ElastiCache Maintenance Window Availability

Question

We are planning to use ElastiCache (Redis) instead of our own redis cluster. However, the "maintenance window" setting creates some questions,

If I use a multi-az replicated cluster, will elasticache failover to available replicas during maintenance windows or does the entire cluster go down during maintenance?

How long does it generally take?

We can also use MemCached instead of Redis, does it have better availability situation during maintenance windows?

How do others handle ElastiCache manintenance windows? Just go woth the downtime?

Thanks!

Answer 1

How long does it generally take?

Under 60 minutes:

"If a "maintenance" event is scheduled for a given week, it will be initiated and completed at some point during the 60 minute maintenance window you identify."

How often:

Software patching occurs infrequently (typically once every few months) and should seldom require more than a fraction of your maintenance window. If you do not specify a preferred weekly maintenance window when creating your Cache Cluster, a 60 minute default value is assigned.

http://aws.amazon.com/elasticache/faqs/

Answer 2

There are usually 2 maintenance AWS does.

1. Continuous managed maintenance updates.
2. Service updates

While creating cluster you need to specify a 60 min maintenance window. Usually all the maintenance updates (1) will happen during that time.

For every service updates you will recieve notifications when there is a scheduled one. Notification will be in the form of email or a notification on the elasticache page etc... Based on the notification you can reschedule the service updates to a comfortable time. If you fail to reschedule it will by default pick you maintenance window and apply the service updates.

Basically during the maintenance updates, AWS will replace your node with a new node with required updates. If you have primary/replica set up with multi az and auto failover set to true, then during maintenance window of the primary node, you replica will be promoted to master and your read/write requests will be served from there. So ideally you don't see any issue during maintenance maybe a few second downtime to promote the replica as master.

If you either don't set up multiaz with auto failover to true or your elasticache has just one node, you will see downtime during maintenance window.

Refer AWS documentation