Shopify not getting rates from custom CarrierService - ruby-on-rails

Question

I'm building a private Shopify app that returns custom shipping rates. Following the API docs, and with some help from this tutorial, I created a functional, proof-of-concept that returns some sample shipping rates in my store.
However, that proof-of-concept app is built in PHP, and the final version must be in Ruby on Rails. So I created a Rails app that returns exactly the same content as the PHP app—but for some reason the rates simply don't show up in the Shopify backend.
The only thing I notice that's different is the HTTP headers (I've tried monkeying around with them to match the PHP app, but to no avail). Is there anything obvious I'm missing here?
Here's a comparison of the HTTP response headers:
PHP:
Connection: Keep-Alive
Content-Encoding: gzip
Content-Length: 186
Content-Type: text/html; charset=UTF-8
Date: Tue, 16 May 2017 13:00:30 GMT
Keep-Alive: timeout=5, max=100
Server: Apache/2.4.18 (Ubuntu)
Vary: Accept-Encoding
Rails:
Cache-Control: max-age=0, private, must-revalidate
Connection: keep-alive
Content-Encoding: gzip
Content-Type: text/html; charset=utf-8
Date: Tue, 16 May 2017 12:57:38 GMT
ETag: W/"ce885edaa10636b3b7459dca958f44dd"
Server: nginx
Transfer-Encoding: chunked
Vary: Accept-Encoding
X-Request-Id: 54e2575e-c86a-4f44-a315-d0a3fbbc13f9
X-Runtime: 0.616974
Again, Shopify handles the PHP response fine, but fails silently on the Rails response. Is there anything in that second (Rails) block that Shopify might be rejecting?
Shopify unfortunately provides no error log or way to debug this type of issue—either they pull the rates from your app or not.
Here is my Rails rates_controller.rb:
class RatesController < ApplicationController
def index
ups_rates = {
rates: [
{
service_name: 'Endertech Overnight',
service_code: 'ETON',
total_price: '000',
currency: 'USD',
min_delivery_date: (DateTime.now + 1.days).strftime('%F %T %z'),
max_delivery_date: (DateTime.now + 2.days).strftime('%F %T %z')
},
{
service_name: 'Endertech Regular',
service_code: 'ETREG',
total_price: '000',
currency: 'USD',
min_delivery_date: (DateTime.now + 3.days).strftime('%F %T %z'),
max_delivery_date: (DateTime.now + 7.days).strftime('%F %T %z')
}
]
}
# Tested returning both application/json (default) or text/html
#render json: ups_rates
render body: ups_rates.to_json, content_type: "text/html"
end
end
I suspected maybe Shopify was caching very aggressively, so I've also tried destroying and recreating both my private app and the carrier service, as well as changing the callback_url. So far nothing has had any effect.

Answer 1

After a whole lot of digging, I found the answer: Shopify requests rates via POST, but my app was only responding to GET.
As a quick fix, I created a static route in routes.rb:
Rails.application.routes.draw do
post 'customrates', to: 'rates#index'
end
In retrospect, this was clearly stated in the documentation, I just hadn't noticed (after all, a rate request intuitively seems like a GET action).
For anyone else encountering this issue, try sending a POST via CURL to your callback_url, just to verify it's returning what it should:
curl -X POST http://yourcallbackurl.com