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Redis Predicate Matching

2021-05-13 by Quinlan Pfiffersoftwareredis

Okay, I just found something pretty cool and I didn’t see a good tutorial example anywhere, so maybe someone somewhere will find this useful later. Redis has some really cool built-in Lua stuff that is useful if you want to do more with your data than just GET and SET it.

My specific problem is that we have some data in a Postgres DB, and some in a Redis instance. There is a reason for this I won’t get into, but basically the Redis instance contains real-time information polled every 60 seconds or so, and the user-facing component of our infrastructure needs to query this data in certain circumstances. We have a bunch of hashmaps in Redis more or less like the following:

localhost:16379> HGETALL x.12345.properties
 1) "property_a"
 2) "true"
 3) "property_b"
 4) "True"
 5) "property_c"
 6) "false"
...

I wanted to get all of the hashes matching the key x.*.properties for which one of the properties (let’s use property_a here) is “true”. Enter Lua! I love Lua, it’s quirky and small and great. Heres how we could use it in our specific case:

local cursor = 0
local keys = {}
repeat
  local result = redis.call('SCAN', cursor, 'MATCH', 'x.*.properties', 'COUNT', ARGV[1])
  local keys = result[2]
  for _, key in ipairs(keys) do
    local hash_result = redis.call('HGET', key, ARGV[2])
    if hash_result == 'true' then
      table.insert(keys, key)
    end
  end
  cursor = tonumber(result[1])
  until cursor == 0
return keys

You could then call this script with EVAL like so:

EVAL "..." 0 1000 property_a

The 0 is important, so don’t forget it. You can see the remaining 1000 and property_a arguments are used by the script. The script itself will return all keys in Redis matching x.*.properties for which property_a is true. It’s fast, non-blocking (Uses SCAN) and pretty easy to grok. Neat!