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Working with binary payloads - AWS IoT Core

To handle your message payload as raw binary data (rather than a JSON object), you can use the * operator to refer to it in a SELECT clause.

When you use * to refer to the message payload as raw binary data, you can add data to the rule. If you have an empty or a JSON payload, the resulting payload can have data added using the rule. The following shows examples of supported SELECT clauses.

• You can use the following SELECT clauses with only a * for binary payloads.

  • SELECT * FROM 'topic/subtopic'

  • SELECT * FROM 'topic/subtopic' WHERE timestamp() % 12 = 0

• You can also add data and use the following SELECT clauses.

  • SELECT *, principal() as principal, timestamp() as time FROM 'topic/subtopic'

  • SELECT encode(*, 'base64') AS data, timestamp() AS ts FROM 'topic/subtopic'

• You can also use these SELECT clauses with binary payloads.

  • The following refers to device_type in the WHERE clause.

    SELECT * FROM 'topic/subtopic' WHERE device_type = 'thermostat'

  • The following is also supported.

    {
    	"sql": "SELECT * FROM 'topic/subtopic'",
    	"actions": [
    		{
    			"republish": {
    				"topic": "device/${device_id}"
    			}
    		}
    	]
    }

The following rule actions don't support binary payloads so you must decode them.

• Some rule actions don't support binary payload input, such as a Lambda action, so you must decode binary payloads. The Lambda rule action can receive binary data, if it's base64 encoded and in a JSON payload. You can do this by changing the rule to the following.

  SELECT encode(*, 'base64') AS data FROM 'my_topic'

• The SQL statement doesn't support string as input. To convert a string input to JSON, you can run the following command.

  SELECT decode(encode(*, 'base64'), 'base64') AS payload FROM 'topic'

Protocol Buffers (protobuf) is an open-source data format used to serialize structured data in a compact, binary form. It's used for transmitting data over networks or storing it in files. Protobuf allows you to send data in small packet sizes and at a faster rate than other messaging formats. AWS IoT Core Rules support protobuf by providing the decode(value, decodingScheme) SQL function, which allows you to decode protobuf-encoded message payloads to JSON format and route them to downstream services. This section details the step-by-step process to configure protobuf decoding in AWS IoT Core Rules.

If you already have your descriptor files, you can skip this step. A descriptor file (.desc) is a compiled version of a .proto file, which is a text file that defines the data structures and message types to be used in a protobuf serialization. To generate a descriptor file, you must define a .proto file and use the protoc compiler to compile it.

1. Create .proto files that define the message types. An example .proto file can look like the following:

   syntax = "proto3";
   
   message Person {
     optional string name = 1;
     optional int32 id = 2;
     optional string email = 3;
   }

   In this example .proto file, you use proto3 syntax and define message type Person. The Person message definition specifies three fields (name, id, and email). For more information about .proto file message formats, see Language Guide (proto3).

2. Use the protoc compiler to compile the .proto files and generate a descriptor file. An example command to create a descriptor (.desc) file can be the following:

   protoc --descriptor_set_out=<FILENAME>.desc \
       --proto_path=<PATH_TO_IMPORTS_DIRECTORY> \
       --include_imports \
       <PROTO_FILENAME>.proto

   This example command generates a descriptor file <FILENAME>.desc, which AWS IoT Core Rules can use to decode protobuf payloads that conform to the data structure defined in <PROTO_FILENAME>.proto.

   • --descriptor_set_out

     Specifies the name of the descriptor file (<FILENAME>.desc ) that should be generated.

   • --proto_path

     Specifies the locations of any imported .proto files that are referenced by the file being compiled. You can specify the flag multiple times if you have multiple imported .proto files with different locations.

   • --include_imports

     Specifies that any imported .proto files should also be compiled and included in the <FILENAME>.desc descriptor file.

   • <PROTO_FILENAME>.proto

     Specifies the name of the .proto file that you want to compile.

   For more information about the protoc reference, see API Reference.

After you create your descriptor files <FILENAME>.desc, upload the descriptor files <FILENAME>.desc to an Amazon S3 bucket, using the AWS API, AWS SDK, or the AWS Management Console.

Important considerations

• Make sure that you upload the descriptor files to an Amazon S3 bucket in your AWS account in the same AWS Region where you intend to configure your Rules.

• Make sure that you grant AWS IoT Core access to read the FileDescriptorSet from S3. If your S3 bucket has server-side encryption (SSE) disabled or if your S3 bucket is encrypted using Amazon S3-managed keys (SSE-S3), no additional policy configurations are required. This can be accomplished with the example bucket policy:

  JSON

  {
  	"Version": "2012-10-17",
  	"Statement": [
  		{
  			"Sid": "Statement1",
  			"Effect": "Allow",
  			"Principal": {
  				"Service": "iot.amazonaws.com"
  			},
  			"Action": "s3:Get*",
                        "Resource": "arn:aws:s3:::<BUCKET NAME>/<FILENAME>.desc"
  		}
  	]
  }
  

• If your S3 bucket is encrypted using an AWS Key Management Service key (SSE-KMS), make sure that you grant AWS IoT Core permission to use the key when accessing your S3 bucket. You can do this by adding this statement to your key policy:

  {
  	"Sid": "Statement1",
  	"Effect": "Allow",
  	"Principal": {
  		"Service": "iot.amazonaws.com"
  	},
  	"Action": [
  		"kms:Decrypt",
  		"kms:GenerateDataKey*",
  		"kms:DescribeKey"
  	],
          "Resource": "arn:aws:kms:us-west-2:111122223333:key/1234abcd-12ab-34cd-56ef-1234567890ab"
  	
  }

After you upload the descriptor files to your Amazon S3 bucket, configure a Rule that can decode your protobuf message payload format using the decode(value, decodingScheme) SQL function. A detailed function signature and example can be found in the decode(value, decodingScheme) SQL function of the AWS IoT SQL reference.

The following is an example SQL expression using the decode(value, decodingScheme) function:

SELECT VALUE decode(*, 'proto', '<BUCKET NAME>', '<FILENAME>.desc', '<PROTO_FILENAME>', '<PROTO_MESSAGE_TYPE>') FROM '<MY_TOPIC>'

In this example expression:

• You use the decode(value, decodingScheme) SQL function to decode the binary message payload referenced by *. This can be a binary protobuf-encoded payload or a JSON string that represents a base64-encoded protobuf payload.

• The message payload provided is encoded using the Person message type defined in PROTO_FILENAME.proto.

• The Amazon S3 bucket named BUCKET NAME contains the FILENAME.desc generated from PROTO_FILENAME.proto.

After you complete the configuration, publish a message to AWS IoT Core on the topic to which the Rule is subscribed.

AWS IoT Core Rules support protobuf with the following limitations:

• Decoding protobuf message payloads within substitution templates is not supported.

• When decoding protobuf message payloads, you can use the decode SQL function within a single SQL expression up to two times.

• The maximum inbound payload size is 128 KiB (1KiB =1024 bytes), the maximum outbound payload size is 128 KiB, and the maximum size for a FileDescriptorSet object stored in an Amazon S3 bucket is 32 KiB.

• Amazon S3 buckets encrypted with SSE-C encryption are not supported.

Here are some best practices and troubleshooting tips.

• Back up your proto files in the Amazon S3 bucket.

  It's a good practice to back up your proto files in case something goes wrong. For example, if you incorrectly modify the proto files without backups when running protoc, this can cause issues in your production stack. There are multiple ways to back up your files in an Amazon S3 bucket. For example, you can use versioning in S3 buckets. For more information about how to back up files in Amazon S3 buckets, refer to the Amazon S3 Developer Guide.

• Configure AWS IoT logging to view log entries.

  It's a good practice to configure AWS IoT logging so that you can check AWS IoT logs for your account in CloudWatch. When a rule's SQL query calls an external function, AWS IoT Core Rules generates a log entry with an eventType of FunctionExecution, which contains the reason field that will help you troubleshoot failures. Possible errors include an Amazon S3 object not found, or invalid protobuf file descriptor. For more information about how to configure AWS IoT logging and see the log entries, see Configure AWS IoT logging and Rules engine log entries.

• Update FileDescriptorSet using a new object key and update the object key in your Rule.

  You can update FileDescriptorSet by uploading an updated descriptor file to your Amazon S3 bucket. Your updates to FileDescriptorSet can take up to 15 minutes to be reflected. To avoid this delay, it's a good practice to upload your updated FileDescriptorSet using a new object key, and update the object key in your Rule.

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