Using memcache.add() instead of set()

Question

Usually I do:

if not memcache.get('mykey'):
   memcache.set('mykey', item)

However, today I saw memcache.add(), which appears to add an item only if it doesn't already exist. So is this equivalent to the code I have above? Can I just replace the code above with memcache.add()?

Also, and more importantly, I'm using AppStats, and under RPC Call Traces, I get to see if my request calls memcache.set() or get() or datastore.put() or get(). When using the 2 lines of code above, I don't see anything for memcache.set(), which is expected. However, using only memcache.add() (without checking if the item already exists) always calls memcache.set(), even though memcache.add() returned false (meaning a new item was not inserted). Why is this the case?

@PaoloMoretti thanks for the link..what exactly am I looking for.. — Snowman
– Snowman, Commented Nov 5, 2012 at 15:27
This question (or one similar) has already been asked on stackoverflow before, I would like to direct you too - stackoverflow.com/questions/2678339/… - serverfault.com/questions/291681/add-vs-set-in-memcached — Matt Seymour
– Matt Seymour, Commented Nov 5, 2012 at 16:02
Thanks. My main question is the second part of the question though. It doesn't make sense that using add() still shows a RPC call for set(). That's what I'm mostly confused about.. — Snowman
– Snowman, Commented Nov 5, 2012 at 16:09

Nick Johnson · Accepted Answer · 2012-11-06 12:08:42Z

5

Your current code has a race condition: between checking for the presence of a value in memcache and inserting it, another process could have inserted a value, which you'll now overwrite. Using memcache.add does not suffer from this race condition.

I'm not sure what you mean by your second question; calling memcache.add should result only in an add call, never a set call. Can you include the code you're running in that case?

answered Nov 6, 2012 at 12:08

Nick Johnson

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1 Comment

Zach Young Over a year ago

To be clear, if you just changed set() to add() there's still the possibility of a key collision, it's just that with add() you'll know about it via its return value?

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