You keep using that word (REST) and I don't think it means what you think it does

A long time ago in a galaxy far, far away ... OK, not quite. But some time ago our industry spent a lot of time and effort on discussion the pros and cons of REST, SOA, SOAP (!) etc. I even had a few things to say on the subject myself. To misquote Jeff Wayne, "minds immeasurably superior to my own" on the topic of REST and HTTP at the time had a lot more to say and do in this area. It's something which is easily Googled these days. Covered in books too. Probably even several video master classes on the topic of REST and HTTP, let alone their relevance to SOA. And yet it seems that some people either have a very short memory, didn't understand what was said, or perhaps didn't do their homework?

I'm specifically talking about how REST plays into the recent microservices wave. Yes, I think there are some problem areas with microservices and yes, one of them is the dogmatic assertion by some that REST (they tend to really mean HTTP) is mandatory. I don't believe that. I do believe that different message exchange patterns may be suitable for a range of microservices and to limit ourselves to just one (HTTP) does not make sense.

Oh and yes, it still frustrates me to this day when people talk about REST and they're really talking about HTTP - my only consolation here is that if I find it frustrating it must really annoy the h*ll out of the RESTafarians! Not all REST is HTTP and likewise not everything which uses HTTP is necessarily REST. Simple, huh? Well you'd think ...

Anyway, putting that aside, what's got me more frustrated recently is that some people are suggesting that REST (really HTTP) can't do async for microservices and therefore can prevent you breaking apart your monolith. I'm not even going to attempt to explain in detail here how that is wrong except to suggest that a) those people should go and read some InfoQ articles from the early 2000's (yes, even theserverside had things to say on the topic), b) do a Google search, c) read something from my friend/colleague Jim Webber, Savas and others on the topic of REST (and maybe Restbucks - hint, hint), d) in case they're confused between REST and HTTP, maybe go and read the HTTP response codes and look at those in the early 200's range (again, another hint). As I said above, I'm not suggesting there aren't some issues with REST/HTTP for microservices. And as I've also said over the last decade or so, maybe there are some issues with building some types of distributed systems with REST/HTTP. But this isn't one of them!

And this then brings me to a related topic. And if he hadn't been saying it around REST, I'm pretty sure Inigo Montoya would've been saying "You keep using that word. I do not think it means what you think it means" about "asynchronous". Yes, some of the authors try to make the distinction between a blocking call within a single address space and a one way interaction with a service across the network, but that really doesn't instil me with confidence that they know what they're talking about. If they threw in a bit of Fischer, Lynch and Patterson, rather than the oft overused reference to CAP, then maybe I'd be slightly less concerned. But that'd require some homework again, which they don't appear to want to do! Oh well, good luck to those who follow them!

Note, I have deliberately not included many links to things in the post in the hopes it will encourage the reader to follow up.

Microservices and events

I wanted to write more on microservices (SOA) and specifically around reactive, event-orientation. We've heard a lot recently about frameworks such as Vert.x or Node.js which are reactive frameworks for Java, JavaScript and other languages (in the case of Vert.x). Developers are using them for a range of applications but because they present a reactive, event driven approach to building services and applications, it turns out they're also useful for microservices. Therefore, I wanted to give an indication of why they are useful for microservices and when I started to put proverbial pen to paper I found I was repeating a lot of what I'd written about before and specifically when I discussed some of the core reasons behind the future adaptive middleware platform.

Now I'm not suggesting that Vert.x, Node.js or any reactive, event-oriented framework or platform fulfills everything I mentioned in that earlier entry because I don't think they do, at least not today. However, they do represent a good basis from which to build. Of course that doesn't preclude existing architectures and associated software stacks from being used to build microservices. In fact I think a combination of these new approaches and existing mature stacks are the right way to go; with each new generation we learn the mistakes of the past and their successes, building upon the latter and hopefully not reproducing the former.

Unfortunately I'm not going to endorse the reactive manifesto, because I'm not entirely convinced that what's encompassed there really represents much more than good fault tolerance and message-driven architecture. For instance, responsive? I'd like to think that any useful platform should be responsive! I think we should add that to the "bleeding obvious" category. My definition of reactive would include fault tolerant, scalable, reliable, message-oriented (supporting synchronous and asynchronous), self-healing (resiliency is only part of that) and event-driven.

Therefore, when I say that reactive, event-oriented is a good fit for microservices (remember ... SOA) I'm talking about something slightly different than reactive as defined within the manifesto. It's all that and more. We've come a long way since the 1970's and 80's when things were typically synchronous and RPCs were the main paradigm. Even those are event driven if you think about it. But since then synchronous interactions fell out of favour, asynchronous rose in interest and so did event-orientation and associated efforts like CEP. Hopefully given what I wrote in that earlier article it should be fairly obvious what kind of platform characteristics I believe are necessary in the future - microservices is just a good approach that can take advantage of it.

As I said earlier though, I don't believe any implementation today really embodies where we need to be. There's a lot we can leverage from the past decades of enterprise software development but there is more work to be done. For instance, we hear a lot about asynchronous systems where in reality what most people are talking about is synchronous interactions performed by a separate thread to the main application (business logic) thread of control; there are known time bounds associated with work or interactions, even in a distributed environment. If we really want to talk about true asynchronous interactions, and I believe we do, then we are looking at unbounded interactions, where you simply do not know when a message is overdue. That causes a number of problems, including failure detection/suspicion (timeouts are a favourite way of trying to determine when a machine may have failed and now you can't use time as a factor), and distributed consensus, which has been proven to be unsolvable in an asynchronous environment.

Of course these may seem like purely theoretical or academic problems. But think about the kinds of environments we have today and which are growing in applicability: cloud, mobile, internet of things with sensors, gateways etc. These are inherently asynchronous, with unpredictable interaction patterns and message exchanges. Whilst synchronous or multi-threaded asynchronous may be a good stop-gap approach, it will inevitably present scalability and resiliency problems. True asynchronous, with all of the problems it represents, is the ultimate goal. But we're not there yet and it's going to take time as well as recognition by developers that further research and development is a necessity.