# System Design
All System Design notes →3.2 Layer 2: Durable Buffer (Kafka)
Layer 2 of the telemetry ingestion pipeline: the Kafka durable buffer — topic design, partitioning strategy, hot-spots, retention, retry/delivery semantics, producer config, consumer lag, and schema evolution.
system-design observability telemetry maang-prep kafka
Retry Policies and the Delivery Semantics They Produce
Every retry decision is made along three axes — trigger, backoff, budget — before delivery semantics even enter the picture. At-least-once, at-most-once, and exactly-once are the accumulated side effect of those decisions at every hop, not a separate design choice.
system-design observability telemetry maang-prep retry-policies delivery-semantics kafka