Friday, September 4, 2020

 AWS Lambda Limits

Serverless application architecture is the cornerstone of cloud IT applications. AWS Lambda has made it possible for developers to concentrate on business logic and set aside the worry of managing server provisioning, OS patching, upgrades and other infrastructure maintenance work.

However, designing serverless applications around AWS Lambda needs special care specially finding workarounds for AWS Lambda limitations. AWS Lambda limits the amount of compute and storage resources that you can use to run and store functions. AWS has deliberately put several Lambda limits that are either soft or hard to ensure that the service is not misused in case of getting into the hands of hackers. It also provides guardrails so that you follow the best practices to design Lambda Function.

In this article, we will take a closer look into all types of Lambda limits defined by AWS and understand how they can affect in a certain use case. Also, we will see what are the workaround and solutions available to overcome these limits for valid use cases.

AWS Lambda limitations are mostly divided into two parts - Soft limits and Hard limits

Soft limits

Soft limits are defined with default values. Lambda soft limits are per-region and can be increased by putting requests to AWS support team.

Concurrent Executions Limit

In Lambda, scaling is achieved using the concurrent execution of Lambda instances. If a Lambda execution environment cannot fulfil all the requests for a given time, it spins off another instance to handle the remaining requests. However, spinning off the new instances infinitely may cause high cost and can be misused so a default AWS Lambda limit concurrency of 1000 has been put for it.

This limit is configured at account level and shared with all the Functions in the account. Having this limit secures from the unintentional use at account level but a Function inside an account may also overuse the concurrency and affect the execution of other Function instances. We will talk about overcoming that in the best practices section.

Function and Layer Storage

When you deploy the Function on Lambda service, it uses the storage to keep the function code with dependencies. Lambda services keep the code for every version. When you update this Function with a newer version, it keeps adding the new version code in the storage. 

AWS has kept the storage limit to 75 GB so ensure you follow the best practice of cleaning up the old version code. 75 GB seems to be a very high number but over the years, it may be exhausted with the frequent update in the code. 

Elastic Network Interface per VPC

 There are use cases where a Lambda Function needs VPC resources like RDS -mysql or so. In that case, you need to configure VPC subnet and AZs for Lambda Function. Lambda Function connects to these VPC resources through Elastic Network Interface (ENI). 

Earlier, each Function instance used to need a new ENI to connect to a VPC resource so there was a chance of hitting the threshold of 250 (default configured by AWS) very easily. But, with the latest feature of Hyperplane, it has improved the VPC networking and requires less number of ENIs for the communication between a Function and VPC resources. Mostly, this threshold is not hit in most of the use cases.

Hard Limits

Hard limits are the ones that cannot be requested to AWS for the increase. These Lambda limits apply to function configuration, deployments, and execution. We will talk about a few of the important limits in detail.

AWS Lambda Memory Limit

AWS Lambda is meant for small functions to execute for short duration so AWS Lambda memory limit has been kept max to 3GB. It starts from 128 MB and can be increased with 64 MB increments. 

This memory is mostly sufficient for event-driven business functions and serves the purpose. However, there are times when you need high CPU intensive or logical calculation based workload and may cause timeout errors due to not being able to complete the execution within time. There are few solutions available to overcome it and we will talk about those in the best practices section.

AWS Lambda Timeout Limit 

As discussed in AWS Lambda memory limit section, a Function may timeout if it doesn’t finish the execution within the time. And, that time is 15 mins (900 seconds). This is a hard limit in which Function has to complete the execution else it will throw a timeout error. 

This limit is very high for synchronous flows as by nature they are supposed to be completed within a few seconds (3-6 seconds). For asynchronous flow, you need to be careful while designing the solution and ensure each function can complete the task within this period. If it cannot, the logic can be broken into smaller Functions to complete within limits.

AWS Lambda Payload Limit

AWS has kept the payload max limit to 6 MB for synchronous flow. It means, you cannot pass more than 6 MB of data as events. So, while designing the Lambda Function, you need to ensure that consumer and downstream systems are not sending very heavy payload requests and responses respectively. If it is, then Lambda is not the correct solution for that use case.

AWS Lambda Deployment Package

AWS Lambda size limit is 50 MB when you upload the code directly to Lambda service. However, if your code deployment package size is more, you have an option to upload it on S3 and download it while triggering the Function invocation. 

Another option is that you use the Lambda layers. If you use layers, then you can have max 250MB size for your package. You can add up to 5 layers for a Function. However, If you are uploading such a huge code, then there is a real problem in your design you should look into. A Function is meant for small logical code. This huge code may cause high cold start time and a latency problem.

Lambda Design Best Practices around Lambda Limits

As we have talked about most of the common Lambda limits, let’s now discuss the workarounds, tips and best practices to design Lambda Function around these limits.

  • Even though AWS has put the concurrent execution limit to 1000 but that is at account level. You must define the concurrency limit at Function level as well so that one Function overuse doesn’t affect the running of other Functions in the account (Bulkhead pattern).

  • Lambda version is a very important feature but continuous update of the Function increases the storage requirement and may hit the threshold limit (75 GB) and that you will come to know suddenly while doing the production deployment. So plan ahead with an automation script that should clean up the old versions of Function. You may decide a number (may be 5-10) of versions to support.

  • For a synchronous flow, keep the timeout limit very low (3-6 seconds) for functions. It ensures that resources are not clogged for a long time unnecessary and saves cost. For asynchronous flow, based on the monitoring metrics decide the average time of execution and configure the timeout with some additional buffer. While deciding timeout configuration, always keep in mind the downstream system’s capacity and SLA for the response.

  • For a CPU intensive logic, allocate more memory to reduce the execution time. However, just keep in mind that having more than 1.8 GB memory along with a single threaded application won’t give better performance beyond a limit. You need to design the logic to use a multi-threaded strategy to use the second core of the CPU.

  • For Batch processes that need more than 15 minutes of time to execute, break the logic in multiple Functions and use the Lambda Destination or  Step Functions to stitch together the events.

  •  Lambda Function has a temporary instance storage /tmp with 512 MB capacity. This will go off once execution is completed and instance is automatically stopped after a certain time period. Don’t use this capacity as Function should be designed for stateless flow.


AWS Lambda limitations are in quite high numbers but most are consciously thought through and applied. These limits are not to restrict you to use Lambda service but to protect you from the unintentional usage and DDoS type of attacks. You just need to make yourself aware of these limits and follow the best practices discussed here to get the best out of Lambda Functions.


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