Temporal Nexus - Java SDK Feature Guide

Use Temporal Nexus to connect Durable Executions within and across Namespaces using a Nexus Endpoint, a Nexus Service contract, and Nexus Operations.

This page shows how to do the following:

Run a development server with Nexus enabled

The first step in working with Temporal Nexus involves starting a Temporal server with Nexus enabled.

Prerequisites:

Install the latest Temporal CLI
Install the latest Temporal Java SDK (v1.26.0 or higher)

Start the Temporal Development Server

Start the Temporal Development Server by using the temporal server start-dev, with system.enableNexus=true. The HTTP port is required for Nexus communications.

temporal server start-dev --http-port 7243 --dynamic-config-value system.enableNexus=true

This command automatically starts the Temporal development server, the Web UI, creates the default Namespace, and uses an in-memory database.

The Temporal Server should be available on localhost:7233 and the Temporal Web UI should be accessible at http://localhost:8233.

Create caller and handler Namespaces

Before setting up Nexus endpoints, create separate Namespaces for the caller and handler.

temporal operator namespace create --namespace my-target-namespace
temporal operator namespace create --namespace my-caller-namespace

The role of my-target-namespace will be to contain the Operation handler. The role of my-caller-namespace will be to attempt to call the Operation handler. We use two namespaces here to demonstrate cross-Namespace Nexus calls.

Create a Nexus Endpoint to route requests from caller to handler

After establishing caller and handler Namespaces, the next step is to create a Nexus Endpoint to route requests.

temporal operator nexus endpoint create \
  --name my-nexus-endpoint-name \
  --target-namespace my-target-namespace \
  --target-task-queue my-handler-task-queue

Define the Nexus Service API contract

Defining a clear API contract for the Nexus Service is crucial for smooth communication between services.

View the source code in the context of the rest of the application code.

git clone https://github.com/temporalio/samples-java.git
cd samples-java

The Nexus Service API contract can be in whatever form works best for your environment. Each Temporal SDK includes and uses a default Data Converter. The default data converter encodes payloads in the following order: Null, Byte array, Protobuf JSON, and JSON. In a polyglot environment, that is where more than one language and SDK is being used to develop a Temporal solution, Protobuf and JSON are common choices. This example uses Java classes serialized into JSON.

In this example, there is a service package that describes the Service and Operation names along with input/output types for caller Workflows to use the Nexus Endpoint.

core/src/main/java/io/temporal/samples/nexus/service/NexusService.java

@Service
public interface NexusService {
  enum Language {
    EN,
    FR,
    DE,
    ES,
    TR
  }

  class HelloInput {
    private final String name;
    private final Language language;

    @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
    public HelloInput(
        @JsonProperty("name") String name, @JsonProperty("language") Language language) {
      this.name = name;
      this.language = language;
    }

    @JsonProperty("name")
    public String getName() {
      return name;
    }

    @JsonProperty("language")
    public Language getLanguage() {
      return language;
    }
  }

  class HelloOutput {
    private final String message;

    @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
    public HelloOutput(@JsonProperty("message") String message) {
      this.message = message;
    }

    @JsonProperty("message")
    public String getMessage() {
      return message;
    }
  }

  class EchoInput {
    private final String message;

    @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
    public EchoInput(@JsonProperty("message") String message) {
      this.message = message;
    }

    @JsonProperty("message")
    public String getMessage() {
      return message;
    }
  }

  class EchoOutput {
    private final String message;

    @JsonCreator(mode = JsonCreator.Mode.PROPERTIES)
    public EchoOutput(@JsonProperty("message") String message) {
      this.message = message;
    }

    @JsonProperty("message")
    public String getMessage() {
      return message;
    }
  }

  @Operation
  HelloOutput hello(HelloInput input);

  @Operation
  EchoOutput echo(EchoInput input);
}

Develop a Nexus Service and Operation handlers

Nexus Operation handlers are typically defined in the same Worker as the underlying Temporal primitives they abstract. Operation handlers can decide if a given Nexus Operation will be synchronous or asynchronous, execute arbitrary code, and invoke underlying Temporal primitives such as a Workflow, Query, Signal, or Update.

The io.temporal.nexus.WorkflowClientOperationHandlers class has builders to create Nexus Operations:

sync - Simple synchronous RPC handlers, such as for Signals
fromWorkflowMethod - Run a Workflow as an asynchronous Nexus Operation

This tutorial starts with a sync example, and then uses fromWorkflowMethod to start a handler Workflow from a Nexus Operation.

Develop a Synchronous Nexus Operation handler

The WorkflowClientOperationHandlers.sync method is for exposing simple RPC handlers. Its handler function is provided with an SDK client that can be used for signaling, querying, and listing Workflows. However, implementations are free to make arbitrary calls to other services or databases, or perform computations such as this one:

core/src/main/java/io/temporal/samples/nexus/handler/NexusServiceImpl.java

// To create a service implementation, annotate the class with @ServiceImpl and provide the
// interface that the service implements. The service implementation class should have methods that
// return OperationHandler that correspond to the operations defined in the service interface.
@ServiceImpl(service = NexusService.class)
public class NexusServiceImpl {
  @OperationImpl
  public OperationHandler<NexusService.EchoInput, NexusService.EchoOutput> echo() {
    // WorkflowClientOperationHandlers.sync is a meant for exposing simple RPC handlers.
    return WorkflowClientOperationHandlers.sync(
        // The method is provided with an SDK client that can be used for arbitrary calls such as
        // signaling, querying,
        // and listing workflows but implementations are free to make arbitrary calls to other
        // services or databases, or
        // perform simple computations such as this one.
        (ctx, details, client, input) -> new NexusService.EchoOutput(input.getMessage()));
  }
// ...
}

Develop an Asynchronous Nexus Operation handler to start a Workflow

Use the WorkflowClientOperationHandlers.fromWorkflowMethod method, which is the easiest way to expose a Workflow as an operation. See alternatives here.

core/src/main/java/io/temporal/samples/nexus/handler/NexusServiceImpl.java

// To create a service implementation, annotate the class with @ServiceImpl and provide the
// interface that the service implements. The service implementation class should have methods that
// return OperationHandler that correspond to the operations defined in the service interface.
@ServiceImpl(service = NexusService.class)
public class NexusServiceImpl {
// ...
  @OperationImpl
  public OperationHandler<NexusService.HelloInput, NexusService.HelloOutput> hello() {
    // Use the WorkflowClientOperationHandlers.fromWorkflowMethod constructor, which is the easiest
    // way to expose a workflow as an operation.
    return WorkflowClientOperationHandlers.fromWorkflowMethod(
        (ctx, details, client, input) ->
            client.newWorkflowStub(
                    HelloHandlerWorkflow.class,
                    // Workflow IDs should typically be business meaningful IDs and are used to
                    // dedupe workflow starts.
                    // For this example, we're using the request ID allocated by Temporal when the
                    // caller workflow schedules
                    // the operation, this ID is guaranteed to be stable across retries of this
                    // operation.
                    //
                    // Task queue defaults to the task queue this operation is handled on.
                    WorkflowOptions.newBuilder().setWorkflowId(details.getRequestId()).build())
                ::hello);
  }
}

Workflow IDs should typically be business meaningful IDs and are used to dedupe Workflow starts. In general business meaningful IDs should be passed in the Operation input as part of the Nexus Service contract.

Register a Nexus Service in a Worker

After developing an asynchronous Nexus Operation handler to start a Workflow, the next step is to register a Nexus Service in a Worker.

core/src/main/java/io/temporal/samples/nexus/handler/HandlerWorker.java

package io.temporal.samples.nexus.handler;

import io.temporal.client.WorkflowClient;
import io.temporal.samples.nexus.options.ClientOptions;
import io.temporal.worker.Worker;
import io.temporal.worker.WorkerFactory;

public class HandlerWorker {
  public static final String DEFAULT_TASK_QUEUE_NAME = "my-handler-task-queue";

  public static void main(String[] args) {
    WorkflowClient client = ClientOptions.getWorkflowClient(args);

    WorkerFactory factory = WorkerFactory.newInstance(client);

    Worker worker = factory.newWorker(DEFAULT_TASK_QUEUE_NAME);
    worker.registerWorkflowImplementationTypes(HelloHandlerWorkflowImpl.class);
    worker.registerNexusServiceImplementation(new NexusServiceImpl());

    factory.start();
  }
}

Develop a caller Workflow that uses the Nexus Service

Import the Service API package, that has the necessary service and operation names and input/output types to execute a Nexus Operation from the caller Workflow:

core/src/main/java/io/temporal/samples/nexus/caller/EchoCallerWorkflowImpl.java

package io.temporal.samples.nexus.caller;

import io.temporal.samples.nexus.service.NexusService;
import io.temporal.workflow.NexusOperationOptions;
import io.temporal.workflow.NexusServiceOptions;
import io.temporal.workflow.Workflow;
import java.time.Duration;

public class EchoCallerWorkflowImpl implements EchoCallerWorkflow {
  NexusService nexusService =
      Workflow.newNexusServiceStub(
          NexusService.class,
          NexusServiceOptions.newBuilder()
              .setOperationOptions(
                  NexusOperationOptions.newBuilder()
                      .setScheduleToCloseTimeout(Duration.ofSeconds(10))
                      .build())
              .build());

  @Override
  public String echo(String message) {
    return nexusService.echo(new NexusService.EchoInput(message)).getMessage();
  }
}

core/src/main/java/io/temporal/samples/nexus/caller/HelloCallerWorkflowImpl.java

package io.temporal.samples.nexus.caller;

import io.temporal.samples.nexus.service.NexusService;
import io.temporal.workflow.NexusOperationHandle;
import io.temporal.workflow.NexusOperationOptions;
import io.temporal.workflow.NexusServiceOptions;
import io.temporal.workflow.Workflow;
import java.time.Duration;

public class HelloCallerWorkflowImpl implements HelloCallerWorkflow {
  NexusService nexusService =
      Workflow.newNexusServiceStub(
          NexusService.class,
          NexusServiceOptions.newBuilder()
              .setOperationOptions(
                  NexusOperationOptions.newBuilder()
                      .setScheduleToCloseTimeout(Duration.ofSeconds(10))
                      .build())
              .build());

  @Override
  public String hello(String message, NexusService.Language language) {
    NexusOperationHandle<NexusService.HelloOutput> handle =
        Workflow.startNexusOperation(
            nexusService::hello, new NexusService.HelloInput(message, language));
    // Optionally wait for the operation to be started. NexusOperationExecution will contain the
    // operation ID in case this operation is asynchronous.
    handle.getExecution().get();
    return handle.getResult().get().getMessage();
  }
}

Register the caller Workflow in a Worker

After developing the caller Workflow, the next step is to register it with a Worker.

core/src/main/java/io/temporal/samples/nexus/caller/CallerWorker.java

package io.temporal.samples.nexus.caller;

import io.temporal.client.WorkflowClient;
import io.temporal.samples.nexus.options.ClientOptions;
import io.temporal.worker.Worker;
import io.temporal.worker.WorkerFactory;
import io.temporal.worker.WorkflowImplementationOptions;
import io.temporal.workflow.NexusServiceOptions;
import java.util.Collections;

public class CallerWorker {
  public static final String DEFAULT_TASK_QUEUE_NAME = "my-caller-workflow-task-queue";

  public static void main(String[] args) {
    WorkflowClient client = ClientOptions.getWorkflowClient(args);

    WorkerFactory factory = WorkerFactory.newInstance(client);

    Worker worker = factory.newWorker(DEFAULT_TASK_QUEUE_NAME);
    worker.registerWorkflowImplementationTypes(
        WorkflowImplementationOptions.newBuilder()
            .setNexusServiceOptions(
                Collections.singletonMap(
                    "NexusService",
                    NexusServiceOptions.newBuilder().setEndpoint("my-nexus-endpoint-name").build()))
            .build(),
        EchoCallerWorkflowImpl.class,
        HelloCallerWorkflowImpl.class);

    factory.start();
  }
}

Develop a starter to start the caller Workflow

To initiate the caller Workflow, a starter program is used.

core/src/main/java/io/temporal/samples/nexus/caller/CallerStarter.java

package io.temporal.samples.nexus.caller;

import io.temporal.client.WorkflowClient;
import io.temporal.client.WorkflowOptions;
import io.temporal.client.WorkflowStub;
import io.temporal.samples.nexus.options.ClientOptions;
import io.temporal.samples.nexus.service.NexusService;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class CallerStarter {
  private static final Logger logger = LoggerFactory.getLogger(CallerStarter.class);

  public static void main(String[] args) {
    WorkflowClient client = ClientOptions.getWorkflowClient(args);

    WorkflowOptions workflowOptions =
        WorkflowOptions.newBuilder().setTaskQueue(CallerWorker.DEFAULT_TASK_QUEUE_NAME).build();
    EchoCallerWorkflow echoWorkflow =
        client.newWorkflowStub(EchoCallerWorkflow.class, workflowOptions);
    logger.info("Workflow result: {}", echoWorkflow.echo("Nexus Echo 👋"));
    logger.info(
        "Started workflow workflowId: {} runId: {}",
        WorkflowStub.fromTyped(echoWorkflow).getExecution().getWorkflowId(),
        WorkflowStub.fromTyped(echoWorkflow).getExecution().getRunId());
    HelloCallerWorkflow helloWorkflow =
        client.newWorkflowStub(HelloCallerWorkflow.class, workflowOptions);
    logger.info("Workflow result: {}", helloWorkflow.hello("Nexus", NexusService.Language.ES));
    logger.info(
        "Started workflow workflowId: {} runId: {}",
        WorkflowStub.fromTyped(helloWorkflow).getExecution().getWorkflowId(),
        WorkflowStub.fromTyped(helloWorkflow).getExecution().getRunId());
  }
}

Make Nexus calls across Namespaces with a development Server

To run the tutorial, follow the steps below to run the Nexus handler Worker, the Nexus caller Worker, and the starter app.

Run Workers connected to a local development server

In separate terminal window, run the Nexus handler Worker:

./gradlew -q execute -PmainClass=io.temporal.samples.nexus.handler.HandlerWorker \
    --args="-target-host localhost:7233 -namespace my-target-namespace"

In another terminal window, run the Nexus caller Worker:

./gradlew -q execute -PmainClass=io.temporal.samples.nexus.caller.CallerWorker \
    --args="-target-host localhost:7233 -namespace my-caller-namespace"

Start a caller Workflow

With the Workers running, the final step in the local development process is to start a caller Workflow.

Run the starter:

./gradlew -q execute -PmainClass=io.temporal.samples.nexus.caller.CallerStarter \
    --args="-target-host localhost:7233 -namespace my-caller-namespace"

This will result in:

[main] INFO  i.t.s.nexus.caller.CallerStarter - Workflow result: Nexus Echo 👋 
[main] INFO  i.t.s.nexus.caller.CallerStarter - Started workflow workflowId: 9b3de8ba-28ae-42fb-8087-bdedf4cecd39 runId: 404a2529-764d-4d1d-9de5-8a9475e40fba 
[main] INFO  i.t.s.nexus.caller.CallerStarter - Workflow result: ¡Hola! Nexus 👋 
[main] INFO  i.t.s.nexus.caller.CallerStarter - Started workflow workflowId: 9cb29897-356a-4714-87b7-aa2f00784a46 runId: 7e71e62a-db50-49da-b081-24b61016a0fc

Make Nexus calls across Namespaces in Temporal Cloud

This section assumes you are already familiar with how connect a Worker to Temporal Cloud. The same source code is used in this section, but the tcld CLI will be used to create Namespaces and the Nexus Endpoint, and mTLS client certificates will be used to securely connect the caller and handler works to their respective Temporal Cloud Namespaces.

Install the latest tcld CLI and generate certificates

To install the latest version of the tcld CLI, run the following command:

brew install temporalio/brew/tcld

If you don’t already have certificates, you can generate them for mTLS Worker authentication using the command below:

tcld gen ca --org $YOUR_ORG_NAME --validity-period 1y --ca-cert ca.pem --ca-key ca.key

These certificates will be valid for one year.

Create caller and handler Namespaces

Before deploying to Temporal Cloud, ensure the appropriate Namespaces are created for both the caller and handler. If you already have these Namespaces, this step is optional.

tcld login

tcld namespace create \
	--namespace <your-caller-namespace> \
	--region us-west-2 \
	--ca-certificate-file 'path/to/your/ca.pem' \
	--retention-days 1

tcld namespace create \
	--namespace <your-target-namespace> \
	--region us-west-2 \
	--ca-certificate-file 'path/to/your/ca.pem' \
	--retention-days 1

Alternatively, you can create Namespace through the UI: https://cloud.temporal.io/Namespaces.

Create a Nexus Endpoint to route requests from caller to handler

To create a Nexus Endpoint you must have a Developer account role or higher, and have NamespaceAdmin permission on the --target-namespace.

tcld nexus endpoint create \
  --name <my-nexus-endpoint-name> \
  --target-task-queue my-handler-task-queue \
  --target-namespace <my-target-namespace.account> \
  --allow-namespace <my-caller-namespace.account> \
  --description-file ./core/src/main/java/io/temporal/samples/nexus/service/description.md

The --allow-namespace is used to build an Endpoint allowlist of caller Namespaces that can use the Nexus Endpoint, as described in Runtime Access Control.

Alternatively, you can create a Nexus Endpoint through the UI: https://cloud.temporal.io/nexus. You can also create a Nexus endpoint through the UI in the development server.

Run Workers Connected to Temporal Cloud

View the source code in the context of the rest of the application code.

Run the handler Worker:

./gradlew -q execute -PmainClass=io.temporal.samples.nexus.handler.HandlerWorker \
  --args="-target-host <your-target-namespace.account>.tmprl.cloud:7233 \
  -namespace <your-target-namespace.account> \
  -client-cert 'path/to/your/ca.pem' \
  -client-key 'path/to/your/ca.key'"

Run the caller Worker:

./gradlew -q execute -PmainClass=io.temporal.samples.nexus.caller.CallerWorker \
  --args="-target-host <your-caller-namespace.account>.tmprl.cloud:7233 \
  -namespace <your-caller-namespace.account> \
  -client-cert 'path/to/your/ca.pem' \
  -client-key 'path/to/your/ca.key'"

Start a caller Workflow

In order to start the caller Workflow, run the starter.

./gradlew -q execute -PmainClass=io.temporal.samples.nexus.caller.CallerStarter \
  --args="-target-host <your-caller-namespace.account>.tmprl.cloud:7233 \
  -namespace <your-caller-namespace.account> \
  -client-cert 'path/to/your/ca.pem' \
  -client-key 'path/to/your/ca.key'"

This will result in:

[main] INFO  i.t.s.nexus.caller.CallerStarter - Workflow result: Nexus Echo 👋 
[main] INFO  i.t.s.nexus.caller.CallerStarter - Started workflow workflowId: 9b3de8ba-28ae-42fb-8087-bdedf4cecd39 runId: 404a2529-764d-4d1d-9de5-8a9475e40fba 
[main] INFO  i.t.s.nexus.caller.CallerStarter - Workflow result: ¡Hola! Nexus 👋 
[main] INFO  i.t.s.nexus.caller.CallerStarter - Started workflow workflowId: 9cb29897-356a-4714-87b7-aa2f00784a46 runId: 7e71e62a-db50-49da-b081-24b61016a0fc

Observability

Web UI

A synchronous Nexus Operation will surface in the caller Workflow as follows, with just NexusOperationScheduled and NexusOperationCompleted events in the caller’s Workflow history:

Observability Sync

An asynchronous Nexus Operation will surface in the caller Workflow as follows, with NexusOperationScheduled, NexusOperationStarted, and NexusOperationCompleted, in the caller’s Workflow history:

Observability Async

Temporal CLI

Use the workflow describe command to show pending Nexus Operations in the caller Workflow and any attached callbacks on the handler Workflow:

temporal workflow describe -w <ID>

Nexus events are included in the caller’s Workflow history:

temporal workflow show -w <ID>

For asynchronous Nexus Operations the following are reported in the caller’s history:

NexusOperationScheduled
NexusOperationStarted
NexusOperationCompleted

For synchronous Nexus Operations the following are reported in the caller’s history:

NexusOperationScheduled
NexusOperationCompleted

note

NexusOperationStarted isn't reported in the caller’s history for synchronous operations.

Run a development server with Nexus enabled​

Start the Temporal Development Server​

Create caller and handler Namespaces​

Create a Nexus Endpoint to route requests from caller to handler​

Define the Nexus Service API contract​

Develop a Nexus Service and Operation handlers​

Develop a Synchronous Nexus Operation handler​

Develop an Asynchronous Nexus Operation handler to start a Workflow​

Register a Nexus Service in a Worker​

Develop a caller Workflow that uses the Nexus Service​

Register the caller Workflow in a Worker​

Develop a starter to start the caller Workflow​

Make Nexus calls across Namespaces with a development Server​

Run Workers connected to a local development server​

Start a caller Workflow​

Make Nexus calls across Namespaces in Temporal Cloud​

Install the latest tcld CLI and generate certificates​

Create caller and handler Namespaces​

Create a Nexus Endpoint to route requests from caller to handler​

Run Workers Connected to Temporal Cloud​

Start a caller Workflow​

Observability​

Web UI​

Temporal CLI​

Run a development server with Nexus enabled

Start the Temporal Development Server

Create caller and handler Namespaces

Create a Nexus Endpoint to route requests from caller to handler

Define the Nexus Service API contract

Develop a Nexus Service and Operation handlers

Develop a Synchronous Nexus Operation handler

Develop an Asynchronous Nexus Operation handler to start a Workflow

Register a Nexus Service in a Worker

Develop a caller Workflow that uses the Nexus Service

Register the caller Workflow in a Worker

Develop a starter to start the caller Workflow

Make Nexus calls across Namespaces with a development Server

Run Workers connected to a local development server

Start a caller Workflow

Make Nexus calls across Namespaces in Temporal Cloud

Install the latest tcld CLI and generate certificates

Create caller and handler Namespaces

Create a Nexus Endpoint to route requests from caller to handler

Run Workers Connected to Temporal Cloud

Start a caller Workflow

Observability

Web UI

Temporal CLI