This is a follow up to our previous article on Managing multi-environment serverless architecture using AWS API Gateway . To see an implementation of the approach described in this article, see the sample project on GitHub . Error handling in or architectures can be tricky. Different components may be integrated using different protocols and be built using different stacks, and yet any client facing error responses should honour the same API contract. microservices serverless The relies on a facade/gated proxy service. API Gateway pattern In this article we are going to look at a simple built using SAM (Serverless Application Model), AWS CloudFormation, API Gateway and Lambda functions written in Go. serverless To-Do app Our goal is to meet the following requirements in relation to error handling: Consistent error responses, regardless of the error’s type or origin Clear separation of external and internal interfaces No leaking of private error details Client facing errors This is our API contract for error responses. The clients expect any non responses to contain an body with this shape. 2XX application/json AWS API Gateway Let’s investigate the request flow with AWS API Gateway and AWS Lambda. As we can see below, there are two sources for errors, the API Gateway itself (Gateway Responses) and the integration function (Integration Responses). Errors can originate from various sources within a serverless application Gateway responses Gateway Responses represent errors that occur before reaching the integration (such as access control errors, internal configuration errors, etc), or when the integration response cannot be mapped to a method response. These can be customised to fit our error schema by using a simple mapping template. Here’s the relevant section of our CloudFormation template. AWS provides a of response types that can be used to define response mappings. full list For the purpose of this example we chose to map only the catch-all and types. In the case of errors we return as our code, and as our message, which will provide validation and access control error details to clients. But in the case of errors we hardcode the code and message in order to avoid leaking internal configuration error details to clients. 4XX 5XX 4XX error.responseType error.messageString 5XX Integration responses The strategy for handling errors returned by a Lambda function is dependent on how the function is integrated with the endpoint, which can be done using either a . The former proxies HTTP requests to the Lambda whereas the latter decouples the function from the original HTTP request further and completely relies on request and response mappings. proxy integration or a custom integration Our Lambda handlers use the custom integration type. This allows us to write handlers with clearly defined inputs and outputs, without any knowledge of the HTTP request initially made to the API gateway. Not only does this keep the function simple, it also makes it easier to invoke and test our command with a simple event payload and inspect the result. We found to be a useful tool during development and leverage it for integration testing, as will be explained later. sam-local Lambda error responses AWS Lambda uses its own error schema which can later be inspected and modified by API Gateway. This is why the following Lambda handler may not do what you expect. Instead of outputting a simple error string, the Go Lambda runtime wraps the message string in a which results in the following response. custom error type The Lambda error response includes the actual type of the original error, see Go source , and the error value . However we want to retain error codes and reliably separate private and public error details. In order to do that our handler needs to return a of an error type that produces a encoded string upon a call of its method, thereby resulting in a integration response. structured error json Error() json-within-json We introduced a type, meant to be used by a Lambda handler function to wrap errors before returning them. lambdaError The error value is the JSON encoded structured error. As you can see, our internal error schema contains a , a and a . The will be useful for matching on the gateway later, the is a human readable string that does not leak any technical details, and the is the detailed error string. code public_message private_message code public_message private_message Invoking our handler now returns the following error response. The handler does not know how this response will be processed by the gateway before being sent to the client. This is not particularly elegant, but it’s the only way to return a structured error message when using custom Lambda integration in API Gateway. Response mappings in API Gateway A response received from the Lambda function then gets mapped by API Gateway in order to conform to our external API contract. To map errors we rely on integration response mappings, which regular expressions, to match error codes. Our example app’s matching strategy is to first create a matching rule for the absence of error (success response) and then for any expected errors that can be proxied to the client, and finally a catch-all for unexpected errors that simply maps to a hardcoded . 500 Internal error Below is a snippet from the template that deals with integration response mapping, which deals with ‘not found’ errors. Note that a 404 method response must also be defined for that endpoint. The regular expression matches a specific error code inside the string value (which happens to be our encoded structured error), then maps it to a status code and uses the to decode the string and fit it to our expected output shape, keeping out the private message, which still gets logged. errorMessage json 404 velocity template language json The resulting error returned by the gateway to our client is just as we expect. The most important thing to note here is that we can manipulate the integration errors in any way that we see fit, for example, changing a code like to if that’s what the client expects. TASK_NOT_FOUND RESOURCE_NOT_FOUND Below is the flow diagram from before, now annotated with the error payloads at different stages: Conclusion The good news is that we were able to fulfil our error handling requirements using AWS API Gateway and Lambda, but we did find that aspects of the solution have some drawbacks. The hack used to return a structured error along with the regex matching seem particularly brittle. This could perhaps be ameliorated by the introduction of support for a protocol such as gRPC on the integration side. json-within-json In the end the trade-off is some clunkiness vs the time that it would take to roll out your own gateway service. One more thing: Integration testing with SAM Local Full integration testing including all gateway mappings, validation, authorisers, etc. would have to be done in a testing CloudFormation stack. However, helps considerably with testing the Lambda responses and gives us a sense of how our handler command will behave when executed in the real Lambda environment. SAM Local Using we can execute a handler cmd in a local environment by providing it with an event file similar to what API Gateway would send it. sam local invoke We decided to automate this process by writing tests that execute with a prebuilt cmd binary and check the response payloads. sam invoke You can find our helper invoke function here. SAM Local is also able to bring up a local API gateway however custom integration is which meant that we couldn’t use that functionality. Note: currently unsupported This investigation was carried out with Christian Klotz at 2PAx (a startup that aims to revolutionise how restaurants allocate covers). Thanks to Christian for helping with this article. References Sample App Source https://github.com/smalleats/serverless-todo-example AWS Lambda Go https://github.com/aws/aws-lambda-go AWS SAM Local https://github.com/awslabs/aws-sam-local Dave Cheney’s error package https://github.com/pkg/errors VTL Reference http://velocity.apache.org/engine/devel/vtl-reference.html Set up API Gateway Request and Response Data Mappings https://docs.aws.amazon.com/apigateway/latest/developerguide/mappings.html API Gateway Mapping Template Reference https://docs.aws.amazon.com/apigateway/latest/developerguide/api-gateway-mapping-template-reference.html Set up Gateway Responses to Customize Error Responses https://docs.aws.amazon.com/apigateway/latest/developerguide/customize-gateway-responses.html Domain errors discussion https://softwareengineering.stackexchange.com/a/351062 API Gateway pattern http://microservices.io/patterns/apigateway.html

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