HTTPDNS:Write user-defined resolution functions

Introduction to the HTTPDNS resolution mechanism

The HTTPDNS resolution process consists of several stages. Software-Defined Name System (SDNS) allows you to call resolution functions that you create in Function Compute among these stages. This way, additional stages are included in the process, and the resolution process of HTTPDNS is changed to implement capabilities based on your business requirements.

The following table describes the capabilities that the user-defined resolution functions must deliver by additional stage.

Input and output of SDNS functions

HTTPDNS passes the runtime context to a Function Compute function that is defined based on your business requirements. Then, the function can process the runtime context and pass the processing result to the next stage in the HTTPDNS resolution process.

Format of input parameters

HTTPDNS passes the runtime context as input parameters to a function in Function Compute. The runtime context is an event of the function and is in the JSON format. The following table describes the fields.

Example of input parameters

{
  "domainName": "www.aliyun.com",
  "clientIp": "192.168.1.4",
  "location": {
    "continent": "asia",
    "country": "china",
    "isp": "bgp",  
    "city": "zhejiang"
  },
  "ips": ["192.168.1.3"],
  "ttl": 60,
  "hookType": "BEFORE_WRITE_CACHE"
}

Format of output parameters

The functions in Function Compute return the processed runtime context to HTTPDNS in the following format to facilitate resolution in HTTPDNS. All fields are optional. If a field is not returned or null is returned for a field, HTTPDNS considers that SDNS does not need to modify the field.

Example of output parameters

{
    "ips": event.ips.concat(['192.168.1.2']),
    "ttl": event.ttl * 2,
    "extra": "some-thing-send-to-user"
    // ,"domainName": "www.alibabacloud.com" This field is valid only in the BEFORE_READ_CACHE stage.
    // ,"cacheKey": "cache-key-001" This field is valid only in the BEFORE_READ_CACHE and BEFORE_WRITE_CACHE stages.
  }

Capabilities of SDNS

SDNS allows you to call resolution functions that you create in Function Compute at specified stages of the HTTPDNS resolution process. SDNS also provides the following capabilities:

1. Obtain the information about the region and ISP that correspond to the IP address of the client.

2. Modify the DNS resolution result and the TTL value.

3. Add custom output and return the custom output together with the resolution result.

Function demo

The programming language of the demo is Node.js 6 or Node.js 8.

'use strict';

exports.handler = function (event, context, callback) {
  // Parse the input parameter.
  event = JSON.parse(event.toString());

  // The first parameter of the callback function is null, and the second parameter is the output parameters. The output parameters are automatically converted into a JSON string.
  callback(null, {
    ips: event.ips.concat(['192.168.1.1']),
    ttl: event.ttl * 2,
    extra: "some-thing-send-to-user"
  });
};

Guidelines for the use of user-defined resolution functions in Function Compute

1. Log on to the  Function Compute console.

2. Create a service.

3. Create a function.

4. Write the function code.

5. Publish a service.

6. Log on to the EMAS console to configure SDNS.

Perform a test in the Function Compute console

To test a function, perform the following steps:

1. Select the function that you want to test and go to the Code tab.

2. Choose Test Function > Configure Test Parameters. In the Configure Test Parameters dialog box, configure the Event Name parameter, paste the JSON file that is provided in the Example of input parameters section, and then click OK.

3. Click Test Function to check whether the output parameters of the function meet your business requirements.