# Spring Cloud Zookeeper

This project provides Zookeeper 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 Zookeeper based components. The patterns provided include Service Discovery and Configuration. Intelligent Routing (Zuul) and Client Side Load Balancing (Ribbon), Circuit Breaker (Hystrix) are provided by integration with Spring Cloud Netflix.

## Install Zookeeper

Please see the installation documentation for instructions on how to install Zookeeper.

## Service Discovery with Zookeeper

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. Curator(A java library for Zookeeper) provides Service Discovery services via Service Discovery Extension. Spring Cloud Zookeeper leverages this extension for service registration and discovery.

### How to activate

Including a dependency on `org.springframework.cloud:spring-cloud-starter-zookeeper-discovery` will enable auto-configuration that will setup Spring Cloud Zookeeper Discovery.

### Registering with Zookeeper

When a client registers with Zookeeper, it provides meta-data about itself such as host and port, id and name.

Example Zookeeper client:

    @SpringBootApplication
    @EnableDiscoveryClient
    @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). If Zookeeper is located somewhere other than `localhost:2181`, the configuration is required to locate the server. Example:

application.yml

    spring:
      cloud:
        zookeeper:
          connect-string: localhost:2181

Caution | If you use Spring Cloud Zookeeper Config, the above values will need to be placed in `bootstrap.yml` instead of `application.yml`.

The default service name, instance id and port, taken from the `Environment`, are `${spring.application.name}`, the Spring Context ID and `${server.port}` respectively.

`@EnableDiscoveryClient` makes the app into both a Zookeeper "service" (i.e. it registers itself) and a "client" (i.e. it can query Zookeeper to locate other services).

### Using the DiscoveryClient

Spring Cloud has support for Feign (a REST client builder) and also Spring `RestTemplate` using the logical service names instead of physical URLs.

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 = discoveryClient.getInstances("STORES");
        if (list != null && list.size() > 0 ) {
            return list.get(0).getUri().toString();
        }
        return null;
    }

## Zookeeper Dependencies

### Using the Zookeeper Dependencies

Spring Cloud Zookeeper gives you a possibility to provide dependencies of your application as properties. As dependencies you can understand other applications that are registered in Zookeeper and which you would like to call via Feign (a REST client builder) and also Spring `RestTemplate`.

You can also benefit from the Zookeeper Dependency Watchers functionality that lets you control and monitor what is the state of your dependencies and decide what to do with that.

### How to activate Zookeeper Dependencies

- Including a dependency on `org.springframework.cloud:spring-cloud-starter-zookeeper-discovery` will enable auto-configuration that will setup Spring Cloud Zookeeper Dependencies.

- If you have to have the `spring.cloud.zookeeper.dependencies` section properly set up - check the subsequent section for more details then the feature is active

- You can have the dependencies turned off even if you’ve provided the dependencies in your properties. Just set the property `spring.cloud.zookeeper.dependency.enabled` to false (defaults to `true`).

### Setting up Zookeeper Dependencies

Let’s take a closer look at an example of dependencies representation:

application.yml

    spring.application.name: yourServiceName
    spring.cloud.zookeeper:
      dependencies:
        newsletter:
          path: /path/where/newsletter/has/registered/in/zookeeper
          loadBalancerType: ROUND_ROBIN
          contentTypeTemplate: application/vnd.newsletter.$version+json
          version: v1
          headers:
            header1:
                - value1
            header2:
                - value2
          required: false
          stubs: org.springframework:foo:stubs
        mailing:
          path: /path/where/mailing/has/registered/in/zookeeper
          loadBalancerType: ROUND_ROBIN
          contentTypeTemplate: application/vnd.mailing.$version+json
          version: v1
          required: true

Let’s now go through each part of the dependency one by one. The root property name is `spring.cloud.zookeeper.dependencies`.

#### Aliases

Below the root property you have to represent each dependency has by an alias due to the constraints of Ribbon (the application id has to be placed in the URL thus you can’t pass any complex path like /foo/bar/name). The alias will be the name that you will use instead of serviceId for `DiscoveryClient`, `Feign` or `RestTemplate`.

In the aforementioned examples the aliases are `newsletter` and `mailing`. Example of Feign usage with `newsletter` would be:

    @FeignClient("newsletter")
    public interface NewsletterService {
            @RequestMapping(method = RequestMethod.GET, value = "/newsletter")
            String getNewsletters();
    }

#### Path

Represented by `path` yaml property.

Path is the path under which the dependency is registered under Zookeeper. Like presented before Ribbon operates on URLs thus this path is not compliant with its requirement. That is why Spring Cloud Zookeeper maps the alias to the proper path.

#### Load balancer type

Represented by `loadBalancerType` yaml property.

If you know what kind of load balancing strategy has to be applied when calling this particular dependency then you can provide it in the yaml file and it will be automatically applied. You can choose one of the following load balancing strategies

- STICKY - once chosen the instance will always be called

- RANDOM - picks an instance randomly

- ROUND_ROBIN - iterates over instances over and over again

#### Content-Type template and version

Represented by `contentTypeTemplate` and `version` yaml property.

If you version your api via the `Content-Type` header then you don’t want to add this header to each of your requests. Also if you want to call a new version of the API you don’t want to roam around your code to bump up the API version. That’s why you can provide a `contentTypeTemplate` with a special `$version` placeholder. That placeholder will be filled by the value of the `version` yaml property. Let’s take a look at an example.

Having the following `contentTypeTemplate`:

    application/vnd.newsletter.$version+json

and the following `version`:

    v1

Will result in setting up of a `Content-Type` header for each request:

    application/vnd.newsletter.v1+json

#### Default headers

Represented by `headers` map in yaml

Sometimes each call to a dependency requires setting up of some default headers. In order not to do that in code you can set them up in the yaml file. Having the following `headers` section:

    headers:
        Accept:
            - text/html
            - application/xhtml+xml
        Cache-Control:
            - no-cache

Results in adding the `Accept` and `Cache-Control` headers with appropriate list of values in your HTTP request.

#### Obligatory dependencies

Represented by `required` property in yaml

If one of your dependencies is required to be up and running when your application is booting then it’s enough to set up the `required: true` property in the yaml file.

If your application can’t localize the required dependency during boot time it will throw an exception and the Spring Context will fail to set up. In other words your application won’t be able to start if the required dependency is not registered in Zookeeper.

You can read more about Spring Cloud Zookeeper Presence Checker in the following sections.

#### Stubs

You can provide a colon separated path to the JAR containing stubs of the dependency. Example

    stubs: org.springframework:foo:stubs

means that for a particular dependencies can be found under:

- groupId: `org.springframework`

- artifactId: `foo`

- classifier: `stubs` - this is the default value

This is actually equal to

    stubs: org.springframework:foo

since `stubs` is the default classifier.

### Configuring Spring Cloud Zookeeper Dependencies

There is a bunch of properties that you can set to enable / disable parts of Zookeeper Dependencies functionalities.

- `spring.cloud.zookeeper.dependencies` - if you don’t set this property you won’t benefit from Zookeeper Dependencies

- `spring.cloud.zookeeper.dependency.ribbon.enabled` (enabled by default) - Ribbon requires explicit global configuration or a particular one for a dependency. By turning on this property runtime load balancing strategy resolution is possible and you can profit from the `loadBalancerType` section of the Zookeeper Dependencies. The configuration that needs this property has an implementation of `LoadBalancerClient` that delegates to the `ILoadBalancer` presented in the next bullet

- `spring.cloud.zookeeper.dependency.ribbon.loadbalancer` (enabled by default) - thanks to this property the custom `ILoadBalancer` knows that the part of the URI passed to Ribbon might actually be the alias that has to be resolved to a proper path in Zookeeper. Without this property you won’t be able to register applications under nested paths.

- `spring.cloud.zookeeper.dependency.headers.enabled` (enabled by default) - this property registers such a `RibbonClient` that automatically will append appropriate headers and content types with version as presented in the Dependency configuration. Without this setting of those two parameters will not be operational.

- `spring.cloud.zookeeper.dependency.resttemplate.enabled` (enabled by default) - when enabled will modify the request headers of `@LoadBalanced` annotated `RestTemplate` so that it passes headers and content type with version set in Dependency configuration. Wihtout this setting of those two parameters will not be operational.

## Spring Cloud Zookeeper Dependency Watcher

The Dependency Watcher mechanism allows you to register listeners to your dependencies. The functionality is in fact an implementation of the `Observator` pattern. When a dependency changes its state (UP or DOWN) then some custom logic can be applied.

### How to activate

Spring Cloud Zookeeper Dependencies functionality needs to be enabled to profit from Dependency Watcher mechanism.

### Registering a listener

In order to register a listener you have to implement an interface `org.springframework.cloud.zookeeper.discovery.watcher.DependencyWatcherListener` and register it as a bean. The interface gives you one method:

        void stateChanged(String dependencyName, DependencyState newState);

If you want to register a listener for a particular dependency then the `dependencyName` would be the discriminator for your concrete implementation. `newState` will provide you with information whether your dependency has changed to `CONNECTED` or `DISCONNECTED`.

### Presence Checker

Bound with Dependency Watcher is the functionality called Presence Checker. It allows you to provide custom behaviour upon booting of your application to react accordingly to the state of your dependencies.

The default implementation of the abstract `org.springframework.cloud.zookeeper.discovery.watcher.presence.DependencyPresenceOnStartupVerifier` class is the `org.springframework.cloud.zookeeper.discovery.watcher.presence.DefaultDependencyPresenceOnStartupVerifier` which works in the following way.

- If the dependency is marked us `required` and it’s not in Zookeeper then upon booting your application will throw an exception and shutdown

- If dependency is not `required` the `org.springframework.cloud.zookeeper.discovery.watcher.presence.LogMissingDependencyChecker` will log that application is missing at `WARN` level

The functionality can be overriden since the `DefaultDependencyPresenceOnStartupVerifier` is registered only when there is no bean of `DependencyPresenceOnStartupVerifier`.

## Distributed Configuration with Zookeeper

Zookeeper provides a hierarchical namespace that allows clients to store arbitrary data, such as configuration data. Spring Cloud Zookeeper Config is an alternative to the Config Server and Client. Configuration is loaded into the Spring Environment during the special "bootstrap" phase. Configuration is stored in the `/config` namespace by default. Multiple `PropertySource` instances are created based on the application’s name and the active profiles that mimicks the Spring Cloud Config order of resolving properties. For example, an application with the name "testApp" and with the "dev" profile will have the following property sources created:

    config/testApp,dev
    config/testApp
    config/application,dev
    config/application

The most specific property source is at the top, with the least specific at the bottom. Properties is the `config/application` namespace are applicable to all applications using zookeeper for configuration. Properties in the `config/testApp` namespace are only available to the instances of the service named "testApp".

Configuration is currently read on startup of the application. Sending a HTTP POST to `/refresh` will cause the configuration to be reloaded. Watching the configuration namespace (which Zookeeper supports) is not currently implemented, but will be a future addition to this project.

### How to activate

Including a dependency on `org.springframework.cloud:spring-cloud-starter-zookeeper-config` will enable auto-configuration that will setup Spring Cloud Zookeeper Config.

### Customizing

Zookeeper Config may be customized using the following properties:

bootstrap.yml

    spring:
      cloud:
        zookeeper:
          config:
            enabled: true
            root: configuration
            defaultContext: apps
            profileSeparator: '::'

- `enabled` setting this value to "false" disables Zookeeper Config

- `root` sets the base namespace for configuration values

- `defaultContext` sets the name used by all applications

- `profileSeparator` sets the value of the separator used to separate the profile name in property sources with profiles