Simple demo Grails applications cluster to show how to integrate with Consul for service registration, health check, cloud config, service discovery and remote service call.

At this point (July 2018), there's no dedicated Grails plugin for Consul integration. Which is not a big issue as Grails is based on Spring (boot) and Spring boot integration with Consul is trivially easy. This Grails example is to show how to integrate with Consul with Grails v3.2 and v3.3. Especially some v3.2 and below specific configuration issues encountered as its underlying (relatively older) Spring cloud version compatibility.

For Grails 3.2, the compatible spring cloud consul version is 1.2.1.RELEASE. Higher GA versions won't work with Grails 3.2 and below.

• In this version of Spring boot, a custom ConsulHealthIndicator is defined to override spring stock version to avoid null-pointer exception during consul health check.
• A similar issue also seen by this thread, where a custom ConsulHealthIndicator overrides the stock one to disable default health check logic.

Both Grails v323 and v335 apps share the same annotation @EnableDiscoveryClient to support service discovery with Consul. Same as Eureka integration, no tricks. For health check is based on SpringBoot Actuator, therefore all framework given health indicators are supported in consul health check. Grails v323 needs a custom healthIndicator as explained above.

Same as Eureka discovery server handshake design, the consul server won't initiate a health check refresh upon a client registration, but stick to its refresh cycle (default 30s) instead. Make sure the local OS hosts file has localhost entry for defined service names, grails323-service, and grails335-service in this case.

sudo vi /etc/hosts

and in hosts file add:

127.0.0.1 grails323-service
127.0.0.1 grails335-service

To work with Consul distributed configuration, the Grails v323 service application has a dummy controller with a @Value annotation to fetch shared configuration property from Consul key-value store.

This example assumes the running consul server agent registers the corresponding property in its key/value store.

In Grails application check grails-app/conf/application.yml file for consul specific settings. For example, for grails323-service:

spring:
    # spring cloud consul integration settings
    # By default, taken from the Environment:
    #  - service name is ${spring.application.name}
    #  - instance id is the Spring Context ID, which by default is
    #    ${spring.application.name}:comma,separated,profiles:${server.port}
    #  - port is ${server.port}
    application:
        name: grails323-service
    cloud:
        consul:
#            host: localhost
#            port: 8500
#            discovery:
#                health-check-path: /health
#                healthCheckInterval: 15s
            ## To enable consul distributed configuration:
            config:
                enabled: true

For this example, a locally running Consul should be running on default port 8500

This depends on your OS, but the process is simple.

For MacOS, the recommended way is install via homebrew.

# check consul bottle info in homebrew
brew info consul
# install
brew install consul

To run consul locally, simply do:

consul agent -dev

This consul server is running in dev mode, which is useful for bringing up a single-node Consul environment quickly and easily with default settings such as port 8500. Check this page for basic consul shell commands.

Make sure docker is installed. To start a consul container:

docker run -d -p 8500:8500 --name dev-consul consul

The above command starts a docker consul server agent listening at port 8500 with container name 'dev-consul'. The running container can be verified by running:

docker ps

or even for runtime resource details:

docker stats dev-consul

Check out this code repo.

The simple way of running both service nodes with consul

./gradlew :grails335-service:bootRun
./gradlew :grails323-service:bootRun

As defined in application.yml, the services are running on port 8090 and 8091 respectively. Change to other values as needed.

Based on SpringBoot profile support, Grails support per environment configuration with JVM option -Dgrails.env. The grails335-service supports two additional custom environment p8190 and p8290 for custom ports (8190 and 8290), so that a cluster of two nodes can be registered to consul under same service grails335-service.

# run these two gradle command in two separate terminals
./gradlew :grails355-service:bootRun -Dgrails.env=p8190
./gradlew :grails355-service:bootRun -Dgrails.env=p8290

The grails323-service now works as a service client to call endpoint grails335-service/hello/greet?name=<some-name>. The service client is based on FeignClient with client-side load balancing built-in.

By calling grails323-service/greet?name=<some-name> multiple times you will find the response shows port number keeps changing between 8190 and 8290 from cloud service. And, if one grails335-service node is shut down, all responses will only come from the other port.

Feign client has circuit breaker functionality built-in basd on Hystrix. Only one property in application.yml is needed to enable it.

feign:
    hystrix:
        enabled: true

With hystrix enabled, the @FeignClient annotation can support fallback method to use a fail-over class that implements the annotated interface. In grails323-service application, such a class is defined as a backup for greet client call when grails3350-service endpoint is down.

Shut down all grails335-service nodes, and try calling grails323-service/hello/greet again, you will receive failover response instead.

Note in Grails, a spring bean is registered in grails-app/conf/resources.groovy file, not by @Component annotation as in pure Spring framework.

curl --request GET http://localhost:8500/v1/agent/services

curl --request PUT \
http://localhost:8500/v1/agent/service/deregister/<obsolete-service-id>

To load a key-value pair to consul KV store:

consul kv put config/grails323-service/hello/name "Consul-KV-Name"

the key above corresponds grails323-service application HelloController's spring @Value annotation with value hello.name.

This key value pair can be removed by:

consul kv delete config/grails323-service/hello/name

• add basic consul distributed configuration property support

• add Feign client Hystrix failover example

• add Feign client example for service discovery and remote call
• rewrite README to make it 'readable'

• simple settings to work with localhost consul agent
• custom ConsulHealthIndicator

Bin Le ([email protected])

Apache License Version 2.0. (http://www.apache.org/licenses/)