Replication: fix the infamous key leakage of writable slaves + EXPIRE.
BACKGROUND AND USE CASEj Redis slaves are normally write only, however the supprot a "writable" mode which is very handy when scaling reads on slaves, that actually need write operations in order to access data. For instance imagine having slaves replicating certain Sets keys from the master. When accessing the data on the slave, we want to peform intersections between such Sets values. However we don't want to intersect each time: to cache the intersection for some time often is a good idea. To do so, it is possible to setup a slave as a writable slave, and perform the intersection on the slave side, perhaps setting a TTL on the resulting key so that it will expire after some time. THE BUG Problem: in order to have a consistent replication, expiring of keys in Redis replication is up to the master, that synthesize DEL operations to send in the replication stream. However slaves logically expire keys by hiding them from read attempts from clients so that if the master did not promptly sent a DEL, the client still see logically expired keys as non existing. Because slaves don't actively expire keys by actually evicting them but just masking from the POV of read operations, if a key is created in a writable slave, and an expire is set, the key will be leaked forever: 1. No DEL will be received from the master, which does not know about such a key at all. 2. No eviction will be performed by the slave, since it needs to disable eviction because it's up to masters, otherwise consistency of data is lost. THE FIX In order to fix the problem, the slave should be able to tag keys that were created in the slave side and have an expire set in some way. My solution involved using an unique additional dictionary created by the writable slave only if needed. The dictionary is obviously keyed by the key name that we need to track: all the keys that are set with an expire directly by a client writing to the slave are tracked. The value in the dictionary is a bitmap of all the DBs where such a key name need to be tracked, so that we can use a single dictionary to track keys in all the DBs used by the slave (actually this limits the solution to the first 64 DBs, but the default with Redis is to use 16 DBs). This solution allows to pay both a small complexity and CPU penalty, which is zero when the feature is not used, actually. The slave-side eviction is encapsulated in code which is not coupled with the rest of the Redis core, if not for the hook to track the keys. TODO I'm doing the first smoke tests to see if the feature works as expected: so far so good. Unit tests should be added before merging into the 4.0 branch.
Showing
想要评论请 注册 或 登录