Database Reference
In-Depth Information
In order to support returning inventory from timed-out carts, we'll need to create an index on
the
status
and
last_modified
properties. Since our query on
last_modified
is an inequal-
ity, we should place it last in the compound index:
>>>
db
.
cart
.
ensure_index
([(
'status'
,
1
), (
'last_modified'
,
1
)])
Error handling
The previous operations do not account for one possible failure situation. If an exception oc-
curs after updating the shopping cart but before updating the inventory collection, then we
have an inconsistent situation where there is inventory “trapped” in a shopping cart.
To account for this case, we'll need a periodic cleanup operation that finds inventory items
that have
carted
items and check to ensure that they exist in some user's active cart, and re-
turn them to available inventory if they do not. Our approach here is to visit each
product
with some
carted
entry older than a specified timestamp. Then, for each SKU found:
1. Load the
cart
that's possibly expired. If it's actually
"active"
, refresh the
carted
entry in the
product
.
2. If an
"active"
cart was not found to match the
carted
entry, then the
carted
is re-
moved and the available inventory is updated:
def
def
cleanup_inventory
(
timeout
):
now
=
datetime
.
utcnow
()
threshold
=
now
-
timedelta
(
seconds
=
timeout
)
# Find all the expiring carted items
for
for
item
iin
db
.
product
.
find
(
{
'carted.timestamp'
: {
'$lt'
:
threshold
}}):
# Find all the carted items that matched
carted
=
dict
(
(
carted_item
[
'cart_id'
],
carted_item
)
for
for
carted_item
iin
item
[
'carted'
]
iif
carted_item
[
'timestamp'
]
<
threshold
)
# First Pass: Find any carts that are active and refresh the carted
# items
for
for
cart
iin
db
.
cart
.
find
(
{
'_id'
: {
'$in'
:
carted
.
keys
() },
'status'
:
'active'
}):
