Caching unexpected behaviour

rubyonrails-talk
1

Hi

I have this RoR (2.3.16) app that has a simple agenda tool. Each customer has their own individual database, and I have one central database with login, passwd and the customer database name.

Something like this:

  • main_database:

  • users_table:

  • login_field

  • passwd_field

  • database_name_field

  • customer_db_1:

  • tables …

  • customer_db_2:

  • tables …

So, after user authentication, I set the correspondent database connection according each customer.

The problem is:

Customer 1 sometimes see records from customer 2, even being on different databases.

I’m totally lost here… I just suspect that this could be a database connection cache issue, since the SQL query is the same for every customer (ie.: select * from agendas;) but the connection string is different for each one.

Most of the time everything runs ok… but some customers are getting this strange behaviour.

Any idea of what could be happening?

Current env:

Passenger: 4.0.21

Apache/2.2.22 (Ubuntu)

ruby 1.8.7 (2012-02-08 patchlevel 358)

Thank you very much in advance for any help!!

Best regards

Paulo

2

Put some debug code in to log stuff in order that you can work out what it going wrong.

Colin

3

Hi Colin

thanks for your reply!! I’ll try it and see if I can find more info to help me…

I was also wondering where can I find how Rails (specially 2.3.x) deals with sql result caching. This could help to diagnose a possible root cause.

Best regards

4

Hi Colin

thanks for your reply!! I’ll try it and see if I can find more info to help me…

I was also wondering where can I find how Rails (specially 2.3.x) deals with sql result caching. This could help to diagnose a possible root cause.

There is an sql cache in Rails, however it is scoped to the current request - you get a fresh cache for each request. When you say that you are setting the DB connection after authentication, what exactly are you doing (and by authentication do you mean when the user signs in or on every request once you have got a user_id from a session/cookie?) ?

Fred

5

Hi Fred, thanks for your reply!

I’m using authlogic gem to authenticate my users.

Inside my Application Controller I created the following code:

prepend_before_filter :database_connection

and this method has the following code:

ActiveRecord::Base.establish_connection({:adapter => “mysql”, :database => “#{current_user.dbname}”, :username => “x”, :password => “x”, :host => “x”, :encoding => “utf8”})

So, for each request I get the current authenticated user database name and set the connection to customer database.

Imagine I have 2 different users requesting the same view: mydomain.com/agenda/view

Since each user had authenticated using their own username/passwd, each request will establish a different connection to the database.

But if Rails doesn’t cache database queries for different time requests, I’m lost about why this issue is happening.

Do you have any clue?

Best regards

6

Are you checking for an error return here in case it fails for some reason, leaving the connection as it was for the last request? Also I presume you are not silently absorbing any exception raised there.

Colin

7

Hi Colin,

I wasn’t able to find any exception referring to this specific line code.

But, in case of an exception, Rails would return a 500 error page and not the correct page with someone else records. Am I right?

Best regards

8

Hi Colin

first of all, thank you very much for your help!! You found the root cause of my problem.

After user session expired, the code:

ActiveRecord::Base.establish_connection({:adapter => “mysql”, :database => “#{current_user.dbname}”, :username => “x”, :password => “x”, :host => “x”, :encoding => “utf8”})

didn’t work, but also didn’t throw any exception. So the requested view got the last opened database connection to retrieve the records. Of course, the records retrieved was from another user (the last successful establish_connection attempt).

Thank you all for the support!!

Best regards

9

I would wonder how this plays with connection pooling. In any case, since you're explicitly opening the connection, I'd add an after_filter to explicitly *release* the connection after using it.

And it might be worth testing whether setting the connection pool size to 1 changes the behavior. Not that that's necessarily a great long-term "solution", but for troubleshooting purposes. :slight_smile:

HTH,

10

I expect it returned an error (hence my suggestion to check for error return). If you look at the docs for establish_connection you will see that it may throw an exception, but under other circumstances will return an error.

Colin

11

Paulo,

You might want to check out Ryan Bigg’s "Multitenancy With Rails"