I'm making a new web app (Rails 6) that I would like to use websockets with. This is the first time I've attempted to use ActionCable with Rails. Ideally to start I would toss the app up on Heroku, since it is generally a fast and convenient way for me to get started with an app project.
If my app is successful I wouldn't be surprised if it had 200 concurrent users. This doesn't seem like a lot to me. I assume many of them would have open other tabs, so my estimate would be roughly 500 websocket connections for 200 users. If my site were more successful, I could see this going to 500 active users so more than 1000 websocket connections.
After preliminary research, it is recommended to use Redis on Heroku for ActionCable. I was dismayed to see that a Redis plan (Redistogo) with 500 websocket connections would cost $75/month and 1000 connections $200/month. More than dismayed, really. Shocked. Am I missing something? Why is this so expensive? It seems like these plans are also linked to large amounts of storage space, something that (my understanding) ActionCable doesn't even need.
I've seen that it's also possible (in theory) to configure ActionCable to use Postgresql as its adapter, though looking online I didn't see any examples of anyone doing this successfully on Heroku. So my questions are these:
Is it possible to use ActionCable on Heroku without paying through the nose? I assume this would mean Postgresql adapter...
How many connections could such a setup handle, what are the limitations, bottlenecks, etc?
I wasn't able to find much information about ActionCable and Postgresql, except that the number of pools has to match. I assume the pool number is not the same as the websocket connection count?
Thanks for any info...
Related
I have a Rails application that I've been developing for quite some time. All this time I tested it locally and on a DEV server. On the DEV server, next to the deployed application, there is also a PG database. And there were no problems with connections. I think there is simply no connection limit, or it is too high - not so important.
Today I started doing deployment to the PROD server. It is similar in power to that for DEV, but BD is already in the DO Database. By the way, the servers themselves are also located in DigitalOcean.
The problem is that DO Database has a limit of 20 connections. And as far as I understand, exceeding this limit - the Rails application gives an error:
ActiveRecord::ConnectionNotEstablished (FATAL: remaining connection slots are reserved for non-replication superuser connections)
The most obvious option is to reduce the number of requests on page load. But this still did not solve the problem if, for example, the number of users increases.
Can you please tell me which way to look? Are there any solutions to the problem other than updating the DO Database power?
You might want to try PG Bouncer (never tried it though, so i can't really tell how it will impact the app).
I have a rails app running on Heroku that makes use of threads, and it occasionally runs into database connection errors. Is this just because I am accessing the database within the threads or does each thread automatically open a database connection? I would like to learn more about threading in rails, and any resources are appreciated.
This question varies largely depending on how many instances you have running, how many requests your receiving, and more importantly your database. Databases can and will have a maximum number of concurrent connections. You can read more about Heroku/concurrent connections here in the Heroku official documentary, it's probably more informative than what I can tell you in a single comment.
That being said, your question was a little vague and it's hard to figure out what's going on. Can you tell us a little more about what error you're getting (like the specific error) and maybe a small backtrace? Are you getting these errors on the same pages or different pages? Would you say your site is particularly high traffic?
I'm experimenting with Rails 4 ActionController::Live and Server Sent Events. I'm using MRI 2.0.0 and Puma.
For what I can see, each connected client keeps an active connection to the server. I was wondering if it is possible to leverage SSEs without keeping all response streams running.
Puma manages multiple connections using threads, and I imagine there is a limit to the number of cuncurrent connections.
What if I want to support a real-world scenario with thousands of clients registering to my Rails app for SSE events?
Is there any example?
Also, I usually run Rails app servers behind an nginx reverse proxy. Would it require any particular setup?
The way that SSEs are built is by the client opening a connection to the server, which is then left open until the server has some data to send. This is part of the SSE spec, and not a thing specific to ActionController::Live. It's effectively the same as long-polling, but with the connection not being closed after the first bit of data is returned, and with the mechanism built into the browser.
As such, the only way it can be implemented is by having multiple open client connections to the webserver which sit there indefinitely. As to what resources are required to deal with them, I'm not sure, as I've not yet tried to benchmark this, but it'll need enough servers for Puma to keep open thousands of connections if you have that many users with a page open.
The default limit for puma is 16 concurrent connections. Several blogs posts about setting up SSEs for Rails mention upping this to a larger value, but none that I've found suggest what this higher value should be. They do mention that the number of DB connections will need to be the same, as each Rails thread keeps one running. Sort of sounds like an expensive way to run things.
"Run a benchmark" is the only answer really.
I can't comment as to reverse proxying as I've not tried it, but as SSEs are done over standard HTTP, I shouldn't think it'll need any special setup.
I got the above error message running Heroku Postgres Basic (as per this question) and have been trying to diagnose the problem.
One of the suggestions is to use connection pooling but it seems Rails has this built in. Another suggestion is that the app is configured improperly and opens too many connections.
My app manages all it's connections through Active Record, and I had one direct connection to the database from Navicat (or at least I thought I had).
How would I debug this?
RESOLUTION
Turns out it was an Heroku issue. From Heroku support:
We've detected an issue on the server running your Basic database.
While we pinpoint this and address it, we would recommend you
provision a new Basic database and migrate over with PGBackups as
detailed here:
https://devcenter.heroku.com/articles/upgrade-heroku-postgres-with-pgbackups
. That should put your database on a new server. I apologize for this
disruption – we're working to fix this issue and prevent it from
occurring in the future.
This has happened a few times on my app -- somehow there is a connection leak, then all of a sudden the database is getting 10 times as many connections as it should. If it is the case that you are getting swamped by an error like this, not traffic, try running this:
heroku pg:killall
That will terminate all connections to the database. If it is dangerous for your situation to possibly cut off queries be careful. I just have a rails app, and if it goes down, losing a couple queries is not a big deal, because the browser requests will have looooooong since timed out anyway.
You might be able to find why you have so many connections by inspecting view pg_stat_activity:
SELECT * FROM pg_stat_activity
Most likely, you have some stray loop that opens new connection(s) without closing it.
To save you the support call, here's the response I got from Heroku Support for a similar issue:
Hello,
One of the limitations of the hobby tier databases is unannounced maintenance. Many hobby databases run on a single shared server, and we will occasionally need to restart that server for hardware maintenance purposes, or migrate databases to another server for load balancing. When that happens, you'll see an error in your logs or have problems connecting. If the server is restarting, it might take 15 minutes or more for the database to come back online.
Most apps that maintain a connection pool (like ActiveRecord in Rails) can just open a new connection to the database. However, in some cases an app won't be able to reconnect. If that happens, you can heroku restart your app to bring it back online.
This is one of the reasons we recommend against running hobby databases for critical production applications. Standard and Premium databases include notifications for downtime events, and are much more performant and stable in general. You can use pg:copy to migrate to a standard or premium plan.
If this continues, you can try provisioning a new database (on a different server) with heroku addons:add, then use pg:copy to move the data. Keep in mind that hobby tier rules apply to the $9 basic plan as well as the free database.
Thanks,
Bradley
I'm thinking of deploying a small Rails app on Heroku. In an effort to save money, I'd like my app to use an external database (to which I have free access), rather than a Heroku-hosted database. The trouble is that the free database only accepts local connections. To access it from Heroku, I'd need to do so via an SSH tunnel.
Is it possible for a Heroku app to persist its data in an external DB accessed via SSH? If so, how?
(For bonus points, here's a second question: is this a good idea? On the one hand, this scheme would save me from paying for a Heroku database. On the other hand, it means having to encrypt all my database traffic. I imagine that this would massively slow down my web dynos, and reduce the number of requests they can serve. Would the money I save on the database get used up paying for more dynos? Am I likely to come out ahead by doing this?)
Yes, you can.
It is possible to set up a tunnel on Heroku to an external database.
You don't want to do it for the reasons the O.P. mentions (to avoid paying for a local database) for the reasons #sgrif mentions (it would be painfully slow and probably not really save anything)
But there are legitimate reasons for wanting to tunnel to an external database, for example if data is residing in a legacy system that you need to analyze.
Rather than simply repeat myself (it's long), here's a link to the recipe that worked for me: SSH tunneling from Heroku
No, and even if it was an option it's a really bad idea as you'd be adding massive latency to every request, since you'd for all intents and purposes have to open a new tunnel for every request.
Your best option is likely to use Heroku's development or starter tiers. The free development tier will work if your database is less than 10,000 rows. Their $15/mo starter tier works for up to 1,000,000 rows.