Re: redis scripting - iterating through arguments

[nghia pham]

It turns out that this is not possible if redis.call not support varargs/

Vào 02:37:39 UTC+7 Thứ ba, ngày 19 tháng sáu năm 2012, nghia pham đã viết:

Hi,

Is there anyway to iterate through the arguments (ARGV) pass to redis ? My case is to use it with some redis command that support multiple argument like SADD

Thanks.

[Dvir Volk]

this might be really useful though.

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[Chris Love]

If you do a type(ARGV) it is a table

This is a simple script to show how to iterate through ARGV

local s = {}

for i=1, #ARGV do // #ARGV is the size of the table

  s[i] = 'Processed' .. i  .. ' ' .. ARGV[i]

end

return cjson.encode(s)

You can test it by doing:

redis-cli --eval test_lua.lau key1 key2 ,  first second

Then do 

redis-cli --eval test_lua.lau key1 key2 ,  first second third fourth

Thanks

Chris

[Daniele Alessandri]

This is more of a trick but Lua's unpack() function, albeit with certain restrictions, can be useful with variadic commands. You can see http://pgl.yoyo.org/luai/i/unpack for reference, but here is a plain stupid example of a Lua script for Redis:

redis.call('sadd', KEYS[1], unpack(ARGV));

That said, unpack() actually has a limit on the size of the table which is defined by LUAI_MAXCSTACK in luaconf.h forcing a maximum number of Lua stack slots that a C function can use. This limit is set to 8000 by default, meaning that your ARGV table can contain up to 8000 elements (it's more than reasonable in this context).

 

--
Daniele Alessandri
http://clorophilla.net/
http://twitter.com/JoL1hAHN

[Chris Love]

Since we are talking stupid redis lua tricks.  We pass in a lot of our data in as JSON, and then decode it and handle it inside of redis.  Since cjson is part of redis, it makes it efficient and easy!

Good catch on the unpack :)

Chris

[nghia pham]

unpack solved my issue.

Thank you!