Reduce AWS Lambda Latencies with Keep-Alive in Python

It starts, as many stories do, with a question. On September 10th, AWS Serverless Hero Luc van Donkersgoed shared his observations on the relationship of reduced latency with increased request rate when using AWS Lambda. This is always an interesting conversation, and sure enough other AWS Heroes like myself are curious about some of the outlier behaviors, and what exactly is going into each request. AWS Data Hero Alex DeBrie, AWS Container Hero Vlad Ionescu both ask excellent questions about the setup and the behaviors, leading Luc to share what he’s seeing with regards to DNS lookups that don’t make sense to him.

After asking a couple of more questions of my own, I rolled up my sleeves and dug into the what, how, and why.

Good to know, helps isolate variables. I'll poke at this a little later to see if I can derive any conclusions.

— Mike Fiedler, Code Gardener (@mikefiedler) September 11, 2022

I dive in to all parts of the stack in use to try and understand why Luc’s code is seeing DNS lookups.
For example, if your function needs to call AWS S3 or a Twilio API, we usually provide the domain name, and have the code or library perform a request to a Domain Name System (DNS) server to return the current IP address, and then communicate using the IP address. This is a network call and can be expensive (in milliseconds) if it’s performed more frequently than the DNS response’s Time To Live (TTL) – kind of like an expiration date. The DNS lookup adds some more latency to your overall call, which is why many systems will cache DNS responses until the TTL is expired, and then make a new call. If you perform DNS lookups when not needed, that’s adding latency unnecessarily. Read the tweet thread for more!

I arrive at two possible solutions:

1. If the Python code calls more than 10 AWS service endpoints, it will trigger a DNS lookup, as urllib3‘s PoolManager will only maintain 10 connections (set by botocore defaults) and will need to recycle if exceeded.
2. Since we’re unlikely to be hitting the limit of 10, something else is at play.
   I found that the default behavior of boto3 is to not use Keep Alive, thus explaining why the occasional connection is reset, triggering a DNS lookup. (Read the tweet thread for the full discovery.)

Using Keep-Alive is nothing new, and was covered quite well by AWS Serverless Hero Yan Cui back in 2019 for Node. It’s even in the official AWS Documentation, citing Yan’s article for the proposed update. Thanks Yan!

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There’s precious little literature on using Keep Alive for Python Lambdas that I could find, leading to issues like Luc’s and reports like this one, so I decided to dig a little further. Knowing now that the default for Keep Alive is off by default for users of the popular boto3 package to interact with AWS services, I wanted to explore what that looks like in practice.

I decided to pattern an app after Yan’s example – a function at receives an event body, and persists it to DynamoDB. All in all, not a too complex operation – we perform a single DNS lookup for the DynamoDB service endpoint, and then use the response IP address to connect over HTTP to put an object into the DynamoDB table.

After re-writing the same function in Python, I was able to test the same kind of behavior that Yan did, running a call to the function once per second, isolating any concurrency concerns, replicating Luc’s test. This should have the benefit of reusing the same Lambda context (no cold starts) and seeing that the latencies range from 7 to 20 milliseconds for the same operation:

So far, so good – pretty much the same. The overall values are lower than Yan’s original experiment, which I attribute to the entire Lambda ecosystem improving, but we can see there’s variance and we often enter double-digit latencies, when we know that the DynamoDB operation is likely to only take 6-7 milliseconds.

As Yan showed in his approach adapted from Matt Levine’s talk snippets, he was able to reconstruct the AWS Config by rebuilding the lowest-level HTTP agent that the library relies on to make the calls, and thereby set the behavior for Keep Alive. This has since been obsoleted by the AWS Node.JS SDK adding an environment variable to enable the keep alive behavior, which is awesome! But what about Python? 🐍

In the recent release of botocore 1.27.84 we can modify the AWS Config passed into the client constructor:

# before:
import boto3
client = boto3.client("dynamodb")

# after:
import boto3
from botocore.config import Config
client = boto3.client("dynamodb", config=Config(tcp_keepalive=True))

With the new configuration in place, if you try this on AWS python3.9 execution runtime, you’ll get this error:
[ERROR] TypeError: Got unexpected keyword argument 'tcp_keepalive'

While the AWS Python runtime includes versions of boto3 and botocore, they do not yet support the new tcp_keepalive parameter – the runtime currently ships:
– boto3 1.20.32
– botocore 1.23.32

So we have to solve another way.

The documentation tells us that we can configure this via a config file in ~/.aws/config, added in version 1.9.17 back in October 2018 – presumably when all the Keep Alive conversations were fresh in folks’ minds.

However, since the Lambda runtime environment disallows writing to that path, we can’t write the config file easily. We might be able to create a custom Docker runtime and place a file in the path, but that’s a bit harder, and we lose some of the benefits of using the AWS prebuilt runtime like startup latency, which when we’re exploring a latency-oriented article, seems like the wrong choice 😁.

Using serverless framework CLI with the serverless-python-requirements (what I’m currently using), or AWS SAM, you can add the updated version of boto3 and botocore, and deploying the updated application allows us to leverage the new setting in a Lambda environment. You may already be using one of these approaches for a more evolved application.
Hopefully 🤞 the Lambda Runtime will be updated to include these versions in the near future, so we don’t have to package these dependencies to get this specific feature.

With the updated packages, we can pass the custom Config with tcp_keepalive enabled (as shown above), and observe more constant performance for the same style of test:

There’s an open request for the config value to be available via environment variable – check it out and give it a 👍 to add your desire and subscribe via GitHub notifications.

Enjoy lower, more predictable latencies with Keep Alive!

Check out the example code here: https://github.com/miketheman/boto3-keep-alive

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Postscript: If you’re interested in pinpointing calls for performance, I recommend checking out Datadog’s APM and associated ddtrace module to see the specifics of every call to AWS endpoints and associated latencies, as well as other parts of your application stack. There’s a slew of other vendors that can help surface these metrics.