Where can I log & debug Velocity Template Language (VTL) in AWS AppSync?

Question

1. Is there any easy way to log or debug VTL coming from Request Mapping Template & Response Mapping Template rather than sending Queries & Mutations to debug & log?

2. Also, is there any Playground to check & play with VTL just like we can do with JavaScript in Web Console?

3. Can we work with AWS AppSync offline & check if everything written in VTL works as expected?

Answer 1

A super nasty way to log and debug is using validate in the response mapping

$util.validate(false, $util.time.nowISO8601().substring(0, 10) ) 

Answer 2

Here's how I logged a value in my VTL resolver:

Add a "$util.error" statement in your request or response template and then make the graphql call.

For example, I wanted to see what was the arguments passed as an input into my resolver, so I added the $util.error statement at the beginning of my template. So, my template was now:

$util.error("Test Error", $util.toJson($ctx)) {     "version" : "2017-02-28",     "operation" : "PutItem",     "key": {         "id": $util.dynamodb.toDynamoDBJson($ctx.arguments.user.id)     },     "attributeValues": {         "name": $util.dynamodb.toDynamoDBJson($ctx.arguments.user.name)     } } 

Then from the "Queries" section of the AWS AppSync console, I ran the following mutation:

mutation MyMutation {   addUser(user: {id: "002", name:"Rick Sanchez"}) {     id     name   } } 

This displayed the log results from my resolver as follows:

{   "data": null,   "errors": [     {       "path": [         "addUser"       ],       "data": null,       "errorType": "{\"arguments\":{\"user\":{\"id\":\"002\",\"name\":\"Rick Sanchez\"}},\"identity\":null,\"source\":null,\"result\":null,\"request\":{\"headers\":{\"x-forwarded-for\":\"112.133.236.59, 130.176.75.151\",\"sec-ch-ua-mobile\":\"?0\",\"cloudfront-viewer-country\":\"IN\",\"cloudfront-is-tablet-viewer\":\"false\",\"via\":\"2.0 a691085135305af276cea0859fd6b129.cloudfront.net (CloudFront)\",\"cloudfront-forwarded-proto\":\"https\",\"origin\":\"https://console.aws.amazon.com\",\"content-length\":\"223\",\"accept-language\":\"en-GB,en;q=0.9,en-US;q=0.8\",\"host\":\"raxua52myfaotgiqzkto2rzqdy.appsync-api.us-east-1.amazonaws.com\",\"x-forwarded-proto\":\"https\",\"user-agent\":\"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/87.0.4280.88 Safari/537.36 Edg/87.0.664.66\",\"accept\":\"*/*\",\"cloudfront-is-mobile-viewer\":\"false\",\"cloudfront-is-smarttv-viewer\":\"false\",\"accept-encoding\":\"gzip, deflate, br\",\"referer\":\"https://console.aws.amazon.com/\",\"x-api-key\":\"api-key-has-been-edited-out\",\"content-type\":\"application/json\",\"sec-fetch-mode\":\"cors\",\"x-amz-cf-id\":\"AvTMLvtxRq9M8J8XntvkDj322SZa06Fjtyhpf_fSXd-GmHs2UeomDg==\",\"x-amzn-trace-id\":\"Root=1-5fee036a-13f9ff472ba6a1211d499b8b\",\"sec-fetch-dest\":\"empty\",\"x-amz-user-agent\":\"AWS-Console-AppSync/\",\"cloudfront-is-desktop-viewer\":\"true\",\"sec-fetch-site\":\"cross-site\",\"sec-ch-ua\":\"\\\"Chromium\\\";v=\\\"87\\\", \\\" Not;A Brand\\\";v=\\\"99\\\", \\\"Microsoft Edge\\\";v=\\\"87\\\"\",\"x-forwarded-port\":\"443\"}},\"info\":{\"fieldName\":\"addUser\",\"parentTypeName\":\"Mutation\",\"variables\":{}},\"error\":null,\"prev\":null,\"stash\":{},\"outErrors\":[]}",       "errorInfo": null,       "locations": [         {           "line": 9,           "column": 3,           "sourceName": null         }       ],       "message": "Test Error"     }   ] }