POI推荐
场景: yelp, 美团, airbnb
Design a system to find nearby restaurants
Design a system to match drivers with riders for Uber
Design a system to compute ETA for food delivery
特点: 如果是event 推荐这种注重实效性、位置性的推荐，event发生后不存在了，所有item可以认为都是冷启动 对于位置的挖掘可采用图特征或模型
1. requirements
products/use cases
objective
connect people with great local businesses
constraint
data
volume
latency
2. ML task & pipeline
预测目标
是否点击
停留时间(dwell time), 可转化为t/(t+1)来逼近sigmoid函数，t很大时接近1；很小时接近0
3. data collection
user
User location: For localized recommendations we need to consider only businesses near the city or neighborhood where the user is located
4. feature
sparse
dense
5. model
retrieval
取决于filter
ranking
rerank
6. evaluation
offline
NDCG
MAP
online: A/B testing holdout canary
7. deploy & serving
batch serving
online serving
8. monitor & maintenance
9. 优化与问答
冷启动的item
双塔可以采用default embedding, 而不是random initial
reference
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