What is application concurrency? - In a multi-user system or application, concurrency is a major issue that the development team must address. Concurrency, in the context of a software application, is the handling of multiple users attempting to access the same data at the same time. |
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In a multi-user system or application, concurrency is a major issue that the development team must address. Concurrency, in the context of a software application, is the handling of multiple users attempting to access the same data at the same time.
Author: Venkatesh Pai
Date: Jul 29, 2009 - 10:59:09 AM
In a
multi-user system or application, concurrency is a major issue that the
development team must address. Concurrency, in the context of a software
application, is the handling of multiple users attempting to access the same
data at the same time.
For
example, consider a multi-user order processing system that allows users to add
and edit orders for customers. Adding orders is not a problem-since each new
order generates a discrete record, several users can simultaneously add orders
to the database without interfering with one another.
Editing
orders, on the other hand, can result in concurrency problems. When a user
opens an order to edit it, a dialog is displayed on the local machine with
information about that order. To accomplish this, the system retrieves the data
from the database and stores it temporarily in the local machine's memory, so
the user can see and change the data. Once changes are made, the data is sent
back to the server, and the database record for that order is updated. Now, if
two users simultaneously have the editing dialog open for the same record, they
both have copies of the data in their local machines' memory, and can make
changes to it. What happens if they both choose to save the data?
The answer
depends on how the application is designed to deal with concurrency. Managing
multi-user access to a shared resource is a challenge. Any resource that can be
accessed by more than one user requires software logic to protect that resource
by managing the way multiple users can access and change it at the same time.
This problem has only become more common since the advent of network file
sharing, relational databases, and client-server computing.
There are
several ways for a software application to deal with concurrency. Among these
are the following:
Pessimistic:
This concurrency model places
locks on data. If one user has a record open and any other users attempt to
read that data in a context that allows editing, the system denies the request.
In the preceding example, the first user to open the order for editing gets the
lock on the order record. Subsequent users attempting to open the order will be
sent a message advising that the order is currently being edited by another
user, and will have to wait until the first user saves the changes or cancels
the operation. This concurrency model is best in situations when it is highly
likely that more than one user will attempt to edit the same data at the same
time. The downside with this model is that others users are prevented from
accessing data that any one user has open, which makes the system less
convenient to use. There is also a certain amount of implementation complexity
when a system must manage record locks.
Optimistic:
In the optimistic concurrency
model, users are always allowed to read the data, and perhaps even to update
it. When the user attempts to save the data, however, the system checks to see
if the data has been updated by anyone else since the user first retrieved it.
If it has been changed, the update fails. This approach allows more users to
view data than does the pessimistic model, and is typically used when it is
unlikely that several users will attempt to edit the same data at the same
time. However, it is inconvenient when a user spends time updating a record
only to find that it cannot be saved. The record must be retrieved anew and the
changes made again.
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