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The simulator is a compiler target that comes provided in Wing for testing applications locally. The simulator does not require you to deploy resources to any cloud providers. Since it's a simulation, the simulator does not provide the same guarantees as running your application on a cloud provider, but it can be useful for testing and debugging your application.

There are two main ways to interact with the simulator:

  • Running inflight unit tests with the wing test command.
  • Interacting with the simulated application through the Wing Console

The simulator is also available as a set of APIs exported through the @winglang/sdk npm package. A quick start guide for using the simulator APIs is provided below.

Note: The simulator APIs are currently experimental and are subject to breaking changes in the future.

Using the low-level simulator APIs (experimental)

Compile a Wing application for the simulator

To use the simulator, you will need a Wing program that has beem compiled to the simulator (with wing compile --target sim <file>). Check out the Wing Getting Started Guide for more information on how to write your first Wing program.

The output will typically be located in the target directory of your Wing project, for example target/my_app.wsim.

Start the simulator

Now, let's try starting the simulator and creating some resource clients to interact with the resources.

First, run npm install @winglang/sdk.

Next, let's create a file in the directory named main.ts:

import { testing } from '@winglang/sdk';

async function main() {
const sim = new testing.Simulator({ simfile: "hello.wsim" });
await sim.start();

// (1)

await sim.stop();

void main();

Here, we create a new Simulator object, passing in the path to our simulator file, and then start the simulator. Inside (1) we can invoke methods on the simulator to get more information about the resources in our application. When we are done, we call sim.stop() to stop the simulator and clean up all resources (files, ports, etc.) that were created on your machine.

The program can be invoked with ts-node main.ts (assuming you have ts-node installed).

Viewing the application tree

To print out a JSON tree view of the application, add the following line:

console.log(JSON.stringify(sim.tree(), null, 2));

The tree contains the IDs of all the resources in the application, as well as details about dependencies between resources.

Interacting with the simulation

Now let's perform operations using a resource client. To obtain a resource's client, get the resource's path (from the JSON tree or elsewhere) and query the simulator with the getResource method. For example:

import { cloud } from '@winglang/sdk';

const fn = sim.getResource("root/Default/cloud.Function") as cloud.IFunctionClient;
const response = await fn.invoke("hello!");


Finally, when you want to understand how Wing resources are working, you may want to debug your application using traces and logs. Simulated resources automatically generate traces that you can inspect by calling listTraces() on your simulator instance.

console.log(JSON.stringify(sim.listTraces(), null, 2));