What is Memory-Centric?
Apache Ignite is based on memory-centric architecture that combines the performance and scale of in-memory computing together with the disk durability and strong consistency in one system.
The main difference between the memory-centric approach and the traditional disk-centric approach is that the memory is treated as a fully functional storage, not just as a caching layer, like most databases do. For example, Apache Ignite can function in a pure in-memory mode, in which case it can be treated as an In-Memory Database (IMDB) and In-Memory Data Grid (IMDG) in one.
On the other hand, when persistence is turned on, Ignite begins to function as a memory-centric system where most of the processing happens in memory, but the data and indexes get persisted to disk. The main difference here from the traditional disk-centric RDBMS or NoSQL system is that Ignite is strongly consistent, horizontally scalable, and supports both SQL and key-value processing APIs.
Collocated vs Client-Server Processing
The disk-centric systems, like RDBMS or NoSQL, generally utilize the classic client-server approach, where the data is brought from the server to the client side where it gets processed and then is usually discarded. This approach does not scale well as moving the data over the network is the most expensive operation in a distributed system.
A much more scalable approach is collocated processing that reverses the flow by bringing the computations to the servers where the data actually resides. This approach allows you to execute advanced logic or distributed SQL with JOINs exactly where the data is stored avoiding expensive serialization and network trips.