Cause analysis: delayed double deletion of redis cache
Aug 24, 2022 pm 05:34 PMRecommended learning: Redis video tutorial
Why is the cache deleted instead of updated?
If it is an update and there is a distributed transaction problem, the cache may be modified and the database modification may fail. If you just delete the cache, even if the database modification fails, the next query will directly fetch the data from the database, and no dirty data will appear.
What is delayed double deletion?
That is, when adding, deleting or modifying an entity class, the cache of the entity class must be cleared. The clearing position is before and after the database operation method.
Adopt proof by contradiction
Only delete first
????
to ensure that the change cache operations of other transactions have been completed before modifying the database -> clearing the cache.
Additional: Why should we delay double deletion to ensure cache consistency
Why To delay double deletion to ensure cache consistency- Before modifying the database data, redis needs to be deleted first: this is to ensure that within the interval between the database data modification and the redis data deletion, If there is a hit, it is guaranteed that this data does not exist in redis. Without this deletion, when the database data has been modified, old data can still be read from redis, resulting in data inconsistency.
- The second deletion is after the database data is modified. At this time, the corresponding data in redis needs to be deleted again. This time it is to delete the first redis deletion and the database data modification. If there is a request, Then the old data will be cached again in redis, but the data in the database will be modified next. If it is not deleted this time, the old data in the database will exist in redis.
- So why do you need to delay deleting redis for a certain period of time after the database is modified the second time?
- In order to wait for the previous read of the database, wait for the data to be written to the cache, and finally delete the dirty data, so it is the time it takes for the data to be sent from the database to the server cache for writing
As long as the cache is deleted in time, other threads can read the latest value.
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