Controlling concurrency


#1

What do you think is the best way to manage concurrency in this (made
up) scenario?

  1. You are selling Orders for items of various ProductVariants,
    belonging to one Product
  2. There may be an upper limit to how many Orders you can take for any
    given Product
  3. For our example today, you can only accept three more orders for
    any combination of ProductVariants of a Product
  4. Two users put in three Orders, at the same time

It’s in the realm of millisecond possibility, then, for this to
happen:

Person 1: Validate that 3 Orders can be made for Product 123 => true
Person 2: Validate that 3 Orders can be made for Product 123 => true
Person 1: Orders.create(:product_variant => 9); Orders.create
(:product_variant => 8); Orders.create(:product_variant => 7)
Person 2: Orders.create(:product_variant => 6); Orders.create
(:product_variant => 5); Orders.create(:product_variant => 4)

Oops! 6 Orders placed!

What do you think is the best way to prevent overselling in this case?
I’ve considered:

  • Table locking (sucks)
  • Setting a lock file per-Product (filesystem-y and not elegantly
    scalable past one machine)
  • Setting a lock variable per-Product in a memcached store
  • A single-worker queue that validates and processes orders in
    sequence
  • A per Product single-worker queue that validates and processes
    orders in sequence
  • Something I haven’t considered?

What do you think?
Steve!


#2

On Apr 14, 10:55 pm, steveluscher removed_email_address@domain.invalid wrote:

orders in sequence

  • Something I haven’t considered?

It sort of depends where the information that only 3 more orders for
Product X lives.
For example if the products table has a stock_remaining column, then
for me the natural thing to do would be to use optimistic locking on
the products table
I wrote up some possibilities at
http://www.spacevatican.org/2008/6/8/dealing-with-concurrency

Fred


#3

On Tue, Apr 14, 2009 at 2:55 PM, steveluscher removed_email_address@domain.invalid
wrote:

  1. You are selling Orders for items of various ProductVariants,
    belonging to one Product
  2. There may be an upper limit to how many Orders you can take for any
    given Product

What do you think is the best way to prevent overselling in this case?

  • Something I haven’t considered?

My wife sells jewelry and many pieces are one-of-a-kind. I have an
inventory table with an entry for each piece, including location: web
site, gallery, etc.

If a customer puts that item in a cart, then the “location” is set to
the
cart id, and no one else can even see the item, much less purchase
it. If the cart times out, the item is returned to inventory and becomes
visible again.

Maybe not directly analogous to your situation, but something to think
about. :slight_smile:

HTH,

Hassan S. ------------------------ removed_email_address@domain.invalid


#4

If you positively cannot have overbooking (oops, airline talk)
over-selling,
then all your orders have to either lock a table (using the database to
enforce serialization), or they have to be serially examined in someway,
such as a single order FIFO queue. You could create an observer, for
example, that monitors the creation of an order, checks to see if stock
(tickets, seats, CD’s, whatever) is available; and gracefully back-out
an
order that “goes over.” Personally, I would be tempted to try a simple
solution, like a queue of some sort first, or perhaps an observer,
rather
than use the database. This is one of those cases where trying to
optimize
too early may hurt. A simple, single, global queue may be all you need.
YMMV.
Cheers–

Charles

On Wed, Apr 15, 2009 at 3:02 AM, Frederick C. <


#5

On Apr 15, 3:28 pm, Charles J. removed_email_address@domain.invalid wrote:

If you positively cannot have overbooking (oops, airline talk) over-selling,
then all your orders have to either lock a table (using the database to
enforce serialization)

I don’t think things have to be quite so drastic, eg a row level lock
is almost certainly enough. It does sort of depend on how the quantity
information is stored (but there is almost always a better way than a
table lock)

Fred

, or they have to be serially examined in someway,


#6

@Fred: The information that only n more products exist will come from
(@product.order_limit - @product.orders.count) >= n. It’s a bit more
complicated than that, because I plan to have optional per-
ProductVariant limits too, but let’s leave it there for now.

@Fred & @Hassan: I don’t really want to have “unsold” ProductVariants
hanging around in the database, because the availability of
ProductVariants is largely procedural. If the order_limit on the
Product as a whole was 20, the order_limit on ProductVariant A was 10,
and ProductVariant B had no order_limit, then I’d end up with a whole
lot of questions as to how many unsold ProductVariants to create of
each type, and which ones to destroy when others were sold. Bah, too
much mess!

@Charles: I like the idea of an OrderTaker global queue, but global
queue means worker process, and worker process means extra messy
infrastructure, right? I’m all for it, I’m just wondering if anyone
has a better mousetrap.

@Fred: Row-level locks are out of the question, since I’m not
interested in blocking updates to a given row, but the creation of
rows that don’t yet exist in the Orders table. That said, I found this
today – named locks in MySQL. It’s a database-specific solution, but
perhaps an easy one?

http://gist.github.com/95977

Named locks in MySQL:
http://dev.mysql.com/doc/refman/5.0/en/miscellaneous-functions.html#function_get-lock
Shopify’s locking implementation:
http://github.com/Shopify/locking/blob/835469ed48f4c2de95856fe2f221eaa624b267a2/lib/locking.rb


#7

On Apr 15, 8:36 pm, steveluscher removed_email_address@domain.invalid wrote:

@Fred: Row-level locks are out of the question, since I’m not
interested in blocking updates to a given row, but the creation of
rows that don’t yet exist in the Orders table. That said, I found this

Actually that’s not true (and mentioned in the blog post I linked).
You can lock the row corresponding to a given product, even if you are
not going to update that product, purely as a synchronization
mechanism (and in that sense it is similar to the named lock stuff).

Fred

today – named locks in MySQL. It’s a database-specific solution, but
perhaps an easy one?