What it is: The number of Lambda executions running at the same time.

What it’s for:

• Control scaling and protect downstream systems (DB/APIs).
• Guarantee capacity for critical functions (reserved concurrency).

Key ideas (exam-friendly):

• Reserved concurrency: sets aside capacity for one function and also limits its max concurrency.
• If concurrency limit is reached, requests can be throttled.

Exam cues:

• “prevent Lambda from overwhelming database” → set reserved concurrency.
• “guarantee this function always has capacity” → reserved concurrency.

Hard words:

• *concurrency* /kənˈkɝːənsi/: chạy song song
• *throttled* /ˈθrɑːtld/: bị giới hạn/bóp lưu lượng
• *downstream* /ˌdaʊnˈstriːm/: hệ phía sau (DB/service nhận tải)