On the mainframe, the big three that immediately come to mind are CICS, IMS, and Db2. When I first started working on z Systems I heard these said together so often but I thought it was two things CICS, IMS, and Db2. But we're talking about three components here there's kicks, or CICS, spelled CICS, IMS, and Db2. Three different things. Let's start with CICS. CICS has been around for a long time and it powers many of the world's transactions such as ATM's, managing insurance policies, utility bills, retail shopping, and others. It does it all securely and reliably day in and day out. A CICS transaction consist of many facets including delivering the user interface, retrieving and updating information, and communicating with other applications and network attached devices. In CICS, a transaction typically lasts a very short time and is associated with one unit of work. CICS takes the work to be processed and lines it up with the program or programs required to complete it. This was the job of the transaction manager, to ensure the work complete successfully according to the process and that there's nothing left hanging out there when we tell the user that it's complete. Here's what's under the hood that lets this all happen. A CICS system manages an area of storage called a region which lives in an address space. You'll hear that term a lot, a CICS region. A CICS region uses domains which contain management nodes for transaction processing. Now, a single region can be defined to perform all the functions necessary to process a transaction. Typically, a CICS installation will have several different regions, each one devoted to a different type of transaction. Think of it like an auto repair center. This area's for body work, over here is for transmission issues, tires and wheels over there. That way, all the tools that are needed are close by and the area isn't crowded with stuff that they don't need. There are three types of domains that supervise and control transaction processing. Remember, these are the domains within the region. We've covered a lot there. So let's make sure we've got the basic order right. A CICS system manages an area of storage called a blank and within that it uses blanks. Let's see if you remember what those are called. So a CICS system manages areas of storage called regions. Then within the regions, it manages domains. There's a transaction manager, the XM, which is responsible for receiving transaction requests and creating the organizing tasks to process them. Then there's the program manager or PG which handles locating and invoking applications required for processing transactions. Then there's these Storage Manager, SM. This domain is responsible for allocating memory resources required for all of this to happen. You can see in this diagram, we have three types of regions. There's the TOR, terminal owning region, which handles connections to terminals and other remote devices. Then there's the AOR for application owning region and the DOR for Data owning region. The glue that connects these regions is VTAM. Now we haven't really spent much time talking about VTAM yet, and we're not going to get deep into it. Just know that it has to do with networking and it's part of comm server. The AORs and DORs talk with other applications like databases, and user written applications. One of those databases could be IMS. Now what's IMS? IMS, Information Management System has a long and interesting history as well which includes some development done for NASA. What makes IMS unique is that it has or is, depending on how you look at it, a hierarchical database management system. It is widely used for manufacturing type of applications but you'll see it in just about any industry. IMS consists of three components. One is the database manager. That's the IMS DB. Then there's the transaction manager. That's the IMS TM. Then there's these system services that provide common services to both the DB and the TM. These work together to deliver data and transactional capabilities in an extremely efficient way. For more complex data handling needs, there's Db2. Big D little b2. Db2 is a database and it offers exceptional availability, massive scalability, flexibility, and installation across platforms and location, and a low total cost of ownership. Db2 is considered to be a relational database. So data exists in one or more constructs known as tables. Within those tables, there are columns and rows and you interact with data by using the structured query language or SQL. Though you can also use Db2 for unstructured data and so much more. Remember those acid properties we've talked about before? Db2 people take each one of those very seriously. They won't chase something that gives them increased availability if it means giving up consistency and vice versa. When it comes to truly enterprise applications, the middleware like CICS, IMS, and Db2 need to step up and deliver time after time after time after time. Can't stop, won't stop. Transaction.
