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Hello Interview guide, topic by topic

Pub/sub inside the box: SONiC daemons talk through Redis. ProducerStateTable/ConsumerStateTable publish a notification and consumers wake up and pop the changed keys ( databases ). This is the same "persist, then notify" rule: the data is in Redis, the notification only says "look".
Streaming to clients: the gNMI server supports Subscribe in SAMPLE and ON_CHANGE modes, and ON_CHANGE is driven by Redis keyspace notifications ( gNMI subscription for YANG data , gRPC telemetry ).
Same caveat as pub/sub: keyspace notifications carry only the key, not the value, so the gNMI layer re-reads the DB and may see several changes collapsed into one.

A study companion to the Hello Interview system design guide. It follows the guide's structure: how you're assessed and the delivery framework, then core concepts, key technologies, common patterns, advanced topics, and question breakdowns.

Concept, technology and pattern topics have the same parts as Key Concepts: the big picture, key concepts, how it works, trade-offs, and pitfalls & follow-up questions.
Question breakdowns follow the delivery framework: requirements, core entities, API, high-level design, then deep dives. The deep dives are collapsed so you can answer them yourself first.
Each topic links to the matching Alex Xu chapter, lists its acronyms, and has a collapsed "How this relates to SONiC" note.

An independent study summary written in our own words. It is not affiliated with Hello Interview and does not quote its content. Read the original guide for the full treatment.

A–Z acronym index

Acronym	Stands for	Topic(s)