Concurrency is a fundamental concept in cloud computing and development, and a critical area of study for any aspiring AWS Certified Developer. The ability to execute multiple sequences of operations simultaneously not only accelerates transaction processing and upgrades the application performance but also comprehends an efficient utilization of hardware resources.

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Foundations of Concurrency in AWS

In the AWS context, concurrency refers to the number of requests that an AWS Lambda function is serving at any given point in time. When more requests come in than the current concurrency capacity can handle, there may be performance implications. This could cause your applications to become slow, unresponsive, or completely fail.

The AWS concurrency model is bound by two main limits:

  1. Per-function concurrency limit: This is a limit set uniquely for an individual lambda function.
  2. Regional concurrency limit: This is set automatically by AWS per region, with a default value of 1000.

AWS Concurrency model

It’s essential to adjust these limits depending on the expected traffic and performance needs of your application. Beyond these limits, new function invocations are throttled.

Decoding Provisioned Concurrency and On-Demand Concurrency

AWS provides two mechanisms to manage concurrency: On-Demand Concurrency and Provisioned Concurrency.

On-Demand Concurrency is the default mechanism. AWS automatically scales the concurrency level based on the incoming traffic, subject to the per-region limit.

On the other hand, with Provisioned Concurrency, you set a specific concurrency level for a lambda function. The provisioned capacity remains ready for your function to execute, ensuring that your application responds predictably even when experiencing sporadic traffic.

The comparison between the two looks like this:

On-Demand Concurrency Provisioned Concurrency
On-start latency Higher (because resources have to be initialized). Lower (since resources are already reserved).
Scaling Automatic, based on incoming traffic. Predefined, based on user settings.
Costs Billed for the actual duration of function execution. Billed for the time your capacity remains available, regardless of function execution.

Why Concurrency Management Matters in AWS

Efficient concurrency management plays a significant role for AWS Certified Developers for several reasons:

  • It ensures application scalability, especially during peak usage times. By managing concurrency well, you ensure that your applications can handle large numbers of simultaneous accesses.
  • It minimises latency and provides a smooth user experience. With provisioned concurrency, you can ensure that your functions respond quickly, without the delay of scaling up resources.
  • It leads to better resource utilisation and can significantly affect the cost. If concurrency settings are not managed efficiently, you may end up paying for unused resources or could fail to serve user requests effectively.


Understanding and managing concurrency is a crucial skill for an AWS Certified Developer. It provides the ability to ensure an application is scalable, performant, and cost-effective. While AWS provides the tools to manage concurrency with instances like AWS Lambda, it’s up to the developer to leverage them effectively. Regardless of whether you are studying for the AWS Certified Developer – Associate (DVA-C02) exam or simply trying to craft a better AWS-based application, mastering concurrency is a necessity.

Practice Test

True/False: AWS supports concurrency at multiple levels.

  • True
  • False

Answer: True

Explanation: AWS offers concurrency support on multiple levels like Lambda, ECS tasks, serverless databases etc. Lambda for example allows you to run code in parallel.

Multiple Choice: Which of the following supports concurrency in AWS?

  • a) Amazon Lambda
  • b) Amazon ECS
  • c) Amazon Athena
  • d) Amazon Redshift

Answer: a) Amazon Lambda, b) Amazon ECS, c) Amazon Athena

Explanation: All these services support concurrency – Lambda runs your code in parallel, ECS maintains multiple simultaneous tasks, Athena allows many queries simultaneously and Redshift also provides an option to run multiple queries at the same time.

True/False: In AWS, a function’s concurrency is the number of instances of that function that are executing simultaneously.

  • True
  • False

Answer: True

Explanation: In AWS, concurrency refers to the multiple instances of a function that can execute at the same time. This is the scalability feature offered by AWS.

Single Select: What is the default safety limit for all function’s concurrency limit in AWS Lambda?

  • a) 500
  • b) 1000
  • c) 700
  • d) 1200

Answer: b) 1000

Explanation: By default, AWS Lambda configures a safety limit of 1000 concurrent executions per region.

Multiple Choice: Which AWS service can you use to schedule capacity in advance for more predictable workloads?

  • a) AWS Elastic Beanstalk
  • b) Amazon S3
  • c) Amazon RDS
  • d) Provisioned Concurrency for AWS Lambda

Answer: d) Provisioned Concurrency for AWS Lambda

Explanation: Provisioned Concurrency for AWS Lambda allows you to reserve capacity in advance for more predictable workloads.

True/False: AWS provides the feature of manual scaling to handle concurrency.

  • True
  • False

Answer: True

Explanation: In addition to auto-scaling, AWS also provides the feature of manual scaling for handling concurrency.

Single Select: Which of the following is not a way to manage concurrency in AWS Lambda?

  • a) Unreserved Concurrency
  • b) Reserved Concurrency
  • c) Provisioned Concurrency
  • d) Allocated Concurrency

Answer: d) Allocated Concurrency

Explanation: AWS Lambda provides three ways to manage concurrency – Unreserved Concurrency, Reserved Concurrency and Provisioned Concurrency. Allocated concurrency is not a feature provided by AWS Lambda.

True/False: AWS X-Ray helps in understanding the behaviour of your applications and troubleshoots concurrency issues.

  • True
  • False

Answer: True

Explanation: AWS X-Ray helps you to visualize and troubleshoot how your application and its underlying services are performing to identify and resolve the root cause of performance issues and errors, including issues related to concurrency.

Multiple Choice: Which AWS services can be used to monitor concurrency?

  • a) AWS CloudTrail
  • b) AWS CloudWatch
  • c) AWS X-Ray
  • d) AWS Config

Answer: a) AWS CloudTrail, b) AWS CloudWatch, c) AWS X-Ray

Explanation: All these services are useful for monitoring various aspects of AWS including concurrency, though their primary focus may differ.

True/False: An API Gateway is another AWS service that supports high concurrency.

  • True
  • False

Answer: True

Explanation: The API Gateway can handle a large number of concurrent API calls, including traffic management, authorization and access control, monitoring, etc.

Interview Questions

What does a concurrency issue refer to in the context of Amazon Web Services (AWS)?

In the context of AWS, a concurrency issue refers to exceeding the limit of the number of concurrently executing Lambda function instances. Often, it might result in throttling where new Lambda invocation requests are dropped or delayed by AWS.

How does AWS Lambda handle simultaneous invocations of your Lambda function?

AWS Lambda automatically scales your applications in response to incoming request traffic. Lambda will run your function in parallel up to your concurrency limit.

What is the default concurrency limit for AWS Lambda functions?

The default concurrency limit for all AWS Lambda functions within a single region is 1000.

What is Provisioned Concurrency in AWS Lambda?

Provisioned Concurrency is a feature in AWS Lambda that keeps your functions initialized and ready to respond in double quick time. It ensures that your functions start without delay and can handle bursts of traffic.

Can you increase your AWS Lambda Concurrency Limit?

Yes. You can request a concurrency limit increase by contacting AWS Support. It is also recommended to manage concurrency by setting individual and reserved concurrency limits.

What is the “Reserve Concurrency” option in AWS Lambda and when would you use it?

Reserve Concurrency is a feature that enables setting a specific number of Lambda function instances that remain ready to respond. It ensures that a certain percentage of the assigned functions always run in parallel. It’s often used when you want to reserve a subset of your account limit for critical functions.

What is AWS Step Functions?

AWS Step Functions is a serverless function orchestrator that makes it easy to sequence AWS Lambda functions and multiple AWS services into business-critical applications.

When do you use AWS SQS vs AWS Step Functions for concurrency control?

AWS SQS is ideal for simple queuing service for reliably sending, storing, and receiving messages between services, ideal for handling varying loads or concurrent processing. AWS Step Functions is suited for orchestrating multiple steps that make up a complex workflow.

How can you reduce the number of concurrent Lambda function executions for a particular feature?

You can set a reserved concurrency limit for a specific Lambda function, which determines the maximum number of simultaneous executions for a function.

What happens if your AWS Lambda function concurrency limit is exceeded?

AWS Lambda will start throttling your function invocations, meaning it will progressively reduce the function’s execution, and further invocation requests will be throttled.

Can you change the concurrency limit for an individual AWS Lambda function?

Yes, you can use the PutFunctionConcurrency API to set a concurrency limit on an individual Lambda function.

How can AWS Step Functions manage concurrency for complex workflows?

AWS Step Functions lets you control the number of different state machine executions with a feature called Execution Throttling. It tries to start an execution but throttles it if the number of active executions is at the maximum.

What happens when a Lambda function reaches its provisioned concurrency limit?

If a Lambda function reaches its provisioned concurrency limit, additional function invocations are served from the free concurrency pool provided they don’t exceed unreserved concurrency.

Can you assign provisioned concurrency to a specific function version or alias?

Yes, provisioned concurrency can be assigned to any function version or alias, allowing precise control over performance at each level.

How can orchestration of AWS Lambda functions be achieved in cases of high concurrency?

High concurrency invocations of AWS Lambda functions can be effectively managed using AWS Step Functions, which provides options to manage function chaining, error handling, complex routing and rollbacks.

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