One of the most challenging aspects in deploying large distributed systems is making sure the right jar files are located where they are needed. For a cluster that contains many nodes (tens and hudreds and even thousands) having the ability to deploy the application code easily is very important.
This problem is not new and indeed there have beens several solutions for it in the past:
The first one is of shared file-system. All hosts have access to a shared drive. A classppath variable is set to this shared drive and upon change this shared drive is the only place to put new jar files. This simple solution is ok when it comes to static deployments and when we know all hosts are on the same physical LAN. Another drawback of this simple solution is that it represents a SPOF in the system. Also, the singularity imposes a contention point that in many cases becomes a bottleneck. Also, it is imperative to make sure the FS is as available as our SLA requirements. This may become very costly.
The second approach is of a codebase server. This is the approach used by RMI and by JINI. As java class loader is a URLClassLoader, all we make sure is that our jars are behind a reliable HTTP server and when the JVM starts it looks for the classes at the codebase server as well. The codebase server is a first step in virtualization. A url is abstracted by a host name controlled by a DNS and can be changed without impacting the application. There are few implementation problems with this approach: The first is that when it comes to downloading the jars, in real life, this may lead to a network storm which will result in system hickups as jars are downloded fully and in real life systems those jars may be very large. Another problem with the codebase server approach, is that the URLClassLoader behavior is static. Once a class has been loaded into the class loader it can not be changed, and in order to replace it the class loader needs to be destroted, which practically means in many cases, the process needs to be restarted.
What we've introducing in 6.5 and is already available in M7, is what we call "Cloud Aware ClassLoader". Once a VM in the cloud requires to load a class that it does not have it's class defition, it calls to the "cloud" classloader to retreive the class definition. This mechanism is fully integrated with the JVM class loader, in many cases the sequence of loading a class is transitive. For example if class A implements interface I, the classloader starts loading class A and as it idetifies other class dependecies such as interface I are missing, it looks for it in the cloud for the interface's class defintion.
The full power of this feature unlocks dynamic capabilities of patterns such as MapReduce and Master Worker. Where is such cases the implementation of the Work object (M/W) can be changed at runtime while the system is running without the need to re-deploy the code on all nodes again.
Cosider the following scenario. Task interface is defined with one execute mehtod. Worker implementations resides on cloud nodes know how to consume a Task object and to call it's execute method. The first client connects to the cloud and submit an implementation of Task which is called TaskA. All he's doing is connecting to the cloud and submitting his task. No need for task deployment on cloud nodes is required. A second client connects to the cloud as well and submits his own implementation of Task named TaskBTask and he's not deploying as well. The beauty of it is that if client A would like to be able to use TaskB the class information of TaskB will be shipped automatically to his JVM upon demand without any configuration.
The funny part, is that we didn't plan in advance to solve this problem in the release, we had other issues to attend. However, as we had several painful experiences in deployment of huge clouds in our lab and on EC2, the Runtime Platform team, decided that they are too spoiled to work with non-elegant solutions, and in few days work, put this great feature into the product.
You're all encourage to give it a spin and share you're thoughts on this with us.