Monday, February 29, 2016

Microservice and Monolithic architectures exemplified

We'll use an example of an application to illustrate the differences between Microservice architecture and Monolithic architecture.  Let us take an e-commerce application which has the purpose of selling products online. In general all applications from this category require functionalities for browsing through available products, purchase them and placing orders which are later managed by administrators. And of course most of the e-commerce websites have some content which must be easily editable at any time from an administration dashboard.

Monolithic architecture

In the below diagram it is presented an e-commerce web application built following a monolithic architecture. As seen in the below diagram the data of the web application is stored in a single database. Products, website content, orders placed by customers and inventory information all stored in the same place. The web application has multiple purposes, first and most important to allow customers to browse through products and allow them to buy, secondly managing orders, content and maybe offering customers the possibility to create an account for managing different settings and subscriptions.
In this case the application built on a monolithic architecture offers both a web interface written using HTML, CSS and JavaScript and an Application Programming Interface which can be used by the clients such as Android or IOS smartphone apps.

E-commerce web application built using Monolithic architecture
E-commerce web application built using Monolithic architecture

Scaling for this kind of applications can be done both horizontally and vertically. The later means that the production machine will get its hardware improved by adding additional RAM memory, disk storage or adding a better CPU. Scaling vertically can be achieved by installing the same version of the application on multiple nodes and putting a load balancer between the nodes and the browser clients or API clients.

Microservice architecture

The below diagram shows a possible way to build the described e-commerce web application in a microservice architecture. As seen below the application was split into multiple small services each having its own purpose, managing orders, managing products and managing content. 
Each small service has its own database which contains the generated or used data by that particular service, so basically we do not have a big database but rather multiple small databases. This represents one of the big advantages of microservices because scaling of the database horizontally becomes possible. Of course that a data consistency issue appears when a complex flow, which passes through multiple services, fails somewhere in the middle of the processing. Assuring data consistency requires now a lot of extra effort, something which was previously easily achieved using database transactions.
The earlier mentioned API Gateway stands in front of the clients and incorporates the API offered by all microservices such that the clients know only about that API and are not aware of the other services behind it. This can be compared to the fa├žade design pattern in which a single entry point for the clients is offered. Following this architecture makes it easier to later change the implementation or the API of the services as long as the API of the Gateway still remains the same.
Using Microservice architecture offers more options for scaling the application, for example one can install half of the services on one node and the other half on the other node. One can install each service on a separate node or even install each service twice on two different nodes, but then most probably a load balancer is needed.

E-commerce web application built using Microservice architecture
E-commerce web application built using Microservice architecture

In future posts we'll have look over the advantages and disadvantages of Microservice architecture and over its use in practice.

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