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Trying to make AWS-Elasticache Redis3.2 as the Master and the redis instances in my EC2 as slaveof for this elasticache. I get this error.

 Connecting to MASTER masterredis.XXXXXXXXXXXXXXXXXXX.amazonaws.com:6379
MASTER <-> SLAVE sync started
Non blocking connect for SYNC fired the event.
Master replied to PING, replication can continue...
Partial resynchronization not possible (no cached master)
Master does not support PSYNC or is in error state (reply: -ERR unknown command 'PSYNC')
Retrying with SYNC...
MASTER aborted replication with an error: ERR unknown command 'SYNC'

....

Navdeep Singh
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2 Answers2

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ElastiCache is a Redis-as-a-Service from AWS. As such, its operator has the liberty to disable certain commands/features - the ability to replicate to an external instance is one of these disabled features and that's the reason for the PSYNC/SYNC errors that you're getting.

Itamar Haber
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  • Okay Thank you Itamar, I was thinking on these lines but could not find any documentation from AWS that mentions this behavior. – Navdeep Singh Nov 22 '16 at 07:59
  • The list of restricted commands can be found in the documentation: https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/RestrictedCommands.html – Dor Serero Mar 21 '23 at 13:35
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Amazon ElastiCache, as noted by Itamar, is a managed service. When using the ElastiCache Redis engine (it also has a memcached option), the interface is 100% open source Redis, but Amazon has done some changes to the underlying code to optimize for the Cloud.

Replication in ElastiCache uses primary nodes and replica nodes. These are similar, but not identical, to the masters and slaves used by Redis Sentinel. Since ElastiCache is always running on AWS EC2, it can use direct memory transfer to make replication faster and failover smoother than the OSS distribution that needs to support a huge number of possible infrastructure stacks. But you cannot mix Sentinel nodes with ElastiCache nodes in the same cluster.

(BTW, I am part of the Managed Databases team at AWS, so feel free to reach out if you'd like more detail, either here on stackoverflow on by email: briskman at amazon dot com).

For documentation, see http://docs.aws.amazon.com/AmazonElastiCache/latest/UserGuide/Replication.html

We don't explicitly state that SYNC/PSYNC are not supported ... but you can find the ElastiCache API reference online at http://docs.aws.amazon.com/AmazonElastiCache/latest/APIReference/Welcome.html . It has details on API calls such as CreateReplicationGroup and such.

Darin Briskman
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