Heroku nodejs app gets 503 error when connecting to heroku-redis - node.js

Question

I have a nodejs API app running on heroku and i get a service unavailable 503 error when this app tries to connect to heroku-redis or another redis server on redislabs.com (I tried both redis servers to see if it'd make a difference). I have a free heroku dev account being used for this nodejs API app and the heroku-redis add-on. Perhaps I need a static IP for the free heroku app to connect to heroku-redis? Not sure.
This is what i see when i look at my app's logs on heroku:
2022-02-05T18:30:24.003600+00:00 app[web.1]: > start
2022-02-05T18:30:24.003601+00:00 app[web.1]: > node app.js
2022-02-05T18:30:24.003601+00:00 app[web.1]:
2022-02-05T18:30:26.254765+00:00 heroku[web.1]: State changed from starting to up
2022-02-05T18:31:03.724181+00:00 app[web.1]: Starting RXNsetup app
2022-02-05T18:31:03.725096+00:00 app[web.1]: Start Redis connection
2022-02-05T18:31:33.716830+00:00 heroku[router]: at=error code=H12 desc="Request timeout" method=POST path="/run" host=rxnsetup.herokuapp.com request_id=c983d257-8773-4fff-a4d1-4b74341366b6 fwd="190.111.231.94" dyno=web.1 connect=0ms service=30176ms status=503 bytes=0 protocol=https
2022-02-05T18:30:00.000000+00:00 app[heroku-redis]: source=HEROKU_REDIS_CRIMSON addon=redis-symmetrical-62029 sample#active-connections=1 sample#load-avg-1m=1.105 sample#load-avg-5m=0.96 sample#load-avg-15m=0.6 sample#read-iops=0 sample#write-iops=0 sample#memory-total=15619136kB sample#memory-free=10625808kB sample#memory-cached=2390936kB sample#memory-redis=333744bytes sample#hit-rate=1 sample#evicted-keys=0
The first log statement above is in reference to another nodejs app that makes a POST HTTP request to the nodejs API app being run on heroku. I am able to connect to my redis server fine when i hit this same nodejs API app locally, but for some reason I get a 503 error when the same nodejs app runs in heroku. I have configured all of the same environment variables in heroku like i've done locally. I've been researching for awhile but am stuck with what could be causing this 503 error.
Update:
The root issue looked to be first that I had multiple instances of heroku-redis running, so I needed to promote one instance to be the primary one. Then another issue was that the REDIS_URL environment variable was being evaluated to undefined at runtime which was causing the 503 service errors I was getting.

Answer 1

I suspect you're looking at a stale connection string.
HEROKU_REDIS_<color> variables are normally only used if you have multiple deployments:
Heroku creates the REDIS_URL config var to store the location of the primary instance. In single-instance setups, your new instance is assigned a REDIS_URL. In cases where REDIS_URL exists, your instance is assigned a HEROKU_REDIS_<color> URL instead.
Normally, even if you have multiple instances, you would still connect to REDIS_URL, not HEROKU_REDIS_<color>. You can promote a HEROKU_REDIS_<color> URL to REDIS_URL by running
heroku redis:promote HEROKU_REDIS_<color>
Update your application to use REDIS_URL, review your addons to see if you have multiple Redis services provisioned, and decide which one(s) to keep. Then promote its variable to REDIS_URL as above.