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Description:

IN AWS Lambda, which is the core of the serverless platform at AWS, the unit of scale is the concurrent execution. This refers to the number of execution of your function code that is happening at any given time.

• Concurrency is the number of requests that your function is serving at any given time

• If the function is invoked again while a request is still being processed, another instance is allocated, which increases the function’s concurrency.

• Concurrency is subject to a Regional quota that is shared by all functions in Region.

There are two types of concurrency controls available:

• Reserved concurrency - Reserved concurrency guarantees the maximum number of concurrent instances for the function. When a function has reserved concurrency, no other function can use that concurrency. There is no charge for configuring reserved concurrency for a function. 

• Provisioned Concurrency - Provisioned concurrency initializes a requested number of execution environments so that they are prepared to respond immediately to your function environments so that they are prepared to respond immediately to your function’s invocations. 

Rationale:

In Lambda functions' concurrency is the number of instances that serves requests at a given time. In a Region, the initial burst of traffic can reach between 500 and 3000, which varies per Region

Note: The burst concurrency quota is not per function; it applies to all of your functions in the Region.

Impact:

 After the configuration there are the following effects:

1. It limits the number of instances of your Lambda function that can be instantiated at any time to the value specified.

2. It makes sure there is always at least enough concurrency capability available in the account to run the number of instances requested.

3. It alow helps to protect from DoS attack

Effects of Reserving concurrency:

• Other functions cannot prevent you function from scaling - all of your account’s functions in the same Region without reserved concurrency share the pool of unreserved concurrency. Without reserved concurrency, other functions can use up all of the available concurrency. This prevents your function from scaling up when needed.

• The functions cannot scale out of control - reserved concurrency also limits your function from using concurrency from the unreserved pool, which caps its maximum concurrency. you can reserve concurrency to prevent your function from using all the available concurrency in the Region, or from overloading downstream resources. 

Default Value:

Burst concurrency quotas

• 3000 - US West(Oregon), US East (N. Virginia), Europe(Ireland )

• 1000 - Asia Pacific(Tokyo), Europe(Frankfurt), US East(Ohio)

• 500 - Other Regions

Lambda sets quotas for the amount of compute and storage

The following quotas apply to function configuration, deployments and execution. They cannot be changed

by default in any lambda function, it looks like

Audit:

Steps 1:Sign in AWS Console Management and go to Lambda console at  https://console.aws.amazon.com/lambda/home#/functions

Steps 2: Click on functions in the left navigation pane

Steps 3: Choose a function to audit

Steps 4: Go to the Configuration tab

Steps 5: Scroll down and then choose Concurrency

Steps 6: In the Concurrency by default  Function concurrency is Use unreserved account concurrency and Unreserved concurrency set 1000 and Provisioned concurrency configurations is zero means it is not created or enable.

You can reserve the concurrency for particular functions. for this follow the implementation steps.

Via CLI Audit

To get a list of provisioned concurrency configurations

aws lambda list-provisioned-concurrency-configs --function-name < give the name of function> 

To view the reserved concurrency setting for a function

aws lambda get-function-concurrency --function-name < give the name of function>

Remediation:

Pre-Requisite:

For configuring the provisioned concurrency you need to create a function alias using the Lambda console and for the alias or version you cannot use $LATEST

If function will not or no any alias created then you can configure reserved concurrency 

Implementation Steps:

Configuring reserved concurrency, To reserve concurrency for a function:

Steps 1:  Sign in AWS Management  Console and go to Lambda console at https://console.aws.amazon.com/lambda/home#/functions

Steps 2: Click on functions in the left navigation pane

Steps 3: Choose a function to configure the concurrency

Steps 4: Go to the Configuration tab

Steps 5: Scroll down and then choose Concurrency

Steps 6: In concurrency click on the Edit button to change the function of concurrency from Use unreserved account concurrency to reserved concurrency and give some value

Steps 7: To select Reserve concurrency click on the radio button in the rectangular box give the value ( for this particular function its reserves for the given concurrent value )

Step 8: Click on Save button

Now selected function is used reserved concurrency

Configuring Provisioned concurrency

To provision concurrency for an alias

Steps 1:  Sign in AWS Management Console and go to Lambda Console at  https://console.aws.amazon.com/lambda/home#/functions

Steps 2: Click on functions in the left navigation pane

Steps 3: Choose a function to configure the concurrency

Steps 4: Go to the Configuration tab

Steps 5: Scroll down and then choose Concurrency

Steps 6: In provisioned concurrency configurations click on click on edit button because here as per prerequisite you already have an alias for this function so that in the concurrency part you have only provisioned concurrency

Steps 7: Set the value of concurrent requests and click on the Save button

                  as per the diagram, you can see the pricing per month before this you always see the pricing 

Via CLI

To configure a reserve concurrency limit for a function

aws lambda put-function-concurrency \
      --function-name <give the function name> \
      --reserved-concurrent-executions < give the value >

If you configure Reserved concurrency follow the implementation steps 1 - 6 after clicking the Edit button you choose to Use unreserved account concurrency and click on the Save button 

If you configure Provisioned concurrency configuration follow  the implementation steps 1-5 and first remove the alias and delete the version 

Via CLI

To remove the reserved concurrent execution limit from a function

aws lambda delete-function-concurrency \
      --function-name < give the function name>

1. Lambda quotas - AWS Lambda

2. AWS Lambda function scaling - AWS Lambda

3. Managing concurrency for a Lambda function - AWS Lambda