What does 'soft-state' in BASE mean?

BAS开发者_JAVA技巧E stands for 'Basically Available, Soft state, Eventually consistent'

So, I've come this far: "Basically Available: the system is available, but not necessarily all items in it at any given point in time" and "Eventually Consistent: after a certain time all nodes are consistent, but at any given time this might not be the case" (please correct me if I'm wrong).

But, what is meant exactly by 'Soft State'? I haven't been able to find any decent explanations on the internet yet.

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This page (originally here, now available only from the web archive) may help:

[soft state] is information (state) the user put into the system that will go away if the user doesn't maintain it. Stated another way, the information will expire unless it is refreshed.

By contrast, the position of a typical simple light-switch is "hard-state". If you flip it up, it will stay up, possibly forever. It will only change back to down when you (or some other user) explicitly comes back to manipulate it.

The BASE acronym is a bit contrived, and most NoSQL stores don't actually require data to be refreshed in this way. There's another explanation suggesting that soft-state means that the system will change state without user intervention due to eventual consistency (but then the soft-state part of the acronym is redundant).

There are some specific usages where state must indeed be refreshed by the user; for example, in the Cassandra NoSQL database, one can give all rows a time-to-live to make them completely soft-state (they will expire unless refreshed), but this is an unusual mode of usage (a transient cache, essentially).

"Soft-state" might also apply to the gossip protocol within Cassandra; a new node can determine the state of the cluster from the gossip messages it receives, and this cluster state must be constantly refreshed to detect unresponsive nodes.

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I was taught in classes that "Soft state" means that the state of the system could change over time (even during times without input), because there may be changes going on due to "eventual consistency". That's why says "soft" state.

Some source: link

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Soft state means data that is not persisted on the disk, yet in case of failure it could be possible to restore it (e.g. recreate a lower quality image from a high quality one). A good article that addresses this and other interesting issues is Cluster-Based Scalable Network Services

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A BASE system gives up on consistency to improve the performance of the database. Hence most of the famous NoSQL databases are highly available and scalable than ACID-compliant relational databases.

Soft state indicates that the state of the system may change over time, even without input. This is because of the eventual consistency model. Eventual consistency indicates that the system will become consistent over time.

For example, Consider two systems such as A and B. If a user writes data to a system A, there will be some delay in reflecting these written data to B usually within milliseconds(depending upon the network speed and the design for syncing).