Function Compute:MNS topic triggers

Overview

MNS is a distributed messaging service that features high scalability, efficiency, reliability, security, and availability. MNS allows application developers to transfer data and notifications between distributed components of an application and build a loosely coupled system. In MNS, messages are published to topics. A publisher can call the PublishMessage operation to publish a message to a topic. Subscribers to the topic receive the message. For more information, see PublishMessage.

Configuring a trigger for an MNS topic is equivalent to registering a function as a subscriber to the MNS topic. When a publisher publishes a message to the MNS topic, the message is passed to the function and used as the event parameter of the function handler to trigger the function. For more information, see Basics.

Integration of MNS and Function Compute provides the following benefits:

• Allows you to perform advanced operations on a message before you send the message in an email or a text message.

• Allows you not to create services at HTTP endpoints.

• Supports abundant custom operations. For example, you can send a message to Slack or persist a specified message.

Before you start

• Function Compute

  • Create a function

• MNS

  • Create a topic

Usage notes

• The MNS topic and the Function Compute function to be associated must reside in the same region.

• You must avoid recursive loops.

  For example, you must avoid the following logic when you write code for a function: Topic A triggers Function B, and Function B publishes a new message to Topic A, which then triggers Function B again. This logic results in an infinite loop of function invocations.

Step 1: Create an MNS topic trigger

1. Log on to the Function Compute console. In the left-side navigation pane, click Functions.

2. In the top navigation bar, select a region. On the Functions page, click the function that you want to manage.

3. On the function details page, click the Configurations tab. In the left-side navigation pane, click Triggers. Then, click Create Trigger.

4. In the Create Trigger panel, configure parameters and click OK.

   Parameter	Description	Example
   Trigger Type	The type of the trigger. Select MNS Triggered by Topic.	MNS Triggered by Topic
   Name	The name of the trigger.	trigger-mns
   Version or Alias	The version or alias of the trigger. Default value: LATEST. If you want to create a trigger for another version or alias, select a version or alias in the upper-right corner of the function details page. For more information about versions and aliases, see Manage versions and Manage aliases.	LATEST
   MNS Region	The region in which the MNS topic resides. The MNS topic must reside in the same region as the function in Function Compute.	China (Chengdu)
   Topic	The name of the existing MNS topic.	Mytopic
   Filter Tag	The tag that is used for message filtering. The function is invoked only when a message that contains the specified filtering tag is received.	tag
   Event Format	The format of the event. Options: STREAM JSON	JSON
   Retry Policy	The retry policy. Options: Backoff Retry Exponential Decay Retry For more information about how to specify the retry policy, see NotifyStrategy.	Backoff Retry
   Role Name	Select AliyunMNSNotificationRole. Note After you configure the preceding parameters, click OK. If you create a trigger of this type for the first time, click Authorize Now in the message that appears.	AliyunMNSNotificationRole

Step 2: Configure the input parameters of the function

1. On the Code tab of the function details page, click the icon next Test Function and select Configure Test Parameters from the drop-down list.

2. In the Configure Test Parameters panel, click the Create New Test Event or Modify Existing Test Event tab, enter the event name and event content, and then click OK.

   Messages that are published to an MNS topic are processed based on the format of the message body. This constructs an event that is used to invoke a function in Function Compute. For more information, see NotifyContentFormat.

   • Examples on the format of the event content if you set the Event Format parameter to STREAM for an MNS topic trigger:

     • The following sample code provides an example on the format of the event content if messages do not contain message attributes:

       Note

       If messages do not contain message attributes, the content of the event is a string in the JSON format.

       # The message body. 
       'hello topic'

     • The following sample code provides an example on the format of the event content if messages contain message attributes:

       Note

       The content of the event contains the key-value pairs that are related to message attributes. For more information, see PublishMessage.

           {
               "body": "hello topic",
               "attrs": {
                   "Extend": "{\\"key\\":\\"value\\"}"
               }
           }                    

   • Examples on the format of the event content if you set the Event Format parameter to JSON for an MNS topic trigger:

     • The following sample code provides an example on the format of the event content if messages do not contain message attributes:

           {
               "TopicOwner": "118620210433****",
               "Message": "hello topic",
               "Subscriber": "118620210433****",
               "PublishTime": 1550216480040,
               "SubscriptionName": "test-fc-subscribe",
               "MessageMD5": "BA4BA9B48AC81F0F9C66F6C909C3****",
               "TopicName": "Mytopic",
               "MessageId": "2F5B3C082B923D4EAC694B76D928****"
           }        
                                   

     • The following sample code provides an example on the format of the event content if messages contain message attributes:

       Note

       The content of the event contains the key-value pairs that are related to message attributes. For more information, see PublishMessage.

           {
               "key": "value",
               "TopicOwner": "118620210433****",
               "Message": "hello topic",
               "Subscriber": "118620210433****",
               "PublishTime": 1550216302888,
               "SubscriptionName": "test-fc-subscribe",
               "MessageMD5": "BA4BA9B48AC81F0F9C66F6C909C3****",
               "TopicName": "Mytopic",
               "MessageId": "2F5B3C281B283D4EAC694B742528****"
           }
                                   

   The following table describes parameters of the event.

   Parameter	Type	Example	Description
   key	String	value	The key-value pairs that are related to message attributes.
   TopicOwner	String	118620210433****	The account ID of the MNS topic owner.
   Message	String	hello topic	The content of the message.
   Subscriber	String	118620210433****	The account ID of the user who subscribes to the MNS topic.
   PublishTime	Int	1550216302888	The time when the message was published.
   SubscriptionName	String	test-fc-subscribe	The name of the subscription.
   MessageMD5	String	BA4BA9B48AC81F0F9C66F6C909C3****	The MD5 hash value of the message body.
   TopicName	String	Mytopic	The name of the MNS topic.
   MessageId	String	2F5B3C281B283D4EAC694B742528****	The message ID.

Step 3: Write function code and test the function

After you create the MNS topic trigger, you can write function code and test the function code to verify whether the code is valid.

1. On the function details page, click the Code tab, enter function code in the code editor, and then click Deploy.

   In this example, the function code is written in Python. The following sample code can be used as a function template for an MNS topic trigger:

   import json
   import logging
   
   def handler(event, context):
     logger = logging.getLogger()
     logger.info("mns_topic trigger event = {}".format(event))
     # For example, you can record an event to Tablestore. 
     return "OK"                     

2. Click Test Function.

   After the function is executed, you can view the results on the Code tab.

More information

To modify or delete an existing trigger, see Manage triggers.