Microservices

Microservices are a variant of software dev technique Service-Oriented architecture. 

• Collection of loosely coupled services
• Increases modularity
• Parallelises development
• Deploy and scale respective services independently
• Small and responsible for one thing
• By single domain OR single feature
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Developing a single application as a suite of small services, each running in its own process and communicating with light weight mechanisms, often an http resource api.

Fine grained SOA(Service oriented Architecture)

• why move to micro services
• New types of clients - smart phone, tablets, tvs,  watches...
• controllers designed for web interfaces are not designed for  new clients
• New persistence technologies
• We have specialized technologies which offer superior performance for specific use cases
• Search - Elastic Search
• Product reviews- can be stored in document store like MongoDB
• Shopping cart- simple key value store - redis
• legacy DB

Monolithic  Challenges

• Single codebase
• Deployment
• versioning 
• team size
• Feature implementation will be slow due to large code base with large team
• fixing and deploying defects
• Advantages
• easy to comprehend but not to comprehend millions of code
• single language
• Disadvanteges
• Language and framework lock
• Digesting large code base is difficult. Too many secrets/hacks, 
• Deploy as single unit

Microservices

• Small and independent applns built around individual functional areas. Each one uses the backend technologies best suited for its need
  
• APi Gateway
• Various client technlogies need to access our services. Having each client access each service will have issues. So we use api gateway
  
• It handles the complexity of talking to services
• Cloud native architechture
• Communicate via apis & not common DB
• Microservices encapsulate business capabilities by business function(cart, catalog, checkout)
  
• not based on technology stack - dao, db, 
• eg: Cart, checkout, REviews, Search
• Services are not Ochestrated but Choregraphed
• Challenges
• Few services may be unavailable
• Design for failure & curcuit breaker
• More monitoring required
• Remote calls expensive than in-process calls
• Must rely on Eventual Consistency over ACID
•  Features span multiple services
• Change management becomes a different challenge
• Refactoring module boundaries
• Network
• should be reliable
• latency 0
• bandwidth is infinete

How to break Monolyth into Micro services

• Primary consideration should be business funcitonality
• Nouns (Carts, customer, catalog)
  
• Verb based(Search, checkout, shipping)
  
• single responsibility service
• Bounded context
  

SOA vs Microservices

• SOA is integration of system. Microservices address individual applns
• soa relies on orchestration, Microservce rely on choregraphy
• soa is smart integration of dumb services, microserviews is dumb integration of smart services eg: pipes, filters in linux
  

Spring Cloud

• Spring cloud config
• Centralized, externalized, secured, easy to reach source of appl config.
• application.yml or application.properties to have location where config file is located
• @EnableConfigServer
  
• Spring Cloud bus
• provides simple way to notify clients about appln config changes
  
• Spring cloud netflix Eureka
• Service discovery- allows applications to register themselves as clients
  
• load balancing
• Circuit breaker
• zuul
• cacheing
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