LokiJS – Lightweight JavaScript in-memory database
lokijs.org
lokijs.org
I'm really no expert, but I think that language-specific persistent in-memory databases are the way to go.
Deep inside, what I really want on my backends is just a data structure. A single big tree (or graph) of objects. Some primitives to make it thread-safe somehow (immutable collections, or mutexes, or just old-fashioned single-threadedness like Node - whatever), and that's it. Requests from the client query and mutate that data. In very many situations, I wouldn't need anything special to make this work fine with an application up to a pretty large number of users. Not all apps do big data analyses. RAMs are gigantic these days, bigger than the SQL datafiles of swathes of webapps.
However, there's that pesky problem of code upgrades and crashing servers. If my server would never crash and I could hot-swap the code, I wouldn't need persistence at all. But I do, so I don't just want an in-memory datastructure, but a persistent in-memory data structure. I want to use this datastructure as easily as the programming language could possibly make it be for me while still guaranteeing some sane level of persistence and fault tolerance.
Redis is a nice idea, but the fact that it's a separate server, with a client and a protocol and data structures that just not precisely map to my programming language's data structures force me to write a whole bunch of boilerplate anyway. Even more so with other databases like Mongo or Postgres.
If I understand it well, Loki doesn't entirely do what I'd want: it does not save the data to disk on every change, but less often, if I get it right. That might be good enough if my problem allows for many little independent Loki databases, but if the dataset is a gigabyte and persistence means flushing that whole gigabyte to disk after every data change, it probably won't work very well.
I'm really curious if other people here have similar ideas, maybe implementations, of these concepts. Maybe Loki could be extended with a snapshot+operation_log type of data storage like Redis has and then it'd be pretty close.
From a cursory glance at the source, it doesn't look like anything internal ever calls .save()/.saveToDisk(), so it is up to you to decide how often you want to be creating your on-disk checkpoints.
EDIT: Seems I am wrong: "In a nodejs/node-webkit environment LokiJS also persists to disk whenever an insert, update or remove is performed." [1]. But I still can't spot where .save[/ToDisk] is called in the source [2]. Anyone?
> That might be good enough if my problem allows for many little independent Loki databases, but if the dataset is a gigabyte and persistence means flushing that whole gigabyte to disk after every data change, it probably won't work very well.
If you're looking at gigabyte+ datasets, then something designed to be in-memory is probably not your best bet. Aside from saving, (one imagines) this would also affect load times (reading->parsing->importing a multi-GB JSON file at launch can't be quick).
From [1]:
> LokiJS is ideal for the following scenarios:
* where a lightweight in-memory db is ideal
* cross-platform mobile apps where you can leverage the power of javascript and avoid interacting with native databases
* data sets are not so large that it wouldn't be a problem loading the entire db from a server and synchronising at the end of the work session
[1] https://github.com/techfort/LokiJS
[2] https://github.com/techfort/LokiJS/blob/master/src/lokijs.js
Why not? I only need to load when my program starts, i.e. after a crash or maybe an upgrade when hot code swapping is not available. Sounds like there may be plenty of persistence schemes that make this not entirely painful (load latest data first, or fallback to disk reads if the data isn't entirely loaded yet, which makes the service slower but still available right after a crash).
Note, I'm really just dreaming here, and I appreciate you dreaming along. Dreaming is good! Some frontend dev dreamed "i just want to rebuild the entire page whenever data changes!" and then React happened.
Thanks for your dig into the code btw, nice findings! I can't find the save() calls either, so I suspect that they used to be there but aren't now, or the other way around. It's alpha, after all.
It would work, but I think it would be dangerous and/or inconvenient.
The danger part comes from having a sync-to-disk operation that lasts any considerable amount of time - the longer it lasts, the more the likelihood that an inopportune crash would leave you with an incomplete (read: corrupted) JSON file. A DB built for fast disk persistence would only update the relevant records, keeping the disk writes as small as possible. I don't think Loki has any option other than writing the entire thing each time (with the current JSON savefiles, that is). Since save() is an expensive op, you also can't be calling it at every update (it wouldn't even work! the first one would lock the file for writing and all subsequent save() ops, until the first one is done, would fail!) so it would be inherently unsafe unless you committed GBs to disk for each update!.
So, this might be somewhat dangerous, but we can mitigate that, right? We'll save frequently, but not too frequently, and somehow version-control our JSON file. Which is GB+ in size. So we'll also compress them? And when we need to restore, we'd.. hmm.. start reading from the latest until a valid one is found? (<--inconvenience)
> Note, I'm really just dreaming here, and I appreciate you dreaming along.
Likewise!
> Sounds like there may be plenty of persistence schemes that make this not entirely painful (load latest data first, or fallback to disk reads if the data isn't entirely loaded yet, which makes the service slower but still available right after a crash).
While those certainly exist, the one currently chosen by Loki can't do any of these things. Since you can't parse half a JSON file (especially in this format, which appends index info etc at the end of the file), you have to read the entire thing from disk, JSON.parse it, and then feed it into Loki. Only after all of that will you know that you are actually restoring from meaningful (and complete) data files and not junk.
The way I see this, it would be great for throwaway-prototypes on the serverside (or for few/unimportant data) but its real value would show on the client side. You basically get a mini-mongodb for 2K LOC that you can embed on anything that speaks ECMA3.
What's even cooler IMHO is the future - on their slideshare presentation they list replication and horizontal scaling on their roadmap. While I have no idea how they have envisaged implementing those, I would love to experiment with the meteor.js concepts and this - providing a full, fast cache of the user's data on the client side and then throw differential updates around.
edit: spelling
However it does throw events, so it would not be hard to implement this behavior yourself with a simple event listener.
Yeah, let's go back to the seventies, and drop everything we learned about having an abstract, sound data model, and multiple access from different services.
A perfect example are, in my mind, some of the reasons why node.js is taking off as opposed to say Java or C#... One, is that the latter tend to follow "enterprise" development practices. That is, they will follow "architect" tooling that implements "design patterns" ... the problem is that this adds its' own kinds of complexity.
In C# (or Java), there are frameworks and tooling that tend to follow design patterns, which is nice. There are abstractions that allow for the use of multiple injection/ioc systems to make code more modular and testable. Which is awesome. It also makes code more complex, and the abstractions are often harder to follow.
In JavaScript, since you can replace just about anything, and even call into functions with differing contexts, this makes testing even easier. Because if this simplicity, with a module system (like commonjs/node/npm), you can write testable code without the need for the use of interface abstractions... It becomes easier to test, write for and use.
There are down sides to using JS over C# etc.. but the same arguments apply to data usage. Most "NoSQL" databases give up some safety, usually for big performance gains. It really depends on the context of the data, which I think in DBA terms is often lost. I've seen some horribly normalized datasets used in SQL to create "flexible" systems, where a NoSQL solution is a much better fit.
It depends on your needs. There are times where I'd lean more towards ElasticSearch, Mongo, Redis, Couch, Cassandra, RethinkDB and others over the likes of MS-SQL, PostgreSQL, Oracle, DB2, Firebird, etc.... all have advantages and disadvantages over eachother. An embedable db abstraction is often a very good thing. Depends on what you need.
I specifically added Firebird above, because I've used it as the core for an embedded/distributed data layer before... This was long before node.js was an option.. now, I might favor something like tfa solution, with a mongodb central db.
New programmers rediscovering the (square) wheel?
>In JavaScript, since you can replace just about anything, and even call into functions with differing contexts, this makes testing even easier.
One should rather draw the opposite conslusion: that the above make testing even more necessary (and code even less robust).
(I contributed to both Minimongo and Miniredis)
> Performs better than similar products (NeDB, TaffyDB, PouchDB etc.) and it's much smaller
It would be interesting to see benchmarks.
From my perspective, with a small collection size, I'll sacrifice performance for expressiveness. I have a number of expensive features that I think do nice things.
[1] https://github.com/techfort/LokiJS/tree/master/benchmark
https://www.npmjs.org/package/data-collection http://thestorefront.github.io/DataCollection.js/
They look fairly similar. What's the test coverage and do you have performance benchmarks?
The feature that looks especially promising is the ability to create Dynamic Views and to listen on notifications on these views.
I'm working on a similar project, that is focused on ease of use combined with distributed/decentralized (concurrency safe) behavior. Http://github.com/amark/gun
This project looks quite interesting. Thanks for sharing. Could be a great solution for single-page web apps (and mobile, Cordova, etc.).
Edit: Actually, reading more about Loki, it looks like the whole point of it is to provide querying/indexing capabilities to data in memory. So I don't know if there would be any value in using it as a replacement for key/value stores.
I'll play around with it myself, but is there anyone who has used this that knows how well it does with memory usage?
The ToDo demo shows the size of the collection [1]. Seems rather efficient.