Service Call

How can I call a remote service in a distributed system where the service is looked up from a service registry of some sorts?

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Use a Service Call acting as a Messaging Gateway for distributed systems, that handles the complexity of calling the service in a reliable manner.

The pattern has the following noteworthy features:

  • Location transparency — Decouples Camel and the physical location of the services using logical names representing the services.

  • URI templating — Allows you to template the Camel endpoint URI as the physical endpoint to use when calling the service.

  • Service discovery - Looks up the service from a service registry of some sort to know the physical locations of the services.

  • Service filter — Allows you to filter unwanted services (for example, blacklisted or unhealthy services).

  • Service chooser — Allows you to choose the most appropriate service based on factors such as geographical zone, affinity, plans, canary deployments, and SLAs.

  • Load balancer — A preconfigured Service Discovery, Filter, and Chooser intended for a specific runtime (these three features combined as one).

In a nutshell, the EIP pattern sits between your Camel application and the services running in a distributed system (cluster). The pattern hides all the complexity of keeping track of all the physical locations where the services are running and allows you to call the service by a name.

Options

The Service Call eip supports 13 options, which are listed below.

Name Description Default Type

expression

Configures the Expression using the given configuration.

ServiceCallExpressionConfiguration

name

Required Sets the name of the service to use.

String

uri

The uri of the endpoint to send to. The uri can be dynamic computed using the org.apache.camel.language.simple.SimpleLanguage expression.

String

component

The component to use.

http

String

pattern

Sets the optional ExchangePattern used to invoke this endpoint.

Enum values:

  • InOnly

  • InOptionalOut

  • InOut

ExchangePattern

configurationRef

Refers to a ServiceCall configuration to use.

String

serviceDiscoveryRef

Sets a reference to a custom ServiceDiscovery to use.

String

serviceFilterRef

Sets a reference to a custom ServiceFilter to use.

String

serviceChooserRef

Sets a reference to a custom ServiceChooser to use.

String

loadBalancerRef

Sets a reference to a custom ServiceLoadBalancer to use.

String

expressionRef

Set a reference to a custom Expression to use.

String

serviceDiscoveryConfiguration

Required Configures the ServiceDiscovery using the given configuration.

ServiceCallServiceDiscoveryConfiguration

serviceFilterConfiguration

Required Configures the ServiceFilter using the given configuration.

ServiceCallServiceFilterConfiguration

loadBalancerConfiguration

Required Configures the LoadBalancer using the given configuration.

ServiceCallServiceLoadBalancerConfiguration

description

Sets the description of this node.

DescriptionDefinition

Using Service Call

The service to call is looked up in a service registry of some sorts such as Kubernetes, Consul, Etcd, Zookeeper, DNS. The EIP separates the configuration of the service registry from the calling of the service.

When calling a service you may just refer to the name of the service in the EIP as shown below:

from("direct:start")
    .serviceCall("foo")
    .to("mock:result");

And in XML:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <serviceCall name="foo"/>
    <to uri="mock:result"/>
  </route>
</camelContext>

Camel will then:

  • search for a service call configuration from the Camel context and registry

  • lookup a service with the name foo from an external service registry

  • filter the servers

  • select the server to use

  • build a Camel URI using the chosen server info

By default, the Service Call EIP uses camel-http so assuming that the selected service instance runs on host myhost.com on port 80, the computed Camel URI will be:

http:myhost.com:80

Mapping Service Name to Endpoint URI

It is often needed to build more complex Camel URI which may include options or paths which is possible through different options:name: value

The service name supports a limited uri like syntax, here some examples

Name Resolution

foo

foo/path

foo/path?foo=bar

from("direct:start")
    .serviceCall("foo/hello")
    .to("mock:result");

If you want to have more control over the uri construction, you can use the uri directive:

Name URI Resolution

foo

undertow:http://foo/hello

undertow:http://host:port/hello

foo

undertow:http://foo.host:foo.port/hello

undertow:http://host:port/hello

from("direct:start")
    .serviceCall("foo", "undertow:http://foo/hello")
    .to("mock:result");

Advanced users can have full control over the uri construction through expressions:

from("direct:start")
    .serviceCall()
        .name("foo")
        .expression()
            .simple("undertow:http://${header.CamelServiceCallServiceHost}:${header.CamelServiceCallServicePort}/hello");

Static Service Discovery

This service discovery implementation does not query any external services to find out the list of services associated to a named service but keep them in memory. Each service should be provided in the following form:

[service@]host:port

The service part is used to discriminate against the services but if not provided it acts like a wildcard so each non named service will be returned whatever the service name is. This is useful if you have a single service so the service name is redundant.

This implementation is provided by camel-core artifact.

Available options:

Name Java Type Description

servers

String

A comma separated list of servers in the form: [service@]host:port,[service@]host2:port,[service@]host3:port

from("direct:start")
    .serviceCall("foo")
        .staticServiceDiscovery()
            .servers("service1@host1:80,service1@host2:80")
            .servers("service2@host1:8080,service2@host2:8080,service2@host3:8080")
            .end()
    .to("mock:result");

And in XML:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <serviceCall name="foo">
      <staticServiceDiscovery>
        <servers>service1@host1:80,service1@host2:80</servers>
        <servers>service2@host1:8080,service2@host2:8080,service2@host3:8080</servers>
      </staticServiceDiscovery>
    </serviceCall>
    <to uri="mock:result"/>
  </route>
</camelContext>

Consul Service Discovery

To leverage Consul for Service Discovery, maven users will need to add the following dependency to their pom.xml

<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-consul</artifactId>
    <!-- use the same version as your Camel core version -->
    <version>x.y.z</version>
</dependency>

Available options:

Name Java Type Description

url

String

The Consul agent URL

datacenter

String

The data center

aclToken

String

Sets the ACL token to be used with Consul

userName

String

Sets the username to be used for basic authentication

password

String

Sets the password to be used for basic authentication

connectTimeoutMillis

Long

Connect timeout for OkHttpClient

readTimeoutMillis

Long

Read timeout for OkHttpClient

writeTimeoutMillis

Long

Write timeout for OkHttpClient

And example in Java

from("direct:start")
    .serviceCall("foo")
        .consulServiceDiscovery()
            .url("http://consul-cluster:8500")
            .datacenter("neverland")
            .end()
    .to("mock:result");

DNS Service Discovery

To leverage DNS for Service Discovery, maven users will need to add the following dependency to their pom.xml

<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-dns</artifactId>
    <!-- use the same version as your Camel core version -->
    <version>x.y.z</version>
</dependency>

Available options:

Name Java Type Description

proto

String

The transport protocol of the desired service, default "_tcp"

domain

String

The user name to use for basic authentication

Example in Java:

from("direct:start")
    .serviceCall("foo")
        .dnsServiceDiscovery("my.domain.com")
    .to("mock:result");

And in XML:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <serviceCall name="foo">
      <dnsServiceDiscovery domain="my.domain.com"/>
    </serviceCall>
    <to uri="mock:result"/>
  </route>
</camelContext>

Etcd Service Discovery

To leverage Etcd for Service Discovery, maven users will need to add the following dependency to their pom.xml

<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-etcd</artifactId>
    <!-- use the same version as your Camel core version -->
    <version>x.y.z</version>
</dependency>

Available options:

Name Java Type Description

uris

String

The URIs the client can connect to

userName

String

The user name to use for basic authentication

password

String

The password to use for basic authentication

timeout

Long

To set the maximum time an action could take to complete

servicePath

String

The path to look for service discovery, default "/services"

type

String

To set the discovery type, valid values are "on-demand" and "watch"

Example in Java:

from("direct:start")
    .serviceCall("foo")
        .etcdServiceDiscovery()
            .uris("http://etcd1:4001,http://etcd2:4001")
            .servicePath("/camel/services")
            .end()
    .to("mock:result");

And in XML:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <serviceCall name="foo">
      <etcdServiceDiscovery uris="http://etcd1:4001,http://etcd2:4001" servicePath="/camel/services"/>
    </serviceCall>
    <to uri="mock:result"/>
  </route>
</camelContext>

Kubernetes Service Discovery

To leverage Kubernetes for Service Discovery, maven users will need to add the following dependency to their pom.xml

<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-kubernetes</artifactId>
    <!-- use the same version as your Camel core version -->
    <version>x.y.z</version>
</dependency>

Available options:

Name Java Type Description

lookup

String

How to perform service lookup. Possible values: client, dns, environment

apiVersion

String

Kubernetes API version when using client lookup

caCertData

String

Sets the Certificate Authority data when using client lookup

caCertFile

String

Sets the Certificate Authority data that are loaded from the file when using client lookup

clientCertData

String

Sets the Client Certificate data when using client lookup

clientCertFile

String

Sets the Client Certificate data that are loaded from the file when using client lookup

clientKeyAlgo

String

Sets the Client Keystore algorithm, such as RSA when using client lookup

clientKeyData

String

Sets the Client Keystore data when using client lookup

clientKeyFile

String

Sets the Client Keystore data that are loaded from the file when using client lookup

clientKeyPassphrase

String

Sets the Client Keystore passphrase when using client lookup

dnsDomain

String

Sets the DNS domain to use for dns lookup

namespace

String

The Kubernetes namespace to use. By default the namespace’s name is taken from the environment variable KUBERNETES_MASTER

oauthToken

String

Sets the OAUTH token for authentication (instead of username/password) when using client lookup

username

String

Sets the username for authentication when using client lookup

password

String

Sets the password for authentication when using client lookup

trustCerts

Boolean

Sets whether to turn on trust certificate check when using client lookup

Example in Java:

from("direct:start")
    .serviceCall("foo")
        .kubernetesServiceDiscovery()
            .lookup("dns")
            .namespace("myNamespace")
            .dnsDomain("my.domain.com")
            .end()
    .to("mock:result");

And in XML:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <serviceCall name="foo">
      <kubernetesServiceDiscovery lookup="dns" namespace="myNamespace" dnsDomain="my.domain.com"/>
    </serviceCall>
    <to uri="mock:result"/>
  </route>
</camelContext>

Using service filtering

The Service Call EIP supports filtering the services using built-in filters, or a custom filter.

Blacklist Service Filter

This service filter implementation removes the listed services from those found by the service discovery. Each service should be provided in the following form:

[service@]host:port

The services are removed if they fully match

Available options:

Name Java Type Description

servers

String

A comma separated list of servers to blacklist: [service@]host:port,[service@]host2:port,[service@]host3:port

Example in Java:

from("direct:start")
    .serviceCall("foo")
        .staticServiceDiscovery()
            .servers("service1@host1:80,service1@host2:80")
            .servers("service2@host1:8080,service2@host2:8080,service2@host3:8080")
            .end()
        .blacklistFilter()
            .servers("service2@host2:8080")
            .end()
    .to("mock:result");

And in XML:

<camelContext xmlns="http://camel.apache.org/schema/spring">
  <route>
    <from uri="direct:start"/>
    <serviceCall name="foo">
      <staticServiceDiscovery>
        <servers>service1@host1:80,service1@host2:80</servers>
        <servers>service2@host1:8080,service2@host2:8080,service2@host3:8080</servers>
      </staticServiceDiscovery>
      <blacklistServiceFilter>
        <servers>service2@host2:8080</servers>
      </blacklistServiceFilter>
    </serviceCall>
    <to uri="mock:result"/>
  </route>
</camelContext>

Custom Service Filter

Service Filters choose suitable candidates from the service definitions found in the service discovery.

The service filter have access to the current exchange, which allows you to create service filters comparing service metadata with message content.

Assuming you have labeled one of the services in your service discovery to support a certain type of requests:

serviceDiscovery.addServer(new DefaultServiceDefinition("service", "127.0.0.1", 1003,
    Collections.singletonMap("supports", "foo")));

The current exchange has a property which says that it needs a foo service:

exchange.setProperty("needs", "foo");

You can then use a ServiceFilter to select the service instances which match the exchange:

from("direct:start")
    .serviceCall()
        .name("service")
        .serviceFilter((exchange, services) -> services.stream()
			.filter(serviceDefinition -> Optional.ofNullable(serviceDefinition.getMetadata()
				.get("supports"))
				.orElse("")
				.equals(exchange.getProperty("needs", String.class)))
			.collect(Collectors.toList()));
        .end()
    .to("mock:result");

Load balancing services

The Service Call EIP comes with its own load-balancer which is instantiated by default if a custom loadbalancer is not configured. It glues Service Discovery, Service Filter, Service Chooser and Service Expression together to load balance requests among the available services.

If you need a more sophisticated load balancer you can use Ribbon by adding camel-ribbon to the mix, maven users will need to add the following dependency to their pom.xml

The RibbonServiceLoadBalancer has no concept of a current Exchange. Service filters therefore receive a dummy exchange when used with Ribbon.
<dependency>
    <groupId>org.apache.camel</groupId>
    <artifactId>camel-ribbon</artifactId>
    <!-- use the same version as your Camel core version -->
    <version>x.y.z</version>
</dependency>

Available options:

Name Java Type Description

clientName

String

The Ribbon client name

properties

List<PropertyDefinition>

Custom client config properties

To leverage Ribbon, it is required to explicit enable it:

Here is how to do that in Java:

from("direct:start")
    .serviceCall("foo")
        .ribbonLoadBalancer()
    .to("mock:result");

And in XML:

<route>
    <from uri="direct:start"/>
    <serviceCall name="foo">
        <ribbonLoadBalancer/>
    </serviceCall>
    <to uri="mock:result"/>
</route>

You can configure Ribbon key programmatically using RibbonConfiguration:

RibbonConfiguration configuration = new RibbonConfiguration();
configuration.addProperty("listOfServers", "localhost:9090,localhost:9091");

from("direct:start")
    .serviceCall("foo")
        .loadBalancer(new RibbonServiceLoadBalancer(configuration))
    .to("mock:result");

Or leveraging XML specific configuration:

<route>
    <from uri="direct:start"/>
    <serviceCall name="foo">
        <ribbonLoadBalancer>
            <properties key="listOfServers" value="localhost:9090,localhost:9091"/>
        </ribbonLoadBalancer>
    </serviceCall>
    <to uri="mock:result"/>
</route>

Shared configurations

The Service Call EIP can be configured straight on the route definition or through shared configurations, here an example with two configurations registered in the CamelContext:

ServiceCallConfigurationDefinition globalConf = new ServiceCallConfigurationDefinition();
globalConf.setServiceDiscovery(
    name -> Arrays.asList(
        new DefaultServiceDefinition(name, "my.host1.com", 8080),
        new DefaultServiceDefinition(name, "my.host2.com", 443))
);
globalConf.setServiceChooser(
    list -> list.get(ThreadLocalRandom.current().nextInt(list.size()))
);

ServiceCallConfigurationDefinition httpsConf = new ServiceCallConfigurationDefinition();
httpsConf.setServiceFilter(
    list -> list.stream().filter((exchange, s) -> s.getPort() == 443).collect(toList())
);

getContext().setServiceCallConfiguration(globalConf);
getContext().addServiceCallConfiguration("https", httpsConf);

Each Service Call definition and configuration will inherit from the globalConf which can be seen as default configuration, then you can reference the httpsConf in your route:

from("direct:start")
    .serviceCall()
        .name("foo")
        .serviceCallConfiguration("https")
    .end()
    .to("mock:result");

This route will leverage the service discovery and service chooser from globalConf and the service filter from `httpsConf, but you can override any of them if needed straight on the route:

from("direct:start")
    .serviceCall()
        .name("foo")
        .serviceCallConfiguration("https")
        .serviceChooser(list -> list.get(0))
    .end()
    .to("mock:result");