Design an algorithm to rank ads in the Google Play Store. Include primary and secondary success metrics.

The ad ranking algorithm for Google Play Store should optimize for a balanced ecosystem, connecting users with relevant apps while maximizing advertiser value and Google's revenue. The core principles are:

1. Relevance: Prioritize ads that closely match user search queries and interests. This involves analyzing user search history, app category preferences, and contextual signals.
2. Performance: Rank ads based on their historical performance, including click-through rates (CTR), conversion rates (e.g., installs, in-app purchases), and user engagement post-install.
3. Quality: Ensure ads lead to high-quality user experiences. This means considering app ratings, user reviews, and retention rates of users acquired through ads.
4. Fairness: Implement mechanisms to prevent bias and ensure smaller or newer advertisers have a fair chance to compete.

Primary Success Metrics:

• User Engagement: Measured by CTR, install rate, and post-install engagement (e.g., session duration, feature usage).
• Advertiser ROI: Measured by conversion rate, cost per acquisition (CPA), and return on ad spend (ROAS).

Secondary Success Metrics:

• Revenue Growth: Overall ad revenue for Google Play.
• App Discovery: Increase in discovery of new and relevant apps by users.
• User Satisfaction: Measured through app store ratings and user feedback related to ad experience.