After a read request is directed to a specific node, it first checks if data is in the memory cache. If so, the data is returned to the client as shown in Figure 20.

Figure 20

Figure 20

If the data is not in memory, it will be retrieved from the disk instead. We need an efficient way to find out which SSTable contains the key. Bloom filter [10] is commonly used to solve this problem.

The read path is shown in Figure 21 when data is not in memory.

Figure 21

Figure 21

1. The system first checks if data is in memory. If not, go to step 2.

2. If data is not in memory, the system checks the bloom filter.

3. The bloom filter is used to figure out which SSTables might contain the key.

4. SSTables return the result of the data set.

5. The result of the data set is returned to the client.