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Purpose of Service Discovery such as Eureka

I don’t understand the reasoning behind service discovery tools. I understand they are used so micro services can connect but why can’t we just store all the urls/load balancers in the config server as properties? Looking for some explanation for eureka over property file with the micro service urls.
Thanks

In your example of putting a URL into a config, clients still needs to do discovery to find other micro services . In this case DNS is acting like a discovery service by converting URLs into IP Addresses (to the load balancer).
using a load balancer as state is "server side load balancing" in which a consumer sends a message to the load balancer and the load balancer sends it to the appropriate back end, in this example the load balancer can become a bottle neck on a very busy service.
An alternative is that the clients do their own load balancing by having a list of instances and choosing which ones to send it to based on reports of how busy certain instances are.
Both of these solutions have strengths and weaknesses, but a discovery service such as Eureka are required for the client side load balancing approach.
Services such as eureka often have other nice non-technical advantages, which DNS does not.
The ability to record metadata against a service such as, which team in the company owns it, what it does.
A central place to look up services when you are developing a new service.
Some discovery services can track which services are looking up other services making it easy to do dependency analysis to work out what might break if a service is changed.
If the load balancer/DNS approach is working for you, a dedicated discovery service like Eureka might not fit your architecture.

Discovery service such as Eureka is used for registry purpose so that client can get the address from the registry prior to making a call to the service so if your architecture fits the model where a client based load balancer is favored, Eureka can be a best fit for that. It's norm in microservices that each service can have multiple instances running at a moment for handling the load.
why can’t we just store all the urls in the config server as
properties?
If you put the address of the services in the property file of config server, how you'll register the address of another instance of the same service at run time and make a call to it. For scalability purpose you need to run more than one instance of service, so it is necessary to have discovery service where varying number (1...N) of instances of individual service get them registered and make themselves available to the clients with the help of discovery service. Of course, you've to use some orchestration service such as Kubernetes to handle how many instances you want to run for the service.

Should Kubernetes Ingress lives together with a Spring Cloud Gateway?

Small architecture and design question please.
Question:
Should Kubernetes Ingress lives together with Spring Cloud Gateway? If not, which one should be preferred?
First, with a Spring Webflux / Spring Cloud Gateway project, I managed to have working route-based forwarding. Meaning, all my clients only need to know this one and only Spring Cloud Gateway endpoint, and the Spring Cloud Gateway will forward to serviceA if the URL contains serviceA, to serviceB if serviceB, etc, straightforward.
I added some more “software level features” such as dynamic configuration (to change the routes at runtime), circuit breaker, rate limit, bulkhead, and few others features, very cool, but really, I ended up using the route forwarding as a main feature.
Then couple of weeks back, I spent time studying Kubernetes, and more precisely Kubernetes Ingress.
I managed to learn Kubernetes Ingress is a very cool and strong thing. I managed to perform at least the route based forwarding.
Meaning, clients needs to only know this one and only Ingress endpoint, and the Ingress will forward to underlying services within the Kubernetes cluster. Where as of now, it forwards everything to Spring Cloud Gateway, which will forward to everything else. I tried, and it could have forwarded to the real business services in the first place).
And this is the moment where I am having doubts.
Did I just duplicate work? (I mean in terms on functionally, I had fun learning both).
Should I consider an architecture where the Spring Cloud Gateway (only him) is really doing the gating?
Should I consider an architecture where both the Ingress and the Software Gateway have full importance, configuring features in both? (Accept the duplicated work?)
Should I remove the Spring Cloud Gateway entirely?
Thank you

In my opinion, Kubernetes must lives together with Spring Cloud Gateway.
Reverse proxies has capabilities like central logging, security, caching, routing, traffic management features etc, but there are also things they cannot do. API Gateways come into play at this point. They have all the features of reverse proxies, plus they have extra capabilities that they can't. For this reason, API Gateways are called enhanced reverse proxies.
So Kubernetes ingress acts like reverse proxy, while Spring Cloud Gateway is implementation of API Gateway pattern. As I mentioned in the definition above, I can say few things you can't do with Kubernetes.
Can you implement API composition? No
Can you implement JWT authentication by Kubernetes? If so, Can you carry JWTs more than 8 KB to downstream service by Kubernetes? No
Can you combine all Swagger/Open API documentation by Kubernetes? No

is Zuul being the unique entrypoint to my app a disadvantge?

i've been using Zuul in my micro-services based application for a while now, and it's been working perfectly.
now i've been asked a question by someone that made me think that micro-services world is not all rainbows and flowers after all.
so the question was " you have Zuul as the sole entrypoint to you app, isn't that some sort of centralization ? what if zuul is down ? "
i just wanted to gather some opinions, and get an answer to that question.
thank you.

Yes it is entry point and it serves many pros having single entry point in your micro service architecture.
Pros:
I can think of on top of mind
Single point of contact for all you API users(Apps). Zuul running at api.yoursite.com and calling it every time is better than calling individual services(service1.yoursite.com, service2.yoursite.com) from you api consumers.
Security point of view, you have to deal with only one server's security and you can hide all the other servers running your services in network and not exposing them on public internet. Amazon AWS and many other providers provides such facility.
You can leverage routing benefits using zuul as your entry point you can route 80% traffic to service1 old version while 20% to your same service1 new version.
Cons:
Great power comes with great responsibility.
If your main zuul server is down your whole app is down.(Edit: only if you have one zuul setup). Please check #lahiru's answer how you can have multiple zuul cluster using eureka registry configuration.

If your Zuul instance is down. You might not be able to direct to other services through the api gateway.
As a solution i would suggest you to go for a clustered solution with two zuul instances.
There you need two client side service registry services (Eureka Servers) .
If you are planning to manage fault tolerance, you can apply sentinel.
Your micro services become eureka clients.
You have to register your micro services in both of the eureka servers and make a eureka server cluster. You can refer to below diagram as a design /architecture diagram for implementation.
Note : You have to maintain two end points for both of the zuul servers. If zuul 1 is down , client can request to zuul 2.
This will help you to achieve zero down time .(You need to make sure that your VM platform is reliable and no unplanned downtimes).

Combination of Spring Cloud and Orchestration Tools Like Docker Swarm and Kubernetes

I have a cloud-native application, which is implemented using Spring Cloud Netflix.
So, in my application, I'm using Eureka service discovery to manage all instances of different services of the application. When each service instance wants to talk to another one, it uses Eureka to fetch the required information about the target service (IP and port for example).
The service orchestration can also be achieved using tools like Docker Swarm and Kubernetes, and it looks there are some overlaps between what Eureka does and what Docker Swarm and Kubernetes can do.
For example, Imagine I create a service in Docker Swarm with 5 instances. So, swarm insures that those 5 instances are always up and running. Additionally, each services of the application is sending a periodic heartbeat to the Eureka internally, to show that it's still alive. It seems we have two layers of health check here, one for Docker and another inside the Spring Cloud itself.
Or for example, you can expose a port for a service across the entire swarm, which eliminates some of the needs to have a service discovery (the ports are always apparent). Another example could be load balancing performed by the routing mesh inside the docker, and the load balancing happening internally by Ribbon component or Eureka itself. In this case, having a hardware load balancer, leads us to a 3-layered load balancing functionality.
So, I want to know is it rational to use these tools together? It seems using a combination of these technologies increases the complexity of the application very much and may be redundant.
Thank you for reading!

If you already have the application working then there's presumably more effort and risk in removing the netflix components than keeping them. There's an argument that if you could remove e.g. eureka then you wouldn't need to maintain it and it would be one less thing to upgrade. But that might not justify the effort and it also depends on whether you are using it for anything that might not be fulfilled by the orchestration tool.
For example, if you're connecting to services that are not set up as load-balanced ('headless services') then you might want ribbon within your services. (You could do this using tools in the spring cloud kubernetes incubator project or its fabric8 equivalent.) Another situation to be mindful of is when you're connecting to external services (i.e. services outside the kubernetes cluster) - then you might want to add load-balancing or rate limiting and ribbon/hystrix would be an option. It will depend on how nuanced your requirements for load-balancing or rate-limiting are.
You've asked specifically about netflix but it's worth stating clearly that spring cloud includes other components and not just netflix ones. And that there's other areas of overlap where you would need to make choices.
I've focused on Kubernetes rather than docker swarm partly because that's what I know best and partly because that's what I believe to be the current direction of travel for the industry - on this you should note that kubernetes is available within docker EE. I guess you've read many comparison articles but https://hackernoon.com/a-kubernetes-guide-for-docker-swarm-users-c14c8aa266cc might be particularly interesting to you.

You are correct in that it does seem redundant. From personal observations, I think that each layer of that architecture should handle load balancing in its' own specific way. It ends up giving you a lot more flexibility for not much more cost. If you want to take advantage of client side load balancing and any failover features, it makes sense to have Eureka. The major benefit is that if you don't want to take advantage of all of the features, you don't have to.
The container orchestration level load balancing has a place for any applications or services that do not conform to your service discovery piece that resides at the application level (Eureka).
The hardware load balancer provides another level that allows for load balancing outside of your container orchestrator.
The specific use case that I ran into was on AWS for a Kubernetes cluster with Traefik and Eureka with Spring Cloud.

Yes, you are correct. We have a similar Spring Cloud Netflix application deployed on Oracle cloud platform and Predix Cloud Foundry. If you use multiple Kubernetes clusters then you have to use Ribbon load balancing because you have multiple instance for services.
I cannot tell you which is better Kubernetes or Docker Swarm. We use Kubernetes for service orchestration as it provides more flexibility.

Using Hystrix to get list of services from Eureka through the circuit breakers

I exploit spring-cloud. As far as I understand, when client of Eureka gets a list of services from Eureka server, it uses the Ribbon for load balancing.
Does the client use Hystrix to get the list of services from Eureka through the circuit breakers?

There is a Gateway Service called Netflix Zuul(You can also call it as edge Service).Client connect to gateway service which in turn queries Eureka Server to get appropriate Micro Service details.
Hystrix basically uses fault tolerance mechanism which can be used in any Micro Service. Its advantage is that, if any API goes down ,it gracefully handles the errors in the Application.

As shankarsh15 says, Hystrix actually provides resilience (e.g. fallbacks) when errors and/or timeouts occur in API calls.
I believe it's actually ribbon-loadbalance (LoadBalancerContext.java -> getServerFromLoadBalancer()) that determines which client to call.
And this ultimately works in a similar way to doing discoveryClient.getInstances("service-name") (aka gets a list of service instances, then uses round robin to pick a service to use)