How to protect against DoS?

Hello, folks.

We have a system that is used by all students on the university. Problem
is,
when the system is overloaded, people keep strinking F5, and eventually
things become too slow to be usable (sometimes, even passenger dies).

How to protect against these cases?

Thanks in advance,
Mauricio

Maurício Szabo wrote:

How to protect against these cases?

Thanks in advance,
Mauricio

Advise administration that they will need to cut down on the number of
students they enroll each year.

It that doesn’t appeal to you, then please resubmit your post with a
little more information including but not limited to caching details,
profile results, cluster design, deployment(s), memcache, etc…

hth

ilan

There’s a great screencast series named “Scaling Rails” that may give
you ideas.

But yeah, as the previous poster implies, your problem could be
anywhere. You may want to try to identify where the main bottleneck is
in your application (whether it’s the rails server or the db server)
and how you can optimize your code or your deployment to reduce the
load.

On Oct 9, 7:38 am, Ilan B. [email protected]

If it is a problem with your db server reaching its limit, you may be
able to optimize your queries to reduce the number required for each
request.

Mauricio,
As one poster suggested, Google ‘Scaling Rails’ for a good overview.
My first option would be to look for page caching opportunities.
Everywhere.
Best,
BrianP.

If the system slows down because of the amount of users you will need
more horsepower or limit the amount of concurrent users that use the
system. In apache you can set the MaxClients setting if you are using
a worker MPM. But you need to find where the bottleneck is so that you
can fix it.

I like to start with the database first. Tune the database to the max
then look at rails then if you still have issues start limiting at
Apache or upgrade/add servers.

If you are using MySQL…check your slow query log
http://dev.mysql.com/doc/refman/5.1/en/slow-query-log.html

This can also help you find slow actions:
http://github.com/jtrupiano/slow-actions

Ok, thanks for the help. The problem is, there is no “slow action”,
it’s just the login page, it sends one query to the database (10
records total), then redirects to a list of matters (it’s just a
"Matter.find(:all)). The problem I’m getting is precisely this: there
is nowhere to optimize…

The application is a “Matter chooser” (something like this, sorry,
english isn’t my primary language): Students login into a page, choose
a couple of matters, and then submit a form. Problem is, each matter
has a limited number of students, so the first one that submits gets
the matter. So, the application stays online for two days, and in the
first 3 hours of the first day, there are a lot of students (200+)
that tries to login, and when passenger timeouts their connections,
keep pressing F5 to update the page… and that is when things get
complicated, because everyone that already logged-in simply gets a
timeout screen. It’s no use trying to optimize anything, because this
happens only in the first 3 hours, for example - after this, things go
back to normal…

Suggestions? And, somebody said I could limit simultaneous connections
in passenger, how do I do this?

Thanks!

Mauricio Szabo wrote:

Ok, thanks for the help. The problem is, there is no “slow action”,
it’s just the login page, it sends one query to the database (10
records total), then redirects to a list of matters (it’s just a
"Matter.find(:all)). The problem I’m getting is precisely this: there
is nowhere to optimize…

How many “matters” are we talking about 10? 1000?.. Are they cached?
Why are you using Matter.find(:all)? Can you scope your query to only
find the ones that may be of interest for a given student?
ilan

About 100 matters. Also, there is no way I can scope the query (but,
with about 100 matters, I hardly think this is an issue).

On Oct 11, 8:55 pm, Ilan B. [email protected]

Mauricio Szabo wrote:

About 100 matters. Also, there is no way I can scope the query (but,
with about 100 matters, I hardly think this is an issue).

On Oct 11, 8:55�pm, Ilan B. [email protected]

So either keep the matters in cache or load them up on application start
and keep them around in memory… You don’t need to get the matters on
each request if you believe that is where the bottleneck lies…

ilan

Ilan B. wrote:

Mauricio Szabo wrote:

About 100 matters. Also, there is no way I can scope the query (but,
with about 100 matters, I hardly think this is an issue).

On Oct 11, 8:55�pm, Ilan B. [email protected]

So either keep the matters in cache or load them up on application start
and keep them around in memory… You don’t need to get the matters on
each request if you believe that is where the bottleneck lies…

ilan

To make life a little clearer, a ‘matter’ is a “subject” or a “topic”
for a course… The bottleneck is that the students all want their first
choice classes so there’s a ridiculous amount of connections being made
and it’s overloading the web server.
I think.
And I only think that because this happened in my college when they
implemented online course selection.

It’s exactly the problem! How did they corrected this issue?

On Oct 12, 3:03 pm, Aldric G. <rails-mailing-l…@andreas-

Welcome to the pointy end of online course registration.

Cache the initial ‘courses’ results.

I don’t know how your institution has its data organized, but I’ll
assume that there are X Courses available. Cache either that initial
page, or cache the Matter.find() results so you aren’t pinging the DB
server for redundant information on every request.

I’ve a feeling that you’re stuck with your current design for now, so
the solution would be to throw more capacity at the issue inexpensively
by changing your web server params or clustering setup to deal with
peaks (max clients, DB connections, etc - Scaling Rails reference).

For the future:

Of course, what would work really well for the students would be to show
the available seats in any course when the course listing is returned,
which means that caching won’t help. But that would be a likely big
design change.

Another option (too late for this at the current time) is to implement a
priority queue for the students. Fourth year students are allowed to
register beginning at 9:00am - on the theory that they should be able to
get into classes first so they can meet graduation requirements. Third
year students are allowed to begin registering starting at 10:00am -
they are under the next strictest set of course requirements, second
year students at 11:00am, and first year students at 12:00. This
forcefully distributes your load a bit, though you’ll still see hourly
peaks.

Another option is to not show everyone everything. No student is
interested in all 100 courses. The odds are that a student has in mind
probably 8 courses (hopefully they are smart enough to plan ahead on the
chance that they won’t get into one of their favorite classes). Put a
search form up front so they can enter their 5 (or however many) classes
and get just those back. That return set could show the actual available
seats in the course at that moment. If a seat is available, the student
can try to get a seat in the course by checking Yes or Register or
whatever and submitting right away. If it’s already filled, the student
knows they have to make another selection.

What you’ll probably find is that each student searches initially for
just those 2 or 3 courses they really truly want, and make the rest of
their course decisions based on how those requests work out.

And given that some popular courses fill up fast, I’d encourage you to
investigate the concept of a “wait list” for courses. Even though the
course is full, a student may want to “wait list” the course just in
case someone already in the class changes their mind and drops out (all
the students are ‘gaming’ their schedules, trying to work out days and
times). If a seat does become available, then the first person on the
wait list should be offered the seat.

Good luck!

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