#Execution Patterns

IndexBus provides four standard execution patterns. Each maps to a specific region layout and set of primitives.

#Router Loop (Fanout)

A single-writer router step that moves events from a producer ring into one or more consumer rings.

                      ┌──────────┐
                  ┌──▶│Consumer 0│
┌────────┐       │   └──────────┘
│Producer│─SPSC─▶│Router│
│        │ queue │(Broadcast│   ┌──────────┐
└────────┘       │ or Work) ├──▶│Consumer 1│
                 │  Queue)  │   └──────────┘
                 └──────────┘

#Modes

#Tuning Knobs

Larger batches increase throughput at the cost of latency. Smaller batches reduce latency but add per-event overhead. Choose a wait strategy to match the deployment: busy spin for best latency with high CPU, backoff for lower CPU, or wake-based for low idle CPU.

#Use Cases

• Market-data fanout
• Edge gateway distribution
• CPU isolation with dedicated router cores

#Sequencer + Gating

Disruptor-style monotonic sequence with per-consumer gating for strict ordering and stage coordination.

┌──────────┐    cursor    ┌───────────┐   gating[0]   ┌──────────┐
│ Producer │───────────▶│ Sequencer  │◀──────────────│Consumer 0│
│          │             │  Region    │               └──────────┘
└──────────┘             │            │   gating[1]   ┌──────────┐
                         │ (barrier)  │◀──────────────│Consumer 1│
                         └───────────┘               └──────────┘

• Producer claims sequence numbers, publishes, and advances the cursor.
• Consumers wait on barriers and advance their gating sequences.
• Wrap prevention: the producer cannot advance more than capacity ahead of the slowest consumer.

#Use Cases

• Low-latency staged flows
• Deterministic coordination
• Multi-stage fanout with ordering guarantees

#Router-Enforced Credits

Credit accounting layered on top of fanout routing. Each consumer has a credit budget (maximum allowed backlog). The router checks credits before delivery. On exhaustion, the configured policy determines behavior.

#Use Cases

• Protect consumers from overwhelm
• Prevent unbounded lag
• Make overload behavior observable and testable

#Journal (Append + Tail/Replay)

Append-only segmented log for fast appends, tailing, and replay-style reads.

┌───────────┐  append   ┌─────────────────────────────────┐
│ Publisher  │─────────▶│ Journal Region                   │
└───────────┘           │ ┌─────────┬─────────┬─────────┐ │
                        │ │Segment 0│Segment 1│Segment 2│ │
                        │ └─────────┴─────────┴─────────┘ │
                        └─────────────────────────────────┘
                                   ▲ tail
                        ┌──────────┴──────────┐
                        │  Subscriber (poll)   │
                        └─────────────────────┘

#Record Format

Each record consists of an 8-byte header (payload_len + flags) followed by an 8-byte-aligned payload. Publication uses two-phase commit with FLAG_COMMITTED. Storage is bounded via fixed-size segments, with overrun detection for slow subscribers.

#Use Cases

• Capture-and-replay debugging
• Late-joiner subscribers
• Inspectable streams

#Quick Chooser