What are Microservices?

In this article written by Gaurav Kumar Aroraa, Lalit Kale, Kanwar Manish, authors of the book Building Microservices with .NET Core, we will start with a brief introduction. Then, we will define its predecessors: monolithic architecture and service-oriented architecture (SOA). After this, we will see how microservices fare against both SOA and the monolithic architecture. We will then compare the advantages and disadvantages of each one of these architectural styles. This will enable us to identify the right scenario for these styles. We will understand the problems that arise from having a layered monolithic architecture. We will discuss the solutions available to these problems in the monolithic world. At the end, we will be able to break down a monolithic application into a microservice architecture. We will cover the following topics in this article:

  • Origin of microservices
  • Discussing microservices

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Origin of microservices

The term microservices was used for the first time in mid-2011 at a workshop of software architects. In March 2012, James Lewis presented some of his ideas about microservices. By the end of 2013, various groups from the IT industry started having discussions on microservices, and by 2014, it had become popular enough to be considered a serious contender for large enterprises.

There is no official introduction available for microservices. The understanding of the term is purely based on the use cases and discussions held in the past. We will discuss this in detail, but before that, let's check out the definition of microservices as per Wikipedia (https://en.wikipedia.org/wiki/Microservices), which sums it up as:

Microservices is a specialization of and implementation approach for SOA used to build flexible, independently deployable software systems.

In 2014, James Lewis and Martin Fowler came together and provided a few real-world examples and presented microservices (refer to http://martinfowler.com/microservices/) in their own words and further detailed it as follows:

The microservice architectural style is an approach to developing a single application as a suite of small services, each running in its own process and communicating with lightweight mechanisms, often an HTTP resource API. These services are built around business capabilities and independently deployable by fully automated deployment machinery. There is a bare minimum of centralized management of these services, which may be written in different programming languages and use different data storage technologies.

It is very important that you see all the attributes James and Martin defined here. They defined it as an architectural style that developers could utilize to develop a single application with the business logic spread across a bunch of small services, each having their own persistent storage functionality. Also, note its attributes: it can be independently deployable, can run in its own process, is a lightweight communication mechanism, and can be written in different programming languages.

We want to emphasize this specific definition since it is the crux of the whole concept. And as we move along, it will come together by the time we finish this book.

Discussing microservices

Until now, we have gone through a few definitions of microservices; now, let's discuss microservices in detail.

In short, a microservice architecture removes most of the drawbacks of SOA architectures. 

Slicing your application into a number of services is neither SOA nor microservices. However, combining service design and best practices from the SOA world along with a few emerging practices, such as isolated deployment, semantic versioning, providing lightweight services, and service discovery in polyglot programming, is microservices. We implement microservices to satisfy business features and implement them with reduced time to market and greater flexibility.

Before we move on to understand the architecture, let's discuss the two important architectures that have led to its existence:

  • The monolithic architecture style
  • SOA

Most of us would be aware of the scenario where during the life cycle of an enterprise application development, a suitable architectural style is decided. Then, at various stages, the initial pattern is further improved and adapted with changes that cater to various challenges, such as deployment complexity, large code base, and scalability issues. This is exactly how the monolithic architecture style evolved into SOA, further leading up to microservices.

Monolithic architecture

The monolithic architectural style is a traditional architecture type and has been widely used in the industry. The term "monolithic" is not new and is borrowed from the Unix world. In Unix, most of the commands exist as a standalone program whose functionality is not dependent on any other program. As seen in the succeeding image, we can have different components in the application such as:

  • User interface: This handles all of the user interaction while responding with HTML or JSON or any other preferred data interchange format (in the case of web services).
  • Business logic: All the business rules applied to the input being received in the form of user input, events, and database exist here.
  • Database access: This houses the complete functionality for accessing the database for the purpose of querying and persisting objects. A widely accepted rule is that it is utilized through business modules and never directly through user-facing components.

Software built using this architecture is self-contained. We can imagine a single .NET assembly that contains various components, as described in the following image:

As the software is self-contained here, its components are interconnected and interdependent. Even a simple code change in one of the modules may break a major functionality in other modules. This would result in a scenario where we'd need to test the whole application. With the business depending critically on its enterprise application frameworks, this amount of time could prove to be very critical.

Having all the components tightly coupled poses another challenge: whenever we execute or compile such software, all the components should be available or the build will fail; refer to the preceding image that represents a monolithic architecture and is a self-contained or a single .NET assembly project. However, monolithic architectures might also have multiple assemblies. This means that even though a business layer (assembly, data access layer assembly, and so on) is separated, at run time, all of them will come together and run as one process. 

A user interface depends on other components' direct sale and inventory in a manner similar to all other components that depend upon each other. In this scenario, we will not be able to execute this project in the absence of any one of these components. The process of upgrading any one of these components will be more complex as we may have to consider other components that require code changes too. This results in more development time than required for the actual change.

Deploying such an application will become another challenge. During deployment, we will have to make sure that each and every component is deployed properly; otherwise, we may end up facing a lot of issues in our production environments.

If we develop an application using the monolithic architecture style, as discussed previously, we might face the following challenges:

  • Large code base: This is a scenario where the code lines outnumber the comments by a great margin. As components are interconnected, we will have to bear with a repetitive code base.
  • Too many business modules: This is in regard to modules within the same system.
  • Code base complexity: This results in a higher chance of code breaking due to the fix required in other modules or services.
  • Complex code deployment: You may come across minor changes that would require whole system deployment.
  • One module failure affecting the whole system: This is in regard to modules that depend on each other.
  • Scalability: This is required for the entire system and not just the modules in it.
  • Intermodule dependency: This is due to tight coupling.
  • Spiraling development time: This is due to code complexity and interdependency.
  • Inability to easily adapt to a new technology: In this case, the entire system would need to be upgraded.

As discussed earlier, if we want to reduce development time, ease of deployment, and improve maintainability of software for enterprise applications, we should avoid the traditional or monolithic architecture.

Service-oriented architecture

In the previous section, we discussed the monolithic architecture and its limitations. We also discussed why it does not fit into our enterprise application requirements. To overcome these issues, we should go with some modular approach where we can separate the components such that they should come out of the self-contained or single .NET assembly.

The main difference between SOA & monolithic is not one or multiple assembly. But as the service in SOA runs as separate process, SOA scales better compared to monolithic.

Let's discuss the modular architecture, that is, SOA. This is a famous architectural style using which the enterprise applications are designed with a collection of services as its base. These services may be RESTful or ASMX Web services. To understand SOA in more detail, let's discuss "service" first.

What is service?

Service, in this case, is an essential concept of SOA. It can be a piece of code, program, or software that provides some functionality to other system components. This piece of code can interact directly with the database or indirectly through another service. Furthermore, it can be consumed by clients directly, where the client may either be a website, desktop app, mobile app, or any other device app. Refer to the following diagram:

Service refers to a type of functionality exposed for consumption by other systems (generally referred to as clients/client applications). As mentioned earlier, it can be represented by a piece of code, program, or software. Such services are exposed over the HTTP transport protocol as a general practice. However, the HTTP protocol is not a limiting factor, and a protocol can be picked as deemed fit for the scenario.

In the following image, Service – direct selling is directly interacting with Database, and three different clients, namely Web, Desktop, and Mobile, are consuming the service. On the other hand, we have clients consuming Service – partner selling, which is interacting with Service – channel partners for database access.

A product selling service is a set of services that interacts with client applications and provides database access directly or through another service, in this case, Service – Channel partner.  In the case of Service – direct selling, shown in the preceding example, it is providing some functionality to a Web Store, a desktop application, and a mobile application. This service is further interacting with the database for various tasks, namely fetching data, persisting data, and so on.

Normally, services interact with other systems via some communication channel, generally the HTTP protocol. These services may or may not be deployed on the same or single servers.

In the preceding image, we have projected an SOA example scenario. There are many fine points to note here, so let's get started. Firstly, our services can be spread across different physical machines. Here, Service-direct selling is hosted on two separate machines. It is a possible scenario that instead of the entire business functionality, only a part of it will reside on Server 1 and the remaining on Server 2. Similarly, Service – partner selling appears to be having the same arrangement on Server 3 and Server 4. However, it doesn't stop Service – channel partners being hosted as a complete set on both the servers: Server 5 and Server 6.

A system that uses a service or multiple services in a fashion mentioned in the preceding figure is called an SOA. We will discuss SOA in detail in the following sections.

Let's recall the monolithic architecture. In this case, we did not use it because it restricts code reusability; it is a self-contained assembly, and all the components are interconnected and interdependent. For deployment, in this case, we will have to deploy our complete project after we select the SOA (refer to preceding image and subsequent discussion). Now, because of the use of this architectural style, we have the benefit of code reusability and easy deployment. Let's examine this in the wake of the preceding figure:

  1. Reusability: Multiple clients can consume the service. The service can also be simultaneously consumed by other services. For example, OrderService is consumed by web and mobile clients. Now, OrderService can also be used by the Reporting Dashboard UI.
  2. Stateless: Services do not persist any state between requests from the client, that is, the service doesn't know, nor care, that the subsequent request has come from the client that has/hasn't made the previous request.
  3. Contract-based: Interfaces make it technology-agnostic on both sides of implementation and consumption. It also serves to make it immune to the code updates in the underlying functionality.
  4. Scalability: A system can be scaled up; SOA can be individually clustered with appropriate load balancing.
  5. Upgradation: It is very easy to roll out new functionalities or introduce new versions of the existing functionality. The system doesn't stop you from keeping multiple versions of the same business functionality.

Summary

In this article, we discussed what the microservice architectural style is in detail, its history, and how it differs from its predecessors: monolithic and SOA. We further defined the various challenges that monolithic faces when dealing with large systems. Scalability and reusability are some definite advantages that SOA provides over monolithic. We also discussed the limitations of the monolithic architecture, including scaling problems, by implementing a real-life monolithic application. The microservice architecture style resolves all these issues by reducing code interdependency and isolating the dataset size that any one of the microservices works upon. We utilized dependency injection and database refactoring for this. We further explored automation, CI, and deployment. These easily allow the development team to let the business sponsor choose what industry trends to respond to first. This results in cost benefits, better business response, timely technology adoption, effective scaling, and removal of human dependency.

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