TKVDB: radix tree persistent key/value storage engine written in ANSI C (opens in new tab)

do … while(0) is a well-know method of writing multi-statement macros:

http://c-faq.com/cpp/multistmt.html

while … while(0) looks like a mistake?

Yes, the first macro is the common trick, and the second is the mistake (wondering, but it works). I update the code. Thanks a lot for pointing it out!

You are correct. At least the do... while statement is a common c macro trick to make sure a macro has its own scope. The idea is that a compiler will optimize the do....while statement away, so it does not have any runtime overhead.

The wile...while(0) statement is new for me (didn't know this is actually legal syntax), but I suspect it's done for similar reasons.

> What made you choose the radix tree?

Library was initially written for use with Netflow collector. More precisely for storing filtered and aggregated traffic information. Typical load for such applications is: a lot of updates in "hot" dataset, fewer inserts, deletions are rare and mostly batched, no needs for fast sequential scan (user can wait for a few seconds or even more to get report). Radix trees are perfectly fits this requirements.

Later we made some tests and decided to use it for IP (plus some other information) lookups without underlying file ("RAM-only" mode).

We have to support some old network hardware (for example almost abandoned Tilera TILE-Gx), that's why we're trying to avoid using OS and platform-specific features in library.

Radix trees has disadvantages, the most notable is that it's hard to implement user-defined sorting order for keys and they required more memory for sparse nodes compared to B-trees (however, this statement is not true for on-disk TKVDB database, we don't store empty pointers)

Red black trees have crap performance due to pointer chasing.

For small maps (compressed) radix trees (patricia, judy) are even faster than hash maps, esp. concurrent.

But but but... every day, many people use implementations written in C to reliably store data. Just look at, say, Linux. The drivers, filesystems and networking stack are all written in C (and maybe some assembler). And once the data reaches the firmware controllers of the mass storage device... Well, the firmware is done in C or assembler.

It is not like implementing a rather thin layer for reliable data storage on the userspace in language x somehow invalidates the contribution of the C language, which the underlying system, all the way from the OS to the device firmware is likely written with.

That said, I do understand the arguments of e.g. Rust being superior in preventing memory bugs, and so on, but whatever gets implemented on the userspace is simply not the whole story regarding reliable data storage. Without system services no data will get replicated, distributed, stored and so on...

If you intend to export the API to the widest possible variety of other languages, C is a good way to go for the initial implementation. The lack of a universally accepted, language-independent "Rosetta stone" ABI means that this is the one job that C still does better than anything else.

Nothing wrong with writing targeted, specialized components in C. It can be done properly, it just frequently isn't.

It's a tradeoff between adoptability, readability, portability, ease of embedding, average familiarity of toolsets, ABI managability, and strictness of rules to access arrays and references in the implementation. C likely wins in all of the above except the last, with C++ in second place, and virtually the only other options are Rust and Go, which only wins in the last point.

It looks like transaction is just a C struct with function pointer fields, not an actual C++ class, so there is no implicit `this` argument.

no "self" in C.

> Can you point out the pros and cons vs above projects?

Briefly speaking, the above projects are way more mature, full-featured and use different data structures for storing data. BDB, LMDB are B-tree-based, LevelDB (and forks), SQLite4 LSM are using log-structured merge-trees.

TKVDB is a lot simpler, a bit more low-level, uses Radix trees and not tested as well as other key-value engines.

> I read this as: only use it single threaded.

If you protect access to data with your OS locks, everything will be fine.

We're using TKVDB in multi threaded environment (in RAM-only mode) for IP lookups with Netmap and DPDK. On 40Gbe full line rate there is only few hundred CPU cycles for decision on network packet, so the locking tricks becomes important.

Intel has guarantees about writing properly aligned data - if one core is writing 1, 2, 4 or 8 bytes, it will be written atomically. We're exploiting this feature when updating our tree - final update operation is 8 byte (4 byte for 32 bit CPU's) aligned pointer update.

So, in some cases you don't need to use locks at all (you may add HW memory barrier (__sync_synchronize()) after tree update, but it will slow down writer). If only one core is writing data, another cores can safely (well, almost) read it. Warning in docs was about this lockless case.