How Reddit Migrated Comments Functionality from Python to Go
Deep Dives
Explore related topics with these Wikipedia articles, rewritten for enjoyable reading:
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Change data capture
12 min read
Linked in the article (7 min read)
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Object–relational mapping
9 min read
The article highlights how Python's ORM had hidden optimizations that caused unexpected database performance issues during migration to Go, which doesn't use an ORM. Understanding ORM architecture and trade-offs provides crucial context for why this migration challenge occurred.
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Race condition
15 min read
The article describes race conditions causing false mismatches in tap compare logs during the migration. A deep understanding of race conditions—their causes, detection, and prevention—illuminates why concurrent system migrations are so challenging to validate.
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Disclaimer: The details in this post have been derived from the details shared online by the Reddit Engineering Team. All credit for the technical details goes to the Reddit Engineering Team. The links to the original articles and sources are present in the references section at the end of the post. We’ve attempted to analyze the details and provide our input about them. If you find any inaccuracies or omissions, please leave a comment, and we will do our best to fix them.
When you upvote a clever comment on Reddit or reply to a discussion thread, you’re interacting with their Comments model. This model is probably the most important and high-traffic model in Reddit’s architectural setup.
Reddit’s infrastructure was built around four Core Models: Comments, Accounts, Posts, and Subreddits.
These models power virtually everything users do on the platform. For years, all four models were served from a single legacy Python service, with ownership awkwardly split across different teams. By 2024, this monolithic architecture had become a problem:
The service suffered from recurring reliability and performance issues.
Maintaining it had become increasingly difficult for all teams involved.
Ownership responsibilities were unclear and fragmented.
In 2024, the Reddit engineering team decided to break up this monolith into modern, domain-specific Go microservices.
They chose comments as their first migration target because it represented Reddit’s largest dataset and handled the highest write throughput of any core model. If they could successfully migrate comments, they would prove their approach could handle anything.
In this article, we will look at how Reddit carried out this migration and the challenges it faced.
The Easy Part: Migrating Read Operations
Before diving into the complex scenario, it’s worth understanding how Reddit approached the simpler part of this migration: read endpoints.
When you view a comment, that’s a read operation. The server fetches data from storage and returns it to you without
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