AWS Lambda Timeout Best Practices

1. Overview
More and more developers and architects are using AWS Lambda to deploy serverless functions to control costs and reduce the burden of server management. Due to the unpredictable nature of end-user demand in today's digital-first world, Lambda functions can help resolve unplanned scaling issues.
However, when I talk to these Lambda developers and architects, I hear that they are struggling with many different types of problems like throttling, timeout, and slow performances. That's the reason there is a lot of emphases these days to apply AWS Lambda Best Practices.

In this article we’re going to take a look at Lambda timeout errors, monitoring timeout errors, and how to apply best practices to handle them effectively.
A Lambda Serverless application is made up of three major components: Event Source, Lambda Function, and Services (DynamoDB, S3, and the third party).

Let’s first, understand what type of limits are defined by AWS around these components:

2. AWS Defined Limits
a. Event Source
 AWS API Gateway has a max timeout of 29 seconds for all integration types, which includes Lambda as well.

It means that any API call coming through API Gateway cannot exceed 29 seconds. It makes sense for most of the APIs except for a few high computational ones.

DynamoDB Streams - has 40,000 write capacity units per table (in most regions).

That means, if DynamoDB is used as a trigger for a Lambda Function, it cannot have more than 40,000 writes per second. This limit is very high, and if it is used with the max configuration, most of the downstream system will not be able to handle that load and could timeout easily.
b. Lambda Function
A Lambda Function Timeout Limit can be set to a maximum of 15 minutes (900 seconds).

It used to be set at 5 minutes, as Lambda was intended only for small, simple functions with event-driven executions. However, it proved to be a roadblock for applications like batch applications and high computation functions which needed more execution time. 

Thankfully AWS increased it, although some purists don't like it as it defeats the purpose of FaaS being run with small logic only. However, this is a very high value for regular HTTP APIs and, if not configured at the code level, may cause timeouts or high cost. We’ll dig deeper into some of those scenarios shortly.
Concurrent Execution - Lambda has a default limit of 1,000 maximum concurrent executions allowed at an account level. 

That means it can spin up 1,000 containers for all the functions in an account. If concurrency is not handled at the function level, it can lead to throttling issues for all other functions as well.

c. Services
DynamoDB - It has 40,000 Read/Write capacity units per table (in most regions).

S3 -  Unlimited objects.
Third Party Services - It depends on the downstream system.

These configurations also need to be handled for each function, otherwise, it can cause frequent timeouts.

Now, let's take a few scenarios and understand how these limits might cause the timeout errors in a serverless application.
Scenario 1
Problem: A REST API implemented through a Lambda Function is exposed through API Gateway.

This API is calling a third-party service to retrieve the data. But for some reason, this third-party service is not responding. The function has a timeout of 15 minutes, so the thread will be kept waiting for the response.

However, the threshold limit for API Gateway is 29 seconds, so the user will receive the timeout error after 29 seconds. Not only is this a poor experience for the user but it will also result in additional cost.
Solution: For APIs, it’s always better to define your own timeouts at the function level, which should be very short - around 3-6 seconds. Setting the short timeout will ensure that we don't wait for an unreasonable time for a downstream response and cause a timeout.
Scenario 2
Problem: REST API is calling multiple services. It's calling a DynamoDB table to retrieve data, calling an API, and then storing the data back in the DynamoDB table.

If the API is not responding, the function will wait for the response until it reaches the timeout set at the function level (let's assume 6s), and then timeout. Here one integration point is causing the whole function to timeout.
Solution: For each integration point, the timeout needs to be set so that the function can handle the timeout error and process the request with the available data and doesn't waste the execution time. So here, for all 3 integrations, the timeout limit has to be defined to handle the response in an effective way.
Scenario 3
Problem: In order to solve the above two problems, most developers use a fixed timeout limit at the function and integration level hardcoded in the code/config. However, it doesn't make full use of the execution time and can cause problems.
  • If it is too short, it doesn't give the request the opportunity to succeed. For example, there's 6s left in the invocation but we had set timeout to 3s at the integration level.
  • If it is too long, the request will timeout at calling the function. For example. there's the only 5s left in the invocation but we had set timeout to 6 seconds at the integration level.
Let's talk about the two general approaches to set timeout values.
In the first approach, the function timeout limit is set as 6s and for each integration call, it is set at 2s. Even though the whole function invocation (including all three calls) can be done within 6s, the API integration call will timeout as it is not able to perform within 2s. It has not been given the best chance to complete the request.

Similarly, in the second approach, if the timeout is set too high for each call, it will cause the function to timeout without giving a chance for recovery. The function has a 6s timeout and each integration call have a 5s timeout. So, the whole execution can take a maximum of 15s + 1s (1s for handling the response at the function level). In this case, requests are allowed too much time to execute and cause the function to timeout.

Solution: To utilize the invocation time better, set the timeout based on the amount of invocation time left. It must also account for the time required to perform recovery steps, like returning a meaningful error or returning a fallback result based on circuit breaker pattern.

Let’s take an example of one programming language to understand better how to do this:

If Nodejs is the programming language of your function, Lambda handler does provide context object as an input. This object has a method, context.getRemainingTimeInMills(), which returns the approximate remaining execution time of the Lambda function that is currently executing.

To set the timeout for the current running function, we can use this code:

var server = app.listen();

And to set the timeout for each API call, we can use this code:'/xxx', function (req, res) {
req.setTimeout( context.getRemainingTimeInMills() - 500 ); // 500ms to account recovery steps

3. Monitoring Timeout Errors
There are two AWS-native solutions to monitor logs for Lambda -  CloudWatch and X-Ray.
Lambda automatically monitors the logs of a function and provide metrics through CloudWatch. CloudWatch provides Duration metrics which tell us how much time a function is taking throughout a particular period. It also tells us the Average Duration which can be used to baseline the function timeout limit.

However, CloudWatch doesn't go deep enough to tell us how much time each downstream call. This information is required to set a timeout limit at the Integration level. That’s where X-Ray can be used. It shows the execution time taken by all downstream systems.

In the example below, it shows the execution time of S3 GET (171ms) and S3 PUT (178ms) requests.  

4. Best Practices To Handle Timeout Errors
  a. Always use short timeout limits. Set them at 3-6 seconds for API calls, while for Kinesis, DynamoDB Streams or SQS you should adjust the limits based on the batch size.
  b. Put monitoring in place using CloudWatch and X-Ray and fine tune the timeout values as applicable.
  c. If timeouts are unavoidable, either return the response with error code & error description or use fallback methods. Fallback methods can use cached data or get data from an alternative source. Spring Cloud provides Hystrix for fallback methods. It also has the Spring Retry library to apply the retry logic. Node.js also provides the oibackoff library for retry.
  d. DynamoDB does have a writing capacity defined. If Lambda function concurrent execution gets increased, it will start timing out if it crosses the writing capacity. The node-rate-limiter library is available in Node.js which can be used to control the number of invocations allowed for DynamoDB.
  e. Find the right balance between performance and cost. To increase performance, Lambda gives only a single option - increase Memory. More memory equals more CPU.
  • If a function logic is CPU intensive, apply more memory to reduce the execution time. It not only saves the cost but reduces timeout errors.
  • If a function spends most of its time on DB operation, there is no point in increasing memory. It doesn't help.
  • AWS does charge for lambda usage down to a fraction of 100ms. So, if average execution time for your lambda function is 110ms, increase the memory to bring it to below 100ms, otherwise, you’ll be charged for 200ms.
  • If your lambda function is taking more time than the timeout value you wish to have, pay attention to the steps that the function is performing. It might be doing too many things in one function, so you can consider Step Functions to break the function into smaller pieces.
5. Conclusion
In this article, we have discussed various scenarios in which timeouts can lead to bad user experience, not to mention adding cost to your account. So, apply common sense. If a function is taking more time than allotted, there could well be a problem that needs proper attention, rather than simply increasing the timeout limit. Monitoring is the best way to identify these gaps and finetune timeout configuration.

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