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Docs / Build / Policy

2.3 Policy

Policies decide whether a request should invoke the defined Controller at all. They are useful to separate validation logic from request-handling logic. For example, they might perform validation on the request parameters, check for a valid User session, or rate-limit a particular client.

Example 1: Reject all requests after business hours

As a simple example, let's say your web service should only be available during business hours, 8am - 5pm. We'll write a Policy that validates this condition, and only forwards the request to our Controller only if the request is made during business hours. We'll use momentjs to help us with the time validation. Go ahead and npm install moment.

Similar to Controllers and Services, we'll create a new Policy using the Trails generator:

Create Policy

yo trails:policy TimePolicy

// api/policies/TimePolicy.js

const moment = require('moment')

module.exports = class TimePolicy extends Policy {
  
  static get businessHours () {
    return {
      open: 8,
      close: 17 // 5pm 
    }
  }

  /**
   * Return true if the current time is during business hours
   */
  isDuringBusinessHours (request) {
    const now = moment()
    const { open, close } = TimePolicy.businessHours

    if (now.hours() < open || now.hours() > close) {
      throw new Error('We are currently closed.')
    }

    return true
  }
}

Configure Policy

Let's say we want this Policy to apply to VersionController.getLatest that we built in the previous section. We configure this Policy as a precondition of the VersionController.getLatest route:

// config/routes.js

module.exports = [
  {
    method: [ 'GET' ],
    path: '/version/{packageName}',
    handler: 'VersionController.getLatest',
    config: {
      pre: [
        'TimePolicy.isDuringBusinessHours'
      ]
    }
  }
]

. . . or

// config/policies.js

module.exports = [
  {
    VersionController: {
      getLatest: ['TimePolicy.isDuringBusinessHours']
    }
  }
]

Now, requests to /version/{packageName} will first be validated by TimePolicy.

Try it!

At 3pm:

  • Request: GET /version/node
  • Response: 
    {
  "node": "v7.5.0"
}

At 9pm:

  • Request: GET /version/node
  • Response: 
    {
  "statusCode": 500,
  "error": "We are currently closed.",
  "message": ""
}

Next: Model