ElasticCache to replace local memcached in iiif machines

[klokan]

In this moment each IIIF machine has it's own memcached. This ensures that the cache is shared between multiple processes of the imageserver on that single machine. If loadbalancer starts multiple IIIF machines - the cache between these is not shared right now.

If a shared cache is applied the service will speed up - and probably save some costs as well (smaller amount of IIIF machines started by the loadbalancer). Cache is important especially for repeatably accessed thumbnails - such as the one on embedr.eu or other often visited large list of thumbnails on other services.

It seems ideal would be to have a single node with a large memcached which will be used across all the iiif machines - Amazon offers for this a service called ElasticCache - which is essentially a machine with MemCached service.

If this is applied we will need to rebuild the IIIF AMI - so it uses the shared single cache from all instances.

So the tasks here are:

• [ ] Configure a single instance of ElasticCache (3 GB or more?)
• [ ] Update the AMI for IIIF load-balancing to use ElasticCache (possibly medium -> small or even micro)

[klokan]

Diagram (compare with older structure mentioned in comments #34):