Every command blocks every other command in Redis, because Redis is single threaded. However the implementation of scripting in Redis does a good use of Lua, so a command implemented in Lua does not run too much slower than a command implemented in C, for simple tasks (since we use a single Lua interpreter, and cache compiled functions). So this is an example of what you can do with Lua scripting:

    redis 127.0.0.1:6379> set foo '{"b":20,"a":10}'
    OK

The user wants to increment the value of "b" with a single query to Redis in an atomic way:

    $ cat /tmp/script.lua
    obj = cjson.decode(redis.call("GET",KEYS[1]))
    obj.b = obj.b + 1
    redis.call("SET",KEYS[1],cjson.encode(obj))
    return obj.b
    $ redis-cli --eval /tmp/script.lua foo
    (integer) 21
    $ redis-cli --eval /tmp/script.lua foo
    (integer) 22
    $ redis-cli GET foo
    "{\"a\":10,\"b\":22}"

No back and forth from client to server, no locking needed. Basically you can implement your specialized, atomic commands, in Lua.

Wavii is using LUA in Redis to implement extremely simple single object atomic consistency. Some but not all of the Redis data structure types support "do XYZ only if the key exists". For example lists support LPUSHX but there is no "X" equivalent for sorted sets. With LUA I can add the ZADDX command in about 4 lines of LUA and 4 lines of Ruby in my Redis driver. We have lots of sorted sets backed by the database. Suppose we add an item to the set. If the set is in Redis we want to do the O(logN) ZADD, if it is not in Redis we need to do an O(N) reload from the DB. The use of ZADDX + transactions in our DB lets multiple writers contend on adding an item to a sorted set and guarantee that at the end all the items are in the sorted set while doing O(logN) operations almost all the time.