I have some scripts that touch a handful of keys. What happens if Elasticache decides to reshard while my script is running? Will it wait for my script to complete before it moves the underlying keys? Or should I assume that it is not the case and design my application with this edge case in mind?
One example would be a script that increment 2 keys at once. I could receive a "cluster error" which means something went wrong and I have to execute my script again (and potentially end up with one key being incremented twice and the other once)
Assuming you are talking about a Lua script, for as long as you're passing the keys in the arguments (and not hardcoded in the script) you should be good. It will be all or nothing. If you are not using a Lua script - consider doing so
From EVAL command:
All Redis commands must be analyzed before execution to determine which keys the command will operate on. In order for this to be true for EVAL, keys must be passed explicitly. This is useful in many ways, but especially to make sure Redis Cluster can forward your request to the appropriate cluster node.
From AWS ElastiCache - Best Practices: Online Cluster Resizing:
During resharding, we recommend the following:
Avoid expensive commands – Avoid running any computationally and I/O intensive operations, such as the KEYS and SMEMBERS commands. We suggest this approach because these operations increase the load on the cluster and have an impact on the performance of the cluster. Instead, use the SCAN and SSCAN commands.
Follow Lua best practices – Avoid long running Lua scripts, and always declare keys used in Lua scripts up front. We recommend this approach to determine that the Lua script is not using cross slot commands. Ensure that the keys used in Lua scripts belong to the same slot.