Graphite

What is using memcache by in graphite

Asked by Marc

What portions of Graphite use memcached? We are setting up a federated system, and I am trying to figure out which portions of the system should have memcached running and configured for their use.

For example, does carbon-cache use it at all, or the relays, or any of the pieces like that, or is it just the web app? When the web front is doing queries for the data to remote systems, does memcache get involved.

One the the architectures I'm considering is to have the web front end on hosts separate from the carbon-cache hosts. Is that a good idea, and if so, then do I just need memcached on the web front end hosts?

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chrismd (chrismd) said : 		#1

memcached is only used by the webapp and if you've got a cluster you want to have a memcached on each server. Configure all the webapps' MEMCACHE_HOSTS to be identical, referring to each memcached instance in the cluster, and use IP addresses instead of hostnames to avoid DNS lookups.

It is possible to separate the frontend and backend but that is rarely the right thing to do. If your system is bottlenecked on I/O, moving the webapp does nothing because the data still has to be fetched from the backend machine's disks. If its a CPU or memory issue then there are probably other ways to better distribute load like tuning apache/mod_wsgi or tuning the carbon pipeline. Also make sure your metrics are evenly distributed throughout the cluster.

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