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Using request and response translators with data for a remote service

With request and response translators, you can change the format of data sent to, and received from, remote services used by external functions.

When Snowflake sends data to a remote service, Snowflake formats the data according to these rules. Similarly, when Snowflake receives data from a remote service, Snowflake expects the data to be formatted according to the same rules.

Many remote services expect to handle data in a different format. With request and response translators, you can conveniently:

• Convert data from Snowflake’s format to the remote service’s native input format (request translator).

• Convert data from the remote service’s native output format to Snowflake’s format (response translator).

To translate data between Snowflake’s format and the remote service’s native input format, you use JavaScript UDFs (user-defined functions). You almost always write a pair of UDFs: one to translate the request and one to translate the response.

Snowflake calls these functions as part of each external function call. For example, for a request to a remote service, Snowflake calls the request translator function, passes it the Snowflake-formatted data, then takes the returned data and sends it to the remote service. When the remote service returns data, Snowflake calls the response translator function to convert the data back to the format that Snowflake understands.

From the user perspective, calling an external function when a translator is converting is the same as calling an external function without a translator. After you specify translators as part of the CREATE EXTERNAL FUNCTION statement, they are called automatically.

An external function can have a maximum of one request translator and one response translator at a time.

The request and response translator UDFs can be secure UDFs.

To specify which user-defined function to use as a translator, include REQUEST_TRANSLATOR and RESPONSE_TRANSLATOR clauses in the CREATE EXTERNAL FUNCTION statement. Each takes the name of the translator function to use at run time.

For example:

CREATE EXTERNAL FUNCTION f(...)
    RETURNS OBJECT
    ...
    REQUEST_TRANSLATOR = my_request_translator_udf
    RESPONSE_TRANSLATOR = my_response_translator_udf
    ...
    AS <url_of_proxy_and_resource>;

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The syntax for specifying translators as part of a CREATE EXTERNAL FUNCTION statement is shown below:

CREATE EXTERNAL FUNCTION f(...)
    RETURNS OBJECT
    ...
    [ REQUEST_TRANSLATOR = <request_translator_udf_name> ]
    [ RESPONSE_TRANSLATOR = <response_translator_udf_name> ]
    ...

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where:

request_translator_udf_name

The name of the request translator function.

response_translator_udf_name

The name of the response translator function.

The REQUEST_TRANSLATOR and RESPONSE_TRANSLATOR parameters each take one parameter of type OBJECT.

You can also specify a request or response translator in an ALTER FUNCTION command. You can:

• Add a translator if the external function does not already have one.

• Replace an existing translator.

• Remove a translator.

Use the SET keyword to add a new translator or to replace an existing translator.

To add or replace a translator:

ALTER FUNCTION ...
    SET [REQUEST_TRANSLATOR | RESPONSE_TRANSLATOR] = <udf_name>;

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where

udf_name

The name of a previously-created JavaScript UDF.

To remove a translator:

ALTER FUNCTION ...
    UNSET [REQUEST_TRANSLATOR | RESPONSE_TRANSLATOR];

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• The name of the translator function in the CREATE EXTERNAL FUNCTION or ALTER FUNCTION statement should be either:

  • A qualified name (e.g. MyDatabase.MySchema.MyJavaScriptUDF).

  • Defined in the same database and schema as the external function that uses them.

• When the translator is specified in a CREATE EXTERNAL FUNCTION or ALTER FUNCTION statement, the translator UDF must already exist. You can’t specify the name first and create the UDF later – even if you don’t call the external function before you create the UDF.

• A UDF used as a translator should not be dropped without first removing it from all external functions that use it. (At the time the external function is called, Snowflake fails with an error if the translator does not exist.)

• If the translator UDF is modified (via ALTER FUNCTION), it must retain the same interface requirements. If it does not retain the interface requirements, an exception is raised before running the external function.

At run time, SQL passes an OBJECT to the translator UDF. The JavaScript code receives this as a JavaScript object.

A translator UDF receives a JavaScript object named event. The object contains the following properties:

• body: The format of the data field is the same as the existing Snowflake rowset batch (i.e. an array of rows).

  For example,

  {
    "body": {
            "data": [
                      [0,"cat"],
                      [1,"dog"]
                    ]
            }
  }
  

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  The existing data is nested under the outer body.

• serviceUrl: The external function’s defined URL to call.

• contextHeaders: An object that contains all the context-related headers, where the names are the field names. For example, the object could contain the field name “SF_CONTEXT_CURRENT_DATABASE”, and the corresponding value would be a string containing the current database name.

The request translator returns an object with fields used to communicate with the external service API gateway. That object has three optional fields:

• body: Defines the actual body to be passed to the service. If this is not defined, there is no body. The body value should be a string or a JSON object in the format that the remote service expects. If the value is a string, that string can contain internal structure (e.g. be JSON-compatible). If the value is a JSON object, that object is converted to a string so that it can be included as part of the HTTP POST command string.

• urlSuffix: Sets the suffix of the service URL, which is added to the end of the serviceUrl value. This suffix is also allowed to contain query parameters. Parameter names and values must be URL encoded. For example, if you want to set a parameter named a to value my param you need to do URL encoding of the space character, so the parameter would be ?a=my%20param.

• translatorData: Passed from the request translator to the response translator. This field can pass context information, such as the input body, the service URL or suffix, or context headers.

All three fields are optional. However, as a practical matter, most request translators return at least the body data.

The input parameter for the response translator function is an object. The example below uses EVENT, which contains two properties:

• body: The response to be decoded from the external service response.

• translatorData: If this field is returned by the request translator, then Snowflake passes it to the response translator.

The response translator response is returned as an object under the body element; the format is the existing external function format (array of rows). For example:

{
  "body": {
          "data": [
                    [0, "Life"],
                    [1, "the universe"],
                    [2, "and everything"]
                  ]
           }
}

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Each translator UDF must meet the following requirements:

• It must be a JavaScript UDF.

• It must take exactly one parameter of type OBJECT, which represents a batch of rows.

• It must return one value of type OBJECT, which also represents a batch of rows.

• It must be a scalar UDF (returning one row for each row (OBJECT) passed in).

  Note

  Although the translator is scalar, the OBJECT passed to the translator can (and usually does) have multiple rows embedded inside the JSON in the OBJECT.

• The number and order of the rows (inside the OBJECT) returned by the response translator UDF must be the same as the number and order of the rows passed to the request translator UDF (inside the OBJECT).

The following example shows a request translator and response translator being used to convert data into the format required by an external service that does sentiment analysis, Amazon Comprehend BatchDetectSentiment. The request translator shapes the HTTP request to match the format that the backend service expects.

To use translators, you’ll need an API gateway. This example uses an API gateway that is already configured to talk to the sentiment analysis service. For more information about how to integrate with an Amazon Web Services (AWS) service as the backend, see Set up an API integration request using the API Gateway console in the AWS documentation.

It is helpful to get your API integration working successfully before adding translators.

Set up a database to hold demo data.

Choose a role that has permission to create external functions:

Specify which warehouse, database and schema to use:

USE WAREHOUSE w;
USE DATABASE a;
USE SCHEMA b;

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Create a table to hold your test sentences:

CREATE TABLE demo(vc varchar);
INSERT INTO demo VALUES('Today is a good day'),('I am feeling mopey');

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This external function doesn’t have a request translator or response translator:

CREATE OR REPLACE EXTERNAL FUNCTION ComprehendSentiment(thought varchar)
RETURNS VARIANT
API_INTEGRATION = aws_comprehend_gateway
AS 'https://<MY_GATEWAY>.execute-api.us-east-1.amazonaws.com/test/comprehend_proxy';

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You can call the external function with your test data from the demo table:

SELECT ComprehendSentiment(vc), vc FROM demo;

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The generated request body uses the Snowflake external function data format:

{"body":{"data:" [[0, "Today is a good day"],[1,"I am feeling mopey"]]}}

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However, the external sentiment analysis service expects a different format that specifies the language and an array of strings:

{"body": { Language: "en", TextList: [ "Today is a good day", "I am feeling mopey"]}}

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The next section describes how you can add a request translator to change the request body to the required format.

By using a request translator, you can convert the default input described above (in the Snowflake data format) to the format that the external service requires.

The following SQL creates an awscomprehendrequest_translator translator function.

CREATE OR REPLACE FUNCTION AWSComprehendrequest_translator(EVENT OBJECT)
RETURNS OBJECT
LANGUAGE JAVASCRIPT AS
'
var textlist = []
for(i = 0; i < EVENT.body.data.length; i++) {
   let row = EVENT.body.data[i];
   // row[0] is the row number and row[1] is the input text.
   textlist.push(row[1]); //put text into the textlist
}
// create the request for the service. Also pass the input request as part of the output.
return { "body": { "LanguageCode": "en", "TextList" : textlist }, "translatorData": EVENT.body }
';

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In the request translator function, the code:

• Loops through each of the input rows. For each row, it adds the string, which is in row[1], to the textlist array. The value at row[0] is the row number and it can be ignored.

• Returns a JSON body that has the language code and text list that matches the requirements of the external service.

• Returns data via the translatorData field. This is used by the response translator. In this example, you are sending the original input data. You will use the length of the input data in the response translator to know how many input requests there were.

You can test the request translator by calling it directly.

SELECT AWSComprehendrequest_translator(parse_json('{"body":{"data": [[0, "I am so happy we got a sunny day for my birthday."], [1, "$$$$$."], [2, "Today is my last day in the old house."]]}}'));

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The request translator puts the body into the shape expected by the external service.

{"body":{
   "LanguageCode": "en",
   "TextList": [
      "I am so happy we got a sunny day for my birthday.",
      "$$$$$.",
      "Today is my last day in the old house."
               ]
         },
   "translatorData": {
      "data": [[0, "I am so happy we got a sunny day for my birthday."],
               [1, "$$$$$."],
               [2, "Today is my last day in the old house."]]
                     }
}

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A response body from the external service looks something like this.

{"body":{
   "ErrorList": [ { "ErrorCode": 57, "ErrorMessage": "Language unknown", "Index": 1} ],
   "ResultList":[ { "Index": 0, "Sentiment": "POSITIVE",
                    "SentimentScore": { "Mixed": 25, "Negative": 5, "Neutral": 1, "Positive": 90 }},
                  { "Index": 2, "Sentiment": "NEGATIVE",
                    "SentimentScore": { "Mixed": 25, "Negative": 75, "Neutral": 30, "Positive": 20 }}
                ]
         }
}

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The response translator processes the results that you get back from the external service. The results contain a combination of errors in the ErrorList and results in the ResultList.

The response translator code combines these results together to make a complete set that matches the order of the rows that were passed to the external service. The response translator returns the results in the Snowflake format.

The following SQL creates an awscomprehendresponse_translator translator function.

CREATE OR REPLACE FUNCTION AWSComprehendresponse_translator(EVENT OBJECT)
RETURNS OBJECT
LANGUAGE JAVASCRIPT AS
'
// Combine the scored results and the errors into a single list.
var responses = new Array(EVENT.translatorData.data.length);
// output format: array of {
// "Sentiment": (POSITIVE, NEUTRAL, MIXED, NEGATIVE, or ERROR),
// "SentimentScore": <score>, "ErrorMessage": ErrorMessage }.
// If error, set ErrorMessage; otherwise, set SentimentScore.
// Insert good results into proper position.
for(i = 0; i < EVENT.body.ResultList.length; i++) {
   let row = EVENT.body.ResultList[i];
   let result = [row.Index, {"Sentiment": row.Sentiment, "SentimentScore": row.SentimentScore}]
   responses[row.Index] = result
}
// Insert errors.
for(i = 0; i < EVENT.body.ErrorList.length; i++) {
   let row = EVENT.body.ErrorList[i];
   let result = [row.Index, {"Sentiment": "Error", "ErrorMessage": row.ErrorMessage}]
   responses[row.Index] = result
}
return { "body": { "data" : responses } };
';

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In the response translator function, the code:

• Initializes an array called responses with the size of the input from the translatorData array length. You sent translatorData from the request translator to the response translator to pass the original list of test strings.

• Loops through each of the non-error results and puts them into the result list.

• Loops through the error results and puts them into the result list. The result list has an index position which tells you what entry it is. The order of the produced results must match the input order. The result list also contains the sentiment information.

After all of the responses have been gathered, they are returned in a JSON body in the format that Snowflake expects.

The following direct test will return a JSON body with the correct format.

SELECT AWSComprehendresponse_translator(
    parse_json('{
        "translatorData": {
            "data": [[0, "I am so happy we got a sunny day for my birthday."],
                    [1, "$$$$$."],
                    [2, "Today is my last day in the old house."]]
                          }
        "body": {
            "ErrorList":  [ { "ErrorCode": 57,  "ErrorMessage": "Language unknown",  "Index": 1 } ],
            "ResultList": [
                            { "Index": 0,  "Sentiment": "POSITIVE",
                              "SentimentScore": { "Mixed": 25,  "Negative": 5,  "Neutral": 1,  "Positive": 90 }
                            },
                            { "Index": 2, "Sentiment": "NEGATIVE",
                              "SentimentScore": { "Mixed": 25,  "Negative": 75,  "Neutral": 30,  "Positive": 20 }
                            }
                          ]
            },
        }'
    )
);

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To the external function, add the request and response translator functions by assigning the function names as values to the request_translator and response_translator parameters. This way, they’ll be called automatically when the external function runs.

CREATE OR REPLACE EXTERNAL FUNCTION ComprehendSentiment(thought varchar)
RETURNS VARIANT
API_INTEGRATION = aws_comprehend_gateway
request_translator = db_name.schema_name.AWSComprehendrequest_translator
response_translator = db_name.schema_name.AWSComprehendresponse_translator
AS 'https://<MY_GATEWAY>.execute-api.us-east-1.amazonaws.com/test/comprehend_proxy';

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You can describe the function to get information about it.

DESCRIBE FUNCTION ComprehendSentiment(VARCHAR);

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Test the external function by calling it with a single sentence.

SELECT ComprehendSentiment('Today is a good day');

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You see the sentiment analysis results.

{"Sentiment": "POSITIVE",
 "SentimentScore":{"Mixed":0.002436627633869648,
                   "Negative":0.0014803812373429537,
                   "Neutral":0.015923455357551575,
                   "Positive": 0.9801595211029053}}

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Test the external function by calling it with multiple sentences. Use the same demo table that you created earlier.

SELECT ComprehendSentiment(vc), vc FROM demo;

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The sentiment analysis results are displayed.

When the external function was called, the request translator automatically converted data into the format required by the external service. Then, the response translator automatically converted the response from the external service back into the format required by Snowflake.

• Test case values are typically OBJECT values (collections of key-value pairs). These should be formatted to meet the requirements in these rules.

• You can start testing your request translator or response translator by passing in an example input converted to a string. For example:

  select my_request_translator_function(parse_json('{"body": {"data": [ [0,"cat",867], [1,"dog",5309] ] } }'));
  

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  (The input to PARSE_JSON() must be a JSON-formatted string.)

• Test with NULL values if appropriate.

  • Include at least one SQL NULL value in your test cases.

  • Include at least one JSON NULL value in your test cases.

• Translating a request and translating a response are often converse processes. Conceptually:

  my_response_translator_udf(my_request_translator_udf(x)) = x
  

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  You can use this characteristic to help test your request translator and response translator if the data formats match. Create a table with good test values, then execute a command similar to:

  SELECT test_case_column
      FROM test_table
      WHERE my_response_translator_udf(my_request_translator_udf(x)) != x;
  

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  The query should not return any rows.

  Note that translating a request and translating a response are not always exactly converse. For an example of where they might not be converse, see the discussion of converse functions in the “Usage Notes” section of the documentation for the TO_JSON() function.

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