what are the different between them about classic-level

[vweevers]

The backstory of abstract-level can be found in the FAQ: What is abstract-level?

The short version, that I hope will answer your more specific questions, is this:

• LevelDB is a C++ project by Google. We're not affiliated with Google.
• leveldown was a Node.js binding for LevelDB
• levelup wrapped leveldown in a more convenient interface, later indeed including promises
• leveldown and levelup are now legacy modules
• classic-level is effectively a merger of leveldown and levelup. It's thus a binding to LevelDB with a convenient interface. I say effectively because the real story is more nuanced and can be found in the FAQ that I mentioned.

If I instead were to explain the relationships to someone that's new to Level (and thus doesn't have to know about leveldown and levelup), then I would say, using OOP terms:

• AbstractLevel is an abstract class for a lexicographically sorted key-value database. It handles state, encodings and other generic behaviors that can be implemented in pure JavaScript.
• ClassicLevel implements that abstract class, using LevelDB for storage.

[vweevers]

In case your question is, should I use classic-level or leveldown? Then the answer is classic-level, without a doubt.

Legacy modules like leveldown are no longer maintained. They work perfectly fine, but won't get new features, performance improvements or other updates. Feature-wise, classic-level has the following additional features:

• Sublevels
• Support of Uint8Array
• iterator.nextv() and iterator.all()
• Built-in TypeScript types

And more features are coming soon:

• Hooks
• Support of AbortSignal
• Explicit snapshots (this one will probably take longer)

[AdaJass]

• AbstractLevel is an abstract class for a lexicographically sorted key-value database. It handles state, encodings and other generic behaviors that can be implemented in pure JavaScript.
• ClassicLevel implements that abstract class, using LevelDB for storage.

Thanks a lot, I saw the compare with levelDB and rocksDB in internet, it is said rocksDb is the better one. By your mention, I could write code with abstractlevel or classiclevel, but if i want in the future, i could just simply replace the require/import package, it is that right?

will you plan to upgrade the rocksdb package to the abstractlevel style?

[vweevers]

We don't have a (new) RocksDB binding yet, see Level/abstract-level#14. I can't say when that will happen, it's low priority atm.

But yes, once that's available, you could replace const { ClassicLevel } = require('classic-level') with const { RocksLevel } = require('rocks-level') and you'll get the same interface.

PS. "Better" is subjective. LevelDB is a good choice (and the most popular among Level users) when you want small binaries, a wide range of supported platforms, and good general performance (that isn't optimized for one particular scenario like SSDs). RocksDB does have more features, but the majority are not exposed in our bindings because those features don't fit in the abstract-level interface and RocksDB receives less attention from us.