An in-memory database (IMDB) is a database whose data is stored in main memory to get faster response times. Currently there are 3 most popular storage systems for data/database, RAM, SSD and HDD.
RAM performs random reads/writes at 100 MB per second. Sequential reads/writes are even faster 1 GB per second and higher. An SSD is slower than RAM. Random access time is a fraction of a millisecond (1e-5 seconds): we can randomly read around 10,000 blocks per second. Random access time is 10 milliseconds (1e-3 seconds), which is 100 times slower than an SSD. If we talk about price on average, each storage type is ten times more expensive than the previous one. An HDD is the cheapest storage type, an SSD is ten times more expensive than an HDD, and RAM is ten times more expensive than an SSD.
So If we need access time of 1 ms or less, choice is RAM. Different database vendor have different size limits. There some popular use cases where In-memory database is being use.
1. Session Cache / Caching
2. Queues3. Real-Time Analytics etc
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