Suggest an editImprove this articleRefine the answer for “Why is Redis an in-memory DB?”. Your changes go to moderation before they’re published.Approval requiredContentWhat you’re changing🇺🇸EN🇺🇦UAPreviewTitle (EN)Short answer (EN)Redis is called an in-memory database because all data is stored and processed right in RAM, rather than on disk like classic DBMSs: every structure (String, List, Set, Hash, ZSet, etc.) lives in memory, and disk is used only for backups or persistence (RDB/AOF). **Key point:** the data volume is limited by RAM, and once it fills up Redis applies an eviction policy (e.g. `allkeys-lru`), removing old keys; data is lost on a power failure unless persistence has been configured.Shown above the full answer for quick recall.Answer (EN)ImageRedis is called an **in-memory database**, because **all data is stored and processed right in RAM**, rather than on disk like classic DBMSs. ### The core idea Redis works entirely out of RAM, to provide: - **instant access** to data (access time in microseconds), - **high throughput** (millions of operations per second), - **low latency** even with a large number of clients. Disk is used **only for backups** or **persistence**, not for the primary work with data. ### How this is set up 1. **Every data structure (String, List, Set, Hash, ZSet, etc.)** lives in RAM. 2. Every operation (`GET`, `SET`, `INCR`, etc.) runs directly on the objects in memory. 3. So data isn't lost on restart, Redis can: - periodically **save a snapshot (an RDB snapshot)** to disk, - or **log every write (AOF, Append Only File)**. On restart, Redis loads that data back into memory. ### Why this matters - RAM is tens of thousands of times faster than disk. - There's no slow I/O, reads and writes go directly against RAM. - That makes Redis ideal for caches, queues, sessions, tokens, counters, and real-time analytics. ### But Redis isn't a "pure cache" for all that Although Redis runs in memory, it **can be made persistent**: - With RDB or AOF enabled, data is saved and restored after a restart. - Redis can also run in **replication and cluster** mode, which makes it resilient to failures. ### Advantages of the in-memory approach - Speed: millions of operations per second. - Minimal latency (<1 ms). - The ability to perform complex operations right in memory (aggregations, sorting, queues). ### Limitations - The data volume is limited by how much RAM is available. - Once it fills up, Redis applies an **eviction policy**, removing old keys (e.g. `volatile-lru`, `allkeys-lru`, and others). - Data is lost on a power failure, unless persistence has been configured. So, Redis is an **in-memory key-value store**, where disk plays a supporting role, and all the work with data happens in RAM to deliver maximum speed.For the reviewerNote to the moderator (optional)Visible only to the moderator. Helps review go faster.