Ask a bunch of Mainframers what makes their platform so powerful and there's a good chance a good number of them will tell you it's sysplex. Simply put, sysplex is a way of making multiple systems work together as a team and this can be LPARs on one physical box, or it can be spread out across many. Before we get into exactly how it does this. We first have to define a couple of basic concepts. First, there's STP, the Server Time Protocol. Systems need to be in exact agreement about time, like exact agreement and that kind of makes sense. If you've got multiple systems writing to the same file milliseconds apart and you're trying to figure out which one got to that piece of data first. Having your watches synchronized is kind of important. Then there's GRS, the Global Resource Serialization. This allows for multiple systems to access the same resources concurrently, serializing where necessary to ensure exclusive access. Without GRS, you have quite a mess. That's a handy rhyme. You should remember that in case you're tested on it later. XCF, the Cross Coupling Facility, this manages communication between applications in a sysplex. This also lets you be logged into one system and issue commands on behalf of another system in that sysplex. This right here is what let's the sysplex communicate as a whole. Next up, the Coupling Links. These connect LPARs to processors. Without these, accessing memory on different physical systems would have to run through one of those LPARs and these allow direct memory access communications between sysplex memory and the memory of an attached system. Finally, there's the CDS, the Coupled Data Set. So imagine you've got a bunch of people working on the same job and let's just say they're tallying up the numbers of different colored candies they're getting from various bags of candies. Bags of candies. Anyway, they're all working together to get one final result, which means they're all working on the same set of data. Now, in order to coordinate this operation so someone doesn't accidentally overwrite someone else's work, there needs to be an additional file that keeps track of who is currently working on a resource. So everyone knows how to wait their turn. That's the CDS. These parts are super important when you're talking about sysplex, because without them you've just got another distributed environment. Those memory links, the tightly coupled resources, and of course, being an absolute lockstep on time is the only way this will all work. We talked a little bit earlier about WLM, the Workload Manager. It's the component that makes sure that high priority work gets run quickly and that all of our targets are met. WLM makes quite a bit of use of sysplex to help balance things out. So if that discussion earlier about performance tuning on the Mainframe was interesting to you, sysplex is something you want to pay close attention to. Sysplex is at its core, a bunch of z/OS operating systems working together. They're able to cooperatively use hardware, software and components to achieve a high availability workload processing environment. This gets us back to the whole physical logical discussion. And when you have systems in a sysplex, when you're talking about how they're going to handle the work, you essentially have one entity that is running the work. It's the sysplex. Even though you know that you've got multiple physical mainframes and within those multiple logical LPARs, you'll often hear people just refer to it all collectively as the sysplex or just the plex. Let's talk about those sysplex configurations. There are three main ways you can configure a sysplex. Now, bit of a riddle, if you've got a sysplex with only one LPAR in it, is it really a sysplex? The answer is yes, and it's called a monoplex. Now, you'll often see a monoplex in testing and staging situations. But beyond that, it doesn't really buy you any additional performance or functionality. Then there's this concept of a base sysplex. In this configuration, every system has a connection to every other system, and they all know about each other, and they all agreed to do work together. It's almost like they started a little band. The most common form of the sysplex though is the parallel sysplex. In this configuration, instead of needing every system connected to every other system. We have this concept of a Coupling Facility. This is a special type of LPAR that you bring up, and typically you have backups of it on other Mainframes too. In a parallel sysplex, the Coupling Facility handles all the complexities and overhead of those systems working together. So you can have up to 32 z/OS images and up to 32 physical Mainframes. That would mean one LPAR on each Mainframe hooked into a 32 way sysplex. Now, most systems aren't scaled out that way. But if you need to, you could do it. Now, a word about availability. You might hear people say that a Mainframe environment hasn't gone down in decades. But how is that possible unless they're running on 20 year old hardware and software. Well, one of the neat things about being in a sysplex is you can do what's called a rolling IPL. An IPL is just booting up an operating system, and a parallel sysplex doesn't require all the systems in it to be up in order to function, just enough to keep it running. So if I've got 16 LPARS in my sysplex, we're just saying, I can bring down four of them, update the operating systems on them, and then bring them back up and put them into the plex. LPAR has got upgraded and the Plex stayed up. I can even do this with physical systems as long as the, some of the members are running elsewhere in other Mainframe. So let's go back to those 16 LPARs, say, they're spread out across for Mainframes, just as an example. And we want to upgrade one of those mainframes to a shiny brand new one. We bring down the four LPARs on that system that we want to replace, physically swap it out with a new one. Cable it up, configure it the way we want to, power it back on. IPL will boot up those four systems on the new hardware, vary them back into the plex and we're good to go. The plex never went down. That is some pretty amazing stuff and you can see why so many people would tell you it's their favorite feature. Although it's hard to pick just one, isn't it?