.2.2")

Next, in our build.sbt , we will enable the plugin on our single module SBT project:

lazy val root = (project in file(".")).enablePlugins(JavaAppPackaging)

We can now specify a Docker registry to which we will publish our image. In our example, we will publish to the LoyaltyOne Docker Hub repository:

dockerRepository := Some("loyaltyone")

We want our image to be published to loyaltyone/theatre-example on DockerHub so we need to ensure our project name is theatre-booking in build.sbt :

name := "theatre-example"

When we publish our image, the build’s version property will be used to tag our image. We would also like the image to be tagged with the latest tag. This tag will always point to the most recent image we publish. To set this behaviour we can set dockerUpdateLatest :

dockerUpdateLatest := true

Publishing Our Image

Now that we’ve configured our build, we can publish our image to DockerHub. In order to do so, we first need to authenticate

$ docker login

You will be asked to enter your username and password for DockerHub

Once logged in, we can publish our image by running

$ sbt docker:publish

Our image is now on DockerHub as loyaltyone/theatre-example:latest

Making Nodes Addressable Across ECS Container Instances

As a default, when Docker containers are launched, they are connected to a virtual bridge device called docker0. This bridge has an IP address range from which it assigns an IP to each container connected to it. Inside a container, it’s eth0 interface will be assigned the IP given by the bridge.

Why is this important? Well, when we run an Akka Cluster node within a Docker container, the default behaviour of the node is to advertise the address of it’s binded interface (eth0) to other nodes.

Let’s assume we have two hosts A and B on the same network, with Docker installed:

Host A: eth0/10.0.1.2, docker0/172.17.0.0/16

Host B: eth0/10.0.1.3, docker0/172.18.0.0/16

For Host A, docker0 will assign IP addresses to containers in the range 172.17.0.0/16 . For Host B, the range will be 172.18.0.0/16

Let’s assume

we have two nodes running on the hosts

each node has a seed list configured with the advertised address of the seed node (we will see how to get this list later)

Containers

Node A: eth0/172.17.0.2 Port Bindings: 2552 -> 2552

Node B: eth0/172.18.0.3 Port Bindings: 2552 -> 2552 (Seed)

Host A does not know about the docker network on Host B hence it is unable to route packets to 172.18.0.3

When Node A tries to join Node B to form a cluster, it will try to reach Node B at 172.18.0.3:2552 . For this to work, Host A would need to know how to route packets to 172.18.0.3 ; which it doesn't. Host A is unaware of the Docker network running on Host B. Unfortunately, ECS does not currently provide a routing mechanism for containers to directly address each other across host machines.

How can we get around this problem? Conveniently, Akka provides a way to advertise nodes on an address different than what they bind to. We need nodes to advertise themselves using the port and IP of their host rather than their container port and IP.

Configuring Akka Remoting

Akka Remoting handles the network level communication for Akka Cluster, so we will configure Akka Remoting. We can achieve the desired behaviour by configuring both a hostname and bind-hostname along with their port counterparts in src/main/resources/application.conf :

akka.remote {

enabled-transports = ["akka.remote.netty.tcp"]

netty.tcp {

bind-hostname = 0.0.0.0

bind-port = 2552

​

hostname = ${?HOST_IP} // Advertise using $HOST_IP

port = ${?HOST_PORT} // Advertise on port $HOST_PORT

}

}

Now all we need to do is get the IP of the host and supply it to the Akka application via the $HOST_IP . Because the HOST_PORT can be left static for all hosts, we can supply it via the Task Definition environment variable as "HOST_PORT": 2552 .

Getting the Host IP

The host, a container instance, is really just an EC2 instance. To retrieve the IP of an EC2 instance Amazon provides a metadata endpoint which we can query to retrieve the IP.

The above call would return the private IP of our instance, for Host A this would be 10.0.1.2

Unfortunately on ECS, the only way to supply the host IP to the container is to hard code it as an environment variable to the container or to mount it into the container as a file. Another option would be to do the metadata query inside the container; this would couple our container heavily to AWS however. This leaves us the option of mounting the host IP as a file. We can do the metadata query, store the IP in a file on the host /etc/hostip and mount this file into the container via the ECS Task Definition.

Revisiting Our Example Scenario

Now that we can supply the correct values for HOST_IP and HOST_PORT , how would our example node communication play out?

Host A: eth0/10.0.1.2, docker0/172.17.0.0/16

Host B: eth0/10.0.1.3, docker0/172.18.0.0/16

​

Containers

Node A: eth0/172.17.0.2 Port Bindings: 2552 -> 2552

Node B: eth0/172.18.0.2 Port Bindings: 2552 -> 2552 (Seed)

Now, when Node A tries to join Node B to form a cluster, it will try to reach Node B at 10.0.1.3:2552 . Host A is now able to send packets to this address since it is in the same network as Host B. On Host B, since we have the Node B container bound to host port 2552, the Docker daemon has set up the necessary routing to direct traffic on this port to our container at 172.18.0.2:2552 . Nodes can now communicate with each other!

So, what have we done so far to achieve node communication?

Retrieve the IP of the host machine

Mount the IP into our containers as a file /etc/hostip via the Task Definition

via the Task Definition Fix our HOST_PORT to 2552 and bind 2552 from our host to 2552 in our containers

to and bind 2552 from our host to 2552 in our containers Configure Akka to advertise on HOST_IP and HOST_PORT while binding to a local container interface

Is this good enough?

This solution works when we want to run a single clustered Akka application on our ECS Cluster, but what happens when we want to run multiple clustered applications on the same cluster?

Since we chose to bind our application to port 2552 on the host, we must choose a different host port to bind to for a second application! In short, to run multiple clustered applications on the same ECS Cluster, we would need to ensure we don't have port conflicts across applications. As you can imagine, keeping track of which port is in use to avoid conflicts would prove to be a maintenance headache!

What if we could let docker choose a random host port to bind to our containers? We know this is possible by binding 0 -> 2552 ; docker will choose a random host port for us. However, there is a slight problem. How can our application know about this randomly selected port? After all, we need to supply this randomly selected port via our HOST_PORT environment variable. The random port is selected at container launch so there would be no way to pass it into the container during launch or mount it in like we did with /etc/hostip

What if, when our containers launch, they could query a local service on the host to figure out their host port bindings and host IP? Such a service would be akin to a mirror for our container, it could see its reflection as seen from the host.

Mirror, Mirror, On the Wall, What is My Host Port?

We decided to create such a service; it is called Docker Mirror. It is intended to be run as a container on every host in the ECS Cluster, in the same bridge network as all containers. Docker Mirror is able to obtain information about containers running on it’s host by mounting in the host’s Docker socket.

When a container needs to see it’s port binding information, it can query Docker Mirror running on it’s host machine to easily obtain this information. Instructions for how to run Docker Mirror in your cluster can be found at it’s Github README. We also have an Amazon CloudFormation template to launch an ECS Cluster with Docker Mirror at ECS Cluster Template

We omit a call to Docker Mirror to retrieve Host IP for brevity

Docker Mirror supports the following two operations:

GET /hostip

GET /container/{container-id}/port/{app-port}

The first call would return the host IP address. For the second call, we supply the container’s ID along with it’s internal port to which the host port is binded to. In our example, host -> internal , internal would be 2552 . What we would get back is the host port, even if it was randomly selected by the Docker daemon!

Updating Our Docker Image to Use Docker Mirror

To use Docker Mirror, we need to update our application’s Docker image to make the necessary calls to Docker Mirror to get its HOST_IP and HOST_PORT . We can create a script which does these calls and run it in our container's entrypoint.

In order to communicate with Docker Mirror, we can direct our requests to the host’s IP and the host port to which Docker Mirror is bound. In our example, Docker Mirror is running on host port 9001 .

How do we get the host IP from the container? It turns out that when Docker launches containers, it sets the container’s default gateway to the IP of the Host in the Docker bridge network. We simply need to get the default gateway address from within our container:

DOCKER_MIRROR_HOST=$(/sbin/ip route | awk '/default/ { print $3 }')

An example script we could run as part of our container entrypoint would look like:

To avoid writing this boilerplate bootstrap logic in all of your container images, we’ve published a base image which contains this logic at loyaltyone/dakka .

Updating our build.sbt

We need to update the base image used for our container and also execute the bootstrap script as part of our container’s entrypoint:

dockerBaseImage := "loyaltyone/dakka:0.5"

dockerEntrypoint := "/usr/local/bin/bootstrap" +: dockerEntrypoint.value

Now, when we launch our container, our application will have access to HOST_IP and HOST_PORT assuming we've supplied APP_PORT=2552 as an environment variable.

Seed Node Discovery

As we discussed in our Cluster Formation Primer, every node in Akka needs to be configured with a list of seed nodes; this is specified by akka.cluster.seed-nodes . Every seed node address in this list consists of the node's IP, port and actor system name. To configure seed-nodes we could hard code a list of host IPs or DNS names but we would also need to hard code ports; this would not work with our random port strategy. This strategy has a number of downsides:

Seed node list is fixed, even if cluster grows or shrinks

Seed node ports need to be fixed, making it easy to have port conflicts when running multiple applications

Requires specialized Task Definitions for seed nodes which force them to always run on the same machines

There are a number of solutions for this problem. Most of them involve some form of centralized seed list stored outside of the cluster nodes themselves. The idea is that when a node starts up, it checks the centralized list, tries to join the nodes in the list and then adds itself to the list. If the list is empty, the node adds itself to the list and joins itself to form the cluster. Writes to this list must be synchronized to avoid cluster formation pitfalls.

Two common options for this problem are:

Both serve a similar purpose but we will use Zookeeper in our example. Setting up a Zookeeper cluster is beyond the scope of this post but we have an AWS CloudFormation template to spin up a cluster in your AWS environment. Please read the README for instructions to setup a Zookeeper cluster.

Adding Constructr

Constructr is an extension for Akka which handles seed node discovery and ultimately cluster formation. It supports both Zookeeper and Etcd as backends.

A simplified interaction with Zookeeper via ConstructR

First, we need to add all necessary constructr dependencies to our applications build.sbt

resolvers += Resolver.bintrayRepo("hseeberger", "maven")

​

libraryDependencies ++= Vector(

"de.heikoseeberger" %% "constructr" % "0.18.0",

"com.lightbend.constructr" %% "constructr-coordination-zookeeper" % "0.4.0",

...

)

Given an existing Zookeeper cluster with the following nodes:

nodea.zk.dev:2181

nodeb.zk.dev:2181

nodec.zk.dev:2181

We will configure Constructr to point to our Zookeeper cluster, or ensemble, as it is idiomatically called.

First we need to enable the Constructr extension in our src/main/resources/application.conf :

extensions = [de.heikoseeberger.constructr.ConstructrExtension]

Finally, we will configure Constructr to contact our Zookeeper nodes:

zookeeper {

nodes = ${?ZOOKEEPER_NODES}

}

​

constructr {

coordination.nodes = ${zookeeper.nodes}

}

With this configuration, assuming we have a Zookeeper ensemble, our nodes will be able to discover eachother and form a cluster if we supply the node list via ZOOKEEPER_NODES .

We will need to publish our new image using sbt docker:publish

Task Definition and Service Definition for Deployment

Now that we have solved the node addressability problem as well as seed node discovery we can configure our ECS Task Definition to run our application nodes as containers.

Of course we need an ECS cluster to deploy our containers to. As we’ve previously mentioned in the post, we will use an existing Cloud Formation template to deploy our cluster with Docker Mirror pre-installed.

For our Task Definition there are a few important configurations:

Ensuring we choose a random host port to bind to our Akka Remoting port (2552)

Supplying a list of Zookeeper nodes to enable seed node discovery

Supply the APP_PORT=2552 environment variable to our container so Docker Mirror can find the matching random host port

Below is a snippet of a Task Definition from our CloudFormation template used to deploy our clustered theatre application:

In our Task Definition, setting HostPort to 0 tells Docker to choose a random host port. We have hardcoded some values for brevity.

We now have a description that ECS can use to run our containers. You’ll notice our container also exposes port 8080 and binds it to a random port; we use this port to service HTTP traffic. We still need a way to specify the size of our cluster, we will do this by defining a Service in ECS.

Service

Recall that a Service in ECS helps maintain a desired number of tasks, nodes in our app. It is also responsible for handling service registration with a Load Balancer.

Below is a snippet of a Service Definition from our CloudFormation template used to run 3 nodes of our theatre application and register them with a load balancer:

We have hardcoded some values here for brevity. Please refer to the original template for more details.

In our service definition we use the Cluster property to specify which ECS cluster to deploy our service to. A cluster with this name must exist before deploying our service. Using the DesiredCount property we can tell ECS to create a 3 node Akka cluster for our application. All 3 nodes will connect to Zookeeper for seed discovery to form a cluster.

We use the LoadBalancers property to register our application's HTTP server port with a load balancer. As part of our cluster template, we create a load balancer for our services to register to. You'll notice we also used HostPort:0 in our Task Definition for 8080 , this is possible because Amazon provides a way to register services to a load balancer with random port assignment. For more information about registering your services with load balancers refer to Amazon's excellent docs.

With the template containing our Task Definitions and Service Definition, we can create a CloudFormation stack which will deploy our 3 node clustered application.

Conclusion

Using the strategies and tools we covered in this post we were able to tackle the challenges associated with deploying Clustered Akka Applications on Amazon ECS. The Task Definition and Service Definition snippets we provided aim to serve as examples for how you can deploy your own services using these strategies. Please refer to the Theatre Booking Application GitHub project for a full example.

At LoyaltyOne we deploy our Task Definitions and ECS Services using CloudFormation templates; we find it easier to manage AWS resources created alongside our services.

Along our ECS journey, we built a couple tools to assist with common deployment needs:

ecs-service — a tool for deploying ECS services using environment agnostic CloudFormation templates

kms-env — a tool for passing application secrets to our containers in a secure manner using AWS KMS.

Using these tools and strategies we discussed in this post we are able to have painless deployments of Clustered Akka applications on Amazon ECS. We hope they will enable you to do the same with your applications. Happy HAkking!