X Algorithm Pipeline

The X recommendation algorithm processes posts through an 8-stage pipeline to generate the "For You" feed. Each stage transforms, filters, or scores the candidate posts.

Pipeline Overview

┌─────────────────────────────────────────────────────────────────────────────┐
│                         X RECOMMENDATION PIPELINE                           │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                             │
│   User Request                                                              │
│        │                                                                    │
│        ▼                                                                    │
│   ┌─────────────┐                                                           │
│   │ 1. Query    │  Hydrate user features, action history, socialgraph       │
│   │   Hydration │                                                           │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│   ┌─────────────┐  Thunder (in-network) + Phoenix (out-of-network)          │
│   │ 2. Sources  │  In-network: Posts from followed accounts                 │
│   │             │  Out-of-network: ML retrieval from all posts              │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│   ┌─────────────┐                                                           │
│   │ 3. Candidate│  Fetch tweet text, author data, visibility status         │
│   │   Hydration │                                                           │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│   ┌─────────────┐                                                           │
│   │ 4. Pre-Score│  Age, duplicates, safety, blocked authors                 │
│   │   Filtering │                                                           │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│   ┌─────────────┐  Phoenix ML → WeightedScorer → AuthorDiversity → OON      │
│   │ 5. Scoring  │  Each scorer adds/adjusts candidate.score                 │
│   │             │                                                           │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│   ┌─────────────┐                                                           │
│   │ 6. Selection│  TopKScoreSelector: Keep top N by final score             │
│   │             │                                                           │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│   ┌─────────────┐                                                           │
│   │ 7. Post-    │  Conversation dedup, previously seen, keywords            │
│   │   Filtering │                                                           │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│   ┌─────────────┐                                                           │
│   │ 8. Side     │  Logging, analytics, impression tracking                  │
│   │   Effects   │                                                           │
│   └──────┬──────┘                                                           │
│          ▼                                                                  │
│      Feed Response                                                          │
│                                                                             │
└─────────────────────────────────────────────────────────────────────────────┘

Stage Details

1. Query Hydration

Enriches the request with user context:

• User features (followed users, blocked users, muted users)
• User action sequence (engagement history for ML)
• Muted keywords
• Subscription status
• Bloom filters for seen posts

2. Sources

Two candidate sources provide posts:

Thunder Source (In-Network)

// home-mixer/sources/thunder_source.rs
// Posts from accounts the user follows
served_type: Some(pb::ServedType::ForYouInNetwork)

• Queries Thunder service with user's following list
• Returns recent posts from followed accounts
• Includes conversation context (ancestors, reply chains)

Phoenix Source (Out-of-Network)

// home-mixer/sources/phoenix_source.rs
fn enable(&self, query: &ScoredPostsQuery) -> bool {
    !query.in_network_only  // Disabled for "Following" tab
}
served_type: Some(pb::ServedType::ForYouPhoenixRetrieval)

• ML-based retrieval using user embedding
• Finds relevant posts from the entire corpus
• Enabled for "For You", disabled for "Following"

3. Candidate Hydration

Fetches full post data:

• Tweet text content
• Author information
• Media metadata (video duration)
• Visibility filtering results
• Subscription requirements

4. Pre-Score Filtering

Removes ineligible candidates before expensive ML scoring:

• AgeFilter - Too old
• DropDuplicatesFilter - Duplicate IDs
• VFFilter - Safety violations
• AuthorSocialgraphFilter - Blocked/muted authors
• CoreDataHydrationFilter - Missing data
• IneligibleSubscriptionFilter - Subscription required

5. Scoring (4 Stages)

a) PhoenixScorer

// home-mixer/scorers/phoenix_scorer.rs
// Calls Phoenix ML to predict engagement probabilities

Produces phoenix_scores with 18 action probabilities.

b) WeightedScorer

// home-mixer/scorers/weighted_scorer.rs
// Combines probabilities into single score
weighted_score = Σ(weight × P(action))

Produces weighted_score from action predictions.

c) AuthorDiversityScorer

// home-mixer/scorers/author_diversity_scorer.rs
// Penalizes multiple posts from same author
multiplier = (1 - floor) × decay^position + floor

Adjusts scores to promote variety.

d) OONScorer

// home-mixer/scorers/oon_scorer.rs
// Adjusts out-of-network post scores
if !in_network: score *= OON_WEIGHT_FACTOR

Balances in-network vs out-of-network content.

6. Selection

// home-mixer/selectors/top_k_score_selector.rs
pub struct TopKScoreSelector;

impl Selector<ScoredPostsQuery, PostCandidate> for TopKScoreSelector {
    fn score(&self, candidate: &PostCandidate) -> f64 {
        candidate.score.unwrap_or(f64::NEG_INFINITY)
    }
    fn size(&self) -> Option<usize> {
        Some(params::TOP_K_CANDIDATES_TO_SELECT)
    }
}

Keeps top K posts by final score.

7. Post-Score Filtering

Fine-grained filtering after selection:

• DedupConversationFilter - One post per conversation
• RetweetDeduplicationFilter - One version per underlying post
• PreviouslySeenPostsFilter - Remove seen posts
• PreviouslyServedPostsFilter - Remove from current session
• MutedKeywordFilter - User keyword mutes
• SelfTweetFilter - Remove own posts

8. Side Effects

Non-blocking operations after response:

• Impression logging
• Analytics events
• Cache updates

Data Flow Summary

Candidates start with:
├── tweet_id, author_id (from Sources)
├── tweet_text, metadata (from Hydration)
├── phoenix_scores (from PhoenixScorer)
├── weighted_score (from WeightedScorer)
├── score (from AuthorDiversity + OON)
└── Final ranking by score

PostCandidate Structure

pub struct PostCandidate {
    pub tweet_id: i64,
    pub author_id: u64,
    pub tweet_text: String,
    pub in_reply_to_tweet_id: Option<u64>,
    pub retweeted_tweet_id: Option<u64>,
    pub retweeted_user_id: Option<u64>,
    pub phoenix_scores: PhoenixScores,      // ML predictions
    pub weighted_score: Option<f64>,         // After WeightedScorer
    pub score: Option<f64>,                  // Final score
    pub served_type: Option<ServedType>,     // Source type
    pub in_network: Option<bool>,            // Following or not
    pub ancestors: Vec<u64>,                 // Conversation context
    pub video_duration_ms: Option<i32>,      // For VQV eligibility
    pub visibility_reason: Option<FilteredReason>,
    pub subscription_author_id: Option<u64>,
    // ...
}

Source Configuration

Tab	Thunder (In-Network)	Phoenix (Out-of-Network)
For You	Enabled	Enabled
Following	Enabled	Disabled

Related Skills

• /x-algo-scoring - Detailed scoring formula
• /x-algo-filters - All filter implementations
• /x-algo-engagement - Action types and signals
• /x-algo-ml - Phoenix ML model architecture