Redis: How do I count the elements in a stream in a certain range?

Question

Bussiness Objective

I'm creating a dashboard that will depend on some time-series and I'll use Redis to implement it. I'm new to using Redis and I'm trying to use Redis-Streams to count the elements in a stream.

XADD conversation:9:chat_messages * id 2583 user_type Bot
XADD conversation:9:chat_messages * id 732016 user_type User
XADD conversation:9:chat_messages * id 732017 user_type Staff
XRANGE conversation:9:chat_messages - +

I'm aware that I can get the total count of the elements using the XLEN command like this:

XLEN conversation:9:chat_messages

but I want to also know the elements in a period, for example:

XLEN conversation:9:chat_messages 1579551316273 1579551321872

I know I can use LUA to count those elements but I want some REALLY fast way to achieve this and I know that using Redis markup will be the fastest way.

Is there any way to achieve this with a straight forward Redis command? Or do I have to write a Lua script to do this?

Additional information

I'm limited by AWS' ElastiCache to use the only Redis 5.0.6, I cannot install other modules such as the RedisTimeSeries module. I'd like to use that module but it's not possible at the moment.

Answer 1

While the Redis Stream data structure doesn't support this, you can use a Sorted Set alongside it for keeping track of message ranges.

Basically, for each message ID you get from XADD - e.g. "1579551316273-0" - you need to do a ZADD conversation:9:ids 0 1579551316273-0. Then, you can use ZLEXCOUNT to get the "length" of a range.