ADR-011: Reputation Decay System (Social Karma v2)

Date: 2025-12-29 Status: Accepted Deciders: Development Team Related: services/reputation-service, docs/DEVELOPMENT_ROADMAP.md

Context

Karma/reputation systems often suffer from:

• Early adopters dominating forever
• Inactive users retaining high scores
• No incentive to keep helping
• "Karma hoarding" behavior

We needed a system where recent helpfulness matters more than past contributions.

Decision

Time-based karma decay with 6-month half-life.

Decay Formula

current_karma = base_karma * (0.5 ^ (months_since_earned / 6))

Example:

• Earned 100 karma today → 100 karma
• After 6 months → 50 karma
• After 12 months → 25 karma
• After 18 months → 12.5 karma

Implementation

Reputation Service (Port 3004):

• Calculates decayed karma on-the-fly
• Stores base_karma and earned_at in database
• Returns current_karma (decay applied)

SELECT
  SUM(points * POW(0.5, EXTRACT(EPOCH FROM (NOW() - created_at)) / (6 * 30 * 24 * 60 * 60))) as current_karma
FROM reputation.karma_records
WHERE user_id = $1 AND community_id = $2;

Configurable Per Community

{
  karma_decay_enabled: true,
  karma_half_life_months: 6  // Communities can adjust
}

Consequences

Positive

• Fair: Recent helpers rank higher than inactive veterans
• Incentive: Must keep helping to maintain score
• Realistic: Matches how human memory works
• Prevents Hoarding: Can't "retire" on old karma
• Community Health: Encourages ongoing participation

Negative

• Query Complexity: Decay calculation on every query
• User Confusion: "Where did my karma go?"
• Migration: Existing high-karma users will see drops
• Performance: More complex SQL queries

Alternatives Considered

Alternative 1: No Decay (Permanent Karma)

• Why rejected: Creates permanent hierarchies, discourages new helpers

Alternative 2: Hard Expiration (Delete After X Months)

• Why rejected: Too harsh, loses all history

Alternative 3: Linear Decay

• Why rejected: Exponential decay more realistic (half-life model)

References

• Reputation service: services/reputation-service/src/services/karmaService.ts
• Decay preview: services/community-service/src/routes/settings.ts
• Database: infrastructure/postgres/init.sql (karma_records table)