In-memory databases (IMDBs) and Solid-State Drive (SSD)-based databases are both prevalent technologies used for data storage and retrieval. However, they differ significantly regarding their performance, cost, persistence, and use-cases.
IMDBs typically offer much faster data access times than SSD-based databases. Because data is stored directly in RAM, access times can be on the order of nanoseconds, compared to SSD access times, which are typically on the order of microseconds.
RAM is significantly more expensive per gigabyte than SSD storage, making in-memory databases more costly to scale up when large amounts of data need to be held in memory.
Data in an IMDB is volatile; it will be lost if the system crashes or is shut down, unless steps are taken to persist it to disk. SSD-based databases do not have this problem as their data is inherently persistent.
Due to their speed, in-memory databases are often used for applications requiring real-time access to data such as caching, session stores, or real-time analytics. On the other hand, due to their lower cost and inherent persistence, SSD-based databases are commonly used for long-term storage and large datasets that don't require extremely fast access times.
Just remember that this is a simplification. For example, it's possible to use techniques like write-behind caching or snapshots to mitigate some of the disadvantages associated with in-memory databases. Similarly, modern SSDs have improved their speed and durability considerably, narrowing the gap with in-memory solutions. Your decision would ultimately depend on the specific needs of your project and budget.