No SQL database is the one thing I disagree with. What if you want to write a SQL query/report? Do you have transactions? As the app gets bigger you might want that.
I'm not saying this approach is a good idea in every case. There are many situations where I would use a proper database. It really depends on what kind of app you're building, what kind of queries you want to run, how big is your dataset, your failover and availability requirements, your programming language, etc.
In my case it was a great decision. The code is simple, and the data is convenient to access since it is already packed in native data structures. And it's really, really fast since all of the "queries" are basically LLVM-optimized machine code.
> What if you want to write a SQL query/report?
I just write a few lines of normal Rust code to do that. Sure, it's not as convenient. But I don't need to do that as often, so I don't care.
> Do you have transactions?
The concept of 'transaction' doesn't really make sense in this approach, since I'm directly modifying the data in memory, every change I make is atomic and can't fail, and I flush the data to the disk atomically as well.
So you don't have mutable data shared between threads then?
I might have worded that one sentence a little better, sorry.
I do technically need to share data between threads, since my event loop runs on multiple threads, so if I want to mutably modify something I do need to wrap it either in a `Mutex` or an `RwLock` and lock it.
What I meant was that it is not the same as a "transaction" in the SQL sense. It can't fail, there is no rollback, I decide on my own what level of granularity I need, and I bake the necessary atomicity into the data structures itself. So there are no SQL-style transactions anywhere - I just use a lock where it's appropriate to directly modify the memory, just as you'd do in any other situation. (Either by wrapping the whole structure in a lock, or wrapping only a single field, or by simply making the field atomic by itself.)
Even simpler. I use mmap only for things that I build offline and that never change during the runtime. Everything else is just kept as your garden variety objects in memory.
(Well, although I guess you could say it is an mmap database, it's just that the mmaped file is the swap, and the OS manages it for me.)
> I like using atomic commands, but I imagine it would become tedious fast to do the extra work of doing the accounting of the transaction manually every time you want to do a non atomic transaction.
In my case it's not really tedious at all. Usually when I want to mutate something it looks something like this:
let user_ref = state.require_login()?;
let user = user_ref.write();
user.set_field(value);
drop(user);
I would love to talk with you and exchange knowledge. Your HN profile has no publicly visible contact info though.
Do you have an email address I can reach you on? Can you put it in your HN profile description?
(Just please note that if I apparently don't reply and if you're using gmail then my reply probably went to spam or gmail's /dev/null, so email me again; lately gmail doesn't like emails that I send and either puts them into spam or just sends them to /dev/null, even when I'm replying to someone that emailed me first! So much for Google having no monopoly, sigh...)
Backups and restore will also have to be DIY. Point in time restore probably won't be possible - and that's a life saver when you need it.
Atomically replacing the whole file instead of updating the data in-place, and using a checksumming filesystem like ZFS should give you such guarantee, no?
> Backups and restore will also have to be DIY.
Well, yeah. A daily cron job that `scp`s the data to an offsite location. Personally I don't really see much problem with that? It's easy to backup, and easy to restore.
No. You need to use fsync to ensure the file is flushed to disk. Also careful not to rename the file across mount points (/tmp is often a different mount and the most common location for people trying this.)
> Well, yeah. A daily cron job that `scp`s the data to an offsite location. Personally I don't really see much problem with that? It's easy to backup, and easy to restore.
That's fine, but your backup and restore granularity is daily. If you introduce a bug that writes the wrong / partial data, you can't recover anything that isn't in the previous day's backup.
With a typical database you have a transaction log and you could restore back to any point during the day.
This has been necessary about a dozen times during my career (once every two years or so on average.)
Are you sure? At least in case of ZFS AFAIK the file wouldn't get corrupted, since it uses copy-on-write internally. Sure, you might get the old data back, but you wouldn't get corruption. And it automatically checksums your files anyway so it will detect any data that did get corrupted, and if you're using RAID-1 (as you should for any important data) it will automatically repair it.
> Also careful not to rename the file across mount points (/tmp is often a different mount and the most common location for people trying this.)
You can't rename across mount points. (Linux's `rename` syscall and the corresponding C API will return an EXDEV error if you try.)
> If you introduce a bug that writes the wrong / partial data, you can't recover anything that isn't in the previous day's backup.
Sure. If I was working in a bank not being able to do that would be totally unacceptable. But in my case that's the tradeoff I'm willing to live with.
As for reports and transactions specifically, noSQL databases support both.
In this case, I don't think OP is even thinking about noSQL databases.
Not really, unless you lack the experience to know what to future proof and what not to future proof.
I've saved days/weeks of development by future proofing certain things, I've also saved days/weeks of development by taking pragmatic YAGNI in the right places.
Obviously this will only surface on projects that last longer than a couple months ;)
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