IMap::merge fails for enabled NearCache

[kirpichik]

Hazelcast version: 5.0

When a NearCache is used for an IMap then merge(key, value, remappingFunction) operation fails reporting a wrong key type.

Exception output:

Exception in thread "main" java.lang.IllegalArgumentException: key cannot be of type Data!
	at com.hazelcast.internal.util.Preconditions.checkNotInstanceOf(Preconditions.java:300)
	at com.hazelcast.internal.nearcache.impl.DefaultNearCache.checkKeyFormat(DefaultNearCache.java:221)
	at com.hazelcast.internal.nearcache.impl.DefaultNearCache.get(DefaultNearCache.java:123)
	at com.hazelcast.client.map.impl.nearcache.NearCachedClientMapProxy.getCachedValue(NearCachedClientMapProxy.java:597)
	at com.hazelcast.client.map.impl.nearcache.NearCachedClientMapProxy.getInternal(NearCachedClientMapProxy.java:117)
	at com.hazelcast.client.impl.proxy.ClientMapProxy.mergeLocally(ClientMapProxy.java:2174)
	at com.hazelcast.client.impl.proxy.ClientMapProxy.merge(ClientMapProxy.java:2167)
	at Test.main(Test.java:32)

Reproducer:

Config config = new Config();
config.setClusterName("dev");
HazelcastInstance hz = Hazelcast.newHazelcastInstance(config);

ClientConfig clientConfig = new ClientConfig();
clientConfig.setClusterName("dev");
clientConfig.getNetworkConfig().addAddress("localhost:5701");

NearCacheConfig nearCacheConfig = new NearCacheConfig();
clientConfig.setNearCacheConfigMap(Collections.singletonMap("my-map", nearCacheConfig));

HazelcastInstance clientHz = HazelcastClient.newHazelcastClient(clientConfig);

IMap<String, String> map = clientHz.getMap("my-map");
map.merge("1", "John", (current, other) -> current + other);
map.merge("1", "Mary", (current, other) -> current + other);

This problem is very similar to the one described in #13371 and the workaround described in original stackoverflow question setting serialize-keys=true helps avoid exception.

Until the bug is fixed, how safe is this workaround to use? What can it affect?

[ahmetmircik]

It is about performance, lookup keys are serialized when serialize-keys=true, this can degrade near-cache read performance, if key serialization is expensive. But best to measure to see.

[kirpichik]

I measured the performance using JMH with and without workaround and indeed, the performance of the get operation drops by 40%:

Benchmark                            Mode  Cnt     Score     Error   Units
HazelcastBenchNotSerializeKeys.get  thrpt    5  3332.311 ± 295.684  ops/ms
HazelcastBenchNotSerializeKeys.put  thrpt    5    45.165 ±   2.337  ops/ms
HazelcastBenchSerializeKeys.get     thrpt    5  2028.248 ±   2.397  ops/ms
HazelcastBenchSerializeKeys.put     thrpt    5    44.982 ±   1.246  ops/ms