Zookeeper Dependencies
The following topics cover how to work with Spring Cloud 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 through
OpenFeign
(a REST client builder), RestTemplate
and WebClient
via Spring Cloud Loadbalancer.
You can also use the Zookeeper Dependency Watchers functionality to control and monitor the state of your dependencies.
Activating Zookeeper Dependencies
Including a dependency on
org.springframework.cloud:spring-cloud-starter-zookeeper-discovery
enables
autoconfiguration that sets up Spring Cloud Zookeeper Dependencies. Even if you provide
the dependencies in your properties, you can turn off the dependencies. To do so, set the
spring.cloud.zookeeper.dependency.enabled
property to false (it defaults to true
).
Setting up Zookeeper Dependencies
Consider the following example of dependency representation:
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
The next few sections 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 as an alias.
This is due to the constraints of Spring Cloud LoadBalancer, which requires that the application ID be placed in the URL.
Consequently, you cannot pass any complex path, suchas /myApp/myRoute/name
).
The alias is the name you use instead of the serviceId
for DiscoveryClient
, Feign
, or
RestTemplate
.
In the previous examples, the aliases are newsletter
and mailing
.
The following example shows Feign usage with a newsletter
alias:
@FeignClient("newsletter")
public interface NewsletterService {
@RequestMapping(method = RequestMethod.GET, value = "/newsletter")
String getNewsletters();
}
Path
The path is represented by the path
YAML property and is the path under which the dependency is registered under Zookeeper.
As described in the
previous section, Spring Cloud LoadBalancer operates on URLs.
As a result, this path is not compliant with its requirement.
That is why Spring Cloud Zookeeper maps the alias to the proper path.
Load Balancer Type
The load balancer type is represented by loadBalancerType
YAML property.
If you know what kind of load-balancing strategy has to be applied when calling this particular dependency, you can provide it in the YAML file, and it is automatically applied. You can choose one of the following load balancing strategies:
-
STICKY: Once chosen, the instance is always called.
-
RANDOM: Picks an instance randomly.
-
ROUND_ROBIN: Iterates over instances over and over again.
Content-Type
Template and Version
The Content-Type
template and version are represented by the contentTypeTemplate
and
version
YAML properties.
If you version your API in the Content-Type
header, you do not want to add this header
to each of your requests. Also, if you want to call a new version of the API, you do not
want to roam around your code to bump up the API version. That is why you can provide a
contentTypeTemplate
with a special $version
placeholder. That placeholder will be filled by the value of the
version
YAML property. Consider the following example of a contentTypeTemplate
:
application/vnd.newsletter.$version+json
Further consider the following version
:
v1
The combination of contentTypeTemplate
and version results in the creation of a
Content-Type
header for each request, as follows:
application/vnd.newsletter.v1+json
Default Headers
Default headers are represented by the headers
map in YAML.
Sometimes, each call to a dependency requires setting up of some default headers. To not
do that in code, you can set them up in the YAML file, as shown in the following example
headers
section:
headers:
Accept:
- text/html
- application/xhtml+xml
Cache-Control:
- no-cache
That headers
section results in adding the Accept
and Cache-Control
headers with
appropriate list of values in your HTTP request.
Required Dependencies
Required dependencies are represented by required
property in YAML.
If one of your dependencies is required to be up when your application boots, you can set
the required: true
property in the YAML file.
If your application cannot localize the required dependency during boot time, it throws an exception, and the Spring Context fails to set up. In other words, your application cannot start if the required dependency is not registered in Zookeeper.
You can read more about Spring Cloud Zookeeper Presence Checker later in this document.
Stubs
You can provide a colon-separated path to the JAR containing stubs of the dependency, as shown in the following example:
stubs: org.springframework:myApp:stubs
where:
-
org.springframework
is thegroupId
. -
myApp
is theartifactId
. -
stubs
is the classifier. (Note thatstubs
is the default value.)
Because stubs
is the default classifier, the preceding example is equal to the following
example:
stubs: org.springframework:myApp
Configuring Spring Cloud Zookeeper Dependencies
You can set the following properties to enable or disable parts of Zookeeper Dependencies functionalities:
-
spring.cloud.zookeeper.dependencies
: If you do not set this property, you cannot use Zookeeper Dependencies. -
spring.cloud.zookeeper.dependency.loadbalancer.enabled
(enabled by default): Turns on Zookeeper-specific custom load-balancing strategies, includingZookeeperServiceInstanceListSupplier
and dependency-based load-balancedRestTemplate
setup. -
spring.cloud.zookeeper.dependency.headers.enabled
(enabled by default): This property registers aFeignBlockingLoadBalancerClient
that automatically appends appropriate headers and content types with their versions, as presented in the Dependency configuration. Without this setting, those two parameters do not work. -
spring.cloud.zookeeper.dependency.resttemplate.enabled
(enabled by default): When enabled, this property modifies the request headers of a@LoadBalanced
-annotatedRestTemplate
such that it passes headers and content type with the version set in dependency configuration. Without this setting, those two parameters do not work.