How about UPDATE impressions SET count = count+1 WHERE id=? .. I would guess this is as atomic as you can get with SQL. Forgive me if my SQL is invalid, it's a little late at night, but you get the point.

I can't say that I'm 100% sure if this statement is guaranteed to be atomic, and if I were really concerned about that, I would approach the problem differently:

• Create a table for timestamped impression transactions. Every impression gets a single row in this table with relevant data plus a timestamp. This will be a volatile and dynamic table, with a lot of update and insert activity.
• Have a batch job run every 5 to 10 minutes. It should collect (then delete) all the transactions stamped before the current time. It calculates the totals and adds them appropriately to your existing table that keeps track of total impressions. This batch job does not run concurrently with anything else, and it should be the only process that modifies the table with the totals. There aren't any race conditions, because if anything is added to the transaction table between the time the batch job reads and deletes, the timestamp of the new record will not be in the affected range.

blokhead

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.