Unique Serial Number Generation and Locking

Rails
1

I need to generate a unique serial number in numerical order, but only
unique for each user.

For example, each user can create a new credit card and each new credit
card
must start at a predetermined number and increment by 1 for each new
credit
card for that user. Thus, credit card numbers are unique for a user but
not
unique for the CreditCard table. And yes, it is absolutely critical that
credit card numbers are always unique for each user.

http://apidock.com/rails/ActiveRecord/Locking/Pessimistic

I’m using PostgreSQL.

Looking through the docs it doesn’t appear that I can use either
pessimistic
locking or optimistic locking. Maybe I’m wrong?

Is my only option to lock the table, find the highest credit card
number,
add 1, update the new credit card number, save, and release the table
lock?

Karl

2

On Jan 31, 2011, at 9:38 PM, Karl S. wrote:

I need to generate a unique serial number in numerical order, but only
unique for each user.

if worst comes to worst, you can always use one sequence per user:

3

Karl S. wrote in post #978730:

I need to generate a unique serial number in numerical order, but only
unique for each user.

For example, each user can create a new credit card and each new credit
card
must start at a predetermined number and increment by 1 for each new
credit
card for that user. Thus, credit card numbers are unique for a user but
not
unique for the CreditCard table. And yes, it is absolutely critical that
credit card numbers are always unique for each user.

I hope this “credit card” scenario is a contrived example. It seems to
me that it would be critical for the credit card number to be unique
across ALL users.

http://apidock.com/rails/ActiveRecord/Locking/Pessimistic

I’m using PostgreSQL.

Looking through the docs it doesn’t appear that I can use either
pessimistic
locking or optimistic locking. Maybe I’m wrong?

You are correct. Row level locking is useless to prevent the duplication
of values between rows due to the race condition during a SQL INSERT.

A unique index (possibly across multiple fields) would prevent such
duplication.

Is my only option to lock the table, find the highest credit card
number,
add 1, update the new credit card number, save, and release the table
lock?

It might be possible for you to use a sequence. But, in your case you
would need a separate sequence for each user if I understand you
correctly. Although this might work it may not be appropriate in your
case.

My concern about locking the entire table would be that if something
went wrong then you might end up in a state where your entire table is
stuck in a locked state. I don’t know for sure if that’s an issue with
PostgreSQL, but something to consider.

There may also be other considerations if you ever have a need to use
more that one database backend for the purposes of scaling.

I would also try to avoid having to use a “max value” query every time
you needed to find the next number in sequence. You could instead create
a table that contains a foreign key to your users table with the next
sequence value stored there. You would then increment that value for
each insert similar to how database sequences work. This way you should
only have to be concerned about concurrent access to the table used for
sequencing.

Good luck, I hope this helps at least a little.

4

On Monday, January 31, 2011 2:16:02 PM UTC-7, Ruby-Forum.com User wrote:

I hope this “credit card” scenario is a contrived example. It seems to
me that it would be critical for the credit card number to be unique
across ALL users.

Yes, it’s made up illustrative purposes.

You are correct. Row level locking is useless to prevent the duplication

of values between rows due to the race condition during a SQL INSERT.

A unique index (possibly across multiple fields) would prevent such
duplication.

Hmm, so you are saying I could use a multi-field index (:user_id,
:credit_card_number). Then, during inserts if a duplicate
:credit_card_number is attempted a ActiveRecord::StatementInvalid would
be
generated, thus giving me an indication to try another
credit_card_number.

Sounds workable, but is it realistic?

PostgreSQL: Documentation: 8.1: CREATE SEQUENCE

I don’t like that, but nice to know.

My concern about locking the entire table would be that if something
went wrong then you might end up in a state where your entire table is
stuck in a locked state. I don’t know for sure if that’s an issue with
PostgreSQL, but something to consider.

Which concerns me as well.

I like that idea. That way I’m only locking that sequence table. This
way I
could use row level (pessimistic) locking.

So, have you are anyone else tried this?

5

Karl S. wrote in post #978748:

Hmm, so you are saying I could use a multi-field index (:user_id,
:credit_card_number). Then, during inserts if a duplicate
:credit_card_number is attempted a ActiveRecord::StatementInvalid would
be
generated, thus giving me an indication to try another
credit_card_number.

Sounds workable, but is it realistic?

Yes, this is really common in database design. There was a time when
composite primary keys were common and this is how those were enforced
to be unique. However, the use of the technique is not limited to
composite keys.

PostgreSQL: Documentation: 8.1: CREATE SEQUENCE

I don’t like that, but nice to know.

Some databases use sequences exclusively for generating primary keys. As
opposed to providing auto-incrmenting column types… Just FYI.

My concern about locking the entire table would be that if something
went wrong then you might end up in a state where your entire table is
stuck in a locked state. I don’t know for sure if that’s an issue with
PostgreSQL, but something to consider.

Which concerns me as well.

I like that idea. That way I’m only locking that sequence table. This
way I
could use row level (pessimistic) locking.

So, have you are anyone else tried this?

Yes, I have used this technique for generating serial sequences on a
number of projects. And you are correct that row level locking is used
to manage concurrent access.