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https://issues.apache.org/jira/browse/CASSANDRA-11592?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15245681#comment-15245681
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Nick Tan commented on CASSANDRA-11592:
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thanks [~slebresne]
yes, i'm using the 'simple int with LWT' approach for now, at least it fulfills
the requirement with putting aside the performance.
> Feature Request - Conditional update on counters
> ------------------------------------------------
>
> Key: CASSANDRA-11592
> URL: https://issues.apache.org/jira/browse/CASSANDRA-11592
> Project: Cassandra
> Issue Type: Bug
> Reporter: Nick Tan
> Priority: Minor
>
> hi folks
> what i'd like to do is have a atomic update on counters if it's value is less
> than some value.
> let's take a example, saying there is a `room`, the capacity is 50 peoples,
> now i'd like to track the number of peoples join the room. if the counter
> exceed the room capacity, i'd like to get error notified.
> table define:
> {code:sql}
> CREATE TABLE IF NOT EXISTS room (
> id uuid,
> name text,
> capacity int,
> PRIMARY KEY(id)
> )
> CREATE TABLE IF NOT EXISTS people_in_room (
> room_id uuid,
> num counter, -- the current number of people in the room
> PRIMARY KEY(room_id)
> );
> {code}
> pseudo update:
> {code}
> UPDATE people_in_room SET num=num+1 WHERE room_id = :room_id IF num <
> :room_capacity;
> {code}
> the expected result is, if current num exceeds the room capacity (aka. the IF
> clause is false), i'd like to get `Applied` column with value of 'false'
> returned immediately.
> in this way, i can perform the atomic update for counters.
> otherwise, i need to 1) read the current counter, 2) increase it and check if
> it exceeds the capacity in the application code. but, however, if there are
> concurrent requests come to the app at the same time, this way is broken.
> thanks
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