LinkedHashMap behaviour with Redis Hashes?

Question

I want to use Hashes data structure in Redis (Jedis client) but also want to maintain the insertion order something like LinkedHashMap in Java. I am totally new to Redis and gone through all the data structures and commands but somehow not able to think of any straight forward solution. Any help or suggestions will be appreciated.

Answer 1

Hashes in Redis do not maintain insertion order. You can achieve the same effect by using a Sorted Set and a counter to keep track of order. Here is a simple example (in Ruby, sorry):

items = {foo: "bar", yin: "yang", some_key: "some_value"}
items.each do |key, value|
  count = redis.incr :my_hash_counter
  redis.hset :my_hash, key, value
  redis.zadd :my_hash_order, count, key
end

Retrieving the values in order would look something like this:

ordered_keys = redis.zrange :my_hash_order, 0, -1
ordered_hash = Hash[
  ordered_keys.map {|key| [key, redis.hget(:my_hash, key)] }
]
# => {"foo"=>"bar", "yin"=>"yang", "some_key"=>"some_value"}

Answer 2

No need to use Sorted Set either counter. Just use a https://redis.io/commands#list, because it keeps the insertion order.

HSET my_hash foo bar
RPUSH my_ordered_keys foo

HSET my_hash yin yang
RPUSH my_ordered_keys yin

HSET my_hash some_key some_value
RPUSH my_ordered_keys some_key

LRANGE my_ordered_keys 0 10
1) "foo"
2) "yin"
3) "some_key"