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Nexus Microservice Development Walkthrough - Java SDK

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caution

This walkthrough covers the Temporal Operation Handler, which is pre-release. APIs are experimental and may change in backwards-incompatible ways.

This walkthrough builds one Nexus Service from nothing to a complete API, adding a single Nexus capability at each step.

A Nexus Service is a contract that one team publishes and other teams call, across Namespace boundaries, without sharing code or a deployment.

The walkthrough problem

This guide is grounded in a purchase approval Workflow — a common Temporal and Nexus use case.

A purchase request needs approval before it can proceed.

Approval is slow and human-driven: someone has to look at the request and decide. The system needs to survive that wait, which may be minutes or weeks. While a request is pending, other systems might need to nudge the approver or attach information to the request. Eventually a decision arrives, and the requesting system needs the outcome.

Concretely, the Service needs to:

• Tell a caller whether a purchase needs approval at all, before any durable work starts
• Start an approval and, eventually, return APPROVED or DENIED
• Accept a nudge that asks the approver again, and count how many have been sent
• Accept supporting information for a purchase, whether or not its approval exists yet
• Accept a decision from the caller and confirm it was recorded
• Send a notification when the decision is final

Each of those needs a different Nexus capability, introduced one step at a time.

One contract, every language

This walkthrough builds the Service in Java. The same contract has a sample implementation in every language the generator supports. The reasoning at each step — what the contract should say, what backs each Operation, which message type to reach for — is the same in all of them.

Working sample code, all built from the one contract:

Language	Sample
Java (this walkthrough)	{sample repo link}
Go	{sample repo link}
Python	{sample repo link}
TypeScript	{sample repo link}

Any caller can call any handler, because the contract is the only thing the two sides share. A Go caller can drive the Python handler; the TypeScript caller can drive the Java handler. Call builds the Java caller and then points at the other languages' samples.

info

Sample repos for each language will land once the docs settle. The idea is that you can run the client from any sample against the handler from any other sample.

The Nexus Client Code Generator takes the contract and emits typed models, validators, and Service definitions for Go, Java, Python, and TypeScript.

Before you start

You need two Namespaces — one for the handler and one for the caller — so the walkthrough crosses a real Namespace boundary. A local development server with two Namespaces is enough through Implement; Publish covers both the development server and Temporal Cloud.

temporal operator namespace create --namespace approval-handler-namespace
temporal operator namespace create --namespace approval-caller-namespace

New to Nexus? Read Nexus Services and Nexus Operations, or work through the shorter Java Nexus quickstart.