Currently MetadataCache.updateCache() grabs a write lock in order to process the UpdateMetadataRequest from controller. And a read lock is needed in order to handle the MetadataRequest from clients. Thus the handling of MetadataRequest and UpdateMetadataRequest blocks each other and the broker can only process such request at a time even if there are multiple request handler threads. Note that broker can not process MetadataRequest in parallel if there is a UpdateMetadataRequest waiting for the write lock, even if MetadataRequest only requires the read lock to e processed.
For large cluster which has tens of thousands of partitions, it can take e.g. 200 ms to process UpdateMetadataRequest and MetadataRequest from large clients (e.g. MM). During the period when user is rebalancinng cluster, the leadership change will cause both UpdateMetadataRequest from controller and also MetadataRequest from client. If a broker receives 10 MetadataRequest per second and 2 UpdateMetadataRequest per second on average, since these requests need to be processed one-at-a-time, it can reduce the request handler thread idle ratio to 0 which makes this broker unavailable to user.
We can address this problem by removing the read lock in MetadataCache. The idea is that MetadataCache.updateCache() can instantiate a new copy of the cache as method local variable when it is processing the UpdateMetadataRequest and replace the class private varaible with newly instantiated method local varaible at the end of MetadataCache.updateCache(). The handling of MetadataRequest only requires access to the read-only class-private variable.