Question 121208 in Redis Question posted in
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How to add a running to Redis?

From several servers I would like to add keys like doc:1 and then doc:2 etc.

Now in order that the different servers do not try to add to, say, doc:2 at the same time, I thought there is some mechanism like "add to doc:{index}" and the index gets automatically bumped.

Is there some way to make that work?

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2

Answers


  1. 0

    You could use a distributed lock and have the winner write

    for each server, attempt to acquire lock until success
on acquire lock
  read numeric value
  increment
  write
  release lock

    docs

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  2. 0

    You could use a key which you increment on every addition: the INCR command does just that, atomically, and returns the incremented value which you can use to generate the new key name.

    > INCR counter
42

> SET doc:42 foobar
OK

    Update: in the event you want the whole set of operations (incrementing the counter and setting the value) to be atomic, you can execute a Lua script instead:

    > EVAL "local value = redis.call('INCR', 'counter'); redis.call('SET', 'doc:'..value, ARGV[1])" 0 foobar
(nil)

    Explanation:

    local value = redis.call('INCR', 'counter');

    Execute INCR counter and store the result in the value variable.

    redis.call('SET', 'doc:'..value, ARGV[1]);

    Execute SET against the key named after the concatenation of doc: and the value variable using the data passed through the first EVAL argument – in our case foobar.

    Finally, should you need to return the incremented counter value to the caller just append a final return value; block to the script.

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