As I said in my last post, scoping all of your variables can be a huge performance improvement for your dated Legacy Code. By explicitly scoping each and every variable reference, within your application, you eliminate the overhead of the system having to ScopeCheck each reference to determine it's scope. Understanding usage of each scope is an important part of this process. In this post, we'll discuss the persistent scopes of an application.

- This scope is valuable in environments where you have a dedicated server for your applications. Use carefully, and sparingly, this is a good place to put crafted objects and variables that can be shared across multiple applications in a shared server. Multi-Site One Codebase setups can get significant use of the SERVER scope, for housing things like utility objects that can be used by each application, without duplicating them in each app. The downside is, any change to a SERVER scoped object or variable will require a server restart to take effect. You can set these variables in onServerStart() method of the Server.cfc.
- This scope is valuable for creating Application wide objects and variables, that can be used regardless of individual user sessions. This is typically a good place for utility objects (if they aren't in the SERVER scope), asset pathing references, logging objects, and more. Typically you initialize these variables in the onApplicationStart() method of your Application.cfc, and only change these variables when making application wide changes, realizing that those changes will affect all current users.
- This scope is used for storing variables only used by a single user's browsing session. This is a good place for storing things like a user object, and shopping carts. Things that are only used for and by a single user, within their current visit to your application. You typically create these variables in the onSessionStart() method of your application's Application.cfc, and you can manipulate SESSION variables during a request, remembering that those changes will affect the remainder of the user's session.
- This scope is used for storing variables only used within the current page request. While a REQUEST variable is available to every template called during the course of a request, it is not best practice to directly access REQUEST variables inside of cfc's or custom tags, but rather to pass them in as a method argument or tag attribute, respectively, in order to maintain encapsulation. These variables are great for storing page specific details for use throughout the request, such as setting a page title, and using it at request end to log the page viewed by the user. Requests typically come in three parts: onRequestStart(), which occurs before the templates execute, onRequestEnd(), which occurs after the requested template executes, and either onRequest() or onRequestCFC(), which allow for some additional pre-template processing, but segment it according to what is being requested. To understand these last two a little further, onRequest() might be used to set a variable to tell the system to log the request on completion, whereas onRequestCFC might set the same variable to tell the system to not log the request (because you don't need to log direct CFC execution, maybe). These last two can be powerful, and confusing, so you might have to play with them a bit, if you want to use them at all. You use any, or all, of those methods within your application's Application.cfc.

SERVER, APPLICATION, SESSION, and REQUEST are ColdFusion's persistent scopes, allowing for the creation and use of variables that can be used across very defined measures of time. In our next post, we'll talk about how you should use the other scopes in CF.

This article is the tenth in a series of articles on bringing life back to your legacy ColdFusion applications. Follow along in the Legacy Code category.