Currently, Elastic Beanstalk provides a Java-based application container with Apache Tomcat as the work horse. All you have to do is to deploy a standard Java WAR file containing your web application.

Simply create a Java web application with Spring MVC, Grails, OSGi, Eclipse RAP, or any other of the numerous Java web frameworks and upload it using the AWS web console.

Additional containers for other platforms such as Ruby, Python or PHP may follow later, but as Tomcat hosts standard Java WAR files, anything with a Java implementation may be run. Ruby apps based on Rails or Rack using Warbler and JRuby, Python apps using Jython, even PHP apps can be made to run on Beanstalk.

In this blog post, I’ll dive into the inner workings of a Beanstalk server instance and poke around its internals. I invite you to come along for the ride.

Accessing your instance
The first step in dissecting a Beanstalk instance is getting access via SSH.
In order to acces your running instance(s), you first need to configure the SSH key pair to be used:

Configuration dialog in Elastic Beanstalk web console

Enter the name of a key pair as configured in your EC2 web console. See the EC2 guide and the description in the Elastic Beanstalk guide for details on creating and configuring a key pair.

Setting the key pair requires a restart of your Beanstalk environment, which may take a couple of minutes.

Finally look up your instance id and get the instance’s hostname. Connecting to the server is now as simple as this:

ssh -i .ec2/mykeypair.pem ec2-user@ec2-184-72-134-2.compute-1.amazonaws.com

Please note, that you need to connect as user ec2-user, root access can be reached using the command

sudo su -

Getting around your instance
The first steps are to collect some interessting facts. The instance uses a Amazon Linux AMI (release 2010.11.1 (beta), README). The AMI id for the ElasticBeanstalk-Tomcat6-32bit is ami-7609f81f, the kernel id is aki-407d9529. The instance is EBS-based and there is no ephemeral storage. Currently, Beanstalk is only available in the US East zone.

Software
The process list reveals: along with Apache Tomcat, Beanstalk uses the venerable Apache Web Server. Additional software includes Bluepill for basic process monitoring, and Amazons own HostManager (see below), which is run within a Thin web server.

Network setup
Elastic Beanstalk scales EC2 instances as needed. Therefore the first target is a load balancer provided by Elastic Load Balancing. Each instance runs Apache as the front end on port 80, with web request being reverse proxied into Tomcat on port 8080. Requests for URI /_hostmanager are forwarded to HostManager on port 8999.

CloudWatch performs health checks by periodically requesting the root page (URI /) of your application. Both health check URI and frequency are configurable. If an instance is no longer available or the load changes, CloudWatch starts or stops instances.

[Image from AWS Elastic Beanstalk Concepts blog post]

HostManager
Local instance management is performed by Amazons HostManager. HostManager is a Ruby application based on Rack and running in a Thin server on port 8999. It receives requests on URI /_hostmanager.

Some examples from the access log:

72.21.217.96 (72.21.217.96) - - [21/Jan/2011:22:00:45 +0000] "POST /_hostmanager/tasks HTTP/1.1" 200 368 "-" "AWS ElasticBeanstalk Health Check/1.0"
10.223.61.43 (10.223.61.43) - - [21/Jan/2011:22:01:05 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
10.122.19.154 (10.122.19.154) - - [21/Jan/2011:22:01:11 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
10.223.61.43 (10.223.61.43) - - [21/Jan/2011:22:01:36 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
10.122.19.154 (10.122.19.154) - - [21/Jan/2011:22:01:42 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
72.21.217.96 (72.21.217.96) - - [21/Jan/2011:22:01:46 +0000] "POST /_hostmanager/tasks HTTP/1.1" 200 368 "-" "AWS ElasticBeanstalk Health Check/1.0"
10.223.61.43 (10.223.61.43) - - [21/Jan/2011:22:02:07 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
10.122.19.154 (10.122.19.154) - - [21/Jan/2011:22:02:13 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
10.223.61.43 (10.223.61.43) - - [21/Jan/2011:22:02:38 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
10.122.19.154 (10.122.19.154) - - [21/Jan/2011:22:02:44 +0000] "GET /_hostmanager/healthcheck HTTP/1.1" 200 90 "-" "ELB-HealthChecker/1.0"
72.21.217.96 (72.21.217.96) - - [21/Jan/2011:22:02:47 +0000] "POST /_hostmanager/tasks HTTP/1.1" 200 368 "-" "AWS ElasticBeanstalk Health Check/1.0"


Files
HostManager is installed in /opt/elasticbeanstalk/srv/hostmanager. A file list can be found here. /opt/elasticbeanstalk/lib also contains a full Ruby 1.9.1 installation.

Some log files can be found at /opt/elasticbeanstalk/var/:

/opt/elasticbeanstalk
/opt/elasticbeanstalk/var
/opt/elasticbeanstalk/var/state
/opt/elasticbeanstalk/var/state/hostmanager.pid
/opt/elasticbeanstalk/var/state/healthcheck
/opt/elasticbeanstalk/var/tmp
/opt/elasticbeanstalk/var/log
/opt/elasticbeanstalk/var/log/LogDirectoryMonitor.log
/opt/elasticbeanstalk/var/log/DaemonManager.log
/opt/elasticbeanstalk/var/log/bluepill.log
/opt/elasticbeanstalk/var/log/hostmanager.log
/opt/elasticbeanstalk/var/log/ApplicationHealthcheck.log
/opt/elasticbeanstalk/var/log/CatalinaLogMonitor.log


Final words
That’s all for today’s blog post. Further posts will take a closer look at HostManager and deployment including the application setup sequence.

I’m looking forward to any feedback and additional information.

Maybe we can even hack HostManager to accept other application containers, e.g. for OSGi applications.

If it proves to be popular (or simply gets slahdotted), the application is in for a first real-world stress test. A single server will be overloaded pretty fast, but hosting the application on a server farm for traffic that might never come will get expensive very soon.

Amazons Elastic Compute Cloud (EC2) to the rescue. EC2 offers great flexibility for running web applications. Pre-created or custom application server images (so called AMI – Amazon Machine Images) based on Linux and Xen allow fine-grained control over available software. The web application might be running on a single server, but if the load is rising, the load may be dispersed by starting new server instances. If the load decreases, the number of servers may be reduced again.

The possibility to provide your own server images (AMIs) allows for server instances with different roles: one or more database servers might provide central storage services for your application servers while another server instance simply processes incoming and outgoing emails. Network security is ensured and can be specified with a fine-grained security policy.

Amazon bills by running instance per hour with three different hardware configurations available starting at 0.10 USD per hour. A single server of the lowest category amounts to roughly 70 USD per month, which is probably more expensive than other hosters. But the flexibility to balance load by starting or stopping server instances is priceless.

Web applications need one or more servers to run on, but what about storage? Each server provided by EC2 offers 160 GB (or more, depending on your hardware configuration) of space, but that storage space is only available as long as the server instance is running. Again Amazon provides a solution: its Simple Storage Service (S3). Space (almost) without limit at 0.15 USD per GB and month (+ data transfer, when accessed from outside Amazon’s network). About the only drawback is that the storage can not be accessed like a ordinary file system. This is due to the distributed architecture, which in turn provides high availability and safety for your precious data.

Communication between your servers may be performed using a third service offered by Amazon: its Simple Queue Service (SQS) allows message to be sent using one or more queues, which provides for an elegant load balancing solution for distributed services.

A recently established service, Amazon SimpleDB offers structured data storage. The service is still in limited beta, but seems to cater for your basic database needs without using a full-blown database server of your own.

All these services can be controlled and accessed through web service APIs with a wide variety of clients available in all major programming languages. For Java, these libraries offer all functionality exposed by Amazon’s services:

• The excellent JetS3t library (Apache license) provides access to Amazon S3
• Typica (Apache license) exposes the functionality of EC2, SQS and SimpleDB for Java developers

Toolkits and support for other languages can be found on the Amazon Resource Center pages.

All in all, Amazons EC2, SQS, and S3 provide the perfect environment for
web applications: great flexibilty for a reasonable price with plenty of room to grow according to your needs.

For an example of deploying distributed J2EE Applications using Amazon EC2 see this article, another example of using Amazon SQS, EC2, and S3 for distributed processing is described in an article by David Kavanagh, author of the excellent Java library Typica (see above).