Tuesday, 30 August 2016

Microservices Part 3 - How to call a Microservice

I'm busy working away on a new Microservices using Spring Boot course for Virtual Pair Programmers. I hope my next blog post will be a draft running order with an estimated release date: in the meantime as promised I'm going to look at how to call a Microservice.

Along the way I'll point out how Spring Boot can help - at the same time this is helping me to decide what needs to be on the new course.

As per last week, my old monolith (which is eventually going to be broken down until nothing remains) is going to call a new Microservice, the "VAT service". It has a single responsiblity, to return the Tax due on an amount, based on the country of residence for a customer.

It's easy to make these things simple on a training course - in real life various governments have conspired to make VAT a living nightmare, so really this service has to deal with IP addresses, Physical Address and location of requesting bank. But that's ok, the microservice still has a single responsibility. Don't be afraid to build microservices which may feel trivially small - that's part of the point (and they tend to grow anyway).

Obviously, we need to call this microservice. How?

Approach 1: The easy way - a naked REST call

Although I advised in the previous blog that you can and should consider other remoting solutions such as gRpc, let's assume we're going with REST. It's kind of standard.

As you're reading this blog, you're probably a Spring fan, so let's also assume that the caller (client, at present the monolith) is using Spring. So the natural choice is to go for the RestTemplate.

We've covered the RestTemplate extensively in our Spring Remoting course, so I won't labour the point. But something like:

String countryRequired = "GBR";
Percentage vatRate = template.getForObject("http://localhost:8039/vat/{country}",Percentage.class,countryRequired);

Almost a no brainer. Things to consider:

  1. We need to get rid of the hardcoded URI - ideally we would have a service discovery solution as part of our architecture. I touched on this briefly last time, but Spring Boot has a plugin which wraps up the very easy to use Eureka, which was originally built by Netflix. Although we use Kubernetes on our live site, I've decided that as Eureka is so tightly integrated with Boot, it would be a shame not to cover it. So it's going to be on the course!
  2. What happens if the VAT service is down? In a Microservice architecture, you must assume that at any one time, at least one service is likely to be unavailable. Again, further Netflix components can help, and Spring can easily integrate with Ribbon (for load balancing) and Hystrix (for Circuit breaking - more on circuit breaking in a future blog post).

Together, these two sub-frameworks can lead to a very robust architecture. I'll be making sure that our practical work on the course explores this in full.

Approach 2: using Feign to hide the remote call

Naked rest calls are all well and good - they're simple - but I always get the feeling that I'm breaking an abstraction. I don't want to feel that I'm making a Http call(*) - as a business programmer, I'm calling a service and that's how I want to think in the code.

(*) Note: this will make some people angry. When working on distributed systems, we must never forget the Fallacies of Distributed Computing, in this case we must never forget that we are making a remote call and it can 1) fail and 2) take a long time. Many argue that by abstracting away the remote call, we are making it easy to forget this. It's a good point which I accept and remain mindful of.

It would be great if I could call this service using idiomatic Java/Spring/Dependency Injection, a little like this:

public class BlahBlah
   private VatService remoteVatService;

   public void billCustomerOrWhatever( .. params .., String countryOfOrigin)
      Percentage vatRate = remoteVatService.findVatRateForCountry(countryOfOrigin);
      // blah blah blah      

And we can! Yet another element of the Spring Cloud library is called "Feign". I admit I didn't know about this until recently (how do you keep up with Spring when it expands faster than my brain cells can work?) - I'll be covering it on the course but it's as simple as declaring the Interface in the usual Java way:

public interface VatService
   public Percentage findVatRateForCountry(String country);

Rather like with Spring Data JPA (which I covered in the Spring Boot course), you do NOT implement this interface - it's done for you via a generated runtime Proxy.

You do need to add a few annotations, so that the generation knows how to translate the Java into REST calls. Cleverly, we use standard SpringMVC annotations (of course usually these annotations are used when defining the server side - this is the first I can think of where I've used the annotations client side!)

public interface VatService
   public Percentage findVatRateForCountry(@PathVariable("country") String country);

Beautifully, this all integrates with the Ribbon load balancer that I mentioned above, so if we've replicated the service on multiple nodes, it's easy to provide failover and fallback behaviour.

Approach 3: Use Messages

The call to the VAT service probably needs to be synchronous, because we absolutely need to know the answer before the user can proceed with what they're doing. But in many cases, a message driven solution is another way of building a robust system.

A working example is that our system needs to record viewing figures. Every time a video is watched by a subscriber on our site, we record this as a "watch". We use the data to decide which courses are a hit, and we can also identify unusual viewing patterns (this is a polite way of saying "we can find out who is using site scrapers").

If we have a microservice which is responsible solely for recording viewing, we can of course use REST to log the view.

Notice the new service has its own private database, as described in part 1. We've chosen MongoDb - it has its detractors, but it will work well for this type of data. We can easily store large blocks of the viewing figures in memory, so it's going to be easy to do fast calculations and aggregations. When this was handled by the monolith in MySQL, doing even basic calculations was grinding the whole system to a halt. One of the joys of microservices is we can make these decisions without too much agony - if it doesn't work out, we can tear down the whole microservice and replace it with a different solution. I call this "ephemerality" but it's a pompous word so it will never catch on.

This call doesn't need to be synchronous - the video can safely play even if we haven't yet logged the viewing. There are ways of making asynchronous REST requests (Spring features an @Asynch annotation which starts a new thread - I've never covered this on any course, but I will maybe get around to that someday).

But this is a great use for messages. Instead of making a call to the service, we could just fire off a message to a queue. We don't care who consumes the message - we just know we've logged that message.

We would then just make the ViewingFigures service either respond to messages on that queue - or, even better, we could use a Topic instead of a Queue. With a Topic, multiple consumers can register an interest. So, in the future, if new services are built which are also interested in the EVENT that a video has been watched, well, they can subscribe to that topic as well.

A Topic has multiple subscribers which can be added over time. Note that on AWS this is implemented using the SNS, Simple Notification Service

This gains us the robustness that we desired above, without the need for extra plumbing such as circuit breakers and load balancers. If the viewing figures service goes down, it's no problem to the monolith as it isn't calling it - it's just sending a message to a queue or topic. The queue will start to backlog the messages until the service comes back up again, and then the service can catch up on its work.

Things to think about: it is essential to ensure the queue has an extremely high uptime. With a few mouse clicks (* see footnote), Amazon SQS automatically provisions a queue which is transparently duplicated across multiple Availability Zones (data centers). You can't assume it will NEVER go down and you must code for this on the calling side. In this case, I would log the exception and carry on, it's no disaster if we miss some viewing records.

Although we've covered messaging in standard JavaEE (and we have a course covering this on WildFly releasing soon), for some reason we've never covered messaging for Spring. So that's going to go on the new course as well!

As always, I'm sorry for the long blog post, I didn't have time to write a shorter one - I'm busy working on the new course!

(* footnote) Edit to add: Ahem, I meant, of course - "with a simple script, under source control, using a tool such as Puppet, Chef or Ansible". That needs to be a course too!

Wednesday, 3 August 2016

Microservices, Part 2 - how to deploy

Here's the next in a short series (I think it will be three parts) where I'm musing about Microservices. My challenge is that VirtualPairProgrammers absolutely needs a course on it, but I'm not sure what form that will take. After writing these first two parts, I'm beginning to think that for development, we just need to extend our existing Spring Boot and JavaEE/Wildfly courses, but a further course on Deploying Microservices will be needed. This blog post will focus on that.

In part 3, I'll return to the "dev" side of things and look at how using events can make your system more loosely coupled.

In Part 1 I described the overall concepts in Microservices, and it turns out to be not too complicated:
  • Services aligned to specific business functions
  • Highly cohesive services and loose coupling between them
  • No integration databases (meaning each service will typical run its own data storage)
  • Automated and continuous deployment.
Actually implementing a microservice is not too hard. Designing an overall architecture where the services collaborate to achieve an overall goal, that's a bit harder - but what's really hard is deploying a microservice architecture. To put another way, the real magic in microservices is in the "Ops" rather than the "Dev".

Unless you're planning on rolling your own infrastructure tools (Netflix did this and they've opened sourced them - more later), you're going to rely on open source tools, and lots of them. There are hundreds - probably more - that you could consider. It's overwhelming, and every day, new tools are emerging. To try to get you started on the Microservices path, this article is going to look at a very simple microservice and the not-so-simple tools needed to get it running.

Note: this article is not intended to be authoritative. These are just the choices we've made and the reasons why. There will be other solutions, and plenty of tools that I've never even heard of. I consider Microservices a journey, and our system is certain to evolve dramatically over the coming years.

Also, I'll be sticking with the tools I know - so for the implementation of the actual service, I'll probably use Spring Framework, JavaEE or associated technologies. If you're coming from other languages, then of course you will have your own equivalents.

Our System at VirtualPairProgrammers.

As described in part 1, our website is deceptively simple - it's a monolith, and behind the facade of the website we're managing well over 20 business functions. It has worked well for us, but it was getting harder and harder to manage. So we decide to migrate to a microservice architecture.

But so much work to do! At least 20 microservices to build! Where do we start?

Well, one appealing thing about microservices (for me) is you don't have to do a big bang migration - you can slowly morph your architecture over time, breaking away parts until the legacy can be retired or left in "hospice" mode.

So we started very simply - we have a business function where we need to calculate the VAT (Value Added Tax*) rate for any country in the world. In the monolith, this code is buried away in a service class somewhere - it's a great candidate to be its own microservice:

Simple to describe, but actually deploying this raises some questions:

How to implement the service?

As stated, the "dev" part isn't too hard - Spring Boot is a perfect fit for Microservices, and we're very experienced with it here at VirtualPairProgrammers. But really there is infinite choice here, you could for example implement this as an EJB in a Wildfly container. Following the guidance in part 1, this service will have its own data store, and it doesn't really matter what that is. For a simple service like this, we might even keep the data in memory and simply de-deploy the service when VAT rules change.

Should the VAT Service be deployed to it's own Machine (Virtual Machine)?

As mentioned in part 1, we want to be able to maintain total separation of the services, but at the same time we don't want to incur the cost of running a separate Virtual Machine. This is where containerization comes in.

A container differs subtly from a Virtual Machine. A VM has it's own operating system, but a container shares the host's operating system. This subtle change has major payoffs, mainly that a container is very lightweight, fast and cheap to startup. Whereas a VM might take minutes to provision and boot, a container is up and running in seconds.
A traditional set of Virtual Machines - each VM has its own Operating System...

...but containers share the host's operating system

The most popular containerization system (quite over hyped at present) is Docker. This book is an excellent introduction, it's a practical book and definitely helped us to get started:

How do we call the now-remote service?

The usual answer here is to expose a REST interface to the VAT service. This is trivial to do using Boot or JavaEE.

But in this specific example, we are NOT exposing this API to end users - it is only going to be called from our own code. So, it's actually not at all necessary to use REST. There will be many disagreements here, but you could certainly consider an RPC call! RPC libraries such as Java's RMI or more generic ones such as gRPC (http://www.grpc.io/) have a bit of a bad name, partly because the binary formats are non-human readable. For service-service APIs, actually RPC is fine - they're high performance and work well.

(Human readable forms, mainly JSON over HTTPs [aka REST if you're not Roy Fielding] are the right choice for APIs that are being called by user interfaces, especially JavaScript frameworks).

(Something to think about here, we've replaced a very fast local call with what is now essentially a network call. Remember this will be an internal network call - see the stackexchange discussion here.)

How does the "client" (the monolith) know where the microservice is?

It wouldn't be a great idea to have code like this:

// Call the VAT service
VATRate rate = rest.get("");
I hope that's obvious - if we change the location of the service (eg change the port it is running on), then the client code will break. So: hardcoding the location of the services is out.

So what can we do? This is where Service Discovery via a Service Registry comes in.

There are many choices of Service Registries. Actually Java had a solution for this back in the 1990's, in the shape of the JINI framework. Sadly that was an idea ahead of its time and never caught on (it still exists as Apache River, but I've never heard of anyone using it).

More popular - Netflix invented one for their Microservice architecture, which is open sourced as Eureka. I haven't used this, but I understand it is quite tied to AWS and is Java only. Do let us know if you've used this at all.

We are using Kubernetes (http://kubernetes.io/) because it provides a service registry (by running a private DNS service), and LOTS more, particularly...

What if the service crashes?

It's no good if the microservice silently falls over and no-one notices for weeks. In our example, it wouldn't be too bad because the failure of the microservice would lead to a failure of the monolith (we'd see lots of HTTP 500's or whatever on the main website - but once we've scaled up to multiple services, this won't be the case). This is where orchestration comes in - in brief this is the technique of automatically managing your containers (orchestration is a bigger concept than this, but for our purposes, it is containers that will be orchestrated). The previously mentioned Kubernetes is a complete Orchestration service, originally built by Google to manage (allegedly) 2 billion containers.

Kubernetes can automatically monitor a service, and if it fails for any reason, get it back running again. Kubernetes also features load balancing, so if we do somehow manage to scale up to Netflix size, we'd ask Kubernete to maintain multiple instances of the VAT service container, on separate physicals instances, and Kubernetes would balance the incoming load between them.

There aren't many books available on Kubernetes (yet) - but at the time of writing, the following book is in early release form:

So the overall message is, you're going to need a lot of tooling, most of it relating to operations rather than deployment. In Part 3 (probably the final part) I'll look at another way that services can communicate, leading to a very loosely coupled solution - this will be Event Based Collaboration...