Ecommerce - MongoDB Applied Design Patterns - page 110

The actual code, then, looks something like the following:

def def expire_carts ( timeout ):

now = datetime . utcnow ()
threshold = now - timedelta ( seconds = timeout )

# Lock and find all the expiring carts
db . cart . update (
{ 'status' : 'active' , 'last_modified' : { '$lt' : threshold } },
{ '$set' : { 'status' : 'expiring' } },
multi = True )

# Actually expire each cart
for for cart iin db . cart . find ({ 'status' : 'expiring' }):

# Return all line items to inventory
for for item iin cart [ 'items' ]:

db . product . update (
{ '_id' : item [ 'sku' ],
'carted.cart_id' : cart [ 'id' ] },
{ '$inc' : { 'qty' : item [ 'qty' ] },
'$pull' : { 'carted' : { 'cart_id' : cart [ 'id' ] } } }) <
db . cart . update (
{ '_id' : cart [ 'id' ] },
{ '$set' : { status ': ' expired ' })

We're using multi=True to “batch up” our cart expiration initial lock.

Un fortunately, we need to handle expiring the carts individually, so this function
can actually be somewhat time-consuming. Note, however, that the expire_carts
function is safely resumable, since we have effectively “locked” the carts needing
expiration by placing them in "expiring" status.

He re, we update the inventory, but only if it still has a carted entry for the cart
we're expiring. Note that without the carted property, our function would become
unsafe to retry if an exception occurred since the inventory could be incremented
multiple times for a single cart.

Fi nally, we fully expire the cart. We could also delete it here if we don't wish to
keep it around any more.