ColdFusion MX is a multithreaded application server. Handling
multiple execution threads simultaneously offers much higher performance than
handling one thread at a time. CFMX also allows developers to store data in
server memory. This lets us cache application-wide data easily, or handle
user-specific data like a shopping cart. Multithreaded execution and persistent
data are useful and powerful features. However, they also have implications
that must be carefully addressed by developers. In this article I will explain
why you need to properly lock reads and writes to shared variables in
ColdFusion MX. In addition, I’ll explore some implications of storing instances
of ColdFusion
(CFCs) in shared scopes.

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Why lock?

Most ColdFusion developers are aware of thread locking on
some level, so I won’t go into great detail explaining why you need to do it.
But a tried and true example is something like this: Say you have some code
that increments an application variable (for anyone who doesn’t know, an
application-scoped variable is a memory-resident variable that is shared
between all users and threads of a specific CF application):

<cfset application.counter = application.counter + 1 />

Seems simple enough, but what if two users run the same page
and the same time? The result can be corrupt data:

  • User A runs the page
  • User B runs the page
  • User A’s thread reaches the counter and reads application.counter which is 25.
  • User B’s thread reaches the counter and reads application.counter which is 25.
  • User A’s thread increments the counter it read
    (25) and sets it to 26.
  • User B’s thread increments the counter it read
    (25) and sets it to 26.

Clearly, the counter will end up being set to 26 (which is incorrect). To avoid
this situation, called a race condition (because two threads are
“racing” to see who gets to the data first), you must specify
single-thread access to this piece of code using the <cflock> tag:

<cflock name="applicationCounterIncrementLock" timeout="10"
      <cfset application.counter = application.counter + 1 />

This will force only one thread at a time to execute that
piece of code. So if User A’s thread reaches this code first, it establishes an
exclusive lock on the code until it has finished with it. If User B’s thread
gets here, it encounters the exclusive lock and will wait until that lock is
released before it establishes its own exclusive lock on the code and performs
its own increment of the application variable. The result is that User B’s
counter would be correctly set to 27.

There are two possible values for the “type”
attribute of the <cflock> tag: “readonly” and
“exclusive”. A read-only lock allows as many threads as necessary to
read the locked code, as long as there is not an exclusive lock on the same
code. Conversely, an exclusive lock takes complete ownership of a piece of
code, and will allow no other read-only or exclusive locks on the same code
until it releases its lock. Typically, read-only locks are used when data is
only being read from a shared scope. Exclusive locks are needed when data is
being modified in a shared scope, as in our counter example above.

Scoped locks and named locks

Locks can be scoped locks or named locks. A scoped lock means
that the lock affects an entire variable scope. It is particularly important to
understand this when using “exclusive” locks. For example, if you
create an exclusive session-scoped lock, this means the code is locked for all
threads from the same user. Scoped locks on the session scope are acceptable
because they are only single-threading access to the code for a single user.

You can also create scoped locks on the application and
server scopes, but this is highly inadvisable. Creating an
“application” scope lock means that all threads for the entire
application are affected by the lock. Even worse, a “server” scope
lock means that all threads for the entire server are affected. This can result
in massive bottlenecks as all threads wait for exclusive locks to be released.

For application and server-scoped variables, a named lock is
more appropriate. In the counter example above, you can see I used an
exclusive, named lock. This is much better than a scoped lock, because it only
single-threads access to other threads trying to access the same named lock.
Thus, we don’t single-thread access for an entire application or server, but
just for reads or writes to the section of code contained within the named
lock. So the basic rule of thumb is to use session-scoped locks for session
variables and named locks for application and server variables. However, use
locks on the application and server scopes with caution.

Shared scopes and CFCs

Most intermediate or advanced CF developers already
understand the previous information. But where many CF’ers seem to be uncertain
relates to shared scopes and instances of ColdFusion Components (CFC).

CFCs are object-like constructs, and they can be
instantiated into any of the shared variable scopes. This has opened a whole
new world to CF developers, but also raises new questions about where and when
to lock code. In particular, confusion seems to center around the difference
between storing a stateless CFC vs. a CFC which is stateful.

A stateless CFC is one which has no internal instance data,
or one which has instance data that is updated very rarely. In other words, its
internal state does not change once it is created. Within CFCs, the
“variables” scope represents instance data, so in practice, this
means that a stateless CFC does not update any internal data in the variables
scope. The “this” scope can also contain instance data, but because it is public it
is often avoided.

A stateful CFC, on the other hand, contains instance data
which changes. Its internal state is in flux, and data within its variables
scope is modified.

The difference between these is very important, as it
dictates the locking approach you must adopt.

Consider the following scenario involving a stateless CFC. A
developer is building an application that must display current company news. He
or she might wish to create a CFC to encapsulate data and behavior related to
news items. Further, to improve performance this CFC could be instantiated into
the application scope:

<cfset application.newsManager = 
createObject( 'component', 'newsManager' ).init() />

Now, instead of creating an instance of the newsManager component every time the
news data is needed, the developer can just call methods on the application
scoped CFC instance. For those familiar with OO design patterns, this is an
implementation of the Singleton pattern.

Locking it

Sounds great; but wait, don’t we have to fit locking in here
somewhere, since we’re using the application scope? Absolutely. However, with
care we can minimize the amount of locking necessary. Let’s think about this.

A single newsManager
instance will be shared between all users of the entire application (as defined
by the name used in the <cfapplication> tag). So it only needs to be
created once, the first time any user runs the application. Thus, we only need
one exclusive named lock on this code, and it must run on application startup:

<cfif not structKeyExists( application, 'newsManager' )>
    <cflock name="loadNewsManagerLock" timeout="10" type="exclusive">
            <cfif not structKeyExists( application, 'newsManager' )>
                  <cfset application.newsManager =
createObject( 'component', 'newsManager' ).init() />

Note that by design, application.newsManager
will only be created one time during the lifetime of an application. Once it is
created, its internal state will not change. This has an important consequence:
it means we don’t need to lock subsequent reads of application.newsManager. There is no possibility of a race
condition. This is because application.newsManager
is a stateless CFC that has no changing instance data—in fact it has no
instance data at all. (Note that with a framework like Mach-II,
the framework itself manages the creation of application-scoped listener

This minimizes the amount of locking we must do, but it also
means we must be careful to adhere to our design decision that newsManager is a stateless CFC. We must
ensure that no other developers accidentally recreate application.newsManager without proper locking. But more subtly, it
means that the application.newsManager
CFC should not contain internal instance data that is modified. Why? Since the newsManager CFC instance exists in the
application scope, all instance data it contains also exists in the application

That means no data inside the CFC should be written to the
“variables” scope (or the “this” scope). Because our newsManager CFC has no instance data, we
can safely avoid the need to lock method calls to this CFC instance. Take a
look at the newsManager code in
Listing A to see for yourself.

Example one

From Listing A, note that in the method getRecentNewsItems() I am careful to define my query variable
qRecentNewsItems” in the “var” scope. This tells CF
that qRecentNewsItems is a
method-local variable. It will only exist for the duration of the method call
and is not instance data.

I cannot stress enough the importance of properly using the var scope within CFC methods! If I had
not used the var scope for the query
variable, CF would default to placing it in the “variables” scope.
This would be instance data, and our CFC would then be exposed to potential
race conditions if multiple threads call it simultaneously. Use the var scope for all method-local variables
and avoid this subtle danger! Failure to do so will result in problems when the
application is placed under load.

Example two

In the newsManager
example we avoid the need for read-only locks on the application scope because
we have made the architectural decision that newsManager would be stateless. But what about a situation where we
know we will have changing data within a CFC that is held in a shared scope? It
means that either your calls to the CFC must be properly locked, or the CFC
itself must take responsibility for locking internally.

As I have said, data stored in the “this” scope
and the “variables” scope is instance data for that CFC. But most
people don’t think about the need to lock these scopes when a CFC is stored in
a shared scope. So let’s make it very obvious. I’ll skip over the
“this” and “variables” scopes for a moment and consider a
CFC that manipulates data stored in the session scope. Remember that a session
variable is a shared-scope variable that is specific to one user. A shopping
cart is an example of this.

We can create an application-scoped cartManager just as we did the newsManager:

<cfif not structKeyExists( application, 'cartManager' )>
      <cflock name="loadCartManagerLock" timeout="10" type="exclusive">
            <cfif not structKeyExists( application, 'cartManager' )>
                  <cfset application.cartManager =
createObject( 'component', 'cartManager' ).init() />

But from here, things are handled differently than our newsManager CFC. The cartManager works as a session façade
that manages interactions with a Cart CFC that we will hold in the session
scope. Remember that session variables are specific to each user. Because the cartManager manipulates a Cart CFC instance
in the session scope, the reads and writes to the Cart CFC should be properly
locked to avoid race conditions. Such a situation could arise if the user is
using the application in multiple browser windows. You can see the cartManager code in
Listing B,
and the Cart code is in
Listing C.

Note that the Cart CFC itself has no locks and doesn’t care
or even know what scope it is being used in. The cartManager takes on these responsibilities. This is a good design
practice that reduces coupling between your business object (the Cart) and the
rest of the application. To avoid any possible race conditions, the cartManager handles locks for reads and
writes to data in the session scope.

Hopefully it is clear that because our manager CFC is
manipulating a variable in the session scope, we should incorporate locking.
But remember that this is also true if a shared-scope CFC holds modifiable
instance data in the “this” or “variables” scopes. Remember
that if the CFC instance is kept in a shared scope, all of its instance data is
also in the shared scope!

A clearer picture

You can see code that runs and tests everything we’ve done
so far in
Listing D. I hope that this
article has helped you build a clearer picture of locking in ColdFusion MX. And
in particular, I hope it has shed some light on proper ways to deal with
locking within shared CFC instances. Please post to the forums or contact me if
you have any other ideas or questions. Thanks
to Sean Corfield for his input regarding this article.