A coarse-grained data model places mutable values inside one large container, so changing an entry requires replacing or re-uploading the whole container. This simplifies the outer structure but can repeat costly updates under concurrency. section 8.2.2
The practical contrast is fine-grained mutability: separate inner MVars let updates target one value without altering the outer map. A coarse-grained map instead constructs new maps for repeated changes to the same key, increasing MVar updates. section 8.2.2
Map replacement versus nested mutability
With CoarseGrainedMap = MVar (Map Key Value), updating one key means re-uploading a new map. A nested structure such as MVar (Map Key (MVar Value)) can alter an inner value directly.
