Why there is no ordered hashmap in Redis? - redis

Question

Redis Data types includes sorted set and other necessary data-structures for key-value storage. But I wonder why it doesn't have any sorted map like Java's TreeMap or C++'s std::map. I think the underlying data-structure would be mostly similar of sorted set as both are supposed to be balanced binary search tree.
There must be some use-cases where we have to store key-value pair in specific order according to key. But current sorted set only serves the purpose of storing key according to score.

Answer 1

There must be some use-cases where we have to store key-value pair in specific order according to key
Since Redis keys are binary strings, I assume that the specific order you mentioned, is lexicographical order (specifically, keys are compared with the memcmp function). In that case, you can easily implement a C++'s std::map with SORTED SET. You can achieve this with 2 steps:
Build a std::set with Redis' Sorted Set
If 2 elements in a SORTED SET have the same score, they're ordered in lexicographical order. So in order to build a std::set, just give all members in a SORTED SET with the same score:
zadd std::set 0 c
zadd std::set 0 a
zadd std::set 0 b
// since all these members have the same score,
// the result is lexicographical ordered:
// a b c
zrange std::set 0 -1
// the following command will fail, since 'c' already exists.
zadd std::set 0 c
Since Redis 2.8, it supports some commands to operate on the lexicographical ranges, so that you can build something similar to std::set::lower_bound, or std::set::upper_bound
// something similar to lower_bound: find all members not less than b
zrangebylex std::set [b +
// something similar to upper_bound: find all members greater than b
zrangebylex std::set (b +
Map each key in the set with a value
Since you already get a std::set, then map the key with a value, you can get a std::map.
set a value_a
set b value_b
set c value_c
Combine these 2 steps together
You can wrap the whole work into a lua script to have a built-in std::map. And use it like this:
redis-cli --eval map.lua map_name , key value