Mocking

Mocking services from API specs (Beta)

Easily turn your API specs and Loadmill tests into a fully working mock server based on Mountebank. You can generate mocks from Swagger or OpenAPI files, run them locally, and connect your Loadmill flows to them.

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What you can do

With this feature you can:

• Generate a Mountebank imposter JSON from:

  • A Swagger / OpenAPI file imported into Loadmill as a suite

  • Any Loadmill test suite (YAML)

  • Any single flow (YAML)

• Run the mock server locally on port 9090 by default.

• Point Loadmill tests, Postman, or any HTTP client at the mock instead of the real service.

• Catch issues in your specs early (for example, mismatched field names) using Loadmill assertions against the mock.

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Prerequisites

Before you start, you need:

• A Swagger / OpenAPI file or existing Loadmill tests.

• A Loadmill account with permission to:

  • Import a Swagger file as a suite.

  • Export suites as YAML.

• Node.js installed on your machine.

• Mountebank (installed through npm or npx).

• Optional: Loadmill private agent or Desktop App if you want to run flows against localhost.

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Generate a mock from a Swagger / OpenAPI file

This is the most common workflow.

1. Import the spec as a Loadmill suite

1. Open Loadmill.

2. Import your Swagger / OpenAPI file as a test suite.

3. Loadmill creates:

   • A flow for each endpoint.

   • JSON schema assertions based on the spec.

4. Open the suite and review:

   • The flows that were created.

   • The Parameters tab where the default origin URL is defined.

2. Export the suite as YAML

1. In the Test Suites list, find the suite generated from your spec.

2. Open the suite menu (three dots).

3. Click Export suite.

4. Save the exported YAML file (this will be your input to the mock generator).

3. Generate a Mountebank imposter

1. Open https://mock.loadmill.com/ in your browser.

2. Upload the exported suite YAML.

3. Click Generate imposter.

4. Wait for the job to complete.

5. Click Download imposter JSON and save the file as something like imposter.json.

The downloaded file is a complete Mountebank configuration that you can run as a mock server.

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Run the mock locally with Mountebank

By default, the generated imposter config uses port 9090 for the mock service.

1. Install Mountebank

You can install Mountebank globally:

npm install -g mountebank

Or run it with npx:

npx mountebank --version

(This also checks that it is available.)

2. Start Mountebank with the generated imposter

From the folder where imposter.json is saved:

mb start --configfile imposter.json

Mountebank will:

• Start its admin API (usually on port 2525).

• Start your mock service on port 9090, as defined in the imposter file.

You can verify that the mock is up by calling it directly:

curl -k https://localhost:9090/health

(Replace /health with any endpoint you have.)

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Use the mock in Loadmill

Once the mock is running, you can run your Loadmill tests against it.

1. Point the suite to the mock server

1. Open the suite you created from your Swagger file.

2. Go to the Parameters tab.

3. Find the origin / base URL parameter.

4. Replace the original host with:

   https://localhost:9090

5. Wait for the cloud icon to show that the suite has been saved.

All flows in the suite that use this origin now target your local mock server.

2. Run flows using a local agent (for localhost)

If your mock is running on your machine (localhost):

1. Start the Loadmill Desktop App or a private agent on your machine.

2. Confirm in Loadmill that your agent is connected.

3. In the suite, pick your private agent in the Run settings (if needed).

4. Run a flow or the entire suite.

Loadmill will:

• Send requests to https://localhost:9090.

• Get responses from the Mountebank mock.

• Validate responses using the assertions created from your spec.

If there is a mismatch between the spec and the responses (or between the spec and the example schema), you will see assertion failures that point to the problem.

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Use the mock from other tools

You can also use the generated mock outside Loadmill:

• Postman collections

• Custom test frameworks

• Frontend or backend services running locally

Just set the base URL to:

https://localhost:9090

and call the same paths that your real service exposes.