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How we built a cost-effective Generative AI application

· 7 min read
Gur Singh

Since its inception, CodeRabbit has experienced steady growth in its user base, comprising developers and organizations. Installed on thousands of repositories, CodeRabbit reviews several thousand pull requests (PRs) daily. We have previously discussed our use of an innovative client-side request prioritization technique to navigate OpenAI rate limits. In this blog post, we will explore how we manage to deliver continuous, in-depth code analysis cost-effectively, while also providing a robust, free plan to open source projects.

CodeRabbit's Product Offering and LLM Consumption

CodeRabbit is an AI-first PR Review tool that uses GPT APIs for various functionalities. CodeRabbit offers the following tiers of service:

  • CodeRabbit Pro: A paid service providing in-depth code reviews for private repositories. It's priced according to the number of developers, starting with a full-featured 7-day free trial.
  • CodeRabbit for Open Source: A free service offering in-depth code reviews for open source (public) repositories.
  • CodeRabbit Free: A free plan for private repositories, providing summarization of code changes in a PR.

Our vision is to offer an affordable, AI-driven code review service to developers and organizations of all sizes while supporting the open source community. We are particularly mindful of open source projects, understanding the challenges in reviewing community contributions. Our goal is to reduce the burden of code reviews for open source maintainers by improving submission quality before the review process begins.

CodeRabbit's review process is automatically triggered when a PR is opened in GitHub or GitLab. Each review involves a complex workflow that builds context and reviews each file using large language models (LLMs). Code review is a complex task that requires an in-depth understanding of the changes and the existing codebase. High-quality review comments necessitate state-of-the-art language models such as gpt-4. However, these models are significantly more expensive than simpler models, as shown by the 10x-30x price difference between gpt-3.5-turbo and gpt-4 models.

ModelContext SizeCost per 1k Input TokensCost per 1k Output Tokens
GPT-4Up to 32k$0.06$0.12
GPT-4Up to 8k$0.03$0.06
GPT-3.5 TurboUp to 16k$0.003$0.004
GPT-3.5 TurboUp to 4k$0.0015$0.002

gpt-4 model is 10-30x more expensive than gpt-3.5-turbo model

Our primary cost driver is using OpenAI's API to generate code review comments. We will share our cost optimization strategies in the following sections. Without these optimizations, our free offering to open source projects would not be feasible.

Let's take a look at the strategies that helped us optimize the cost and improve user experience.


1. Dual-models: Summarize & Triage Using Simpler Models

For less complex tasks such as summarizing code diffs, simpler models such as gpt-3.5-turbo are adequate. As an initial optimization, we use a mix of models, as detailed in our earlier blog post. We use gpt-3.5-turbo to compress large code diffs into concise summaries, which are then processed by gpt-4 for reviewing each file. This dual-model approach significantly reduces costs and enhances review quality, enabling us to manage PRs with numerous files and extensive code differences.

Additionally, we implemented triage logic to skip trivial changes from the review process. We use the simpler model to classify each diff as either trivial or complex, as part of the same prompt used for code diff summarization. Low-risk changes such as documentation updates, variable renames, and so on, are thus excluded from the thorough review process. This strategy has proven effective, as simpler models can accurately identify trivial changes.

By using this dual-model approach for summarization and filtering out trivial changes, we save almost 50% on costs.

2. Rate-limiting: Enforcing Fair Usage

Upon launching our free service for open source projects, we noticed individual developers using it as a coding co-pilot by making hundreds of incremental commits for continuous feedback. CodeRabbit, designed for thorough code reviews unlike tools such as GitHub Copilot, incurs high costs when used in this manner. Therefore, we implemented hourly rate-limits on the number of files and commits reviewed per user, to control excessive usage without compromising user experience. These limits vary across different product tiers. For example, we set more aggressive limits for open source users compared to trial and paid users.

To implement these rate-limits, we evaluated various options for Serverless environments. We opted for FluxNinja Aperture for its simplicity and policy sophistication. We were already using Aperture for managing OpenAI rate limits, making it a natural choice for our rate-limiting needs as well.

In FluxNinja Aperture, policies are decoupled from application logic through labels, enabling new policy additions without altering application code. We apply labels in FluxNinja Aperture, wrap the review workload with its SDK, and write policies that enforce limits on those labels. For example, we enforce a 3 reviews per hour limit (1 review every 20 minutes) for open source users, allowing a burst of 2 back-to-back reviews, as shown in the screenshots below.

Integration with FluxNinja Aperture SDK

Rate limiting commits per hour for open source users Rate limiting commits per hour for open source users

Wait time feedback to the user in a comment Wait time feedback to the user in a comment

Given the high cost and capacity constraints of state-of-the-art models such as gpt-4, rate-limiting is an essential requirement for any AI application. By implementing fair-usage rate limits, we are saving almost 20% on our costs.

Rate limit metrics for open source users Rate limit metrics for open source users

3. Caching: Avoid Re-generating Similar Review Comments

We believe that building user habits around AI involves seamlessly augmenting existing workflows. Therefore, AI code reviews must be continuous: they should trigger as soon as a PR is opened and incrementally update the summary and generate review comments as more commits are added.

However, this approach can become expensive and generate repetitive feedback, as similar review comments are re-generated for each commit. We observed that most incremental commits involve minor adjustments or bug fixes in the initial implementation. To address this, we implemented a caching layer to avoid re-generating similar review comments for incremental commits.

Fortunately, Aperture also provides a simple caching mechanism for summaries from previous commits, using the same API call where we implemented rate limits. During each incremental review, we use the simpler model for a semantic comparison of the code changes described in both summaries. If the changes are similar, we skip the review for those files to prevent re-generating similar review comments. This method differs from vector similarity-based caching techniques, as we use an LLM model for comparing summaries. Vector similarity-based approaches wouldn't be effective in our case, as the summaries require semantic comparison. We have integrated this method into the same prompt used for code diff summarization and triage.

By using the more cost-effective gpt-3.5-turbo model as an advanced similarity filter before invoking the more expensive gpt-4 model for the same file, we have saved almost 20% of our costs by avoiding the generation of similar review comments.


Conclusion

In this blog post, we briefly discussed how state-of-the-art LLMs such as gpt-4 can be expensive in production. We also shared our strategy of using a combination of simpler models, rate limits, and caching to optimize operational costs. We hope our experiences can assist other AI startups in optimizing their costs and developing cost-effective AI applications.