CODEWITHSUNDEEP
luaredis
geoyws
geoyws

Reputation: 3697

What's the recommended way to iterate SCANs in Lua for Redis?

I've been looking around for the recommended way to iterate over SCANs using Lua and was wondering if there was a "best practice" so to speak for it. The script below was the best I could find so far. Does anyone have anything to add? Is it wise to iterate using COUNT 1000000000? Wouldn't that be blocking?

local ans, has, cursor = {}, {}, "0";
repeat
    local t = redis.call("SCAN", cursor, "MATCH", KEYS[1], "COUNT", 1000000000);
    local list = t[2];
    for i = 1, #list do
        local s = list[i];
        if has[s] == nil then has[s] = 1; ans[#ans + 1] = s; end;
    end;
    cursor = t[1];
until cursor == "0";
return #ans; --or return ans;

Taken from: https://github.com/antirez/redis/issues/3190#issuecomment-214022437

Upvotes: 3

Views: 2628

Answers (1)

Itamar Haber
Itamar Haber

Reputation: 50112

The script will block while it is running and until it ends - in your example that will happen once SCANning is complete. The bigger the COUNT hint to the SCAN command, the more results each call to it will return, resulting in more RAM needed for storing the reply (the variable t). This could lead to an out of memory condition, so you should refrain from using such large COUNT values. I recommend sticking with the default (100), and just omitting the COUNT hint.

Upvotes: 1

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