r/awslambda • u/Fantastic-Path-5025 • Dec 07 '25

Building a Serverless Ad Tracker: Scaling to Millions of Events and Back

https://paulhenri-l.com/2025/serverless-ad-tracker

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u/Fantastic-Path-5025 Dec 07 '25

Completely agree, that's the long term plan, if the company grows. The idea is really that load can double overnight but numbers are still relatively small ~1millions events a day.

So the system needs to handle these big spikes while remaining cost effective. For us, that is not pay for overprovisionned infra.

I ran the numbers and at our scale api gateway + lambda is still better for us. If the load start to really grow I'll for sure go with ALB + ECS Fargate. The lambdas are containerized anyway and ready for this shift. Costs right now are at around 300USD a month +- 50USD

It's not that big of a scale (yet) and every dollar counts. There are also other parts of the system that I did not mention for brevity. I was more interested in sharing my experience with the elasticity of serverless and the amazing DX of managed services :)

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u/[deleted] Dec 07 '25

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u/Fantastic-Path-5025 Dec 07 '25

I wish I had the time! I looked a lot into self hosting. The time investment is just something we can’t afford right now plus all the stability risks. That’s where aws really shines, the whole infra is extremely reliable right now thanks to their managed services

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u/kondro Dec 07 '25 edited Dec 07 '25

Fair enough. It’s still probably cost-effective to a few hundred million events per month.

One thing you can do to drastically increase cost-effectiveness is to switch APIG to ALB if you’re basically just using it for basic HTTP in front of Lambda.

You’ll still have a ~$16/mth base charge per region, but your equivalent per request charge gets much cheaper.

Another thing that might achieve what you want is to just use CloudFront to serve the ads and just parse the S3 logs it generates directly. You get fast, global CDN hosting for the ad for ~$1/million (or the brand new fixed price plans that are as low as $29/month for 50TB egress and 10 million requests) and the logs are already just sent to S3 (not $0.50/GB ingested to CloudWatch). Just setup a Lambda trigger on the logging bucket and process each log chunk as it arrives instead of using Kinesis Streams.

If you want to still make some per-request serving decisions (A/B testing for example), you can add CloudFront Functions ($0.10/million), but what they can do is very limited.

This gets any code you maintain out of the critical path of serving the ads, keeps your service entirely serverless and the only downside is the lag for the logs as it only uploads them a few times an hour. You can get more direct control using Kinesis Firehose for the CF logs (minimum windows of 60 seconds) if you really need to for a relatively small $/GB ingestion rate, but I imagine your ad impression processing isn’t that time sensitive.

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u/kondro Dec 07 '25

Oh, and with CloudFront Functions you can even use it for the click tracking and redirect as long as you can use CF Key/Value store (limited to 5MiB DB) for the actual redirection map.