Can redis Lua script contain key determined at runtime?

Question

Look at this lua script:

        local clientIds = redis.call('ZRANGEBYSCORE', KEYS[1], '-inf', ARGV[1], 'LIMIT', '0', ARGV[2]);
        local prefix = 'lock:';
        local lockedClientIds = {};
        for _, value in ipairs(clientIds)
        do
           lockal key = prefix .. tostring(value)
           if redis.call('EXISTS', key) == 0 then
              redis.call('SET', key, 'PX', ARGV[3]);
              table.insert(lockedClientIds, value)
           end
        end
        redis.pcall('ZREM', KEYS[1], unpack(lockedClientIds));
        return lockedClientIds;

It takes some values from the sorted set and uses them to create keys (after some simple concatenation). I'm not sure if this is OK because according to Redis Lua tutorials, all keys should be provided in the KEYS array so should be known in compile-time, not the runtime.

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. Note this rule is not enforced in order to provide the user with opportunities to abuse the Redis single instance configuration, at the cost of writing scripts not compatible with Redis Cluster.

So does it mean, there is a risk that this will only work with a single node and when redis is distributed across many nodes it won't work?

sazzad sazzad · Accepted Answer · 2021-06-14T16:19:50

YES, it is (highly) possible that the script will not work in cluster mode.

It will continue to work even in cluster mode only if the keys are in same hash slot. The idea of hash tags can be used for this purpose.

Note: I'm assuming, by "redis is distributed across many nodes", you are meaning Redis Cluster mode.

Can redis Lua script contain key determined at runtime?

1 Answers