- The History of COM, MTS/COM+, VB, and SQL Server
- The New World of .NET
- COM+ 1.5 Component Services
- Visual Basic .NET
- C#
- SQL Server 2000
- What's Next
- References
This chapter is from the book
This chapter is from the book
This chapter is from the book
COM+ 1.5 Component Services
Windows XP and Windows .NET Server will contain COM+ 1.5 and with it a lot of new functionality, including:
Process recycling
Configurable isolation level
Applications as Windows services
Memory gates
Pause/disable applications
Process dumping
Moving and copying components
Public/private components
Application partitions
COM+ applications exposed as XML Web Services
We will discuss each of these features in the rest of this section.
Process Recycling
A simple way of escaping the problems with memory leakage is to restart the server process now and then. With process recycling, you can declaratively tell how and when this should happen—for example, after a certain amount of requests or every night.
NOTE
I once wrote a Web application that had a memory leak. The customer thought it was a cheap solution for them to just reboot the machine every weekend because they had people working then anyway. I preferred to try to solve the problem instead of using a Band-Aid, but they didn't see this as a problem at all. Of course, process recycling would have been more convenient for them.
Configuration Isolation Level
In COM+ 1.0, the transaction isolation level was always SERIALIZABLE for COM+ transactions against SQL Server. (Oracle gets the same request to set the isolation level to SERIALIZABLE, but neglects this and uses READ COMMITTED instead. This is because Oracle uses versioning instead of locking for concurrency control, and then READ COMMITTED is used most often enough.)3 In the past, this has been a major constraint to keep in mind when designing for COM+, but in COM+ 1.5, it will be possible to configure the needed isolation level.
Applications as Windows Services
Some of the component services, such as queued components, need a started COM+ application to work. In COM+ 1.5 it is possible to run COM+ applications as Windows services. This also comes in handy if you like to run the application as the local system account, to make it possible to act with the privileges from that user. This is also a solution to the short delay that might otherwise be apparent when the first request is made to a COM+ server application after system boot (or after the application has been shut down).
Memory Gates
The Memory Gates feature prevents the creation of objects when the amount of free memory falls below a certain level. The reason for this functionality is to avoid adding objects when there's not enough memory to support their operations. Strange errors occur when a system runs out of memory, and it is hard to trap for errors in components in this environment. In fact, most components aren't even tested for low memory conditions, so it is better that they don't get called at all if the system has run out of memory.
Pause/Disable Applications
In COM+ 1.0, you can't pause a COM+ application. Instead, you have to delete the component if, for any reason, you don't want anybody to use it for a certain amount of time. In COM+ 1.5, you can both pause and disable applications.
Process Dumping
To make debugging easier in COM+ 1.5, it will be possible to take a snapshot of the process without having to stop the process. This snapshot can then be used with a debugger to examine the exact state when the problem occurred.
Moving and Copying Components
Often, it is useful to be able to let the same component appear several times, in several applications. That is especially the case for helper components. Assume that one helper component is used by five different COM+ applications. In COM+ 1.0, all five applications have to be upgraded at the same time to use the new version of the helper. I generally prefer to upgrade step-by-step instead. Therefore, it's common that helpers are reused in source-code form instead of as binary components. In COM+ 1.5, it will be possible to configure the same physical component several times and in different ways.
Public/Private Components
In COM+ 1.0, all components are public. In COM+ 1.5, it will be possible to set a component to be private to the current application. The less you expose externally, the easier it is for you to make changes in the future to the application.
Application Partitions
The purpose of application partitions is to partition a machine so that applications don't disturb each other. A typical situation when application partitions are of great use is in data centers. Consider the possibility of running several instances of SQL Server 2000 in one single machine.
COM+ Applications Can Be Exposed as XML Web Services
If you want to expose some functionality of a COM+ application as XML Web Services, COM+ 1.5 makes that easy. You can achieve that automatically with the help of the Component Services Explorer.
NOTE
Now that I have sounded like a salesperson for a while, I'd like to say that I was told by Microsoft that there are no plans to add the functionality of COM+ 1.5 to Windows 2000. Only time will tell.