get_multi / set_multi atomic?

Question

In the official FAQ of Memcached i read:

"All individual commands sent to memcached are absolutely atomic."

However this is still unclear to me when it comes to get_multi and set_multi. I'd like to know whether get_multi and set_multi are atomic in the following sense:

• All writes performed by set_multi will be performed together atomically.
• All reads performed by get_multi will be performed together atomically.

For example these situations should be impossible:

1)

• Initially contents of the cache is {'a': 0, 'b': 0}
• machine A calls set_multi({'a': 1, 'b': 1})
• machine B calls get_multi(['a', 'b']) and receives {'a': 1, 'b': 0}

2)

• Initially contents of the cache is {'a': 0, 'b': 0}
• machine A calls `set({'a': 1})
• machine A calls `set({'b': 2})
• machine B calls get_multi(['a', 'b']) and receives {'a': 0, 'b': 2}

This question is just so important for my design, that I thought I'd better ask for confirmation.

Answer 1

As I read this section, get_multi issues multiple requests that run in parallel, the idea being that for large requests, get_multi allows the total amount of time to get all results to be reduced. I don't see any guarantee or mention that the independent requests, done together, are collectively atomic. The same rule likely applies to set_multi (i.e. the individual requests are atomic, but the collection of them is not).

There also appears to be no mention of transactions.