# Spring Cloud Netflix

This project provides Netflix OSS integrations for Spring Boot apps through autoconfiguration and binding to the Spring Environment and other Spring programming model idioms. With a few simple annotations you can quickly enable and configure the common patterns inside your application and build large distributed systems with battle-tested Netflix components. The patterns provided include Service Discovery (Eureka), Circuit Breaker (Hystrix), Intelligent Routing (Zuul) and Client Side Load Balancing (Ribbon).

## Service Discovery: Eureka Clients

Service Discovery is one of the key tenets of a microservice based architecture. Trying to hand configure each client or some form of convention can be very difficult to do and can be very brittle. Eureka is the Netflix Service Discovery Server and Client. The server can be configured and deployed to be highly available, with each server replicating state about the registered services to the others.

### Registering with Eureka

When a client registers with Eureka, it provides meta-data about itself such as host and port, health indicator URL, home page etc. Eureka receives heartbeat messages from each instance belonging to a service. If the heartbeat fails over a configurable timetable, the instance is normally removed from the registry.

Example eureka client:

    @Configuration
    @ComponentScan
    @EnableAutoConfiguration
    @EnableEurekaClient
    @RestController
    public class Application {

        @RequestMapping("/")
        public String home() {
            return "Hello world";
        }

        public static void main(String[] args) {
            new SpringApplicationBuilder(Application.class).web(true).run(args);
        }

    }

(i.e. utterly normal Spring Boot app). In this example we use `@EnableEurekaClient` explicitly, but with only Eureka available you could also use `@EnableDiscoveryClient`. Configuration is required to locate the Eureka server. Example:

application.yml

    eureka:
      client:
        serviceUrl:
          defaultZone: http://localhost:8761/eureka/

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 registry to locate other services). The instance behaviour is driven by `eureka.instance.*` configuration keys, but the defaults will be fine if you ensure that your application has a `spring.application.name` (this is the default for the Eureka service ID, or VIP).

See EurekaInstanceConfigBean and EurekaClientConfigBean for more details of the configurable options.

### Status Page and Health Indicator

The status page and health indicators for a Eureka instance default to "/info" and "/health" respectively, which are the default locations of useful endpoints in a Spring Boot Actuator application. You need to change these, even for an Actuator application if you use a non-default context path or servlet path (e.g. `server.servletPath=/foo`) or management endpoint path (e.g. `management.contextPath=/admin`). Example:

application.yml

    eureka:
      instance:
        statusPageUrlPath: ${management.contextPath}/info
        healthCheckUrlPath: ${management.contextPath}/health

These links show up in the metadata that is consumers by clients, and used in some scenarios to decide whether to send requests to your application, so it’s helpful if they are accurate.

### Eureka Metadata for Instances and Clients

It’s worth spending a bit of time understanding how the Eureka metadata works, so you can use it in a way that makes sense in your platform. There is standard metadata for things like hostname, IP address, port numbers, status page and health check. These are published in the service registry and used by clients to contact the services in a straightforward way. Additional metadata can be added to the instance registration in the `eureka.instance.metadataMap`, and this will be accessible in the remote clients, but in general will not change the behaviour of the client, unless it is made aware of the meaning of the metadata. There are a couple of special cases described below where Spring Cloud already assigns meaning to the metadata map.

#### Using Eureka on Cloudfoundry

Cloudfoundry has a global router so that all instances of the same app have the same hostname (it’s the same in other PaaS solutions with a similar architecture). This isn’t necessarily a barrier to using Eureka, but if you use the router (recommended, or even mandatory depending on the way your platform was set up), you need to explicitly set the hostname and port numbers (secure or non-secure) so that they use the router. You might also want to use instance metadata so you can distinguish between the instances on the client (e.g. in a custom load balancer). For example:

application.yml

    eureka:
      instance:
        hostname: ${vcap.application.uris[0]}
        nonSecurePort: 80
        metadataMap:
          instanceId: ${vcap.application.instance_id:${spring.application.name}:${spring.application.instance_id:${server.port}}}

Depending on the way the security rules are set up in your Cloudfoundry instance, you might be able to register and use the IP address of the host VM for direct service-to-service calls. This feature is not (yet) available on Pivotal Web Services (PWS).

#### Using Eureka on AWS

If the application is planned to be deployed to an AWS cloud, then the Eureka instance will have to be configured to be Amazon aware and this can be done by customizing the EurekaInstanceConfigBean the following way:

    @Bean
    @Profile("!default")
    public EurekaInstanceConfigBean eurekaInstanceConfig() {
      EurekaInstanceConfigBean b = new EurekaInstanceConfigBean();
      AmazonInfo info = AmazonInfo.Builder.newBuilder().autoBuild("eureka");
      b.setDataCenterInfo(info);
      return b;
    }

#### Making the Eureka Instance ID Unique

By default a eureka instance is registered with an ID that is equal to its host name (i.e. only one service per host). Using Spring Cloud you can override this by providing a unique identifier in `eureka.instance.metadataMap.instanceId`. For example:

application.yml

    eureka:
      instance:
        metadataMap:
          instanceId: ${spring.application.name}:${spring.application.instance_id:${random.value}}

With this metadata, and multiple service instances deployed on localhost, the random value will kick in there to make the instance unique. In Cloudfoundry the `spring.application.instance_id` will be populated automatically in a Spring Boot Actuator application, so the random value will not be needed.

### Using the DiscoveryClient

Once you have an app that is `@EnableEurekaClient` you can use it to discover service instances from the Eureka Server. One way to do that is to use the native `com.netflix.discovery.DiscoveryClient` (as opposed to the Spring Cloud `DiscoveryClient`), e.g.

    @Autowired
    private DiscoveryClient discoveryClient;

    public String serviceUrl() {
        InstanceInfo instance = discoveryClient.getNextServerFromEureka("STORES", false);
        return instance.getHomePageUrl();
    }

Tip | Don’t use the `DiscoveryClient` in `@PostConstruct` method or in a `@Scheduled` method (or anywhere where the `ApplicationContext` might not be started yet). It is initialized in a `SmartLifecycle` (with `phase=0`) so the earliest you can rely on it being available is in another `SmartLifecycle` with higher phase.

### Alternatives to the native Netflix DiscoveryClient

You don’t have to use the raw Netflix `DiscoveryClient` and usually it is more convenient to use it behind a wrapper of some sort. Spring Cloud has support for Feign (a REST client builder) and also Spring `RestTemplate` using the logical Eureka service identifiers (VIPs) instead of physical URLs. To configure Ribbon with a fixed list of physical servers you can simply set `<client>.ribbon.listOfServers` to a comma-separated list of physical addresses (or hostnames), where `<client>` is the ID of the client.

You can also use the `org.springframework.cloud.client.discovery.DiscoveryClient` which provides a simple API for discovery clients that is not specific to Netflix, e.g.

    @Autowired
    private DiscoveryClient discoveryClient;

    public String serviceUrl() {
        List<ServiceInstance> list = client.getInstances("STORES");
        if (list != null && list.size() > 0 ) {
            return list.get(0).getUri();
        }
        return null;
    }

### Why is it so Slow to Register a Service?

Being an instance also involves a periodic heartbeat to the registry (via the client’s `serviceUrl`) with default duration 30 seconds. A service is not available for discovery by clients until the instance, the server and the client all have the same metadata in their local cache (so it could take 3 hearbeats). You can change the period using `eureka.instance.leaseRenewalIntervalInSeconds` and this will speed up the process of getting clients connected to other services. In 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 (e.g. using spring-cloud-starter-eureka-server to set up the classpath):

    @SpringBootApplication
    @EnableEurekaServer
    public class Application {

        public static void main(String[] args) {
            new SpringApplicationBuilder(Application.class).web(true).run(args);
        }

    }

The server has a home page with a UI, and HTTP API endpoints per the normal Eureka functionality under `/eureka/*`.

Eureka background reading: see flux capacitor and google group discussion.

Tip

Due to Gradle’s dependency resolution rules and the lack of a parent bom feature, simply depending on spring-cloud-starter-eureka-server can cause failures on application startup. To remedy this the Spring dependency management plugin must be added and the Spring cloud starter parent bom must be imported like so:

build.gradle

    buildscript {
      dependencies {
        classpath "io.spring.gradle:dependency-management-plugin:0.4.0.RELEASE"
      }
    }

    apply plugin: "io.spring.dependency-management"

    dependencyManagement {
      imports {
        mavenBom 'org.springframework.cloud:spring-cloud-starter-parent:1.0.0.RELEASE'
      }
    }

### 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 registrations 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.

See also below for details of Ribbon support on the client side for Zones and Regions.

### 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.

### Prefer IP Address

In some cases, it is preferable for Eureka to advertise the IP Adresses of services rather than the hostname. Set `eureka.instance.preferIpAddress` to `true` and when the application registers with eureka, it will use its IP Address rather than its hostname.

## Circuit Breaker: Hystrix Clients

Netflix has created a library called Hystrix that implements the circuit breaker pattern. In a microservice architecture it is common to have multiple layers of service calls.

Figure 1. Microservice Graph

A service failure in the lower level of services can cause cascading failure all the way up to the user. When calls to a particular service reach a certain threshold (20 failures in 5 seconds is the default in Hystrix), the circuit opens and the call is not made. In cases of error and an open circuit a fallback can be provided by the developer.

Figure 2. Hystrix fallback prevents cascading failures

Having an open circuit stops cascading failures and allows overwhelmed or failing services time to heal. The fallback can be another Hystrix protected call, static data or a sane empty value. Fallbacks may be chained so the first fallback makes some other business call which in turn falls back to static data.

Example boot app:

    @SpringBootApplication
    @EnableCircuitBreaker
    public class Application {

        public static void main(String[] args) {
            new SpringApplicationBuilder(Application.class).web(true).run(args);
        }

    }

    @Component
    public class StoreIntegration {

        @HystrixCommand(fallbackMethod = "defaultStores")
        public Object getStores(Map<String, Object> parameters) {
            //do stuff that might fail
        }

        public Object defaultStores(Map<String, Object> parameters) {
            return /* something useful */;
        }
    }

The `@HystrixCommand` is provided by a Netflix contrib library called "javanica". Spring Cloud automatically wraps Spring beans with that annotation in a proxy that is connected to the Hystrix circuit breaker. The circuit breaker calculates when to open and close the circuit, and what to do in case of a failure.

To configure the `@HystrixCommand` you can use the `commandProperties` attribute with a list of `@HystrixProperty` annotations. See here for more details. See the Hystrix wiki for details on the properties available.

### Propagating the Security Context or using Spring Scopes

If you want some thread local context to propagate into a `@HystrixCommand` the default declaration will not work because it executes the command in a thread pool (in case of timeouts). You can switch Hystrix to use the same thread as the caller using some configuration, or directly in the annotation, by asking it to use a different "Isolation Strategy". For example:

    @HystrixCommand(fallbackMethod = "stubMyService",
        commandProperties = {
          @HystrixProperty(name="execution.isolation.strategy", value="SEMAPHORE")
        }
    )
    ...

The same thing applies if you are using `@SessionScope` or `@RequestScope`. You will know when you need to do this because of a runtime exception that says it can’t find the scoped context.

In particular you might be interested

### Health Indicator

The state of the connected circuit breakers are also exposed in the `/health` endpoint of the calling application.

    {
        "hystrix": {
            "openCircuitBreakers": [
                "StoreIntegration::getStoresByLocationLink"
            ],
            "status": "CIRCUIT_OPEN"
        },
        "status": "UP"
    }

### Hystrix Metrics Stream

To enable the Hystrix metrics stream include a dependency on `spring-boot-starter-actuator`. This will expose the `/hystrix.stream` as a management endpoint.

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>

## Circuit Breaker: Hystrix Dashboard

One of the main benefits of Hystrix is the set of metrics it gathers about each HystrixCommand. The Hystrix Dashboard displays the health of each circuit breaker in an efficient manner.

Figure 3. Hystrix Dashboard

To run the Hystrix Dashboard annotate your Spring Boot main class with `@EnableHystrixDashboard`. You then visit `/hystrix` and point the dashboard to an individual instances `/hystrix.stream` endpoint in a Hystrix client application.

### Turbine

Looking at an individual instances Hystrix data is not very useful in terms of the overall health of the system. Turbine is an application that aggregates all of the relevant `/hystrix.stream` endpoints into a combined `/turbine.stream` for use in the Hystrix Dashboard. Individual instances are located via Eureka. Running Turbine is as simple as annotating your main class with the `@EnableTurbine` annotation (e.g. using spring-cloud-starter-turbine to set up the classpath). All of the documented configuration properties from the Turbine 1 wiki apply. The only difference is that the `turbine.instanceUrlSuffix` does not need the port prepended as this is handled automatically unless `turbine.instanceInsertPort=false`.

The configuration key `turbine.appConfig` is a list of eureka serviceIds that turbine will use to lookup instances. The turbine stream is then used in the Hystrix dashboard using a url that looks like: `http://my.turbine.sever:8080/turbine.stream?cluster=<CLUSTERNAME>;` (the cluster parameter can be omitted if the name is "default"). The `cluster` parameter must match an entry in `turbine.aggregator.clusterConfig`. Values returned from eureka are uppercase, thus we expect this example to work if there is an app registered with Eureka called "customers":

    turbine:
      aggregator:
        clusterConfig: CUSTOMERS
      appConfig: customers

The `clusterName` can be customized by a SPEL expression in `turbine.clusterNameExpression` with root an instance of `InstanceInfo`. The default value is `appName`, which means that the Eureka serviceId ends up as the cluster key (i.e. the `InstanceInfo` for customers has an `appName` of "CUSTOMERS"). A different example would be `turbine.clusterNameExpression=aSGName`, which would get the cluster name from the AWS ASG name. Another example:

    turbine:
      aggregator:
        clusterConfig: SYSTEM,USER
      appConfig: customers,stores,ui,admin
      clusterNameExpression: metadata['cluster']

In this case, the cluster name from 4 services is pulled from their metadata map, and is expected to have values that include "SYSTEM" and "USER".

To use the "default" cluster for all apps you need a string literal expression (with single quotes):

    turbine:
      appConfig: customers,stores
      clusterNameExpression: 'default'

Spring Cloud provides a `spring-cloud-starter-turbine` that has all the dependencies you need to get a Turbine server running. Just create a Spring Boot application and annotate it with `@EnableTurbine`.

### Turbine AMQP

In some environments (e.g. in a PaaS setting), the classic Turbine model of pulling metrics from all the distributed Hystrix commands doesn’t work. In that case you might want to have your Hystrix commands push metrics to Turbine, and Spring Cloud enables that with AMQP messaging. All you need to do on the client is add a dependency to `spring-cloud-netflix-hystrix-amqp` and make sure there is a Rabbit broker available (see Spring Boot documentation for details on how to configure the client credentials, but it should work out of the box for a local broker or in Cloud Foundry).

On the server side Just create a Spring Boot application and annotate it with `@EnableTurbineAmqp` and by default it will come up on port 8989 (point your Hystrix dashboard to that port, any path). You can customize the port using either `server.port` or `turbine.amqp.port`. If you have `spring-boot-starter-web` and `spring-boot-starter-actuator` on the classpath as well, then you can open up the Actuator endpoints on a separate port (with Tomcat by default) by providing a `management.port` which is different.

You can then point the Hystrix Dashboard to the Turbine AMQP Server instead of individual Hystrix streams. If Turbine AMQP is running on port 8989 on myhost, then put `http://myhost:8989` in the stream input field in the Hystrix Dashboard. Circuits will be prefixed by their respective serviceId, followed by a dot, then the circuit name.

Spring Cloud provides a `spring-cloud-starter-turbine-amqp` that has all the dependencies you need to get a Turbine AMQP server running. You need Java 8 to run the app because it is Netty-based.

## Customizing the AMQP ConnectionFactory

If you are using AMQP there needs to be a `ConnectionFactory` (from Spring Rabbit) in the application context. If there is a single `ConnectionFactory` it will be used, or if there is a one qualified as `@HystrixConnectionFactory` (on the client) and `@TurbineConnectionFactory` (on the server) it will be preferred over others, otherwise the `@Primary` one will be used. If there are multiple unqualified connection factories there will be an error.

Note that Spring Boot (as of 1.2.2) creates a `ConnectionFactory` that is not `@Primary`, so if you want to use one connection factory for the bus and another for business messages, you need to create both, and annotate them `@*ConnectionFactory` and `@Primary` respectively.

## Client Side Load Balancer: Ribbon

Ribbon is a client side load balancer which gives you a lot of control over the behaviour of HTTP and TCP clients. Feign already uses Ribbon, so if you are using `@FeignClient` then this section also applies.

A central concept in Ribbon is that of the named client. Each load balancer is part of an ensemble of components that work together to contact a remote server on demand, and the ensemble has a name that you give it as an application developer (e.g. using the `@FeignClient` annotation). Spring Cloud creates a new ensemble as an `ApplicationContext` on demand for each named client using `RibbonClientConfiguration`. This contains (amongst other things) an `ILoadBalancer`, a `RestClient`, and a `ServerListFilter`.

### Customizing the Ribbon Client

You can configure some bits of a Ribbon client using external properties in `<client>.ribbon.*`, which is no different than using the Netflix APIs natively, except that you can use Spring Boot configuration files. The native options can be inspected as static fields in `CommonClientConfigKey` (part of ribbon-core).

Spring Cloud also lets you take full control of the client by declaring additional configuration (on top of the `RibbonClientConfiguration`) using `@RibbonClient`. Example:

    @Configuration
    @RibbonClient(name = "foo", configuration = FooConfiguration.class)
    public class TestConfiguration {
    }

In this case the client is composed from the components already in `RibbonClientConfiguration` together with any in `FooConfiguration` (where the latter generally will override the former).

Warning

The `FooConfiguration` has to be `@Configuration` but take care that it is not in a `@ComponentScan` for the main application context, otherwise it will be shared by all the `@RibbonClients`. If you use `@ComponentScan` (or `@SpringBootApplication`) you need to take steps to avoid it being included (for instance put it in a separate, non-overlapping package, or specify the packages to scan explicitly in the `@ComponentScan`).

Spring Cloud Netflix provides the following beans by default for ribbon (`BeanType` beanName: `ClassName`):

- `IClientConfig` ribbonClientConfig: `DefaultClientConfigImpl`

- `IRule` ribbonRule: `ZoneAvoidanceRule`

- `IPing` ribbonPing: `NoOpPing`

- `ServerList<Server> ribbonServerList: `ConfigurationBasedServerList`

- `ServerListFilter<Server>` ribbonServerListFilter: `ZonePreferenceServerListFilter`

- `ILoadBalancer` ribbonLoadBalancer: `ZoneAwareLoadBalancer`

Creating a bean of one of those type and placing it in a `@RibbonClient` configuration (such as `FooConfiguration` above) allows you to override each one of the beans described. Example:

    @Configuration
    public class FooConfiguration {
        @Bean
        public IPing ribbonPing(IClientConfig config) {
            return new PingUrl();
        }
    }

This replaces the `NoOpPing` with `PingUrl`.

### Using Ribbon with Eureka

When Eureka is used in conjunction with Ribbon the `ribbonServerList` is overridden with an extension of `DiscoveryEnabledNIWSServerList` which populates the list of servers from Eureka. It also replaces the `IPing` interface with `NIWSDiscoveryPing` which delegates to Eureka to determine if a server is up. The `ServerList` that is installed by default is a `DomainExtractingServerList` and the purpose of this is to make physical metadata available to the load balancer without using AWS AMI metadata (which is what Netflix relies on). By default the server list will be constructed with "zone" information as provided in the instance metadata (so on the client set `eureka.instance.metadataMap.zone`), and if that is missing it can use the domain name from the server hostname as a proxy for zone (if the flag `approximateZoneFromDomain` is set). Once the zone information is available it can be used in a `ServerListFilter` (by default it will be used to locate a server in the same zone as the client because the default is a `ZonePreferenceServerListFilter`).

### Example: How to Use Ribbon Without Eureka

Eureka is a convenient way to abstract the discovery of remote servers so you don’t have to hard code their URLs in clients, but if you prefer not to use it, Ribbon and Feign are still quite amenable. Suppose you have declared a `@RibbonClient` for "stores", and Eureka is not in use (and not even on the classpath). The Ribbon client defaults to a configured server list, and you can supply the configuration like this

application.yml

    stores:
      ribbon:
        listOfServers: example.com,google.com

### Example: Disable Eureka use in Ribbon

Setting the property `ribbon.eureka.enabled = false` will explicitly disable the use of Eureka in Ribbon.

application.yml

    ribbon:
      eureka:
       enabled: false

### Using the Ribbon API Directly

You can also use the `LoadBalancerClient` directly. Example:

    public class MyClass {
        @Autowired
        private LoadBalancerClient loadBalancer;

        public void doStuff() {
            ServiceInstance instance = loadBalancer.choose("stores");
            URI storesUri = URI.create(String.format("http://%s:%s", instance.getHost(), instance.getPort()));
            // ... do something with the URI
        }
    }

## Declarative REST Client: 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

    @Configuration
    @ComponentScan
    @EnableAutoConfiguration
    @EnableEurekaClient
    @EnableFeignClients
    public class Application {

        public static void main(String[] args) {
            SpringApplication.run(Application.class, args);
        }

    }

StoreClient.java

    @FeignClient("stores")
    public interface StoreClient {
        @RequestMapping(method = RequestMethod.GET, value = "/stores")
        List<Store> getStores();

        @RequestMapping(method = RequestMethod.POST, value = "/stores/{storeId}", consumes = "application/json")
        Store update(@PathParameter("storeId") Long storeId, Store store);
    }

In the `@FeignClient` annotation the String value ("stores" above) is an arbitrary client name, which is used to create a Ribbon load balancer (see below for details of Ribbon support). You can also specify a URL using the `url` attribute (absolute value or just a hostname).

The Ribbon client above will want to discover the physical addresses for the "stores" service. If your application is a Eureka client then it will resolve the service in the Eureka service registry. If you don’t want to use Eureka, you can simply configure a list of servers in your external configuration (see above for example).

## External Configuration: Archaius

Archaius is the Netflix client side configuration library. It is the library used by all of the Netflix OSS components for configuration. Archaius is an extension of the Apache Commons Configuration project. It allows updates to configuration by either polling a source for changes or for a source to push changes to the client. Archaius uses Dynamic<Type>Property classes as handles to properties.

Archaius Example

    class ArchaiusTest {
        DynamicStringProperty myprop = DynamicPropertyFactory
                .getInstance()
                .getStringProperty("my.prop");

        void doSomething() {
            OtherClass.someMethod(myprop.get());
        }
    }

Archaius has its own set of configuration files and loading priorities. Spring applications should generally not use Archaius directly., but the need to configure the Netflix tools natively remains. Spring Cloud has a Spring Environment Bridge so Archaius can read properties from the Spring Environment. This allows Spring Boot projects to use the normal configuration toolchain, while allowing them to configure the Netflix tools, for the most part, as documented.

## Router and Filter: Zuul

Routing in an integral part of a microservice architecture. For example, `/` may be mapped to your web application, `/api/users` is mapped to the user service and `/api/shop` is mapped to the shop service. Zuul is a JVM based router and server side load balancer by Netflix.

Netflix uses Zuul for the following:

- Authentication

- Insights

- Stress Testing

- Canary Testing

- Dynamic Routing

- Service Migration

- Load Shedding

- Security

- Static Response handling

- Active/Active traffic management

Zuul’s rule engine allows rules and filters to be written in essentially any JVM language, with built in support for Java and Groovy.

### Embedded Zuul Reverse Proxy

Spring Cloud has created an embedded Zuul proxy to ease the development of a very common use case where a UI application wants to proxy calls to one or more back end services. This feature is useful for a user interface to proxy to the backend services it requires, avoiding the need to manage CORS and authentication concerns independently for all the backends.

To enable it, annotate a Spring Boot main class with `@EnableZuulProxy`, and this forwards local calls to the appropriate service. By convention, a service with the Eureka ID "users", will receive requests from the proxy located at `/users` (with the prefix stripped). The proxy uses Ribbon to locate an instance to forward to via Eureka, and all requests are executed in a hystrix command, so failures will show up in Hystrix metrics, and once the circuit is open the proxy will not try to contact the service.

To skip having a service automatically added, set `zuul.ignored-services` to a list of service id patterns. If a service matches a pattern that is ignored, but also included in the explicitly configured routes map, then it will be unignored. Example:

application.yml

     zuul:
      ignoredServices: *
      routes:
        users: /myusers/**

In this example, all services are ignored except "users".

To augment or change the proxy routes, you can add external configuration like the following:

application.yml

     zuul:
      routes:
        users: /myusers/**

This means that http calls to "/myusers" get forwarded to the "users" service (for example "/myusers/101" is forwarded to "/101").

To get more fine-grained control over a route you can specify the path and the serviceId independently:

application.yml

     zuul:
      routes:
        users:
          path: /myusers/**
          serviceId: users_service

This means that http calls to "/myusers" get forwarded to the "users_service" service. The route has to have a "path" which can be specified as an ant-style pattern, so "/myusers/*" only matches one level, but "/myusers/**" matches hierarchically.

The location of the backend can be specified as either a "serviceId" (for a Eureka service) or a "url" (for a physical location), e.g.

application.yml

     zuul:
      routes:
        users:
          path: /myusers/**
          url: http://example.com/users_service

These simple url-routes doesn’t get executed as HystrixCommand nor can you loadbalance multiple url with Ribbon. To achieve this specify a service-route and configure a Ribbon client for the serviceId (this currently requires disabling Eureka support in Ribbon: see above for more information), e.g.

application.yml

    zuul:
      routes:
        users:
          path: /myusers/**
          serviceId: users

    ribbon:
      eureka:
        enabled: false

    users:
      ribbon:
        listOfServers: example.com,google.com

To add a prefix to all mappings, set `zuul.prefix` to a value, such as `/api`. The proxy prefix is stripped from the request before the request is forwarded by default (switch this behaviour off with `zuul.stripPrefix=false`). You can also switch off the stripping of the service-specific prefix from individual routes, e.g.

application.yml

     zuul:
      routes:
        users:
          path: /myusers/**
          stripPrefix: false

In this example requests to "/myusers/101" will be forwarded to "/myusers/101" on the "users" service.

The `zuul.routes` entries actually bind to an object of type `ProxyRouteLocator`. If you look at the properties of that object you will see that it also has a "retryable" flag. Set that flag to "true" to have the Ribbon client automatically retry failed requests (and if you need to you can modify the parameters of the retry operations using the Ribbon client configuration).

The `X-Forwarded-Host` header added to the forwarded requests by default. To turn it off set `zuul.addProxyHeaders = false`. The prefix path is stripped by default, and the request to the backend picks up a header "X-Forwarded-Prefix" ("/myusers" in the examples above).

An application with the `@EnableZuulProxy` could act as a standalone server if you set a default route ("/"), for example `zuul.route.home: /` would route all traffic (i.e. "/**") to the "home" service.

### Uploading Files through Zuul

If you `@EnableZuulProxy` you can use the proxy paths to upload files and it should just work as long as the files are small. For large files there is an alternative path which bypasses the Spring `DispatcherServlet` (to avoid multipart processing) in "/zuul/". I.e. if `zuul.routes.customers=/customers/`* then you can POST large files to "/zuul/customers/*". The servlet path is externalized via `zuul.servletPath`. Extremely large files will also require elevated timeout settings if the proxy route takes you through a Ribbon load balancer, e.g.

application.yml

    hystrix.command.default.execution.isolation.thread.timeoutInMilliseconds: 60000
    ribbon:
      ConnectTimeout: 3000
      ReadTimeout: 60000

Note that for streaming to work with large files, you need to use chunked encoding in the request (which some browsers do not do by default). E.g. on the command line:

    $ curl -v -H "Transfer-Encoding: chunked" \
        -F "file=@mylarge.iso" localhost:9999/zuul/simple/file

### Plain Embedded Zuul

You can also run a Zuul server without the proxying, or switch on parts of the proxying platform selectively, if you use `@EnableZuulServer` (instead of `@EnableZuulProxy`). Any beans that you add to the application of type `ZuulFilter` will be installed automatically, as they are with `@EnableZuulProxy`, but without any of the proxy filters being added automatically.

In this case the routes into the Zuul server are still specified by configuring "zuul.routes.*", but there is no service discovery and no proxying, so the "serviceId" and "url" settings are ignored. For example:

application.yml

     zuul:
      routes:
        api: /api/**

maps all paths in "/api/**" to the Zuul filter chain.

### Disable Zuul Filters

Zuul for Spring Cloud comes with a number of `ZuulFilter` beans enabled by default in both proxy and server mode. See the zuul filters package for the possible filters that are enabled. If you want to disable one, simply set `zuul.<SimpleClassName>.<filterType>.disable=true`. By convention, the package after `filters` is the Zuul filter type. For example to disable `org.springframework.cloud.netflix.zuul.filters.post.SendResponseFilter` set `zuul.SendResponseFilter.post.disable=true`.

### Polyglot support with Sidecar

Do you have non-jvm languages you want to take advantage of Eureka, Ribbon and Config Server? The Spring Cloud Netflix Sidecar was inspired by Netflix Prana. It includes a simple http api to get all of the instances (ie host and port) for a given service. You can also proxy service calls through an embedded Zuul proxy which gets its route entries from Eureka. The Spring Cloud Config Server can be accessed directly via host lookup or through the Zuul Proxy. The non-jvm app should implement a health check so the Sidecar can report to eureka if the app is up or down.

To enable the Sidecar, create a Spring Boot application with `@EnableSidecar`. This annotation includes `@EnableCircuitBreaker`, `@EnableDiscoveryClient`, and `@EnableZuulProxy`. Run the resulting application on the same host as the non-jvm application.

To configure the side car add `sidecar.port` and `sidecar.health-uri` to `application.yml`. The `sidecar.port` property is the port the non-jvm app is listening on. This is so the Sidecar can properly register the app with Eureka. The `sidecar.health-uri` is a uri accessible on the non-jvm app that mimicks a Spring Boot health indicator. It should return a json document like the following:

health-uri-document

    {
      "status":"UP"
    }

Here is an example application.yml for a Sidecar application:

application.yml

    server:
      port: 5678
    spring:
      application:
        name: sidecar

    sidecar:
      port: 8000
      health-uri: http://localhost:8000/health.json

The api for the `DiscoveryClient.getInstances()` method is `/hosts/{serviceId}`. Here is an example response for `/hosts/customers` that returns two instances on different hosts. This api is accessible to the non-jvm app (if the sidecar is on port 5678) at `http://localhost:5678/hosts/{serviceId}`.

/hosts/customers

    [
        {
            "host": "myhost",
            "port": 9000,
            "uri": "http://myhost:9000",
            "serviceId": "CUSTOMERS",
            "secure": false
        },
        {
            "host": "myhost2",
            "port": 9000,
            "uri": "http://myhost2:9000",
            "serviceId": "CUSTOMERS",
            "secure": false
        }
    ]

The Zuul proxy automatically adds routes for each service known in eureka to `/<serviceId>`, so the customers service is available at `/customers`. The Non-jvm app can access the customer service via `http://localhost:5678/customers` (assuming the sidecar is listening on port 5678).

If the Config Server is registered with Eureka, non-jvm application can access it via the Zuul proxy. If the serviceId of the ConfigServer is `configserver` and the Sidecar is on port 5678, then it can be accessed at http://localhost:5678/configserver

Non-jvm app can take advantage of the Config Server’s ability to return YAML documents. For example, a call to http://sidecar.local.spring.io:5678/configserver/default-master.yml might result in a YAML document like the following

    eureka:
      client:
        serviceUrl:
          defaultZone: http://localhost:8761/eureka/
      password: password
    info:
      description: Spring Cloud Samples
      url: https://github.com/spring-cloud-samples