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Bill commented on GEODE-5845:
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To point (1): it isn't clear to me, why drawing key values from a set of named
constants would, in general, be a bad idea. Is it bad because we envision that
set (of keys) to be relatively small?
I don't see anything particularly wrong with a key having a small number of
distinct values. It seems perfectly reasonable to create a region to hold days
of the week, months of the year, or countries on planet earth.
Point (2) is on point. I have a couple minor suggestions:
I think this would be clearer:
"The hash of an enum is derived from a local memory address, resulting in
different hash values on different servers."
And it seems to me the good advice should be something like:
* don't use an {{enum}} type as a key (for a region)
* don't use the {{hashCode()}} of an {{enum}} type in computing the hash code
of a (non-enum) key type
(Geode could conceivably enforce that first bit of advice at runtime. The
second would be a lot harder.)
Then there's the [{{Region}} class
Javadoc|https://geode.apache.org/releases/latest/javadoc/index.html?org/apache/geode/cache/Region.html]:
{quote}Region entries are identified by their key. Any Object can be used as a
key as long as the key Object is region-wide unique and implements both the
equals and hashCode methods. For regions with distributed scope, the key must
also be Serializable.
{quote}
That Javadoc is correct. But as GEODE-5666 illustrates, it's incomplete. Per
the existing Javadoc: the key type {{K}}, used in a {{Region<K,V>}} must define
{{equals()}} and {{hashCode()}} methods. What we need to stipulate,
additionally is: if the keys are for a partitioned region, then {{hashCode()}}
must _return the same results when applied to the same key value, in every JVM_.
As alluded to in the first comment, this would not be an insurmountable problem
(with {{enum}} types) if the user could override the {{hashCode()}} method on
an {{enum}} type. Alas, the {{hashCode()}} method on an {{enum}} type is
{{final}}—it cannot be overridden. As a result {{enum}} types should not be
used as key types for regions.
> Document that an enum should never be used for keys
> ---------------------------------------------------
>
> Key: GEODE-5845
> URL: https://issues.apache.org/jira/browse/GEODE-5845
> Project: Geode
> Issue Type: Improvement
> Components: docs
> Reporter: Karen Smoler Miller
> Assignee: Karen Smoler Miller
> Priority: Major
>
> Maybe it should be obvious, but an enum type should never be used for an
> entry key. To make this explicitly clear, add this advice to the docs.
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