Build Rails app for production environment

[NathanielRN]

Description

In our Soak Tests, we found out that a Rails app instrumented with OTel Ruby in development mode will use an unbounded amount of memory when receiving several requests.

CloudWatch showed a development Rails app using 11 GB of memory after just 2 hours:

We asked on the OTel Ruby CNCF Slack channel and got the great feedback that we should use a production environment.

This PR does that, and when we did that the Memory Usage was less than 1 GB after 5 hours:

However, to allow for a production environment, the rails app requires a "secret_base_key". Otherwise it will spam the log output with warnings which is really troublesome.

To solve this, we added dummy credentials which don't do anything. Because this is an example, we directly commit the security credentials in the Dockerfile but this is NOT GOOD PRACTICE FOR REAL PRODUCTION ENVIRONMENTS. We allow it to be like this because we want this demo example to work.

You can confirm the credentials work and view the encoded contents of by doing the following command:

$ cd sample-apps/manual-instrumentation/ruby-on-rails
$ RAILS_MASTER_KEY=<KEY_IN_SAMPLE-APP_DOCKERFILE> bin/rails credentials:edit

This will show the following contents:

# NOTE: DO NOT USE THIS IN PRODUCTION ENVIRONMENTS, WE ONLY SET THIS TO SIMULATE A REAL RAILS APP.

# Used as the base secret for all MessageVerifiers in Rails, including the one protecting cookies.
secret_key_base: DO_NOT_STORE_A_SECRET_THIS_IS_JUST_FOR_AN_EXAMPLE

We cannot use something like RAILS_MASTER_KEY=BOGUS_KEY but the tests immediately fail to start the app 😕:

app_1 | 2022-02-04 22:18:51 +0000 Rack app ("GET /outgoing-http-call" - (172.18.0.4)): #<ActiveSupport::MessageEncryptor::InvalidMessage: ActiveSupport::MessageEncryptor::InvalidMessage>

[srprash]

Thanks for figuring this out and making it work!
Just out of curiosity, if the RAILS_MASTER_KEY is just a dummy credential, does it need to be in a certain format to work or can we simply replace it with something like "dummy_credential"?

[NathanielRN]

@srprash Thanks for that prompt!!

I actually did test this with something like RAILS_MASTER_KEY=BOGUS_KEY but the tests immediately fail to start the app 😕:

app_1             | 2022-02-04 22:18:51 +0000 Rack app ("GET /outgoing-http-call" - (172.18.0.4)): #<ActiveSupport::MessageEncryptor::InvalidMessage: ActiveSupport::MessageEncryptor::InvalidMessage>

It's a good idea to post this in the README and PR description though so I've done that!

[srprash]

LGTM!