AWS AppSync Resolvers Lambda Function vs Velocity Template Language (VTL)

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I have been looking into AWS AppSync to create a managed GraphQL API with DynamoDB as the datastore. I know AppSync can use Apache Velocity Template Language as a resolver to fetch data from dynamoDB. However, that means I have to introduce an extra language to the programming stack, so I would prefer to write the resolvers in Javascript/Node.js

Is there any downside of using a lambda function to fetch data from DynamoDB? What reasons are there to use VTL instead of a lambda for resolvers?

aws-appsync graphql aws-lambda amazon-web-services
1 Answers

There are pros and cons to using lambda functions as your AppsSync resolvers (although note you'll still need to invoke your lambdas from VTLs):

Pros

  • Easier to write and maintain
  • More powerful for marshalling and validating requests and responses
  • Common functionality can be more DRY than possible with VTLs (macros are not supported)
  • More flexible debugging and logging
  • Easier to test
  • Better tooling and linting available
  • If you need to support long integers in your DynamoDB table (DynamoDB number types do support long, but AppSync resolvers only support 32-bit integers. You can get around this if you use a lambda, for example by serializing longs to a string before transport through the AppSync resolver layer) - See (currently) open Feature Request: https://github.com/aws/aws-appsync-community/issues/21

Cons

  • Extra latency for every invocation
  • Cold starts = even more latency (although this can usually be minimised by keeping your lambdas warm if this is a problem for your use case)
  • Extra cost
  • Extra resources for each lambda, eating up the fixed 200 limit

If you're doing a simple vanilla DynamoDB operation it's worth giving VTLs a go. The docs from AWS are pretty good for this: https://docs.aws.amazon.com/appsync/latest/devguide/resolver-mapping-template-reference-dynamodb.html

If you're doing anything mildly complex, such as marshalling fields, looping, or generally hacky non-DRY code, then lambdas are definitely worth considering for the speed of writing and maintaining your code provided you're comfortable with the extra latency and cost.

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