"Read-Preference"
- MongoDB Replica Set Read Preference：DB 層 causal session vs cache 層 freshness token MongoDB read preference 五擇一 + read concern + causal consistency session 機制；DB 層機制解 cluster 內 read-your-own-write、cache 層 freshness token 解跨層 read-after-write、大規模 OLTP 必須兩層合用