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Clarify ORMap docs on replacing entries

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James Roper 2019-10-01 14:41:32 +02:00 • committed by Patrik Nordwall
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akka-docs/src/main/paradox/typed/distributed-data.md
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@ -397,6 +397,14 @@ you have not seen. This is the same semantics as for the `ORSet`.
If an entry is concurrently updated to different values the values will be merged, hence the
requirement that the values must be `ReplicatedData` types.
While the `ORMap` supports removing and re-adding keys any number of times, the impact that this has on the values
can be non-deterministic. A merge will always attempt to merge two values for the same key, regardless of whether that
key has been removed and re-added in the meantime, an attempt to replace a value with a new one may not have the
intended effect. This means that old values can effectively be resurrected if a node, that has seen both the remove and
the update,gossips with a node that has seen neither. One consequence of this is that changing the value type of the
CRDT, for example, from a `GCounter` to a `GSet`, could result in the merge function for the CRDT always failing.
This could be an unrecoverable state for the node, hence, the types of `ORMap` values must never change for a given key.
It is rather inconvenient to use the `ORMap` directly since it does not expose specific types
of the values. The `ORMap` is intended as a low level tool for building more specific maps,
such as the following specialized maps.