How does 'Read Preference' differ from 'Read Concern'?

Read Preference and Read Concern address two different dimensions of read operations in MongoDB. Read Preference is about routing — it tells the driver which replica set members are eligible to handle the read operation (e.g., primary, secondary, nearest). Read Concern is about consistency — it controls how the data is returned, ensuring a certain level of isolation and durability . The official MongoDB documentation is explicit: "不要将读关注与读偏好相混淆，后者处理从何处读取数据的问题" (Do not confuse read concern with read preference; the latter deals with the question of where to read data from) .