Redis Lua scripts - Amazon ElastiCache for Redis
Services or capabilities described in Amazon Web Services documentation might vary by Region. To see the differences applicable to the China Regions, see Getting Started with Amazon Web Services in China (PDF).

Redis Lua scripts

Redis supports more than 200 commands, including those to run Lua scripts. However, when it comes to Lua scripts, there are several pitfalls that can affect memory and availability of Redis.

Unparameterized Lua scripts

Each Lua script is cached on the Redis server before it runs. Unparameterized Lua scripts are unique, which can lead to the Redis server storing a large number of Lua scripts and consuming more memory. To mitigate this, ensure that all Lua scripts are parameterized and regularly perform SCRIPT FLUSH to clean up cached Lua scripts if needed.

The following example shows how to use parameterized scripts. First, we have an example of an unparameterized approach that results in three different cached Lua scripts and is not recommended:

eval "return redis.call('set','key1','1')" 0
eval "return redis.call('set','key2','2')" 0
eval "return redis.call('set','key3','3')" 0

Instead, use the following pattern to create a single script that can accept passed parameters:

eval "return redis.call('set',KEYS[1],ARGV[1])" 1 key1 1 
eval "return redis.call('set',KEYS[1],ARGV[1])" 1 key2 2 
eval "return redis.call('set',KEYS[1],ARGV[1])" 1 key3 3

Long-running Lua scripts

Lua scripts can run multiple commands atomically, so it can take longer to complete than a regular Redis command. If the Lua script only runs read-only operations, you can stop it in the middle. However, as soon as the Lua script performs a write operation, it becomes unkillable and must run to completion. A long-running Lua script that is mutating can cause the Redis server to be unresponsive for a long time. To mitigate this issue, avoid long-running Lua scripts and test the script out in a pre-production environment.

Lua script with stealth writes

There are a few ways a Lua script can continue to write new data into Redis even when Redis is over maxmemory:

  • The script starts when the Redis server is below maxmemory, and contains multiple write operations inside

  • The script's first write command isn't consuming memory (such as DEL), followed by more write operations that consume memory

  • You can mitigate this problem by configuring a proper eviction policy in Redis server other than noeviction. This allows Redis to evict items and free up memory in between Lua scripts.