Caching unexpected behaviour


#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

On 19 November 2013 19:55, Paulo H. Leite de Castro
removed_email_address@domain.invalid wrote:

- passwd_field

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

Any idea of what could be happening?

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

Em quarta-feira, 20 de novembro de 2013 05h34min08s UTC-2, Colin L.
escreveu:


#4

On Wednesday, November 20, 2013 4:19:40 PM UTC, Paulo H. Leite de
Castro wrote:

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

Em quarta-feira, 20 de novembro de 2013 14h36min11s UTC-2, Frederick
Cheung
escreveu:


#6

On 20 November 2013 16:52, Paulo H. Leite de Castro
removed_email_address@domain.invalid wrote:

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

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

Em quarta-feira, 20 de novembro de 2013 14h58min40s UTC-2, Colin L.
escreveu:


#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

Em quarta-feira, 20 de novembro de 2013 15h09min18s UTC-2, Paulo
Henrique
Leite de Castro escreveu:


#9

On Wed, Nov 20, 2013 at 8:52 AM, Paulo H. Leite de Castro
removed_email_address@domain.invalid wrote:

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.

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,

Hassan S. ------------------------ removed_email_address@domain.invalid
http://about.me/hassanschroeder
twitter: @hassan


#10

On 20 November 2013 17:40, Paulo C. removed_email_address@domain.invalid wrote:

didn’t work, but also didn’t throw any exception.

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 B.s “Multitenancy With Rails”

https://leanpub.com/multi-tenancy-rails