Heartbeat

Heartbeat

The heartbeat (HB) feature in VIS ensures that both the system and critical integration components—such as IO cards—remain responsive. While VIS is self-aware and can detect internal states of functionality, it may not be able to notify external systems (the "listening party") if the integration layer fails or if there is a major system-wide malfunction. To address this, the heartbeat periodically sends a signal (bit) to the listening party. If the heartbeat signal fails to arrive, the listening party can infer a failure and take appropriate action—such as halting operation or triggering a fail-safe response.

Purpose

The heartbeat serves two distinct but related purposes:

1. External signaling: A bit-level toggle is periodically transmitted to the IO module to signal ongoing healthy operation.
2. Internal liveness check: VIS monitors individual Vislets (processing pipelines) to determine if they are still functioning. If no output is received from a Vislet during a heartbeat interval, that Vislet is considered non-functional or "dead."

Behavior

• After each frame is processed by a Vislet, the output is sent to a shared pool. As long as messages are received from a Vislet during the heartbeat interval, it is assumed to be alive.
• If a Vislet stops its grab → detect → interpret → emit loop, it stops sending messages. The heartbeat component detects this and marks the Vislet as unavailable.
• When at least one Vislet is marked dead (depending on configuration), the heartbeat signal to the IO module may stop, indicating a system-level failure.

Auto-Resurrection

If a Vislet becomes responsive again, the heartbeat automatically resumes. This behavior is not configurable—any recovered Vislet re-enables its contribution to the heartbeat mechanism without requiring manual intervention.

Configuration

The heartbeat supports two configuration parameters:

• period (int): The interval in seconds between each call to the toggle method. Default is 5 seconds.
• partial_death (bool): Determines how strictly the system interprets Vislet failures.
  • If false (default): The system declares death if any Vislet is lost.
  • If true: The system tolerates partial Vislet failures and continues operation unless a broader failure occurs.