counter implementation

rubyonrails-talk
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'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

3

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.

4

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.

5

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

Jason Roelofs 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?

7

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.

8

Maurício Linhares 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.

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.

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.