samples of back pressure which can easily happen

grails-datastore-gorm-rx-mongodb/src/test/groovy/grails/gorm/rx/mongodb/RxBackpressurePersistenceSpec.groovy

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+package grails.gorm.rx.mongodb
+
+import grails.gorm.rx.mongodb.domains.Simple
+import org.grails.datastore.rx.mongodb.RxMongoDatastoreClient
+import rx.Observable
+import spock.lang.Specification
+
+import java.util.concurrent.atomic.AtomicInteger
+
+class RxBackpressurePersistenceSpec extends Specification {
+
+    void setup() {
+//        def client = new RxMongoDatastoreClient("mongodb:https://192.168.99.100", "test", Simple)
+        def client = new RxMongoDatastoreClient("test", Simple)
+        client.dropDatabase()
+
+        Simple s = Observable.range(0, 25000).flatMap {
+            new Simple(name: "Fred $it").save(failOnError: true)
+        }.toBlocking().last()
+
+        assert s, "the entities wasn't persisted properly"
+    }
+
+    void "Test basic persistence of MongoDB using RX - plain with backpressure"() {
+        when:"All Freds are listed"
+        AtomicInteger count = new AtomicInteger(0)
+        Simple.where {
+            name ==~ ~/Fred.+/
+        }
+        .toObservable()
+        .doOnNext { println "Now we have $it.name on the stage..."}
+        // this is the ideal state - nothing fancy here and it works out of the box
+        .subscribe {
+            // emulates show processing
+            Thread.sleep(10)
+            count.incrementAndGet()
+        }
+
+        then:"The results are correct"
+        count.get() == 25000
+    }
+
+    void "Test basic persistence of MongoDB using RX - on backpressure buffered"() {
+        when:"All Freds are listed"
+        AtomicInteger count = new AtomicInteger(0)
+        Simple.where {
+            name ==~ ~/Fred.+/
+        }
+        .toObservable()
+        // this often causes OOME (if the underlying implementation wouldn't fail with too many connections)
+        .onBackpressureBuffer()
+        .doOnNext { println "Now we have $it.name on the stage..."}
+        .subscribe {
+            // emulates show processing
+            Thread.sleep(10)
+            count.incrementAndGet()
+        }
+
+        then:"The results are correct"
+        count.get() == 25000
+    }
+
+    void "Test basic persistence of MongoDB using RX - buffered to prevent backpressure"() {
+        when:"All Freds are listed"
+        AtomicInteger count = new AtomicInteger(0)
+        Simple.where {
+            name ==~ ~/Fred.+/
+        }
+        .toObservable()
+        .doOnNext { println "Now we have $it.name on the stage..."}
+        // buffer requests from the observable by 50 each time which should prevent the backpressure of well-behaved
+        // observables
+        .buffer(50)
+        .subscribe {
+            // emulates show processing
+            Thread.sleep(10)
+            count.addAndGet(it.size())
+        }
+
+        then:"The results are correct"
+        count.get() == 25000
+    }
+}