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spring-cloud-netflix
Commit dd967120 by Dave Syer
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Add eureka peer awareness docs

It works quite well locally actually. I tested with hostnames 127.0.0.1 and 127.0.1.1 and 2 servers running on localhost. See gh-28
parent 7c67fe28
Showing with 120 additions and 21 deletions
docs/src/main/asciidoc/spring-cloud-netflix.adoc
......@@ -14,7 +14,8 @@ When clients register with Eureka, they provide eureka with meta-data about them
Example eureka client:
```
[source,java,indent=0]
----
@Configuration
@ComponentScan
@EnableAutoConfiguration
......@@ -32,20 +33,26 @@ public class Application {
}
}
```
----
(i.e. utterly normal Spring Boot app). Configuration is required to locate the Eureka server. Example:
```
.application.yml
----
eureka:
client:
serviceUrl:
defaultZone: http://localhost:8761/eureka/
```
----
The default application name, virtual host and non-secure port are taken from the `Environment` is
`${spring.application.name}`, `${spring.application.name}.mydomain.net` and `${server.port}` respectively.
where "defaultZone" is a magic string fallback value that provides the
service URL for any client that doesn't express a preference
(i.e. it's a useful default).
The default application name (service ID), virtual host and non-secure
port, taken from the `Environment`, are `${spring.application.name}`,
`${spring.application.name}` and `${server.port}` respectively.
`@EnableEurekaClient` makes the app into both a Eureka "instance"
(i.e. it registers itself) and a "client" (i.e. it can query the
......@@ -70,11 +77,13 @@ production it's probably better to stick with the default because
there are some computations internally in the server that make
assumptions about the lease renewal period.
== Service Discovery: Eureka Server
Example eureka server:
```
[source,java,indent=0]
----
@Configuration
@EnableAutoConfiguration
@EnableEurekaServer
......@@ -85,8 +94,7 @@ public class Application {
}
}
```
----
The server has a home page with a UI, and HTTP API endpoints per the
normal Eureka functionality under `/eureka/*`.
......@@ -134,6 +142,92 @@ springBoot {
----
====
=== High Availability, Zones and Regions
The Eureka server does not have a backend store, but the service
instances in the registry all have to send heartbeats to keep their
resistrations up to date (so this can be done in memory). Clients also
have an in-memory cache of eureka registrations (so they don't have to
go to the registry for every single request to a service).
By default every Eureka server is also a Eureka client and requires
(at least one) service URL to locate a peer. If you don't provide it
the service will run and work, but it will shower your logs with a lot
of noise about not being able to register with the peer.
=== Standalone Mode
The combination of the two caches (client and server) and the
heartbeats make a standalone Eureka server fairly resilient to
failure, as long as there is some sort of monitor or elastic runtime
keeping it alive (e.g. Cloud Foundry). In standalone mode, you might
prefer to switch off the client side behaviour, so it doesn't keep
trying and failing to reach its peers. Example:
.application.yml (Standalone Eureka Server)
----
server:
port: 8761
eureka:
instance:
hostname: localhost
client:
registerWithEureka: false
fetchRegistry: false
serviceUrl:
defaultZone: http://${eureka.instance.hostname}:${server.port}/eureka/
----
Notice that the `serviceUrl` is pointing to the same host as the local
instance.
=== Peer Awareness
Eureka can be made even more resilient and available by running
multiple instances and asking them to register with each other. In
fact, this is the default behaviour, so all you need to do to make it
work is add a valid `serviceUrl` to a peer, e.g.
.application.yml (Two Peer Aware Eureka Servers)
----
---
spring:
profiles: peer1
eureka:
instance:
hostname: peer1
client:
serviceUrl:
defaultZone: http://peer2/eureka/
---
spring:
profiles: peer2
eureka:
instance:
hostname: peer2
client:
serviceUrl:
defaultZone: http://peer1/eureka/
----
In this example we have a YAML file that can be used to run the same
server on 2 hosts (peer1 and peer2), by running it in different
Spring profiles. You could use this configuration to test the peer
awareness on a single host (there's not much value in doing that in
production) by manipulating `/etc/hosts` to resolve the host names. In
fact, the `eureka.instance.hostname` is not needed if you are running
on a machine that knows its own hostname (it is looked up using
`java.net.InetAddress` by default).
You can add multiple peers to a system, and as long as they are all
connected to each other by at least one edge, they will synchronize
the registrations amongst themselves. If the peers are physically
separated (inside a data centre or between multiple data centres) then
the system can in principle survive split-brain type failures.
== Circuit Breaker: Hystrix Clients
Netflix has created a library called https://github.com/Netflix/Hystrix[Hystrix] that implements the http://martinfowler.com/bliki/CircuitBreaker.html[circuit breaker pattern]. In a microservice architecture it is common to have multiple layers of service calls.
......@@ -150,7 +244,7 @@ Having an open circuit stops cascading failures and allows overwhelmed or failin
Example boot app:
```
----
@Configuration
@EnableAutoConfiguration
@EnableHystrix
......@@ -175,7 +269,7 @@ public class StoreIntegration {
}
}
```
----
The `@HystrixCommand` is provided by a Netflix contrib library called
"javanica". Spring Cloud automatically wraps Spring beans with that
......@@ -201,7 +295,9 @@ Looking at an individual instances Hystrix data is not very useful in terms of t
https://github.com/Netflix/feign[Feign] is a declarative web service client. It makes writing web service clients easier. To use Feign create an interface and annotate it. It has pluggable annotation support including Feign annotations and JAX-RS annotations. Feign also supports pluggable encoders and decoders. Spring Cloud adds support for Spring MVC annotations and for using the same `HttpMessageConverters` used by default in Spring Web. Spring Cloud integrates Ribbon and Eureka to provide a load balanced http client when using Feign.
Example spring boot app
```
[source,java,indent=0]
----
@Configuration
@ComponentScan
@EnableAutoConfiguration
......@@ -218,10 +314,11 @@ public class Application extends FeignConfigurer {
}
}
```
----
StoreClient.java
```
.StoreClient.java
[source,java,indent=0]
----
public interface StoreClient {
@RequestMapping(method = RequestMethod.GET, value = "/stores")
Stores getStores();
......@@ -229,13 +326,14 @@ public interface StoreClient {
@RequestMapping(method = RequestMethod.POST, value = "/stores/{storeId}", consumes = "application/json")
Store update(@PathParameter("storeId") Long storeId, Store store);
}
```
----
== Client Side Load Balancer: Ribbon
Usage of `LoadBalancerClient` directly:
```
[source,java,indent=0]
----
public class MyClass {
@Autowired
private LoadBalancerClient loadBalancer;
......@@ -246,11 +344,12 @@ public class MyClass {
// ... do something with the URI
}
}
```
----
Indirect usage via `RestTemplate`.
```
[source,java,indent=0]
----
public class MyClass {
@Autowired
private RestTemplate restTemplate;
......@@ -260,7 +359,7 @@ public class MyClass {
return results;
}
}
```
----
== External Configuration: Archaius
......
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