Why so many connections are used by Spring reactive with Mongo - spring

Question

I got the exception 'MongoWaitQueueFullException' and I realize the number of connections that my application is using. I use the default configuration of Spring boot (2.2.7.RELEASE) with reactive MongoDB (4.2.8). Transactions are used.
Even when running an integration test that basically creates a bit more than 200 elements then groups them (200 groups). 10 connections are used. When this algorithm is executed over a real data-set, this exception is thrown. The default limit of the waiting queue (500) was reached. This does not make the application scalable.
My question is: is there a way to design a reactive application that helps to reduce the number of connections?
This is the output of my test. Basically, it scans all translations of bundle files and them group them per translation key. An element is persisted per translation key.
return Flux
.fromIterable(bundleFile.getFiles())
.map(ScannedBundleFileEntry::getLocale)
.flatMap(locale ->
handler
.scanTranslations(bundleFileEntity.toLocation(), locale, context)
.index()
.map(indexedTranslation ->
createTranslation(
workspaceEntity,
bundleFileEntity,
locale.getId(),
indexedTranslation.getT1(), // index
indexedTranslation.getT2().getKey(), // bundle key
indexedTranslation.getT2().getValue() // translation
)
)
.flatMap(bundleKeyTemporaryRepository::save)
)
.thenMany(groupIntoBundleKeys(bundleFileEntity))
.then(bundleKeyTemporaryRepository.deleteByBundleFile(bundleFileEntity.getId()))
.then(Mono.just(bundleFileEntity));
The grouping function:
private Flux<BundleKeyEntity> groupIntoBundleKeys(BundleFileEntity bundleFile) {
return this
.findBundleKeys(bundleFile)
.groupBy(BundleKeyGroupKey::new)
.flatMap(bundleKeyGroup ->
bundleKeyGroup
.collectList()
.map(bundleKeys -> {
final BundleKeyGroupKey key = bundleKeyGroup.key();
final BundleKeyEntity entity = new BundleKeyEntity(key.getWorkspace(), key.getBundleFile(), key.getKey());
bundleKeys.forEach(entity::mergeInto);
return entity;
})
)
.flatMap(bundleKeyEntityRepository::save);
}
The test output:
560 [main] INFO o.s.b.t.c.SpringBootTestContextBootstrapper - Neither #ContextConfiguration nor #ContextHierarchy found for test class [be.sgerard.i18n.controller.TranslationControllerTest], using SpringBootContextLoader
569 [main] INFO o.s.t.c.s.AbstractContextLoader - Could not detect default resource locations for test class [be.sgerard.i18n.controller.TranslationControllerTest]: no resource found for suffixes {-context.xml, Context.groovy}.
870 [main] INFO o.s.b.t.c.SpringBootTestContextBootstrapper - Loaded default TestExecutionListener class names from location [META-INF/spring.factories]: [org.springframework.boot.test.mock.mockito.MockitoTestExecutionListener, org.springframework.boot.test.mock.mockito.ResetMocksTestExecutionListener, org.springframework.boot.test.autoconfigure.restdocs.RestDocsTestExecutionListener, org.springframework.boot.test.autoconfigure.web.client.MockRestServiceServerResetTestExecutionListener, org.springframework.boot.test.autoconfigure.web.servlet.MockMvcPrintOnlyOnFailureTestExecutionListener, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverTestExecutionListener, org.springframework.test.context.web.ServletTestExecutionListener, org.springframework.test.context.support.DirtiesContextBeforeModesTestExecutionListener, org.springframework.test.context.support.DependencyInjectionTestExecutionListener, org.springframework.test.context.support.DirtiesContextTestExecutionListener, org.springframework.test.context.transaction.TransactionalTestExecutionListener, org.springframework.test.context.jdbc.SqlScriptsTestExecutionListener, org.springframework.test.context.event.EventPublishingTestExecutionListener, org.springframework.security.test.context.support.WithSecurityContextTestExecutionListener, org.springframework.security.test.context.support.ReactorContextTestExecutionListener]
897 [main] INFO o.s.b.t.c.SpringBootTestContextBootstrapper - Using TestExecutionListeners: [org.springframework.test.context.support.DirtiesContextBeforeModesTestExecutionListener#4372b9b6, org.springframework.boot.test.mock.mockito.MockitoTestExecutionListener#232a7d73, org.springframework.boot.test.autoconfigure.SpringBootDependencyInjectionTestExecutionListener#4b41e4dd, org.springframework.test.context.support.DirtiesContextTestExecutionListener#22ffa91a, org.springframework.test.context.transaction.TransactionalTestExecutionListener#74960bfa, org.springframework.test.context.jdbc.SqlScriptsTestExecutionListener#42721fe, org.springframework.test.context.event.EventPublishingTestExecutionListener#40844aab, org.springframework.security.test.context.support.WithSecurityContextTestExecutionListener#1f6c9cd8, org.springframework.security.test.context.support.ReactorContextTestExecutionListener#5b619d14, org.springframework.boot.test.mock.mockito.ResetMocksTestExecutionListener#66746f57, org.springframework.boot.test.autoconfigure.restdocs.RestDocsTestExecutionListener#447a020, org.springframework.boot.test.autoconfigure.web.client.MockRestServiceServerResetTestExecutionListener#7f36662c, org.springframework.boot.test.autoconfigure.web.servlet.MockMvcPrintOnlyOnFailureTestExecutionListener#28e8dde3, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverTestExecutionListener#6d23017e]
1551 [background-preinit] INFO o.h.v.i.x.c.ValidationBootstrapParameters - HV000006: Using org.hibernate.validator.HibernateValidator as validation provider.
1677 [main] INFO b.s.i.c.TranslationControllerTest - Starting TranslationControllerTest on sgerard with PID 538 (started by sgerard in /home/sgerard/sandboxes/github-oauth/server)
1678 [main] INFO b.s.i.c.TranslationControllerTest - The following profiles are active: test
3250 [main] INFO o.s.d.r.c.RepositoryConfigurationDelegate - Bootstrapping Spring Data Reactive MongoDB repositories in DEFAULT mode.
3747 [main] INFO o.s.d.r.c.RepositoryConfigurationDelegate - Finished Spring Data repository scanning in 493ms. Found 9 Reactive MongoDB repository interfaces.
5143 [main] INFO o.s.c.s.PostProcessorRegistrationDelegate$BeanPostProcessorChecker - Bean 'org.springframework.security.config.annotation.method.configuration.ReactiveMethodSecurityConfiguration' of type [org.springframework.security.config.annotation.method.configuration.ReactiveMethodSecurityConfiguration] is not eligible for getting processed by all BeanPostProcessors (for example: not eligible for auto-proxying)
5719 [main] INFO org.mongodb.driver.cluster - Cluster created with settings {hosts=[localhost:27017], mode=SINGLE, requiredClusterType=UNKNOWN, serverSelectionTimeout='30000 ms', maxWaitQueueSize=500}
5996 [cluster-ClusterId{value='5f42490f1c60f43aff9d7d46', description='null'}-localhost:27017] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:1, serverValue:4337}] to localhost:27017
6010 [cluster-ClusterId{value='5f42490f1c60f43aff9d7d46', description='null'}-localhost:27017] INFO org.mongodb.driver.cluster - Monitor thread successfully connected to server with description ServerDescription{address=localhost:27017, type=REPLICA_SET_PRIMARY, state=CONNECTED, ok=true, version=ServerVersion{versionList=[4, 2, 8]}, minWireVersion=0, maxWireVersion=8, maxDocumentSize=16777216, logicalSessionTimeoutMinutes=30, roundTripTimeNanos=12207332, setName='rs0', canonicalAddress=4802c4aff450:27017, hosts=[4802c4aff450:27017], passives=[], arbiters=[], primary='4802c4aff450:27017', tagSet=TagSet{[]}, electionId=7fffffff0000000000000013, setVersion=1, lastWriteDate=Sun Aug 23 12:46:30 CEST 2020, lastUpdateTimeNanos=384505436362981}
6019 [main] INFO org.mongodb.driver.cluster - Cluster created with settings {hosts=[localhost:27017], mode=SINGLE, requiredClusterType=UNKNOWN, serverSelectionTimeout='30000 ms', maxWaitQueueSize=500}
6040 [cluster-ClusterId{value='5f42490f1c60f43aff9d7d47', description='null'}-localhost:27017] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:2, serverValue:4338}] to localhost:27017
6042 [cluster-ClusterId{value='5f42490f1c60f43aff9d7d47', description='null'}-localhost:27017] INFO org.mongodb.driver.cluster - Monitor thread successfully connected to server with description ServerDescription{address=localhost:27017, type=REPLICA_SET_PRIMARY, state=CONNECTED, ok=true, version=ServerVersion{versionList=[4, 2, 8]}, minWireVersion=0, maxWireVersion=8, maxDocumentSize=16777216, logicalSessionTimeoutMinutes=30, roundTripTimeNanos=1727974, setName='rs0', canonicalAddress=4802c4aff450:27017, hosts=[4802c4aff450:27017], passives=[], arbiters=[], primary='4802c4aff450:27017', tagSet=TagSet{[]}, electionId=7fffffff0000000000000013, setVersion=1, lastWriteDate=Sun Aug 23 12:46:30 CEST 2020, lastUpdateTimeNanos=384505468960066}
7102 [nioEventLoopGroup-2-2] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:3, serverValue:4339}] to localhost:27017
11078 [main] INFO o.s.b.a.e.web.EndpointLinksResolver - Exposing 1 endpoint(s) beneath base path ''
11158 [main] INFO o.h.v.i.x.c.ValidationBootstrapParameters - HV000006: Using org.hibernate.validator.HibernateValidator as validation provider.
11720 [main] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:4, serverValue:4340}] to localhost:27017
12084 [main] INFO o.s.s.c.ThreadPoolTaskScheduler - Initializing ExecutorService 'taskScheduler'
12161 [main] INFO b.s.i.c.TranslationControllerTest - Started TranslationControllerTest in 11.157 seconds (JVM running for 13.532)
20381 [nioEventLoopGroup-2-3] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:5, serverValue:4341}] to localhost:27017
20408 [nioEventLoopGroup-2-2] INFO b.s.i.s.w.WorkspaceManagerImpl - Synchronize, there is no workspace for the branch [master], let's create it.
20416 [nioEventLoopGroup-2-3] INFO b.s.i.s.w.WorkspaceManagerImpl - The workspace [master] alias [e3cea374-0d37-4c57-bdbf-8bd14d279c12] has been created.
20421 [nioEventLoopGroup-2-3] INFO b.s.i.s.w.WorkspaceManagerImpl - Initializing workspace [master] alias [e3cea374-0d37-4c57-bdbf-8bd14d279c12].
20525 [nioEventLoopGroup-2-2] INFO b.s.i.s.i18n.TranslationManagerImpl - A bundle file has been found located in [server/src/main/resources/i18n] named [exception] with 2 file(s).
20812 [nioEventLoopGroup-2-4] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:6, serverValue:4342}] to localhost:27017
21167 [nioEventLoopGroup-2-8] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:10, serverValue:4345}] to localhost:27017
21167 [nioEventLoopGroup-2-6] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:8, serverValue:4344}] to localhost:27017
21393 [nioEventLoopGroup-2-5] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:7, serverValue:4343}] to localhost:27017
21398 [nioEventLoopGroup-2-7] INFO org.mongodb.driver.connection - Opened connection [connectionId{localValue:9, serverValue:4346}] to localhost:27017
21442 [nioEventLoopGroup-2-2] INFO b.s.i.s.i18n.TranslationManagerImpl - A bundle file has been found located in [server/src/main/resources/i18n] named [validation] with 2 file(s).
21503 [nioEventLoopGroup-2-2] INFO b.s.i.s.i18n.TranslationManagerImpl - A bundle file has been found located in [server/src/test/resources/be/sgerard/i18n/service/i18n/file] named [file] with 2 file(s).
21621 [nioEventLoopGroup-2-2] INFO b.s.i.s.i18n.TranslationManagerImpl - A bundle file has been found located in [front/src/main/web/src/assets/i18n] named [i18n] with 2 file(s).
22745 [SpringContextShutdownHook] INFO o.s.s.c.ThreadPoolTaskScheduler - Shutting down ExecutorService 'taskScheduler'
22763 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:4, serverValue:4340}] to localhost:27017 because the pool has been closed.
22766 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:9, serverValue:4346}] to localhost:27017 because the pool has been closed.
22767 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:6, serverValue:4342}] to localhost:27017 because the pool has been closed.
22768 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:8, serverValue:4344}] to localhost:27017 because the pool has been closed.
22768 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:5, serverValue:4341}] to localhost:27017 because the pool has been closed.
22769 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:10, serverValue:4345}] to localhost:27017 because the pool has been closed.
22770 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:7, serverValue:4343}] to localhost:27017 because the pool has been closed.
22776 [SpringContextShutdownHook] INFO org.mongodb.driver.connection - Closed connection [connectionId{localValue:3, serverValue:4339}] to localhost:27017 because the pool has been closed.
Process finished with exit code 0

Answer 1

Spring Reactive is asynchronous. Imagine you have 3 items in your dataset. It opens a connection for the save of the first item. But it won't wait for it to finish and use for the second save. Instead, it opens a second connection as soon as possible. Thus you'll end up overloading all the possible connections in the pool.