[ffdjjjffjj]

I’m also a big fan of rails, but I’ve experienced a lot of problems scaling it. A lot of the problems ultimately came down to the simple fact that Ruby is really, really slow. At a certain scale you end up forced to develop infrastructure on a different stack to keep up with the CPU load. I never ran into that so quickly when building similar systems with java, go, and C#.

I wanna say something nice about rails too so I’ll say I have never seen a team so quickly deliver high quality web app features than a well oiled rails team. It’s something to behold.

[dasil003]

This very much depends on the use case. If you are truly CPU-bound than yes, ruby is a bad choice. Similarly, if you are IO-bound or otherwise need a lot of concurrency ruby is not great. I also think it's problematic for large code bases with hundreds of developers. However, there is a huge swath of web apps and APIs that just need to pull a bit of data from a DB or two, munge it together and send it to the client. Rails is perfectly horizontally scalable and plenty fast for this even at scale given Amdahl's law.

That said, there are definitely some self-inflicted wounds people run into here. Of course there are the oft-cited expert beginner problems like n+1 queries and missing indices, but there's a more subtle issue as well: ActiveRecord itself is so convenient that it discourages writing baseline efficient code even when you need it. Basically any time you need to process a medium to large amount of data, ActiveRecord acts like a sort of super boxed type (to use Java terminology) adding a huge amount of overhead. Yes it's true that ruby doesn't even have true primitive types that can achieve the performance of Go or Java, but often times ruby primitives are plenty fast, you just have to be willing to write a bit of extra code, and ActiveRecord as an ORM has plenty of escape hatches at various levels of granularity to facilitate this (eg. pluck, find_by_sql, update_all, execute, etc).

[ryanbrunner]

I definitely agree that when writing Rails at scale, it's extremely important to view ActiveRecord as just another tool in the toolbox, and not always effective for every use case. I've worked in places that had a dogmatic aversion to ever writing raw SQL, and to a one they always suffered for it in performance sensitive code.

At the places who did handle this well, the "high-performance code" that needs to be hand-tuned SQL is usually much smaller than you think (a few queries here and there), and ActiveRecord is still great for your simple queries or for smallish tables.

[Lio]

Could you go into more detail about what you see as issues with IO bound tasks?

My understanding is that MRI Ruby provides non-block IO operations if you wrap them in a thread and that it is only CPU bound tasks that are blocked by the GVL.

Is there some other issue related to that?

(JRuby provides fully multi-threading for cpu bound tasks without GVL).

[joshmn]

Rails makes it really easy to do something 10 different ways to get the same result. Unfortunately, most of which aren't the most performant way. In my 10 years of building Rails apps of all different sizes, and seeing some very mature apps in production, this is the most common culprit I've seen.

I currently work on a rather large Rails app for a site that most of us here use. A common pattern for our performance pitfalls are things like this:

  Tag.all.map(&:name)

versus

  Tag.all.pluck(:name)

Using `#map` will instantiate a `Tag` object and do all the slow(er) metaprogramming to get you the syntactic sugar that makes interacting with an ActiveRecord object a treat. It does this, and then you only hit `#name`, completely wasting all that effort.

`#pluck` will change the SQL from `select * from tags` to `select tags.name from tags` and never instantiate a `Tag` object, instead short-circuiting and directly fetching the resulting SQL rows — which comes back as an array. It's something along the lines of:

  ActiveRecord::Base.connection.exec_query(Tag.select(:id).to_sql)

Another one I see:

  ProgrammingLanguage.where(tag_id: @user.tags.select(&:language_tag?).map(&:id))

versus

  ProgrammingLanguage.where(tag_id: @user.tags.select(:id).where(type: 'LanguageTag'))

The first example loops over the loaded `@user.tags`, loads them if they're not already `#loaded?`, selects ones that are `type == 'LanguageTag'`, only to grab the `#id`.

The second example joins the two resulting SQL statements and calls `#to_sql` on the second statement, building one query from two queries.

Are these times when the first example would be preferred? Yeah, plenty! If your result is already `#loaded?`, then you probably don't need to hit the database again. But for this example and the ones I'm stumbling across while getting our company up-to-speed on "good ways", these are the the commonalities.

Save for only very recently, the company I work for hasn't put emphasis on real-world Ruby/Rails skills, instead "if you can code at a high level for any language, we think you can make reasonable contributions to our codebase." This has lead to hiring skilled developers that just don't know that there's a more preferred way of doing things in Rails for different contexts.

Double-edged sword, really.

[thirdusername]

I would refactor your second example into an exists using Arel, because at best the IN will result in the same performance. At worst it will be significant my slower. There are also particular issues with NOT IN and NULL. This is at least true in PG.

I also deal with a lot of scale, the issues people have here doesn’t match my reality. I think people have issues and rather than looking at what is fundamentally happening with their call patterns, they jump to calling out rails itself.

Rails does have some specific issues, but you’d have to go pretty deep to see them and boot times are terrible.

[JeremyNT]

The general problem you describe - n + 1 queries caused by needless iterating over / instantiating Ruby objects when a simple SQL query would do - is certainly a very common newbie mistake, but it's just that: a newbie mistake.

Confusion here simply shouldn't be a problem for even a moderately seasoned developer, and if they do make such a mistake (because hey, we all make mistakes...) in performance-sensitive code they could quickly recognize it for what it is - a bug - and fix it.

If you're hiring junior developers, on the other hand? Sure! But you should know what you're getting, and your code review / mentoring process should get them straight.

I'm not sure I really understand how this is Ruby's or Rails' fault, unless your premise is "ORMs considered harmful" - in which case, ActiveRecord is far from alone here, and that's a different sort of conversation.

[treis]

Your examples aren't really doing the same thing in two different ways. Map (and select, collect, each, and some others) iterate over an enumerable. Pluck and select are active record methods that generate SQL.

It's a good example of choosing magic/brevity over expressiveness. You don't know that Tag.all.map calls SQL because it's not something you explicitly tell it to do. That's the real issue with Ruby & Rails. The magic lets you do some powerful stuff but sometimes it's hard to tell what exactly is happening.

[ohgodplsno]

That's a consequence of using ORMs. ORMs are a horrible performance mess. Just straight up write SQL. I'm sure Ruby has enough metaprogramming magic to not have to deal with cursors manually.

[adrianpike]

Would love to hear what you're doing that's got Ruby pegging the CPU. I've ran a few rather large Rails deployments over the years and it's rare to crush the app tier first, so I wonder if there's something unique here that we can make Rails better at.

[dwheeler]

Have you used fragment caching (in "Russian Doll" style)? In my experience, that's the key to making Rails applications fast. Ruby is slow, so there are definitely situations where it's not a good choice, but in many "basic applications" caching makes that irrelevant. Language speed is irrelevant when you don't run much code :-).

[erikrothoff]

Curious what part of Rails you didn’t find scalable? Our Rails frontend servers run on cheap VMs that are easily scaled horizontally.

[jon-wood]

Rails is great if what you’ve got is read-mostly cachable content being served up. In an environment with high rates of incoming data, most of which can’t be reliably cached, things get more interesting and you start needing to scale horizontally a lot more and leaning on the database.

[awestroke]

Scaling horizontally is not cheap. A faster language needs fewer replicas.

[prh8]

CPU load being the ceiling in Rails is typically the sign of sloppy development. Most well done apps it will be the database.

[JohnBooty]

Yes, this matches my experience. I've been doing Rails full-time for about 6 years and any "slowness" has been the result of some problem, not Rails itself. 99% of the time this is an uncached n+1 query situation, or some beastly slow database operation that needs to be moved to background processing, things that would be problems in any framework or even some sort of bare-metal asm solution that nonetheless relies upon an external storage layer. =)

In a CRUD app the Rails layer should be extremely thin and the storage layer(s) should be doing nearly all of the heavy lifting.

There is a level of traffic at which even a "properly" thin Rails layer becomes the bottleneck, relative to many other frameworks.

TechEmpower benchmarks suggest it is around 2,500 requests per second in their "multiple queries" benchmark. In a more real-world scenario that might be 1,000 req/sec or less.

https://www.techempower.com/benchmarks/#section=data-r19&hw=...

If one is attempting to serve more requests than this per minute then yes, perhaps Rails is the bottleneck. Admittedly, Rails' large memory consumption relative to other frameworks means it can be tough (well, technically easy, but expensive) to scale horizontally at this point.

[jes5199]

it seems like a lot of my career has been optimizing SQL queries in Rails apps... which is often just adding the correct indexes. Kind of a lot of Rails devs just don’t know to do that

[stanislavb]

"the simple fact that Ruby is really, really slow"... it's that slow that Shopify and Stripe don't mind building their global scale products in Ruby.

[Nextgrid]

Slowness seems to be an overblown issue when it comes to the backend. Hardware is cheap compared to developers and app servers scale horizontally by default; if a language is actually slow but otherwise provides good productivity it's almost always cheaper to throw more hardware at it than move to a "faster" language (there's a reason we don't typically write web apps in C).