Facebook News Feed — Detailed (Pull-on-read aggregator model)

flowchart TB
  subgraph Clients
    Web
    Mobile
  end

  subgraph Edge
    DNS
    LB[L7 LB]
    GW[GraphQL Gateway]
  end

  subgraph Write[Compose path]
    COMP[Compose Service]
    MEDIA[Media Service]
    OBJ[(Haystack / S3)]
    POSTS[(Posts store<br/>sharded MySQL)]
    TAO[(TAO<br/>distributed graph cache)]
    INGEST[[Kafka post events]]
  end

  subgraph Read[Read path - aggregator]
    FEED([News Feed Aggregator])
    LEAF[Leaf services<br/>per friend / per source]
    RANK([Ranker - DL model])
    SCORE[Scoring service]
    HYD([Hydrator])
    PAG[Cursor pagination]
  end

  subgraph Sources[Candidate sources]
    FRIENDS[Friends posts]
    PAGES[Pages followed]
    GROUPS[Groups]
    SUGG([Recommendations / video])
    ADS[Ads]
  end

  subgraph Signals
    AFF[Affinity scores]
    DECAY[Time decay]
    ENG[Past engagement features]
    EMB([Embeddings store])
  end

  subgraph Safety
    INTEG[Integrity / hate-speech filter]
    DEDUP[Dedup]
    FATIGUE[Source diversity / fatigue]
  end

  subgraph Realtime
    NOTIF[Notification Service]
    PUSH((APNS / FCM))
  end

  subgraph Storage
    LIKES[(Likes / reactions)]
    COMM[(Comments)]
    ACT[(Activity log)]
  end

  Clients --> DNS --> LB --> GW
  GW --> COMP --> POSTS
  COMP --> MEDIA --> OBJ
  COMP --> TAO
  COMP --> INGEST --> NOTIF --> PUSH
  GW --> FEED --> LEAF
  LEAF --> Sources
  Sources --> TAO
  Sources --> POSTS
  LEAF --> SCORE --> RANK
  Signals --> RANK
  RANK --> Safety --> HYD --> PAG --> Clients
  LIKES --- HYD
  COMM --- HYD
  ACT --- LEAF

    classDef client fill:#dbeafe,stroke:#1e40af,stroke-width:1px,color:#0f172a;
    classDef edge fill:#cffafe,stroke:#0e7490,stroke-width:1px,color:#0f172a;
    classDef service fill:#fef3c7,stroke:#92400e,stroke-width:1px,color:#0f172a;
    classDef datastore fill:#fee2e2,stroke:#991b1b,stroke-width:1px,color:#0f172a;
    classDef cache fill:#fed7aa,stroke:#9a3412,stroke-width:1px,color:#0f172a;
    classDef queue fill:#ede9fe,stroke:#5b21b6,stroke-width:1px,color:#0f172a;
    classDef compute fill:#d1fae5,stroke:#065f46,stroke-width:1px,color:#0f172a;
    classDef storage fill:#e5e7eb,stroke:#374151,stroke-width:1px,color:#0f172a;
    classDef external fill:#fce7f3,stroke:#9d174d,stroke-width:1px,color:#0f172a;
    classDef obs fill:#f3e8ff,stroke:#6b21a8,stroke-width:1px,color:#0f172a;
    class LB,GW edge;
    class COMP,MEDIA,LEAF,SCORE,PAG,FRIENDS,PAGES,GROUPS,ADS,AFF,DECAY,ENG,INTEG,DEDUP,FATIGUE,NOTIF service;
    class POSTS,TAO,LIKES,COMM,ACT datastore;
    class INGEST queue;
    class FEED,RANK,HYD,SUGG,EMB compute;
    class OBJ storage;
    class PUSH external;

Aggregator pattern

• Pull-on-read: at feed open, fan-out queries to leaves (one per source / friend bucket).
• Each leaf returns top-K candidates (recent + popular per source).
• Aggregator merges → candidate pool → ranker → top-N.
• Cache final feed page in user's recents for back-press / pagination.

TAO (The Associations and Objects)

• Read-through write-through graph cache over MySQL.
• Objects (type, id) and Associations (from, type, to) with counts.
• Read-mostly; eventual consistency across regions.

Ranking

• Thousands of features per candidate.
• Multi-task DNN: predicts P(like), P(comment), P(share), P(meaningful interaction).
• Combined into a value model with policy weights.

Caveats vs Twitter

• Facebook leans pull (large per-friend variance, fewer celebs in friend graph).
• Twitter leans push for normal users (very long-tail celeb followers).

Glossary & fundamentals

Concepts referenced in this design. Each row links to its canonical page; the tag column shows whether it is a high-level (HLD) or low-level (LLD) concept.

Tag	Concept	What it is	Page
HLD	Load balancer / GSLB	L4/L7 traffic distribution and failover	load-balancer
HLD	Cache strategies	cache-aside, read/write-through, eviction	caching-strategies
HLD	Pub/Sub & message brokers	topics, consumer groups, delivery semantics	pub-sub-pattern
HLD	CAP / PACELC	C vs A under partition; L vs C otherwise	cap-pacelc
LLD	REST API design	verbs, statuses, pagination, errors	rest-api-design