cache invalidation

// read data
val = cache.get(key)
if val == nil {
    val = db.get(key)
    cache.put(key, val)
}
return val

// write data
db.put(key, val)
cache.put(key, val)

这会造成dual write conflict

如果需要的只是eventaul consistency，那么通过dbus来进行cache invalidation是最有效的

https://martinfowler.com/bliki/TwoHardThings.html