Rails auto-scaling on Heroku

We are investigating moving our medium-small-ish Rails app to heroku.

We looked at both the Rails Autoscale add-on available on heroku marketplace, and the hirefire.io service which is not listed on heroku marketplace and I almost didn’t realize it existed.

I guess hirefire.io doesn’t have any kind of a partnership with heroku, but still uses the heroku API to provide an autoscale service. hirefire.io ended up looking more fully-featured and lesser priced than Rails Autoscale; so the main service of this post is just trying to increase visibility of hirefire.io and therefore competition in the field, which benefits us consumers.

Background: Interest in auto-scaling Rails background jobs

At first I didn’t realize there was such a thing as “auto-scaling” on heroku, but once I did, I realized it could indeed save us lots of money.

I am more interested in scaling Rails background workers than I a web workers though — our background workers are busiest when we are doing “ingests” into our digital collections/digital asset management system, so the work is highly variable. Auto-scaling up to more when there is ingest work piling up can give us really nice inget throughput while keeping costs low.

On the other hand, our web traffic is fairly low and probably isn’t going to go up by an order of magnitude (non-profit cultural institution here). And after discovering that a “standard” dyno is just too slow, we will likely be running a performance-m or performance-l anyway — which likely can handle all anticipated traffic on it’s own. If we have an auto-scaling solution, we might configure it for web dynos, but we are especially interested in good features for background scaling.

There is a heroku built-in autoscale feature, but it only works for performance dynos, and won’t do anything for Rails background job dynos, so that was right out.

That could work for Rails bg jobs, the Rails Autoscale add-on on the heroku marketplace; and then we found hirefire.io.

Pricing: Pretty different

hirefire

As of now January 2021, hirefire.io has pretty simple and affordable pricing. $15/month/heroku application. Auto-scaling as many dynos and process types as you like.

hirefire.io by default can only check into your apps metrics to decide if a scaling event can occur once per minute. If you want more frequent than that (up to once every 15 seconds), you have to pay an additional $10/month, for $25/month/heroku application.

Even though it is not a heroku add-on, hirefire does advertise that they bill pro-rated to the second, just like heroku and heroku add-ons.

Rails autoscale

Rails autoscale has a more tiered approach to pricing that is based on number and type of dynos you are scaling. Starting at $9/month for 1-3 standard dynos, the next tier up is $39 for up to 9 standard dynos, all the way up to $279 (!) for 1 to 99 dynos. If you have performance dynos involved, from $39/month for 1-3 performance dynos, up to $599/month for up to 99 performance dynos.

For our anticipated uses… if we only scale bg dynos, I might want to scale from (low) 1 or 2 to (high) 5 or 6 standard dynos, so we’d be at $39/month. Our web dynos are likely to be performance and I wouldn’t want/need to scale more than probably 2, but that puts us into performance dyno tier, so we’re looking at $99/month.

This is of course significantly more expensive than hirefire.io’s flat rate.

Metric Resolution

Since Hirefire had an additional charge for finer than 1-minute resolution on checks for autoscaling, we’ll discuss resolution here in this section too. Rails Autoscale has same resolution for all tiers, and I think it’s generally 10 seconds, so approximately the same as hirefire if you pay the extra $10 for increased resolution.

Configuration

Let’s look at configuration screens to get a sense of feature-sets.

Rails Autoscale

web dynos

To configure web dynos, here’s what you get, with default values:

The metric Rails Autoscale uses for scaling web dynos is time in heroku routing queue, which seems right to me — when things are spending longer in heroku routing queue before getting to a dyno, it means scale up.

worker dynos

For scaling worker dynos, Rails Autoscale can scale dyno type named “worker” — it can understand ruby queuing libraries Sidekiq, Resque, Delayed Job, or Que. I’m not certain if there are options for writing custom adapter code for other backends.

Here’s what the configuration options are — sorry these aren’t the defaults, I’ve already customized them and lost track of what defaults are.

You can see that worker dynos are scaled based on the metric “number of jobs queued”, and you can tell it to only pay attention to certain queues if you want.

Hirefire

Hirefire has far more options for customization than Rails Autoscale, which can make it a bit overwhelming, but also potentially more powerful.

web dynos

You can actually configure as many Heroku process types as you have for autoscale, not just ones named “web” and “worker”. And for each, you have your choice of several metrics to be used as scaling triggers.

For web, I think Queue Time (percentile, average) matches what Rails Autoscale does, configured to percentile, 95, and is probably the best to use unless you have a reason to use another. (“Rails Autoscale tracks the 95th percentile queue time, which for most applications will hover well below the default threshold of 100ms.“)

Here’s what configuration Hirefire makes available if you are scaling on “queue time” like Rails Autoscale, configuration may vary for other metrics.

I think if you fill in the right numbers, you can configure to work equivalently to Rails Autoscale.

worker dynos

If you have more than one heroku process type for workers — say, working on different queues — Hirefire can scale the independently, with entirely separate configuration. This is pretty handy, and I don’t think Rails Autoscale offers this. (update i may be wrong, Rails Autoscale says they do support this, so check on it yourself if it matters to you).

For worker dynos, you could choose to scale based on actual “dyno load”, but I think this is probably mostly for types of processes where there isn’t the ability to look at “number of jobs”. A “number of jobs in queue” like Rails Autoscale does makes a lot more sense to me as an effective metric for scaling queue-based bg workers.

Hirefire’s metric is slightly difererent than Rails Autoscale’s “jobs in queue”. For recognized ruby queue systems (a larger list than Rails Autoscale’s; and you can write your own custom adapter for whatever you like), it actually measures jobs in queue plus workers currently busy. So queued+in-progress, rather than Rails Autoscale’s just queued. I actually have a bit of trouble wrapping my head around the implications of this, but basically, it means that Hirefire’s “jobs in queue” metric strategy is intended to try to scale all the way to emptying your queue, or reaching your max scale limit, whichever comes first. I think this may make sense and work out at least as well or perhaps better than Rails Autoscale’s approach?

Here’s what configuration Hirefire makes available for worker dynos scaling on “job queue” metric.

Since the metric isn’t the same as Rails Autosale, we can’t configure this to work identically. But there are a whole bunch of configuration options, some similar to Rails Autoscale’s.

The most important thing here is that “Ratio” configuration. It may not be obvious, but with the way the hirefire metric works, you are basically meant to configure this to equal the number of workers/threads you have on each dyno. I have it configured to 3 because my heroku worker processes use resque, with resque_pool, configured to run 3 resque workers on each dyno. If you use sidekiq, set ratio to your configured concurrency — or if you are running more than one sidekiq process, processes*concurrency. Basically how many jobs your dyno can be concurrently working is what you should normally set for ‘ratio’.

Hirefire not a heroku plugin

Hirefire isn’t actually a heroku plugin. In addition to that meaning separate invoicing, there can be some other inconveniences.

Since hirefire only can interact with heroku API, for some metrics (including the “queue time” metric that is probably optimal for web dyno scaling) you have to configure your app to log regular statistics to heroku’s “Logplex” system. This can add a lot of noise to your log, and for heroku logging add-ons that are tired based on number of log lines or bytes, can push you up to higher pricing tiers.

If you use paperclip, I think you should be able to use the log filtering feature to solve this, keep that noise out of your logs and avoid impacting data log transfer limits. However, if you ever have cause to look at heroku’s raw logs, that noise will still be there.

Support and Docs

I asked a couple questions of both Hirefire and Rails Autoscale as part of my evaluation, and got back well-informed and easy-to-understand answers quickly from both. Support for both seems to be great.

I would say the documentation is decent-but-not-exhaustive for both products. Hirefire may have slightly more complete documentation.

Other Features?

There are other things you might want to compare, various kinds of observability (bar chart or graph of dynos or observed metrics) and notification. I don’t have time to get into the details (and didn’t actually spend much time exploring them to evaluate), but they seem to offer roughly similar features.

Conclusion

Rails Autoscale is quite a bit more expensive than hirefire.io’s flat rate, once you get past Rails Autoscale’s most basic tier (scaling no more than 3 standard dynos).

It’s true that autoscaling saves you money over not, so even an expensive price could be considered a ‘cut’ of that, and possibly for many ecommerce sites even $99 a month might a drop in the bucket (!)…. but this price difference is so significant with hirefire (which has flat rate regardless of dynos), that it seems to me it would take a lot of additional features/value to justify.

And it’s not clear that Rails Autoscale has any feature advantage. In general, hirefire.io seems to have more features and flexibility.

Until 2021, hirefire.io could only analyze metrics with 1-minute resolution, so perhaps that was a “killer feature”?

Honestly I wonder if this price difference is sustained by Rails Autoscale only because most customers aren’t aware of hirefire.io, it not being listed on the heroku marketplace? Single-invoice billing is handy, but probably not worth $80+ a month. I guess hirefire’s logplex noise is a bit inconvenient?

Or is there something else I’m missing? Pricing competition is good for the consumer.

And are there any other heroku autoscale solutions, that can handle Rails bg job dynos, that I still don’t know about?

update a day after writing djcp on a reddit thread writes:

I used to be a principal engineer for the heroku add-ons program.

One issue with hirefire is they request account level oauth tokens that essentially give them ability to do anything with your apps, where Rails Autoscaling worked with us to create a partnership and integrate with our “official” add-on APIs that limits security concerns and are scoped to the application that’s being scaled.

Part of the reason for hirefire working the way it does is historical, but we’ve supported the endpoints they need to scale for “official” partners for years now.

A lot of heroku customers use hirefire so please don’t think I’m spreading FUD, but you should be aware you’re giving a third party very broad rights to do things to your apps. They probably won’t, of course, but what if there’s a compromise?

“Official” add-on providers are given limited scoped tokens to (mostly) only the actions / endpoints they need, minimizing blast radius if they do get compromised.

You can read some more discussion at that thread.

2 thoughts on “Rails auto-scaling on Heroku

  1. This is a well-done and fair comparison. I appreciate the write-up and feedback.

    Thanks also for the correction regarding multiple worker processes. That’s on me for not having it documented (which I have since updated).

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