Written in Rust, released under the MIT license. Built at pafin Hackathon 2026, our second in-house hackathon.

pafin Inc., the company behind cryptact, a crypto tax platform with more than 200,000 users in Japan, has released fakestream as open source under the MIT license, which allows commercial use and modification. fakestream emulates Amazon Kinesis Data Streams on a developer's machine or in a CI job. It is the first project to come out of pafin Hackathon 2026, the in-house hackathon we held to accelerate AI adoption across the company.

Background

We have used Kinesis for about eight years. It moves transaction data between our services: the crawlers that fetch trades from exchanges and blockchains, the translation layer that normalizes them, and the ledger engine that computes gains and losses.

Reproducing that pipeline on a developer's machine and in CI, however, had been a problem for a long time. We ran LocalStack, with its JVM-based engine enabled to get the throughput we needed. Even with several gigabytes of heap it ran out of memory on large wallets, and every restart of the stack waited on the JVM to start. In CI, one instance per test job limited how many jobs fit on a runner. As DeFi grew and our data volume grew with it, that cost rose year after year.

In March 2026 LocalStack ended its Community edition. The remaining free tier requires an account and an auth token and is limited to non-commercial use. That change was what pushed us to solve a problem we had been working around.

Why we built it

After looking at the existing alternatives, we measured what we actually call. It came to ten Kinesis operations: managing streams and shards, and reading and writing records. With a surface that small, we decided to use the hackathon week to write our own server in Rust, with no runtime like the JVM underneath it, and to see whether it could replace Kinesis in our development environment.

The replacement was finished during the hackathon, and fakestream now runs in our local development stacks every day.

Results

Idle memory is about 2 MB, down from the multi-gigabyte heap the previous setup needed. Startup takes milliseconds, and the out-of-memory failures on large wallets are gone. In CI, each test job costs less, so a single runner fits more jobs in parallel.

On performance: on a developer laptop, replaying our own ingestion traffic at about twelve times its peak rate, fakestream sustained 171 MB/s of writes and 227 MB/s of reads. It had headroom left at that point; the load generator was the limit.

Scope and caveats

fakestream runs as a single binary, and using it is a matter of pointing your AWS SDK's Kinesis endpoint at it. SQS, DynamoDB, S3 and the rest of your setup stay as they are.

It is a development and CI tool. It has no authentication, so it should run on trusted networks only: localhost, a CI job or a private container network. It is not a replacement for Kinesis in production.

Technical details — the operations it supports, the Kinesis limits it enforces, the persistence option and the full benchmarks — are published on the project site.

Getting started

For full setup instructions, see the repository or the project site.

• Repository: https://github.com/pafin-inc/fakestream

• Project site: https://fakestream.pafin.com/

fakestream is released under the MIT license. It is free to use, including commercially, and requires no account and no auth token. It can also be modified and redistributed, with no restrictions on building it into your own development environment or CI.

About pafin Hackathon 2026

pafin Hackathon 2026 was our second in-house hackathon. We are accelerating AI adoption across the company, and the hackathon brought engineers and business teams together to build a wide range of ideas with AI, from internal tooling improvements and new cryptact features to entirely new services.

fakestream is not affiliated with Amazon Web Services, Inc. Amazon Kinesis is a trademark of Amazon.com, Inc.