Spring Boot - Different systems( eureka , zuul, ribbon, nginx,) used for what?

Eureka can be used in conjunction with NGINX, which leads to very powerful combination.

I am using it on AWS EC2 environment. Previously instead of NGINX I was using Spring Cloud Gateway and before that Zuul. Depending of the load Spring Cloud Gateway was running on AWS t3.medium or t3.large instances. After moving to NGINX I am using t3.micro (8 times less memory) instance. I am almost sure that I can do the trick and with t3.nano (16 times less memory) instance, but I wanted to be sure that there will be no surprises.

Below are the high level steps what you have to do in order to plug NGINX in the Eureka ecosystem. More details you can find in NGINX With Eureka Instead of Spring Cloud Gateway or Zuul article.

• Create a service which can read the configuration of all applications from Eureka and to 'translate' it to NGINX configuration.
• Create a cronjob entry which at certain period will read the configuration from the above service and will call the NGINX hot reload
• NGINX which will consume the configuration produced from the service and the cronjob and will work as API Gateway

without having an API gateway is there any use with this service registry ?

Yes. For example you can use it to locate (IP and port) of all your microservices. This comes in handy for devops type work. For example, at one project I worked on, we used Eureka to find all instances of our microservices and ping them for their status (/health, /info).

I understand ZUUL can be used as API gateway which is basically a load balancer , that calls appropriate microservice corresponding to request URL. iS that assumption correct?

Yes but it can do a lot more. Essentially because Zuul is more of a framework/library that you turn into a microservice, you can code it to implement any kind of routing logic you can come up with. It is very powerful in that sense. For example, lets say you want to change how you route based on time of day or any other external factors, with Zuul you can do it.

will the api gateway interact with Eureka for getting the appropriate microservice?

Yes. You configure Zuul to point to Eureka. It becomes a client to Eureka and even subscribes to Eureka for realtime updates (which instances have joined or left).

I have read NGINX can also be used as API gateway? Also i read some where else like NGINX can be used as a service registry , that is as an alternate for Eureka ! Thus which is right? Api gateway or service registry or both?

Nginx is pretty powerful and can do API gateway type work. But there are some major differences. AFAIK, microservices cannot dynamically register with Nginx, please correct me if I am wrong... as they can with Eureka. Second, while I know Nginx is highly (very highly) configurable, I suspect its configuration abilities do not come close to Zuul's routing capabilities (due to having the whole Java language at your disposal within Zuul to code your routing logic). It could be the case that there are service discovery solutions that work with Nginx. So Nginx will take care of the routing and such, but service discovery will still require a solution.

Is this can also be used as an alternate for ZUUL?

Yes AWS API Gateway can be used as a Zuul replacement of sorts. The issue here, just like Nginx, is service discovery. AWS API Gateway lets you apply logic to your routing... though not as open ended as Zuul.

for what ribbon is used?

While you can use the Ribbon library directly, for the most part consider it as an internal dependency of Zuul. It helps Zuul do the simple load balancing that it does. Please note that this project is in maintenance mode and not recommended any more.

This can also be used for load balancing. Thus do we need ZUUL if we have AWS ALB?

You can use ALB with ECS (elastic container service) to replace Eureka/Zuul. ECS will take care of the service discover for you and will map all instances of a particular service to a Target Group. Your ALB routing table can then route to Target Groups based on simple routing rules. The routing rules in ALB are very simple though, but improving over time.

Different systems which can be used for the working of microservices, that comes along with spring boot:

1. Eureka: Probably the first microservice to be UP. Eureka is a service registry, means , it knows which ever microservices are running and in which port. Eureka is deploying as a sperate application and we can use @EnableEurekaServer annotation along with @SpringBootAPplication to make that app a eureka server. So our eureka service registery is UP and running. From now on all microservices will be registered in this eureka server by using @EnableDiscoveryClient annotation along with @SpringBootAPplication in all deployed microservices.

2. Zuul: ZUUL is a load balancer , routing application and reverse proxy server as well. That is before we were using apache for reverse proxy things , now , for microservices we can use ZUUL. Advantage is, in ZUUL we can programatically set configurations, like if /customer/* comes go to this microservice like that. Also ZUUL can act as a load balancer as well , which will pick the appropriate microservice in a round robin fashion. SO how does the ZUUL knows the details of microservices, the answer is eureka. It will work along with eureka to get microservice details. And in fact this ZUUL is also a Eureka client where we should mark using @EnableDiscoveryClient, thats how these 2 apps(Eureka and zuul) linked.

3. Ribbbon: Ribbon use for load balancing. This is already available inside ZUUL, in which zuul is using Ribbon for load balancing stuff. Microservices are identified by service-name in properties file. IF we run 2 instances of one microservices in different port, this will be identified by Eureka and along with Ribbon(Inside zuul), requests will be redirected in a balanced way.

4. Aws ALB , NGINX , AWS Api gateway etc: There are alternatives for all the above mentioned things. Aws is having own load balancer, service discovery , api gateway etc . Not only AWS all cloud platofrms ,like Azure, have these. Its depends which one to use.

5. Adding a general question as well , How these microservices communicate each other: Using Resttemplate or Feignclient actual rest API can be called or Message queues like Rabbit MQ etc can be used .