Is it safe to use HashSet as a very simple cache in a multi-threaded environment?

Question

I realize there are similar questions/answers to this, but I can't seem to find exactly what I'm looking for.

I'm looking to implement an in-memory cache that caches the result of a call to the DB, which is doing an existence check. This existence check is quite expensive and once the object exists in the DB it will never be removed, so a very simple cache in-memory (in-process even) is all I need. Once the DB call is made the process remembers the result of the existence check for that ID and shouldn't call the DB again.

(Maybe another thing to mention is if the object doesn't exist in the DB, it will be created).

I'm using a HashSet (java) for this and just adding the ID to the set when the check/create is done, however this is a highly concurrent environment and I am unsure about the implications of HashSet's lack-of-threadsafeness.

The code only ever uses the add() and contains() methods (no iteration).

I don't really care about a cache miss (and a resulting extra DB call) here and there, but what I'm wondering is if this pattern of add() and contains() being called on the set in concurrent threads could lead to more catastrophic errors.

No, it isn't safe. You read and write concurrently to a non-thread-safe collection. That is never safe. — JB Nizet, May 28 '19 at 20:36
Out of curiosity, what kind of errors could we expect to see in this particular scenario? NPE's and such? — TR1096, May 28 '19 at 21:02

score 2 · Accepted Answer · answered May 28 '19 at 20:38

2

You can use ConcurrentHashMap for multi threaded access and write operation. If you want only HashSet, you can ConcurrentHashSet derived from ConcurrentHashMap. You can use like this.

Set<String> myConcurrentSet = ConcurrentHashMap.newKeySet();

answered May 28 '19 at 20:38

Sambit

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This is correct for Java 8+. Prior to Java 8: `Collections.newSetFromMap(map)` – Miss Chanandler Bong May 28 '19 at 20:48
I was worried about performance if all operations are synchronized but from some reading it seems that at least for reads there is no locking. Are there any performance concerns here? – TR1096 May 28 '19 at 21:02
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ConcurrentHashMap is highly scalable and we should not worry about performance. – Sambit May 28 '19 at 21:06
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That's a good solution. CHM works on segments. So performance will be better at anytime – RoyalTiger Mar 07 '20 at 18:41

score 0 · Answer 2 · answered May 28 '19 at 20:42

No. If you want to use a Map in a multithreaded environment, use Collections.synchronizedMap(<map object>) or ConcurrentHashMap.newKeySet();:

Set<String> concurrentSet = Collections.synchronizedSet(new HashSet<>());

or...

Set<String> concurrentSet = ConcurrentHashMap.newKeySet();

Is it safe to use HashSet as a very simple cache in a multi-threaded environment?

2 Answers2