Having the height balanced means the same number of values in each bucket is maintained.
高度平衡意味着每个桶中的值的数量是相同的。
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What's more, if some asset in the AWS network also decides to kick the bucket, you can bet that your mission-critical messages won't be lost — they'll still exist on any number of other machines.
An implementation of size would have to sequentially acquire the lock for each bucket, count the number of nodes in that bucket, and release the lock and move on to the next bucket.