Counter implementation

Rails
1

Greetings all,

I have a model which holds a counter field, the field have to be
incremented frequently to record the access frequency of the specific
object:

@obj = Temp.find(id)
@obj.counter += 1
@obj.save

The implementation seems quite simple, but I am worrying about the
race condition (if there are any, I do not know whether rails is
thread-safe) since these statements may be interleaved. Could anyone
help about this? Thanks very much!

Difei

2

You should be doing it like this:

Temp.increment_counter( :counter, id )

This will increment the property “:counter” for the model Temp with
“id”, this column must have a default value of 0 (zero) or else this
call won’t work.

On Wed, Sep 10, 2008 at 12:59 PM, Difei Z.
[email protected] wrote:

–
Maurício Linhares
http://alinhavado.wordpress.com/ (pt-br) | http://blog.codevader.com/
(en)
João Pessoa, PB, +55 83 8867-7208

3

On Wed, Sep 10, 2008 at 11:59 AM, Difei Z.
[email protected] wrote:

The implementation seems quite simple, but I am worrying about the
race condition (if there are any, I do not know whether rails is
thread-safe) since these statements may be interleaved. Could anyone
help about this? Thanks very much!

Difei

You’re correct to worry about a race condition, but for the wrong
reason. Rails deployments up to now and for a while yet into the
future are clusters of mongrels / thins / etc, and as such nothing is
threaded. The race condition in who (as in, which Mongrel) gets to
save to the database first. As for protecting against this, you’ll
need to make sure to lock the row you working with, update the
counter, save, then unlock. You’ll want to do this as quick as
possible of course, but it still could cause some slowdown if a lot of
processes are trying to do the same thing.

If there are better ways around this, I’d definitely be interested in
hearing them.

Jason

4

Jason R. wrote:

On Wed, Sep 10, 2008 at 11:59 AM, Difei Z.
[email protected] wrote:

The implementation seems quite simple, but I am worrying about the
race condition (if there are any, I do not know whether rails is
thread-safe) since these statements may be interleaved. Could anyone
help about this? Thanks very much!

Difei

You’re correct to worry about a race condition, but for the wrong
reason. Rails deployments up to now and for a while yet into the
future are clusters of mongrels / thins / etc, and as such nothing is
threaded. The race condition in who (as in, which Mongrel) gets to
save to the database first. As for protecting against this, you’ll
need to make sure to lock the row you working with, update the
counter, save, then unlock. You’ll want to do this as quick as
possible of course, but it still could cause some slowdown if a lot of
processes are trying to do the same thing.

If there are better ways around this, I’d definitely be interested in
hearing them.

Maurício proposed the increment_counter method, may I assume that is
atomic?

Jason

5

On 10 Sep 2008, at 17:04, Maurício Linhares wrote:

You should be doing it like this:

Temp.increment_counter( :counter, id )

This will increment the property “:counter” for the model Temp with
“id”, this column must have a default value of 0 (zero) or else this
call won’t work.

this works because it does

update foos set counter = counter + 1

and then it’s the databases problem to worry about concurrent queries
and so on.
If what you showed was a simplification of what you’re actually doing
(and you can’t reduce it to increment_counter) then you’ll need to
look into locking (either optimistic or pessimistic)

Fred

6

On Wed, Sep 10, 2008 at 1:42 PM, Difei Z.
[email protected] wrote:

Maurício proposed the increment_counter method, may I assume that is
atomic?

This method does something like this:

"update model_table_name set counter_column = counter_column + 1 where
id = ‘your row id’ "

So, it’s up to your database (and the isolation level that it’s using)
to handle the atomicity of this call.

Put simply, it will usually work, unless your database has some
serious concurrency issues.

–
Maurício Linhares
http://alinhavado.wordpress.com/ (pt-br) | http://blog.codevader.com/
(en)
João Pessoa, PB, +55 83 8867-7208

7

Maurício Linhares wrote:

You should be doing it like this:

Temp.increment_counter( :counter, id )

This will increment the property “:counter” for the model Temp with
“id”, this column must have a default value of 0 (zero) or else this
call won’t work.

Thanks a lot! This look like what I am looking for.

On Wed, Sep 10, 2008 at 12:59 PM, Difei Z.
[email protected] wrote:

–
Maur�cio Linhares
http://alinhavado.wordpress.com/ (pt-br) | http://blog.codevader.com/
(en)
Jo�o Pessoa, PB, +55 83 8867-7208

8

Maurício Linhares wrote:

On Wed, Sep 10, 2008 at 1:42 PM, Difei Z.
[email protected] wrote:

Maur�cio proposed the increment_counter method, may I assume that is
atomic?

This method does something like this:

"update model_table_name set counter_column = counter_column + 1 where
id = ‘your row id’ "

So, it’s up to your database (and the isolation level that it’s using)
to handle the atomicity of this call.

Put simply, it will usually work, unless your database has some
serious concurrency issues.

yeah, as you and Fred said, I noticed the SQL UPDATE statement in the
log.

–
Maur�cio Linhares
http://alinhavado.wordpress.com/ (pt-br) | http://blog.codevader.com/
(en)
Jo�o Pessoa, PB, +55 83 8867-7208

9

One thing to notice is that a loaded model WILL NOT be updated by this
call, if you have a model loaded and call this method to perform the
increment, the model that is already loaded will not be updated, you
will have to call “reload” on it to get the latest counter value.

On Wed, Sep 10, 2008 at 1:53 PM, Difei Z.
[email protected] wrote:

yeah, as you and Fred said, I noticed the SQL UPDATE statement in the
log.

–
Maurício Linhares
http://alinhavado.wordpress.com/ (pt-br) | http://blog.codevader.com/
(en)
João Pessoa, PB, +55 83 8867-7208

10

Maurício Linhares wrote:

One thing to notice is that a loaded model WILL NOT be updated by this
call, if you have a model loaded and call this method to perform the
increment, the model that is already loaded will not be updated, you
will have to call “reload” on it to get the latest counter value.

Thanks for the information.

On Wed, Sep 10, 2008 at 1:53 PM, Difei Z.
[email protected] wrote:

yeah, as you and Fred said, I noticed the SQL UPDATE statement in the
log.

–
Maur�cio Linhares
http://alinhavado.wordpress.com/ (pt-br) | http://blog.codevader.com/
(en)
Jo�o Pessoa, PB, +55 83 8867-7208