Lambda Power Tuning advanced features

This section describes some advanced features of this project, as well as some considerations related to security and cost.

Error handling

If something goes wrong during the initialization or execution states, the CleanUpOnError step will be executed. All temporary versions and aliases will be deleted as expected (the same happens in the Cleaner step).

You can customize the totalExecutionTimeout parameter at deploy time (up to 15min). This parameter will be used both for Lambda function timeouts and Step Function tasks timeouts. In case the Executor raises a timeout error, you will see a States.Timeout error. Keep in mind that the timeout you configure will vary whether you're setting parallelInvocation to true or false. When you enable parallel invocation, all the function executions will run concurrently (rather than in series) so that you can keep that timeout lower and your overall state machine execution faster.

In all other error cases, you will see a Lambda.Unknown error, which corresponds to unhandled errors in Lambda such as out-of-memory errors and hitting the concurrent Lambda invoke limit. If you encounter it as input of CleanUpOnError, it's very likely that something went wrong with the function you're power-tuning.

Retry policy

The executor will retry twice in case any invocation fails. This is helpful in case of execution timeouts or memory errors. You will find the failed execution's stack trace in the CleanUpOnError state input.

How do I know which executor failed and why?

You can inspect the "Execution event history" and look for the corresponding TaskStateAborted event type.

Additionally, you can inspect the CleanUpOnError state input. Here you will find the stack trace of the error.

Security

All the IAM roles used by the state machine adopt the least privilege best practice, meaning that only a minimal set of Actions are granted to each Lambda function.

For example, the Executor function can only call lambda:InvokeFunction. The Analyzer function doesn't require any permission at all. On the other hand, the Initializer, Cleaner, and Optimizer functions require a broader set of actions.

By default, the Executor function is allowed to invoke any Lambda function in your account, in any region. This happens because the default resource defined in the IAM role is "*", but you can change this value at deploy-time, via the the lambdaResource CloudFormation parameter.

For example, you could use a mix of the following:

Same-region prefix: arn:aws:lambda:us-east-1:*:function:*
Function name prefix: arn:aws:lambda:*:*:function:my-prefix-*
Function name suffix: arn:aws:lambda:*:*:function:*-dev
By account ID: arn:aws:lambda:*:ACCOUNT_ID:function:*

Execution cost

There are three main costs associated with AWS Lambda Power Tuning:

AWS Step Functions cost: it corresponds to the number of state transitions; this cost depends on the number of tested power values, and it's approximately 0.000025 * (6 + N) where N is the number of power values; for example, if you test the 6 default power values, the state machine cost will be $0.0003
AWS Lambda cost it relates to your function's executions and depends on three factors: 1) number of invocations that you configure as input (num), the number of tested power configurations (powerValues), and the average invocation time of your function; for example, if you test all the default power configurations with num: 100 and all invocations take less than 100ms, the Lambda cost will be approximately $0.001
AWS Lambda cost related to Initializer, Executor, Cleaner, and Analyzer: for most cases it's negligible, especially if you enable parallelInvocation: true; this cost is not included in the results.stateMachine output to keep the state machine simple and easy to read and debug

State Machine Internals

The AWS Step Functions state machine is composed of five Lambda functions:

Initializer: define all the versions and aliases that need to be created (see Publisher below)
Publisher: create a new version and aliases corresponding to one of the power values provided as input (e.g. 128MB, 256MB, etc.)
IsCountReached: go back to Publisher until all the versiona and aliases have been created
Executor: execute the given Lambda function num times, extract execution time from logs, and compute average cost per invocation
Cleaner: delete all the previously generated aliases and versions
Analyzer: compute the optimal power value (current logic: lowest average cost per invocation)
Optimizer: automatically set the power to its optimal value (only if autoOptimize is true)

Initializer, Cleaner, Analyzer, and Optimizer are invoked only once, while the Publisher and Executor are invoked multiple times. Publisher is used in a loop to create all the required versions and aliases, which depend on the values of num, powerValues, and onlyColdStarts. Executor is used by N parallel branches - one for each configured power value. By default, the Executor will invike the given Lambda function num consecutive times, but you can enable parallel invocation by setting parallelInvocation to true.

Weighted Payloads

Important

Your payload will only be treated as a weighted payload if it adheres to the JSON structure that follows. Otherwise, it's assumed to be an array-shaped payload.

Weighted payloads can be used in scenarios where the payload structure and the corresponding performance/speed could vary a lot in production and you'd like to include multiple payloads in the tuning process.

You may want to use weighted payloads also in case of functions with side effects that would be hard or impossible to test with the very same payload (for example, a function that deletes records from a database).

You configure weighted payloads as follows:

{
    ...
    "num": 50,
    "payload": [
        { "payload": {...}, "weight": 5 },
        { "payload": {...}, "weight": 15 },
        { "payload": {...}, "weight": 30 }
    ]
}

In the example above, the weights 5, 15, and 30 are used as relative weights. They will correspond to 10% (5 out of 50), 30% (15 out of 50), and 60% (30 out of 50) respectively - meaning that the corresponding payload will be used 10%, 30% and 60% of the time.

For example, if num=100 the first payload will be used 10 times, the second 30 times, and the third 60 times.

To simplify these calculations, you could use weights that sum up to 100.

Note: the number of weighted payloads must always be smaller or equal than num (or num >= count(payloads)). For example, if you have 50 weighted payloads, you'll need to set at least num: 50 so that each payload will be used at least once.

Pre/Post-processing functions

Sometimes you need to power-tune Lambda functions that have side effects such as creating or deleting records in a database. In these cases, you may need to execute some pre-processing or post-processing logic before and/or after each function invocation.

For example, imagine that you are power-tuning a function that deletes one record from a downstream database. Since you want to execute this function num times you'd need to insert some records in advance and then find a way to delete all of them with a dynamic payload. Or you could simply configure a pre-processing function (using the preProcessorARN input parameter) that will create a brand new record before the actual function is executed.

Here's the flow in pseudo-code:

function Executor:
  iterate from 0 to num:
    [payload = execute Pre-processor (payload)]
    results = execute Main Function (payload)
    [execute Post-processor (results)]

Please also keep in mind the following:

You can configure a pre-processor and/or a post-processor independently
The pre-processor will receive the original payload
If the pre-processor returns a non-empty output, it will overwrite the original payload
The post-processor will receive the main function's output as payload
If a pre-processor or post-processor fails, the whole power-tuning state machine will fail
Pre/post-processors don't have to be in the same region of the main function
Pre/post-processors don't alter the statistics related to cost and performance

S3 payloads

In case of very large payloads above 256KB, you can provide an S3 object reference (s3://bucket/key) instead of an inline payload.

Your state machine input will look like this:

{
    "lambdaARN": "your-lambda-function-arn",
    "powerValues": [128, 256, 512, 1024],
    "num": 50,
    "payloadS3": "s3://your-bucket/your-object.json"
}

Please note that the state machine will require IAM access to your S3 bucket, so you might need to redeploy the Lambda Power Tuning application and configure the payloadS3Bucket parameter at deployment time. This will automatically generate a custom IAM managed policy to grant read-only access to that bucket. If you want to narrow down the read-only policy to a specific object or pattern, use the payloadS3Key parameter (which is * by default).

S3 payloads work fine with weighted payloads too.

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