$sampleRate

Emits each document with the given probability. Useful for lightweight sampling or throttling without shuffling the pipeline.

---

Syntax

{ "$sampleRate": <rate> }

{ "$sampleRate": { "rate": <rate>, "seed": <number> } }

• rate must be between 0 and 1. A document is kept when a random draw is less than or equal to the rate.
• seed (optional) makes the sampling deterministic, which is handy for tests.

---

Example

Input

[
  { "eventId": 1 },
  { "eventId": 2 },
  { "eventId": 3 },
  { "eventId": 4 }
]

Stage

{ "$sampleRate": { "rate": 0.5, "seed": 42 } }

Output

[
  { "eventId": 2 },
  { "eventId": 3 },
  { "eventId": 4 }
]

With the provided seed, events 2, 3, and 4 are retained. Without a seed, the sample varies across executions.

---

Tips

• Apply $match or $project before sampling to reduce payload size.
• Use after $sort if you need sampling from a specific ordering of documents.