CODEWITHSUNDEEP
ruby-on-railsactiverecordtransactionslocking
m33lky
m33lky

Reputation: 7275

reserve a row for a specific time using ActiveRecord

I'd like to be able to "reserve" an element similar to how an airplane seat is locked for a short period of time before it's actually paid for. I think the best way is to do it through the database and preferably at the ORM layer.

Here's an example:

ActiveRecord::Base.transaction do
  bar = Bar.find(1, :lock => true)
  # do my stuff
end

I need a more flexible solution though.

Here's how I am imagining it to work conceptually:

# action1: 
# put an expiring lock (30s) on an element (don't block unrelated code) 
# other code

# action2 (after payment):
# come back to the locked element to claim ownership of it

UPDATE: Anyone trying to do this in Rails should try using built-in optimistic locking functionality first.

Upvotes: 0

Views: 136

Answers (2)

m33lky
m33lky

Reputation: 7275

I could have a separate table specifically for this purpose called potential_owner. It would have a timestamp, so that one can figure out the timing. Basically it would work something like that:

# lock the table
# check latest record to see if the element is available
# add a new record or die

This is pretty simple to implement, however locking is not fine-grained. The table describes potential ownership of different elements, and a simple check locks down the whole table. In Tass's solution only the row for a particular element is locked.

Upvotes: 0

Reactormonk
Reactormonk

Reputation: 21700

Add an additional column locked_until - but beware of concurrency. I'd probably do that down on the db layer.

Upvotes: 2

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