requirements.txt

1) GET:
    return value of key
    if key not exists then return "nil".
    if value is not string then raise exception.

2) SET:
    Set key to hold the string value. 
    If key already holds a value, it is overwritten, regardless of its type. 
    Any previous time to live associated with the key is discarded on successful SET operation.

    options:
    EX seconds -- Set the specified expire time, in seconds.
    PX milliseconds -- Set the specified expire time, in milliseconds.
    NX -- Only set the key if it does not already exist.
    XX -- Only set the key if it already exist.
    KEEPTTL -- Retain the time to live associated with the key.

    return:
    Simple string reply: OK if SET was executed correctly. 
    Null reply: a Null Bulk Reply is returned if the SET operation was not 
    performed because the user specified the NX or XX option but the condition was not met.

3) EXPIRE:
    Set a timeout on key. 
    After the timeout has expired, the key will automatically be deleted.
    return:
    1 if the timeout was set.
    0 if key does not exist.

4) ZADD:
    Adds all the specified members with the specified scores to the sorted set stored at key. 

    options:
    XX: Only update elements that already exist. Never add elements.
    NX: Don't update already existing elements. Always add new elements.
    CH: Modify the return value from the number of new elements added, to the total number of elements changed 
        (CH is an abbreviation of changed). Changed elements are new elements added and elements already existing 
        for which the score was updated. So elements specified in the command line having the same score as they 
        had in the past are not counted. Note: normally the return value of ZADD only counts the number of new 
        elements added.
    INCR: When this option is specified ZADD acts like ZINCRBY. Only one score-element pair can be specified in 
        this mode.

    return:
        integer reply, specifically:

        The number of elements added to the sorted set, not including elements already existing for which the score 
        was updated.
        If the INCR option is specified, the return value will be Bulk string reply:

        The new score of member (a double precision floating point number) represented as string, or nil 
        if the operation was aborted (when called with either the XX or the NX option).

5) ZRANK:
    Returns the rank of member in the sorted set stored at key (0 based index)

    return
        If member exists in the sorted set, Integer reply: the rank of member.
        If member does not exist in the sorted set or key does not exist, Bulk string reply: nil.

6) ZRANGE:
    Returns the specified range of elements in the sorted set stored at key. (0 based index)
    need to handle negative value of range.

    return:
        Array reply: list of elements in the specified range (optionally with their scores, 
        in case the WITHSCORES option is given).