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Problem Statement
Microservice architecture structures an application as a set of loosely coupled microservices and each service should be developed independently in agile manner to enable continous delivery/deployment. When a large, complex application is to be built using microservice architecture, the major problem is how to design loosely coupled microservices or to break a large application into small loosely coupled services?
Solution
We can define a microservice using strangler pattern. A strangler application has two types of services −
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Existing Behavior − These services exhibits the behavior that previously resides in Monolith.
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New Functionalities − These services implements new behavior.
So over the time of development, microservices increases and monolith shrinks with features moving out from monolith to Strangler Application.
Example
Consider an example of an Online Book Store. Initially we have only developed Book Catalog management service and other services are supported in legacy monolith application. During the course of development, more and more services are developed and functionalities are moved away from a monolith.
So when a new service is developed, the monolith is strangled, the old component is decommissioned and new microservice is deployed and supports the new functionality. A strangler pattern can be implemented using three steps −
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Transformation − Develop the microservices independently to implement a particular functionality of a monolith.
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Co-Exist − Both Monolith and Microservices will work. User can access functionality from both components.
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Elliminate − Once the newly developed functionality is production ready, remove the functionality from the monolith.
Advantages
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Test Driven Development − As services are developed in chunks, we can use TDD for business logic and ensure the code quality.
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Independent Teams − Teams can work in parallel fashion on both monolith and microservices thus making a robust delivery mechanism.
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