Cloud, Big Data and Mobile: Part 3: Understanding Amazon ElastiCache Internals: Auto Discovery

Cloud, Big Data and Mobile: Part 3: Understanding Amazon ElastiCache Internals: Auto Discovery

When your online user base grows, naturally the cache size also grows for a cache dependent application. AWS understands this very well and have designed their Amazon ElastiCache with a core idea that you can elastically increase and decrease the number of cache nodes. Though it is easy to add new nodes in the cluster, this activity has some impacts on client side -> Memcached client library.

Imagine you have an online application which has following system components:

• 25+ Java based web/app servers deployed on Amazon EC2's in Availability Zone -A
• 5+ Amazon ElastiCache nodes in a single cluster in Availability Zone -A
• Web/App Servers use Java based memcached client ( example spymemcached client) to connect to the Amazon ElastiCache(Memcached) nodes

Now you are planning to run an online sales promotion and choose to add few ElastiCache nodes in the cluster to match your growing memory and performance requirements. The java based memcached clients cannot immediately identify and recognize the new ElastiCache nodes, you would have to add the cache end points manually in the client configuration and restart the the web/application server process(in this case 25+ web/app EC2's). This re-initialization action can result in temporary disruption of some services and even downtime on some architectures.
Amazon ElastiCache has recently introduced an Auto discovery feature on the spymemcached client(java based) to eliminate this complexity. You can download this patch from Github. Soon AWS will make this feature available for all popular memcached clients. 
Using Amazon ElastiCache Auto Discovery feature, customer applications now transparently adapt to the addition/ deletion of cache nodes in the cache clusters. The applications are automated to react quickly to changes in your cache cluster without downtime. Amazon ElastiCache clusters now include a unique Configuration Endpoint(DNS Record). This record contains the DNS names of each of the cache nodes that belong to the cluster. Amazon ElastiCache service will ensure that the Configuration Endpoint always points to at least one such "target" cache node. A query to the target cache node then returns endpoints for all the nodes in the cluster. AWS team has implemented this config command(which queries the target node) as an extension to the Memcached ASCII protocol. Since ElastiCache remains 100% Memcached-compatible, you can keep using your existing Memcached client libraries with new and existing clusters, but to take advantage of Auto Discovery you must use an Auto Discovery-capable client provided by AWS.

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