LUA Scripting help

[Kevin Burton]

I have a script:

local count = 0

for k,v in ipairs(KEYS) do

   if redis.call('exists', v) then

       count = count + 1

   end

end

return count

When I run this script it always returns 20 (in my case that is the number if keys that were passed in). But when I run KEYS <pattern> I get an empty set. I seems that my LUA script is not detecting existence corrently. Ideas?

[Itamar Haber]

What <pattern> are you calling KEYS with? Use `KEYS *` to get all keys in a non-production environment.

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[Jan-Erik Rediger]

redis.call('exists', v) returns 0 if the key does not exist and 1 if it
does.
So you need to test if it returns 1 (0 is not a falsy value in Lua)

[Kevin Burton]

That is the point KEYS * returns an empty list so redis-cli connecting to my server says there are no keys but the LUA script seems to indicate there are.

[Itamar Haber]

Thanks for putting it differently - now I see the problem.

TL;DR - redis.call('exists') returns 0 or 1, depending on the key's existance. However, 'if 0' in Lua is true, i.e.:

foo@bar:~$ redis-cli eval 'if 0 then return 1 end' 0

(integer) 1

To solve do 'if redis.call('exists', v) == 1 then`.

Longer version: I changed the script slightly and used my very own redis-lua-debugger (https://github.com/redislabs/redis-lua-debugger) to verify that behavior. Below is the modified script and the debugger's output - note how e is set to 0 and doesn't change until the end:

foo@bar:~$ cat t.lua 

rld.start()

local count = 0

local e

for k,v in ipairs(KEYS) do

   e = redis.call('exists', v)

   if e then 

       count = count + 1

   end

end

return count

foo@bar:~$ redis-cli flushall

OK

foo@bar:~$ redis-cli --eval t.lua foo bar baz qaz

(integer) 4

foo@bar:~$ cat /var/log/redis_6379

...

[970] 01 Mar 17:34:38.564 # [rld] -- rld v0.1.1 started

[970] 01 Mar 17:34:38.564 # [rld] KEYS[1] = 'foo'

[970] 01 Mar 17:34:38.564 # [rld] KEYS[2] = 'bar'

[970] 01 Mar 17:34:38.564 # [rld] KEYS[3] = 'baz'

[970] 01 Mar 17:34:38.564 # [rld] KEYS[4] = 'qaz'

[970] 01 Mar 17:34:38.564 # [rld] at Lua:@user_script (line: 2)

[970] 01 Mar 17:34:38.565 # [rld] at Lua:@user_script (line: 3)

[970] 01 Mar 17:34:38.565 # [rld] new [count] = 0

[970] 01 Mar 17:34:38.565 # [rld] at Lua:@user_script (line: 5)

[970] 01 Mar 17:34:38.565 # [rld] call to function C:ipairs

[970] 01 Mar 17:34:38.565 # [rld] return from function C:ipairs

[970] 01 Mar 17:34:38.565 # [rld] call to function C:(for generator)

[970] 01 Mar 17:34:38.565 # [rld] return from function C:(for generator)

[970] 01 Mar 17:34:38.565 # [rld] at Lua:@user_script (line: 6)

[970] 01 Mar 17:34:38.565 # [rld] new [k] = 1

[970] 01 Mar 17:34:38.565 # [rld] new [v] = 'foo'

[970] 01 Mar 17:34:38.565 # [rld] call to function C:call

[970] 01 Mar 17:34:38.565 # [rld] return from function C:call

[970] 01 Mar 17:34:38.566 # [rld] at Lua:@user_script (line: 7)

[970] 01 Mar 17:34:38.566 # [rld] new [e] = 0

[970] 01 Mar 17:34:38.566 # [rld] at Lua:@user_script (line: 8)

[970] 01 Mar 17:34:38.566 # [rld] at Lua:@user_script (line: 5)

[970] 01 Mar 17:34:38.566 # [rld] changed [count] = 1 (was 0)

[970] 01 Mar 17:34:38.566 # [rld] call to function C:(for generator)

[970] 01 Mar 17:34:38.566 # [rld] return from function C:(for generator)

[970] 01 Mar 17:34:38.566 # [rld] at Lua:@user_script (line: 6)

[970] 01 Mar 17:34:38.566 # [rld] changed [k] = 2 (was 1)

[970] 01 Mar 17:34:38.566 # [rld] changed [v] = 'bar' (was foo)

[970] 01 Mar 17:34:38.566 # [rld] call to function C:call

[970] 01 Mar 17:34:38.567 # [rld] return from function C:call

[970] 01 Mar 17:34:38.567 # [rld] at Lua:@user_script (line: 7)

[970] 01 Mar 17:34:38.567 # [rld] at Lua:@user_script (line: 8)

[970] 01 Mar 17:34:38.567 # [rld] at Lua:@user_script (line: 5)

[970] 01 Mar 17:34:38.567 # [rld] changed [count] = 2 (was 1)

[970] 01 Mar 17:34:38.567 # [rld] call to function C:(for generator)

[970] 01 Mar 17:34:38.567 # [rld] return from function C:(for generator)

[970] 01 Mar 17:34:38.567 # [rld] at Lua:@user_script (line: 6)

[970] 01 Mar 17:34:38.567 # [rld] changed [k] = 3 (was 2)

[970] 01 Mar 17:34:38.567 # [rld] changed [v] = 'baz' (was bar)

[970] 01 Mar 17:34:38.567 # [rld] call to function C:call

[970] 01 Mar 17:34:38.568 # [rld] return from function C:call

[970] 01 Mar 17:34:38.568 # [rld] at Lua:@user_script (line: 7)

[970] 01 Mar 17:34:38.568 # [rld] at Lua:@user_script (line: 8)

[970] 01 Mar 17:34:38.569 # [rld] at Lua:@user_script (line: 5)

[970] 01 Mar 17:34:38.569 # [rld] changed [count] = 3 (was 2)

[970] 01 Mar 17:34:38.569 # [rld] call to function C:(for generator)

[970] 01 Mar 17:34:38.569 # [rld] return from function C:(for generator)

[970] 01 Mar 17:34:38.569 # [rld] at Lua:@user_script (line: 6)

[970] 01 Mar 17:34:38.569 # [rld] changed [k] = 4 (was 3)

[970] 01 Mar 17:34:38.569 # [rld] changed [v] = 'qaz' (was baz)

[970] 01 Mar 17:34:38.569 # [rld] call to function C:call

[970] 01 Mar 17:34:38.569 # [rld] return from function C:call

[970] 01 Mar 17:34:38.569 # [rld] at Lua:@user_script (line: 7)

[970] 01 Mar 17:34:38.569 # [rld] at Lua:@user_script (line: 8)

[970] 01 Mar 17:34:38.569 # [rld] at Lua:@user_script (line: 5)

[970] 01 Mar 17:34:38.570 # [rld] changed [count] = 4 (was 3)

[970] 01 Mar 17:34:38.570 # [rld] call to function C:(for generator)

[970] 01 Mar 17:34:38.570 # [rld] return from function C:(for generator)

[970] 01 Mar 17:34:38.570 # [rld] at Lua:@user_script (line: 11)

[970] 01 Mar 17:34:38.570 # [rld] return from function Lua:@user_script

[970] 01 Mar 17:34:38.570 # [rld] -- profiler summary:

[970] 01 Mar 17:34:38.570 # [rld] --   C:(for generator) called x5 times

[970] 01 Mar 17:34:38.570 # [rld] --   C:ipairs called x1 times

[970] 01 Mar 17:34:38.570 # [rld] --   C:call called x4 times

[970] 01 Mar 17:34:38.570 # [rld] -- rld exited: end of script.

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Itamar Haber | Chief Developers Advocate
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