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Queue Node ​

Purpose & Use Cases ​

The queue node buffers incoming messages, enforces a minimum interval between outputs, and discards stale entries. It helps coordinate bursty producers with slower consumers by smoothing throughput and avoiding overload.

Common Scenarios:

  • Throttle high-frequency image sources before storage or slow analysis stages
  • Prevent downstream rate limits from being exceeded when batch processing arrays
  • Provide simple back-pressure where only a fixed number of messages can be buffered
  • Drop stale detections or frames that are no longer relevant after a timeout

Input/Output Specification ​

Inputs ​

  • Incoming Message: Any Node-RED message to be enqueued.

Outputs ​

  • Output 1: Messages forwarded in FIFO order while respecting the configured interval.
  • Output 2: Messages dropped due to timeout or overflow, with metadata:
    • msg.meta.queueDropped - Always true
    • msg.meta.queueDropReason - Either "timeout" or "overflow"
    • msg.meta.queuedDuration - Time spent in queue before timeout (timeout drops only)
    • msg.meta.queueTimeout - Configured timeout value in ms (timeout drops only)
    • msg.meta.queueSize - Queue size when overflow occurred (overflow drops only)
    • msg.meta.queueMaxSize - Configured max queue size (overflow drops only)

Configuration Options ​

Mode ​

  • Type: Dropdown (Queue Size Limit / Timeout)
  • Default: Queue Size Limit
  • Purpose: Select whether the node enforces a hard buffer capacity or lets messages expire after a timeout; the controls shown below change depending on the selected mode.

Queue Size Limit ​

  • Type: Number (integer)
  • Default: 0 (no limit)
  • Purpose: Available only in queue-size mode. Controls how many messages may be enqueued before excess messages are ignored.

Timeout (ms) ​

  • Type: Number (integer)
  • Default: 0 (no timeout)
  • Purpose: Available only in timeout mode. Messages that stay longer than this duration are removed before they reach the output.

Interval (ms) ​

  • Type: Number (integer)
  • Default: 0 (no enforced delay)
  • Purpose: Minimum time that must elapse between forwarded messages. When set to 0, messages flow out as soon as they are available.

Behavior & Status ​

  • Messages are processed in first-in-first-out order.
  • The node emits messages immediately when the interval requirement has already been satisfied.
  • If necessary, the node waits the remaining interval before releasing the next message.
  • When messages expire due to timeout (and timeout mode is active) they are sent to output 2 with metadata and a warning is logged.
  • When the queue overflows (queue-size mode) excess messages are sent to output 2 with metadata.
  • Node status indicates whether the queue is idle, holding messages, or actively sending.

Best Practices ​

  • Pair with Array nodes to control the rate of batch processing pipelines.
  • Use timeouts to keep sensor readings or camera frames current.
  • Connect output 2 to logging, alerting, or fallback processing to handle dropped messages.
  • Use msg.meta.queueDropReason to differentiate between timeout and overflow scenarios in downstream logic.