May I ask the question about memcache?

Ask the hero, memcache the question
I set a key, if not write the expiration time, what is the default?

There is also the assumption that I set the expiration after 10 minutes, but in the middle I need to change the value. But I also want him to expire in the first 10 minutes, which means I just want to change his value, when it expires or when. What should I fill out at this time of the set expiration?
------Solution--------------------
1, by default, never expires, but may be deleted due to memory pressure
2, the Memcache agreement does not take this expiration time, but can use a workaround, that is, the expiration time of this key is also stored, the new setting to remove this value for the calculation

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