Create a topic with SMTs

This document explains how to create a Pub/Sub topic with Single Message Transforms (SMTs).

Topic SMTs allow for lightweight modifications to message data and attributes directly within Pub/Sub. This feature enables data cleaning, filtering, or format conversion before the messages are published to the topic.

To create a topic with SMTs, you can use the Google Cloud console, the Google Cloud CLI, the client library, or the Pub/Sub API.

Before you begin

Learn about the Pub/Sub service and its terminology .
Learn about SMTs .

Required roles and permissions

To get the permissions that you need to create a topic with SMTs, ask your administrator to grant you the Pub/Sub Editor ( roles/pubsub.editor ) IAM role on your project. For more information about granting roles, see Manage access to projects, folders, and organizations .

This predefined role contains the permissions required to create a topic with SMTs. To see the exact permissions that are required, expand the Required permissionssection:

Required permissions

The following permissions are required to create a topic with SMTs:

Grant the create a topic permission on the project: pubsub.topics.create

You might also be able to get these permissions with custom roles or other predefined roles .

You can configure access control at the project level and at the individual resource level.

Create a topic with SMTs

Before you create a topic with SMTs, review the documentation for Properties of a topic .

To create a Pub/Sub with one or more SMTs, perform the following steps.

Console

In the Google Cloud console, go to the Pub/Sub Topicspage.

Go to Topics
Click Create topic.
In the Topic IDfield, enter an ID for your topic. For more information about naming topics, see the naming guidelines .
Under Transforms, click Add a transform.
Enter a function name. For example: redactSSN .
If you don't want the SMT to be active immediately, select Disable transform. When this option is selected, the SMT is created with the topic, but isn't executed on incoming messages. After the topic is created, you can edit the topic to enable the SMT.

In the text area, enter the code for the SMT. For example:

  function 
  
 redactSSN 
 ( 
 message 
 , 
  
 metadata 
 ) 
  
 { 
  
 const 
  
 data 
  
 = 
  
 JSON 
 . 
 parse 
 ( 
 message 
 . 
 data 
 ); 
  
 delete 
  
 data 
 [ 
 'ssn' 
 ]; 
  
 message 
 . 
 data 
  
 = 
  
 JSON 
 . 
 stringify 
 ( 
 data 
 ); 
  
 return 
  
 message 
 ; 
 }

Optional. To validate the SMT, click Validate. If the SMT is valid, the message "Validation passed" is displayed. Otherwise, an error message is displayed.
To add another transform, click Add a transformand repeat the previous steps.

To arrange the SMTs in a specific order, click Move upor Move down. To remove an SMT, click Delete.
Optional. To test an SMT on a sample message, perform the following steps:
1. Click Test transforms.
2. In the Test transformwindow, select the function that you want to test.
3. In the Input messagewindow, enter a sample message.
4. To add an attribute to the message, click Add an attributeand enter the attribute's key and value. You can add multiple attributes.
5. Click Test. The result of applying the SMT on the message is displayed under Output message.
6. To close the Test transformswindow, click Close.
If you create more than one SMT, you can test the entire sequence of transforms as follows:
1. Test the first SMT in the sequence, as described in the previous steps.
2. Select the next SMT. The input message is pre-populated with the output message from the previous test.
3. Continue testing the SMTs in order, to make sure the entire sequence works as expected.
To create the topic, click Create.

gcloud

In the Google Cloud console, activate Cloud Shell.

Activate Cloud Shell

At the bottom of the Google Cloud console, a Cloud Shell session starts and displays a command-line prompt. Cloud Shell is a shell environment with the Google Cloud CLI already installed and with values already set for your current project. It can take a few seconds for the session to initialize.

Create a YAML or JSON file that defines one or more SMTs. If you have more than one SMT, they are executed on messages in the order that you list them.

Here is an example of a YAML transform file:

  - 
  
 javascriptUdf 
 : 
  
 code 
 : 
  
>  
 function redactSSN(message, metadata) { 
  
 const data = JSON.parse(message.data); 
  
 delete data['ssn']; 
  
 message.data = JSON.stringify(data); 
  
 return message; 
  
 } 
  
 functionName 
 : 
  
 redactSSN

Optional. To validate an SMT, run the gcloud pubsub message-transforms validate command:
```
 gcloud  
pubsub  
message-transforms  
validate  
 \ 
  
--message-transform-file = 
 TRANSFORM_FILE 
 
```
Replace the following:
- TRANSFORM_FILE : The path to a YAML or JSON file that defines a single SMT. If you are creating multiple SMTs, you must validate them individually.
Optional. To test one or more SMTs on a sample Pub/Sub message, run the gcloud pubsub message-transforms test command:
```
 gcloud  
pubsub  
message-transforms  
 test 
  
 \ 
  
--message-transforms-file = 
 TRANSFORMS_FILE 
  
 \ 
  
--message = 
 MESSAGE 
  
 \ 
  
--attribute = 
 ATTRIBUTES 
 
```
Replace the following:
- TRANSFORMS_FILE : The path to a YAML or JSON file that defines one or more SMTs.
- MESSAGE : The body of the sample message.
- ATTRIBUTES : Optional. A comma-separated list of message attributes. Each attribute is a key-value pair formatted as KEY =" VALUE " .
The command executes the SMTs in order, using the output from each SMT as the input to the next. The command outputs the results of each step.
To create the topic, run the gcloud pubsub topics create command:
```
 gcloud  
pubsub  
topics  
create  
 TOPIC_ID 
  
 \ 
  
--message-transforms-file = 
 TRANSFORMS_FILE 
 
```
Replace the following:
- TOPIC_ID : The ID or name of the topic you want to create. For guidelines on how to name a topic, see Resource names . The name of a topic is immutable.
- TRANSFORMS_FILE : The path to a YAML or JSON file that defines one or more SMTs.

Java

Before trying this sample, follow the Java setup instructions in Quickstart: Using Client Libraries . For more information, see the Pub/Sub Java API reference documentation .

  import 
  
 com.google.api.gax.rpc. AlreadyExistsException 
 
 ; 
 import 
  
 com.google.cloud.pubsub.v1. TopicAdminClient 
 
 ; 
 import 
  
 com.google.pubsub.v1. JavaScriptUDF 
 
 ; 
 import 
  
 com.google.pubsub.v1. MessageTransform 
 
 ; 
 import 
  
 com.google.pubsub.v1. Topic 
 
 ; 
 import 
  
 com.google.pubsub.v1. TopicName 
 
 ; 
 import 
  
 java.io.IOException 
 ; 
 public 
  
 class 
 CreateTopicWithSmtExample 
  
 { 
  
 public 
  
 static 
  
 void 
  
 main 
 ( 
 String 
 ... 
  
 args 
 ) 
  
 throws 
  
 Exception 
  
 { 
  
 // TODO(developer): Replace these variables before running the sample. 
  
 String 
  
 projectId 
  
 = 
  
 "your-project-id" 
 ; 
  
 String 
  
 topicId 
  
 = 
  
 "your-topic-id" 
 ; 
  
 createTopicWithSmtExample 
 ( 
 projectId 
 , 
  
 topicId 
 ); 
  
 } 
  
 public 
  
 static 
  
 void 
  
 createTopicWithSmtExample 
 ( 
 String 
  
 projectId 
 , 
  
 String 
  
 topicId 
 ) 
  
 throws 
  
 IOException 
  
 { 
  
  TopicName 
 
  
 topicName 
  
 = 
  
  TopicName 
 
 . 
 of 
 ( 
 projectId 
 , 
  
 topicId 
 ); 
  
 // UDF that removes the 'ssn' field, if present 
  
 String 
  
 code 
  
 = 
  
 "function redactSSN(message, metadata) {" 
  
 + 
  
 "  const data = JSON.parse(message.data);" 
  
 + 
  
 "  delete data['ssn'];" 
  
 + 
  
 "  message.data = JSON.stringify(data);" 
  
 + 
  
 "  return message;" 
  
 + 
  
 "}" 
 ; 
  
 String 
  
 functionName 
  
 = 
  
 "redactSSN" 
 ; 
  
  JavaScriptUDF 
 
  
 udf 
  
 = 
  
  JavaScriptUDF 
 
 . 
 newBuilder 
 (). 
  setCode 
 
 ( 
 code 
 ). 
  setFunctionName 
 
 ( 
 functionName 
 ). 
 build 
 (); 
  
  MessageTransform 
 
  
 transform 
  
 = 
  
  MessageTransform 
 
 . 
 newBuilder 
 (). 
  setJavascriptUdf 
 
 ( 
 udf 
 ). 
 build 
 (); 
  
 try 
  
 ( 
  TopicAdminClient 
 
  
 topicAdminClient 
  
 = 
  
  TopicAdminClient 
 
 . 
 create 
 ()) 
  
 { 
  
  Topic 
 
  
 topic 
  
 = 
  
 topicAdminClient 
 . 
 createTopic 
 ( 
  
  Topic 
 
 . 
 newBuilder 
 () 
  
 . 
 setName 
 ( 
 topicName 
 . 
  toString 
 
 ()) 
  
 // Add the UDF message transform 
  
 . 
 addMessageTransforms 
 ( 
 transform 
 ) 
  
 . 
 build 
 ()); 
  
 System 
 . 
 out 
 . 
 println 
 ( 
 "Created topic with SMT: " 
  
 + 
  
 topic 
 . 
  getName 
 
 ()); 
  
 } 
  
 catch 
  
 ( 
  AlreadyExistsException 
 
  
 e 
 ) 
  
 { 
  
 System 
 . 
 out 
 . 
 println 
 ( 
 topicName 
  
 + 
  
 "already exists." 
 ); 
  
 } 
  
 } 
 }

Python

Before trying this sample, follow the Python setup instructions in Quickstart: Using Client Libraries . For more information, see the Pub/Sub Python API reference documentation .

  from 
  
 google.cloud 
  
 import 
 pubsub_v1 
 from 
  
 google.pubsub_v1.types 
  
 import 
  JavaScriptUDF 
 
 , 
  MessageTransform 
 
 , 
  Topic 
 
 # TODO(developer) 
 # project_id = "your-project-id" 
 # topic_id = "your-topic-id" 
 code 
 = 
 """function redactSSN(message, metadata) { 
 const data = JSON.parse(message.data); 
 delete data['ssn']; 
 message.data = JSON.stringify(data); 
 return message; 
 }""" 
 udf 
 = 
 JavaScriptUDF 
 ( 
 code 
 = 
 code 
 , 
 function_name 
 = 
 "redactSSN" 
 ) 
 transforms 
 = 
 [ 
 MessageTransform 
 ( 
 javascript_udf 
 = 
 udf 
 )] 
 publisher 
 = 
 pubsub_v1 
 . 
  PublisherClient 
 
 () 
 topic_path 
 = 
 publisher 
 . 
 topic_path 
 ( 
 project_id 
 , 
 topic_id 
 ) 
 request 
 = 
 Topic 
 ( 
 name 
 = 
 topic_path 
 , 
 message_transforms 
 = 
 transforms 
 ) 
 topic 
 = 
 publisher 
 . 
 create_topic 
 ( 
 request 
 = 
 request 
 ) 
 print 
 ( 
 f 
 "Created topic: 
 { 
 topic 
 . 
 name 
 } 
 with SMT" 
 )

Go

The following sample uses the major version of the Go Pub/Sub client library (v2). If you are still using the v1 library, see the migration guide to v2 . To see a list of v1 code samples, see the deprecated code samples .

Before trying this sample, follow the Go setup instructions in Quickstart: Using Client Libraries . For more information, see the Pub/Sub Go API reference documentation .

  import 
  
 ( 
  
 "context" 
  
 "fmt" 
  
 "io" 
  
 "cloud.google.com/go/pubsub/v2" 
  
 "cloud.google.com/go/pubsub/v2/apiv1/pubsubpb" 
 ) 
 // createTopicWithSMT creates a topic with a single message transform function applied. 
 func 
  
 createTopicWithSMT 
 ( 
 w 
  
 io 
 . 
 Writer 
 , 
  
 projectID 
 , 
  
 topicID 
  
 string 
 ) 
  
 error 
  
 { 
  
 // projectID := "my-project-id" 
  
 // topicID := "my-topic" 
  
 ctx 
  
 := 
  
 context 
 . 
 Background 
 () 
  
 client 
 , 
  
 err 
  
 := 
  
 pubsub 
 . 
 NewClient 
 ( 
 ctx 
 , 
  
 projectID 
 ) 
  
 if 
  
 err 
  
 != 
  
 nil 
  
 { 
  
 return 
  
 fmt 
 . 
 Errorf 
 ( 
 "pubsub.NewClient: %w" 
 , 
  
 err 
 ) 
  
 } 
  
 defer 
  
 client 
 . 
 Close 
 () 
  
 code 
  
 := 
  
 `function redactSSN(message, metadata) { 
 const data = JSON.parse(message.data); 
 delete data['ssn']; 
 message.data = JSON.stringify(data); 
 return message; 
 }` 
  
 transform 
  
 := 
  
& pubsubpb 
 . 
 MessageTransform 
 { 
  
 Transform 
 : 
  
& pubsubpb 
 . 
 MessageTransform_JavascriptUdf 
 { 
  
 JavascriptUdf 
 : 
  
& pubsubpb 
 . 
 JavaScriptUDF 
 { 
  
 FunctionName 
 : 
  
 "redactSSN" 
 , 
  
 Code 
 : 
  
 code 
 , 
  
 }, 
  
 }, 
  
 } 
  
 topic 
  
 := 
  
& pubsubpb 
 . 
 Topic 
 { 
  
 Name 
 : 
  
 fmt 
 . 
 Sprintf 
 ( 
 "projects/%s/topics/%s" 
 , 
  
 projectID 
 , 
  
 topicID 
 ), 
  
 MessageTransforms 
 : 
  
 [] 
 * 
 pubsubpb 
 . 
 MessageTransform 
 { 
 transform 
 }, 
  
 } 
  
 topic 
 , 
  
 err 
  
 = 
  
 client 
 . 
 TopicAdminClient 
 . 
 CreateTopic 
 ( 
 ctx 
 , 
  
 topic 
 ) 
  
 if 
  
 err 
  
 != 
  
 nil 
  
 { 
  
 return 
  
 fmt 
 . 
 Errorf 
 ( 
 "CreateTopic: %w" 
 , 
  
 err 
 ) 
  
 } 
  
 fmt 
 . 
 Fprintf 
 ( 
 w 
 , 
  
 "Created topic with message transform: %v\n" 
 , 
  
 topic 
 ) 
  
 return 
  
 nil 
 }

What's next

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License , and code samples are licensed under the Apache 2.0 License . For details, see the Google Developers Site Policies . Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2026-01-13 UTC.

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