Amplify Mock Commands

Pre-requisite

Java is required on your development workstation to use Local Mocking in Amplify

Which package is used?#

If you go into the $HOME_DIR/.amplify/plugins.json file (where $HOME_DIR is your home directory), which lists all the plugins available on your system, you might notice the following lines at the end:

"mock": [
      {
        "packageName": "amplify-util-mock",
        "packageVersion": "3.27.7",
        "packageLocation": "$HOME_DIR/.config/yarn/global/node_modules/amplify-util-mock",
        "manifest": {
          "name": "mock",
          "type": "util",
          "commands": [
            "api",
            "function",
            "mock",
            "storage",
            "help"
          ],
          "eventHandlers": []
        }
      }
    ]

This indicates that the amplify-util-mock package is used when we invoke mocking commands.

What commands are available?#

As per the above JSON property, we can infer that the following commands are available for invocation:

# To mock the AWS AppSync API
amplify mock api

# To mock AWS Lambda Functions
amplify mock function

# To mock all available local resources
amplify mock

# To mock storage solutions (Amazon S3 or Amazon DynamoDB)
amplify mock storage

# Help!
amplify mock help

Folder Structure#

# /amplify-util-mock/src

├── amplify-plugin-index.ts # Handles the entry point
├── api                     # AppSync Mocking
├── CFNParser               # CloudFormation Template Parser
├── commands                # Handles the mock commands
├── __e2e__                 # E2E Tests
├── func                    # For mocking Lambda functions
├── index.ts                # Entry point of all commands
├── mockAll.ts              # Mocking all the available resources
├── storage                 # Mocking Storage
├── __tests__               # Tests
└── utils                   # Utility Programs

Details of files and folders#

export async function executeAmplifyCommand(context: any) {
  const commandPath = path.normalize(
    path.join(__dirname, "commands", pluginName, context.input.command)
  );
  const commandModule = await import(commandPath);
  await commandModule.run(context);
}

The executeAmplifyCommand determines the sub-command and calls the run() function in the appropriate file located in the commands/mock directory.

# amplify-util-mock/src/api

├── api.ts
├── index.ts
├── lambda-arn-to-config.ts
├── resolver-overrides.ts
└── run-graphql-transformer.ts

Files:

api.ts : It exposes a class called APITest responsible for starting the Local AppSync simulator, as well as starting the GraphQL transformer. It also starts a local DynamoDB server which uses SQLite.
index.ts : It exposes the global start() function which is called when amplify mock api is executed.
lambda-arn-to-config.ts : Attempts to match an arn object against the array of lambdas configured in the project.
run-graphql-transformer.ts : Executes the GraphQL transformer from the $TSContext object.

# amplify-util-mock/src/commands/mock

├── api.ts
├── function.ts
├── help.ts
├── mock.ts
└── storage.ts

This folder exposes a run() for each function which is called by amplify mock <command>.

Files:

api.ts : For amplify mock api
fucntion.ts : For amplify mock function <functionName>
help.ts : For amplify mock help
mock.ts : For amplify mock
storage.ts : For amplify mock storage

# amplify-util-mock/src/func

└── index.ts

The file exposes the start() function which is responsible to mock Lambda functions locally. It uses the package amplify-category-function in order to execute operations.

The function mockAllCategories() first searches for all the locally available resources which can be mocked and then executes each mocking utility to mock each mockable resource.

# amplify-util-mock/src/storage

├── index.ts
└── storage.ts

Files:

index.ts : It exposes the start() function which is called when amplify mock storage is executed.
storage.ts : It exposes a class called StorageTest which is responsible for mocking S3 resources and start a mock storage server.