> Datomic uses DataLog as part of its query language, but that's pretty much where the comparison should end. Things like "treating the database as a value", and features such as data accretion that Rich talks about have nothing to do with DataLog.
I'm not sure I totally agree with this. I think that apart from talking about the design of Datomic, he also has a more general point against what he calls PLOP (PLace Oriented Programming), which Datalog does address.
For example in plain Racket a value is lost if something else is put in its place:
Hickey is also mentioning how git doesn't do PLOP in that it doesn't throw out your commit history (without you asking it to do so).
> The reasons I mention Redis is that the HN app is very well suited to it. HN only keeps 'x' amount of data in memory. And in Redis the data lives in memory. Also Redis allows you to set expiry times on data for auto eviction [6].
Interesting. Just checked news.arc, and yes `initload*` is set to 15000. Interesting idea from Redis with expiry times. I'll check it out. I hadn't considered the scenario of storing text enough to max out on memory, because it would probably be premature optimisation, but good to keep in mind. I'd like to give Redis/Rackdis a try; thanks for the suggestion. I've been hosting an Etherpad Lite instance, and Redis was painless to setup.
> I'm pointing this out because it seems to me that you're doing (or are going to be doing) a lot of work that may not be worth it for what you're trying to accomplish.
Yes, my priorities here are definitely to make the code as brief and simple as possible, and to not have to do to much work. With plain Datalog it's very little work to timestamp a fact, and it's also kind of necessary, e.g. to figure out which fact is most recent, when previous facts are not removed. I'm just trying to get the gist of Hickey's ideas here.
> PLOP (PLace Oriented Programming), which Datalog does address.
Yeah, I was thinking more along the lines that Datomic has built-in functionality to address the caching, cache eviction, and indexing that goes along with all that data accumulation. But you're correct, DataLog does accumulate facts.
> Interesting. Just checked news.arc, and yes `initload*` is set to 15000.
I did the same thing, about 6 or 7 years ago, that you're doing now. I ported HN to Clojure (which is actually how I learned Clojure). If memory serves me correctly when I was doing the work I realized I needed a real DB if I wanted to support load balancing. i.e. I needed to centralize the data for the authentication and fnid session info. I think Arc calls them fnids... You probably know better than I do now, but Arc has all this code to expire these session fnids and so, for me, Redis was just a good fit for that task.
Anyways, I'll be sure to take a look at the final result of your work.
> I did the same thing, about 6 or 7 years ago, that you're doing now. I ported HN to Clojure (which is actually how I learned Clojure).
Cool!
> I needed to centralize the data for the authentication and fnid session info. I think Arc calls them fnids... You probably know better than I do now, but Arc has all this code to expire these session fnids and so, for me, Redis was just a good fit for that task.
The Racket web server is quite "batteries included" and comes with these different managers[0] for dealing with expiration of sessions/continuations, such as the LRU manager:
> The memory limit is set to `memory-threshold` bytes. Continuations start with 24 life points. Life points are deducted at the rate of one every 10 minutes, or one every 5 seconds when the memory limit is exceeded. Hence the maximum life time for a continuation is 4 hours, and the minimum is 2 minutes.
> If the load on the server spikes—as indicated by memory usage—the server will quickly expire continuations, until the memory is back under control. If the load stays low, it will still efficiently expire old continuations.
When I was referring to load balancing and centralizing the data I was referring to many web servers sharing a centralized/external source for auth/session data.
I'm unfamiliar with racket's web server 'servlets'. The docs are little unclear (at least to me). Can these servlets live on a separate server so that the data can be shared between web servers? I'm guessing that was/is not a requirement for you, but I'm just interested in knowing if that's how it can work.
Uh oh, you're getting me interested in Racket now. I can't have that... I have too many projects :)
edit: I guess at the end of the day these servlets are web-servers right, so you can, even if you have to do it over http and build an api.
> Can these servlets live on a separate server so that the data can be shared between web servers?
Probably. I assume that serializable continuations[0] from stateless servlets can just be stored wherever, like in Redis or something, instead of in the memory of one server.
> I ported HN to Clojure
If that is something you have published, it'd be fun to see, whether it's finished or not.
> Uh oh, you're getting me interested in Racket now.
My impression is that Clojure is faster, less verbose partly due to clever syntax and provides more immutable data structures than Racket. But when it comes to documentation and error messages, I find Racket more coherent and comprehensible.
Say, if I wanted to connect to a SQL databse, with Racket I'd use the DB module[1], end of discussion. But with Clojure there's Korma, ClojureQL, Persist, HoneySQL, Yesql, a JDBC wrapper from Clojure contrib, SQLingvo, oj, Suricatta, aggregate, Hyperion, HugSQL, and probably a few more[2][3]. That multitude of libraries with similar purpose may be useful in some cases, sure, but also potentially a bit overwhelming for beginners, so I guess that's why I found it easier to get started with Racket.
> If that is something you have published, it'd be fun to see, whether it's finished or not.
I actually tried to look it out the other day during this conv, but it's buried somewhere unavailable right now. If I find/get to it I'll post.
> That multitude of libraries with similar purpose may be useful in some cases, sure, but also potentially a bit overwhelming for beginners, so I guess that's why I found it easier to get started with Racket.
Agreed. Navigating the volume libraries and the options available is a real pain in the beginning, but once you get past that, then it's not bad at all. At the same time, take a look at the quality of Clojure's Redis Carmine Library vs. Racket's Redis Libraries. Miles apart.
