Alright, let's give a concrete example.
I'm still working on my game application.
Each gamesession holds 4 players and each player updates the shared 
gamession entity once every 2 sec.
Basically every time a player requests the gamesession entity, he/she needs 
the most up2date entity.

I don't quite understand what you mean with the durability of writes, as 
English is not my primary language.

Do you mean with durability the amount of time the entity lives in the 
memcache?
In that case: the entity should stay alive for no less then 6 minutes, after 
6 minutes it'l be deleted from the datastore anyway.

If you meant durability as in the likely hood of the request failing to 
update in memcache? 
That would be catastrophic i guess(however a few lines of could should 
change that).

Now in the case of delaying writes to the datastore(with a Task Queue i 
presume?)
A risk would be that:

Player0 sends an update.
Memcache is updated.
Task is made.
Memcache entity bites the dust.
Player1 asks for an update.
Memcache picks up the entity in the datastore, wich hasn't been adjusted yet 
from the task.
Player1 gets an old update, and there is an temporary desync in the client.

Now that would be an unwanted but tolerable situation.
I guess it all comes down to, how long my entity will  live in memcache.
And as i've read the docs, memcache is unreliable. 
Yet is this unreliability after some amount of time or could an entity get 
lost after 0:05 seconds of creation(so to speak).
Maybe even more important, what kind of error margin would i be looking at.

After reading this, could you conclude anything about whether to use 
memcache in my case?
I'm guessing delaying writes would not be a smart option in my case?

I hope my story is understandable.

Much appreciated,

Rohan




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