# Nexus Developer Experience - .NET SDK feature guide > For the complete documentation index, see [llms.txt](https://docs.temporal.io/llms.txt). > Any documentation page is available as raw Markdown by appending `.md` to its URL. > Build a Nexus Service in .NET with the pre-release APIs - the Temporal Operation Handler, Activity-backed Operations, and a single Service contract. Use [Temporal Nexus](/evaluate/nexus) to connect Temporal Applications within and across Namespaces using a Nexus Endpoint, a Nexus Service contract, and Nexus Operations. > **💡 Tip:** > > New to Nexus? Start with the [Nexus .NET Quickstart](/develop/dotnet/nexus/quickstart). > This page shows how to do the following: - [Run a development Temporal Service with Nexus enabled](#run-the-temporal-nexus-development-server) - [Create caller and handler Namespaces](#create-caller-handler-namespaces) - [Create a Nexus Endpoint to route requests from caller to handler](#create-nexus-endpoint) - [Define the Nexus Service contract](#define-nexus-service-contract) - [Develop a Nexus Service and Operation handlers](#develop-nexus-service-operation-handlers) - [Develop a caller Workflow that uses a Nexus Service](#develop-caller-workflow-nexus-service) - [Make Nexus calls across Namespaces with a development Server](#nexus-calls-across-namespaces-dev-server) - [Make Nexus calls across Namespaces in Temporal Cloud](#nexus-calls-across-namespaces-temporal-cloud) > **📝 Note:** > > This documentation uses source code derived from the > [.NET Nexus sample](https://github.com/temporalio/samples-dotnet/tree/main/src/NexusSimple). > ## Run the Temporal Development Server with Nexus enabled Prerequisites: - [Install the latest Temporal CLI](https://learn.temporal.io/getting_started/dotnet/dev_environment/#set-up-a-local-temporal-service-for-development-with-temporal-cli) (v1.3.0 or higher recommended) - [Install the latest Temporal .NET SDK](https://learn.temporal.io/getting_started/dotnet/dev_environment/#install-the-temporal-net-sdk) (v1.18.0 or higher recommended) The first step in working with Temporal Nexus involves starting a Temporal server with Nexus enabled. ``` temporal server start-dev ``` This command automatically starts the Temporal development server with the Web UI, and creates the `default` Namespace. It uses an in-memory database, so do not use it for real use cases. The Temporal Web UI should now be accessible at [http://localhost:8233](http://localhost:8233), and the Temporal Server should now be available for client connections on `localhost:7233`. ## Create caller and handler Namespaces Before setting up Nexus endpoints, create separate Namespaces for the caller and handler. ``` temporal operator namespace create --namespace nexus-simple-handler-namespace temporal operator namespace create --namespace nexus-simple-caller-namespace ``` `nexus-simple-handler-namespace` will contain the Nexus Operation handler, and we will use a Workflow in `nexus-simple-caller-namespace` to call that Operation handler. We use different namespaces 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 nexus-simple-endpoint \ --target-namespace nexus-simple-handler-namespace \ --target-task-queue nexus-simple-handler-sample ``` You can also use the Web UI to create the Namespaces and Nexus endpoint. ## Define the Nexus Service contract Defining a clear contract for the Nexus Service is crucial for smooth communication. 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. You can hand-write that package, but the preferred way is to generate it with the [Nexus Code Generator](https://github.com/temporalio/nex-gen). You write the contract once as a JSON definition file and run `nexgen` against it, and it emits the typed models, runtime validators, and the Service definition itself. This is what makes a Nexus Service polyglot. Both sides generate from the same definition file: the handler implements the Service, the caller invokes its Operations, and neither hand-writes a request or response type. A Python handler and a Go caller share no code, but they both run off that same service contract - so they interoperate with no coordination between the teams beyond the contract itself. The generated validators check every payload against the contract, when a value is parsed off the wire and again when it is serialized onto it, so bad data is rejected at the boundary rather than reaching your Workflow. A value validates identically in every language, which is what lets a caller and a handler written in different languages trust the same contract. See the [`chat.nexusrpc.yaml`](https://github.com/temporalio/nex-gen/blob/main/samples/schemas/chat.nexusrpc.yaml) sample contract and the [Definition files](https://github.com/temporalio/nex-gen#definition-files) section of the `nexgen` README for the file format. ## 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. They can invoke underlying Temporal primitives such as a Query, Signal, or Update using the Temporal SDK Client, or run other reliable code. Use a synchronous Nexus Operation only when its complete execution path is highly reliable, has predictably low latency, and finishes well within the [10-second handler deadline](/cloud/limits#nexus-operation-request-timeout). Use an asynchronous Nexus Operation when latency or availability is uncertain, the work might exceed the handler deadline, or execution depends on a potentially unreliable service or database. Handlers should be reliable since the [circuit breaker](/nexus/operations#circuit-breaking) trips after 5 consecutive retryable errors, blocking all Operations from the caller to that Endpoint. Every Operation is written with [`TemporalOperationHandler`](/nexus/temporal-operation-handler). Mark a method `[TemporalOperation]` and the method body itself becomes the start handler, receiving three things: a `TemporalOperationStartContext`, an `ITemporalNexusClient`, and the Operation input. The Operation the method handles is matched by method name to the corresponding `[NexusOperation]` method on the Service interface. What you do with the Client decides what backs the Operation: - **Synchronous.** Return `TemporalOperationResult.SyncResult(...)` and the Operation completes during the handler call. The caller has its result as soon as the call returns. - **Asynchronous.** Call `StartWorkflowAsync`, `StartActivityAsync`, or `StartWorkflowUpdateAsync` on the Client. The handler returns as soon as that Execution has started, and the Operation stays open until the Execution finishes, which may be days later. Its result is delivered to the caller through the Nexus completion callback. This is what lets an Operation outlive the [Nexus request timeout](/cloud/limits#nexus-operation-request-timeout). A handler can perform any number of synchronous side effects, such as sending a Signal, but at most one asynchronous backing per invocation. ### Develop a Synchronous Nexus Operation handler Return a synchronous result when the Operation can answer immediately. The handler computes the answer and returns it, and the Operation completes during the call. Handlers should be reliable to avoid tripping the [circuit breaker](/nexus/operations#circuit-breaking), and the whole call has to finish inside the [Nexus request timeout](/cloud/limits#nexus-operation-request-timeout). ```csharp using NexusRpc.Handlers; using Temporalio.Nexus; [NexusServiceHandler(typeof(IHelloService))] public class HelloService { [TemporalOperation] public Task> Echo( TemporalOperationStartContext ctx, ITemporalNexusClient client, IHelloService.EchoInput input) => Task.FromResult(TemporalOperationResult.SyncResult( new(input.Message))); } ``` ### Use the Temporal Client for Signals, Queries, and Updates A common pattern is to reach a Workflow that is already running. Query it or Signal it from a synchronous Operation, or use Signal-With-Start to make sure the Workflow exists before the Signal arrives. Those calls complete during the handler call, so they have to finish inside the [Nexus request timeout](/cloud/limits#nexus-operation-request-timeout). Updates are the exception. Do not wait for one inside the handler. Start it with `StartWorkflowUpdateAsync` and it backs the Operation. The handler returns straight away, and the Operation completes when the Update does, however long it takes. The [NexusMessaging](https://github.com/temporalio/samples-dotnet/tree/main/src/NexusMessaging) sample shows a Nexus Service that Queries and Signals a running Workflow from synchronous Operations, and backs an Operation with a Workflow Update. The Client your handler receives is not an ordinary Temporal Client. It propagates [bidirectional links](/nexus/execution-debugging#bi-directional-linking) and request Ids on every call, so the caller-side and handler-side Executions are connected in the UI without wiring anything. Reach the Workflow Client through `client.TemporalClient` rather than constructing your own. In this example the Workflow Id is derived from an identifier carried in the Operation input, so the caller only needs the identifier it cares about: ```csharp private static string WorkflowIdForUser(string userId) => $"GreetingWorkflow_for_{userId}"; [TemporalOperation] public async Task> Approve( TemporalOperationStartContext ctx, ITemporalNexusClient client, INexusGreetingService.ApproveInput input) { var handle = client.TemporalClient.GetWorkflowHandle( WorkflowIdForUser(input.UserId)); await handle.SignalAsync(wf => wf.ApproveAsync(input)); return TemporalOperationResult.SyncResult(new()); } ``` There are two examples of messaging through Nexus in the sample code: the [caller pattern](https://github.com/temporalio/samples-dotnet/tree/main/src/NexusMessaging/CallerPattern) and the [on-demand pattern](https://github.com/temporalio/samples-dotnet/tree/main/src/NexusMessaging/OnDemandPattern). The caller pattern shows how to send messages to an existing Workflow, while the on-demand pattern shows how to start a Workflow through Nexus and then send Signals to it. ### Develop an Asynchronous Nexus Operation handler to start a Workflow Call `StartWorkflowAsync` on the Client. The Operation completes when the Workflow returns, and the Workflow's return value is delivered to the caller as the Operation's result. ```csharp [NexusServiceHandler(typeof(IHelloService))] public class HelloService { [TemporalOperation] public Task> SayHello( TemporalOperationStartContext ctx, ITemporalNexusClient client, IHelloService.HelloInput input) => client.StartWorkflowAsync( (HelloHandlerWorkflow wf) => wf.RunAsync(input), // Workflow IDs should typically be business meaningful IDs and are used to dedupe // workflow starts. Task queue defaults to the operation's task queue when omitted. new() { Id = $"hello-{input.Name}-{input.Language}" }); } ``` Workflow IDs should typically be business-meaningful IDs and are used to dedupe Workflow starts. In general, the ID should be passed in the Operation input as part of the Nexus Service contract. > **💡 Tip:** > RESOURCES > > [Attach multiple Nexus callers to a handler Workflow](/nexus/operations#attaching-multiple-nexus-callers) with a > Conflict-Policy of Use-Existing. > #### Map a Nexus Operation input to multiple Workflow arguments A Nexus Operation can only take one input parameter. To start a Workflow that takes several, pass them as separate arguments in the `RunAsync` lambda: ```csharp client.StartWorkflowAsync( (HelloHandlerWorkflow wf) => wf.RunAsync(input.Language, input.Name), new() { Id = $"hello-{input.Name}-{input.Language}" }); ``` ### 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. [NexusSimple/Program.cs](https://github.com/temporalio/samples-dotnet/blob/main/src/NexusSimple/Program.cs) ```csharp async Task RunHandlerWorkerAsync() { // Run worker until cancelled logger.LogInformation("Running handler worker"); using var worker = new TemporalWorker( await ConnectClientAsync("nexus-simple-handler-namespace"), new TemporalWorkerOptions(taskQueue: "nexus-simple-handler-sample"). AddNexusService(new HelloService()). AddWorkflow()); try { await worker.ExecuteAsync(tokenSource.Token); } catch (OperationCanceledException) { logger.LogInformation("Handler worker cancelled"); } } ``` Nexus Service handlers also support dependency injection through the generic-host Worker. See [Use dependency injection with a Nexus Service handler](/develop/dotnet/nexus/feature-guide#dependency-injection). ## Develop a caller Workflow that uses the Nexus Service Import the Service interface that has the necessary Operation names and input/output types to execute a Nexus Operation from the caller Workflow: [NexusSimple/Caller/EchoCallerWorkflow.workflow.cs](https://github.com/temporalio/samples-dotnet/blob/main/src/NexusSimple/Caller/EchoCallerWorkflow.workflow.cs) ```csharp using Temporalio.Workflows; [Workflow] public class EchoCallerWorkflow { [WorkflowRun] public async Task RunAsync(string message) { var output = await Workflow.CreateNexusWorkflowClient(IHelloService.EndpointName). ExecuteNexusOperationAsync(svc => svc.Echo(new(message))); return output.Message; } } ``` [NexusSimple/Caller/HelloCallerWorkflow.workflow.cs](https://github.com/temporalio/samples-dotnet/blob/main/src/NexusSimple/Caller/HelloCallerWorkflow.workflow.cs) ```csharp using Temporalio.Workflows; [Workflow] public class HelloCallerWorkflow { [WorkflowRun] public async Task RunAsync(string name, IHelloService.HelloLanguage language) { var output = await Workflow.CreateNexusWorkflowClient(IHelloService.EndpointName). ExecuteNexusOperationAsync(svc => svc.SayHello(new(name, language))); return output.Message; } } ``` ### Register the caller Workflow in a Worker After developing the caller Workflow, the next step is to register it with a Worker. [NexusSimple/Program.cs](https://github.com/temporalio/samples-dotnet/blob/main/src/NexusSimple/Program.cs) ```csharp async Task RunCallerWorkerAsync() { // Run worker until cancelled logger.LogInformation("Running caller worker"); using var worker = new TemporalWorker( await ConnectClientAsync("nexus-simple-caller-namespace"), new TemporalWorkerOptions(taskQueue: "nexus-simple-caller-sample"). AddWorkflow(). AddWorkflow()); try { await worker.ExecuteAsync(tokenSource.Token); } catch (OperationCanceledException) { logger.LogInformation("Caller worker cancelled"); } } ``` ### Develop a starter to start the caller Workflow To initiate the caller Workflow, a starter program is used. [NexusSimple/Program.cs](https://github.com/temporalio/samples-dotnet/blob/main/src/NexusSimple/Program.cs) ```csharp async Task ExecuteCallerWorkflowAsync() { logger.LogInformation("Executing caller echo workflow"); var client = await ConnectClientAsync("nexus-simple-caller-namespace"); var result1 = await client.ExecuteWorkflowAsync( (EchoCallerWorkflow wf) => wf.RunAsync("Nexus Echo 👋"), new(id: "nexus-simple-echo-id", taskQueue: "nexus-simple-caller-sample")); logger.LogInformation("Workflow result: {Result}", result1); logger.LogInformation("Executing caller hello workflow"); var result2 = await client.ExecuteWorkflowAsync( (HelloCallerWorkflow wf) => wf.RunAsync("Temporal", IHelloService.HelloLanguage.Es), new(id: "nexus-simple-hello-id", taskQueue: "nexus-simple-caller-sample")); logger.LogInformation("Workflow result: {Result}", result2); } ``` ## Make Nexus calls across Namespaces with a development Server 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 Run the Nexus handler Worker: ```bash dotnet run handler-worker ``` In another terminal window, run the Nexus caller Worker: ```bash dotnet run caller-worker ``` ### 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: ```bash dotnet run caller-workflow ``` This will show the two workflows started and their results. ### Canceling a Nexus Operation To cancel a Nexus Operation from within a Workflow, cancel the cancellation token passed to the operation call. Only asynchronous operations can be canceled in Nexus, since cancellation is sent using an operation token. The Workflow or other resources backing the operation may choose to ignore the cancellation request. If ignored, the operation may enter a terminal state. When a Nexus operation is started, the caller can specify different cancellation types that control how the caller reacts to cancellation: - `Abandon` - Do not request cancellation of the operation. - `TryCancel` - Initiate a cancellation request and immediately report cancellation to the caller. Note that this type doesn't guarantee that cancellation is delivered to the operation handler if the caller exits before the delivery is done. - `WaitCancellationRequested` - Request cancellation of the operation and wait for confirmation that the request was received. Doesn't wait for actual cancellation. - `WaitCancellationCompleted` - Wait for operation completion. Operation may or may not complete as cancelled. The default is `WaitCancellationCompleted`. Users can set a different option for `CancellationType` in `NexusWorkflowOperationOptions` when starting an operation. Once the caller Workflow completes, the caller's Nexus Machinery stops attempting to cancel operations that have not yet been canceled, letting them run to completion. It's okay to leave operations running in some use cases. To ensure cancellations are delivered, wait for all pending operations to deliver their cancellation requests before exiting the Workflow. See the [Nexus cancellation sample](https://github.com/temporalio/samples-dotnet/tree/main/src/NexusCancellation) for reference. ## Make Nexus calls across Namespaces in Temporal Cloud This section assumes you are already familiar with how to connect a Worker to Temporal Cloud. The `tcld` CLI is used to create Namespaces and the Nexus Endpoint, and mTLS client certificates will be used to securely connect the caller and handler Workers 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 (on MacOS): ``` 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 that the appropriate Namespaces are created for both the caller and handler. If you already have these Namespaces, you don't need to do this. ``` tcld login tcld namespace create \ --namespace \ --region us-west-2 \ --ca-certificate-file 'path/to/your/ca.pem' \ --retention-days 1 tcld namespace create \ --namespace \ --region us-west-2 \ --ca-certificate-file 'path/to/your/ca.pem' \ --retention-days 1 ``` Alternatively, you can create Namespaces through the UI: [https://cloud.temporal.io/Namespaces](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 nexus-simple-endpoint \ --target-task-queue nexus-simple-handler-sample \ --target-namespace \ --allow-namespace \ --description-file endpoint_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](https://cloud.temporal.io/nexus). ## 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 Event history: ![Observability Sync](/img/cloud/nexus/go-sdk-observability-sync.png) An asynchronous Nexus Operation will surface in the caller Workflow as follows, with `NexusOperationScheduled`, `NexusOperationStarted`, and `NexusOperationCompleted`, in the caller's Event history: ![Observability Async](/img/cloud/nexus/go-sdk-observability-async.png) ### 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 ``` Nexus events are included in the caller's Event history: ``` temporal workflow show -w ``` 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. > ## Learn more - Read the high-level description of the [Temporal Nexus feature](/evaluate/nexus) and watch the [Nexus keynote and demo](https://youtu.be/qqc2vsv1mrU?feature=shared&t=2082). - Learn how Nexus works in the [Nexus deep dive talk](https://www.youtube.com/watch?v=izR9dQ_eIe4) and [Encyclopedia](/nexus). - Deploy Nexus Endpoints in production with [Temporal Cloud](/cloud/nexus).