72. Zookeeper Dependencies

72.1 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.

72.2 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 ).

72.3 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 .

72.3.1 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();
}

72.3.2 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.

72.3.3 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

72.3.4 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

72.3.5 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.

72.3.6 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.

72.3.7 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.

72.4 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.