CODEWITHSUNDEEP
springmongodbmongo-java-driver
Sydney
Sydney

Reputation: 12212

MongoDB change stream replica set recovery

I implemented MongoDB change streams with Spring and it works fine when the replica set primary node is up.

@Service
public class ChangeEventService {
    private static final Logger logger = LoggerFactory.getLogger(ChangeEventService.class);
    private final MongoClient mongoClient;
    public ChangeEventService(MongoClient mongoClient) {
        this.mongoClient = mongoClient;
    }
    @PostConstruct
    public void subscribe() {
        MongoDatabase db = mongoClient.getDatabase("experiment");
        MongoCollection<Document> collection = db.getCollection("debug");
        Block<ChangeStreamDocument<Document>> printBlock = changeStreamDocument -> {
            logger.info("Received: {}", changeStreamDocument.getFullDocument().toString());
            BsonDocument resumeToken = changeStreamDocument.getResumeToken();
        };
        collection.watch().forEach(printBlock);
        logger.info("Consumer is ready to process");
    }
}

Then I shut down the primary node of the replica set. I was expecting the change stream to wait for the replica set to elect a new primary and continue to get the data changes. The actual behavior is an application crash.

From the logs I can see that the connection to the primary (27000) is closed which is expected, then it seems to try to open a connection to one of the secondary (27001) but can't because the pool has been closed.

From the documentation: "The change stream is bound to a collection and change stream documents are iterated with a cursor. This cursor remains open until it is explicitly closed, as long as a connection to the MongoDB deployment remains open and the collection exists."

2018-05-02 12:03:03.424  INFO 9560 --- [           main] c.e.m.service.ChangeEventService         : Received: Document{{_id=5ae98cd7dcc8921c94d5f9e5, _class=com.mongodb.BasicDBObject, uuid=4f836d00-efc3-4d48-956a-af4dbfed90e7, now=Wed May 02 12:03:03 CEST 2018}}
2018-05-02 12:03:06.500  WARN 9560 --- [           main] org.mongodb.driver.connection            : Got socket exception on connection [connectionId{localValue:4, serverValue:8}] to localhost:27000. All connections to localhost:27000 will be closed.
2018-05-02 12:03:06.501  INFO 9560 --- [           main] org.mongodb.driver.connection            : Closed connection [connectionId{localValue:4, serverValue:8}] to localhost:27000 because there was a socket exception raised by this connection.
2018-05-02 12:03:07.502  INFO 9560 --- [           main] org.mongodb.driver.connection            : Closed connection [connectionId{localValue:6}] to localhost:27000 because there was a socket exception raised by this connection.
2018-05-02 12:03:07.504  WARN 9560 --- [           main] ationConfigEmbeddedWebApplicationContext : Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'changeEventService': Invocation of init method failed; nested exception is com.mongodb.MongoSocketOpenException: Exception opening socket
2018-05-02 12:03:07.505  INFO 9560 --- [localhost:27000] org.mongodb.driver.cluster               : Exception in monitor thread while connecting to server localhost:27000
com.mongodb.MongoSocketOpenException: Exception opening socket
    at com.mongodb.connection.SocketStream.open(SocketStream.java:62) ~[mongodb-driver-core-3.6.3.jar:na]
    at com.mongodb.connection.InternalStreamConnection.open(InternalStreamConnection.java:126) ~[mongodb-driver-core-3.6.3.jar:na]
    at com.mongodb.connection.DefaultServerMonitor$ServerMonitorRunnable.run(DefaultServerMonitor.java:128) ~[mongodb-driver-core-3.6.3.jar:na]
    at java.lang.Thread.run(Thread.java:748) [na:1.8.0_161]
Caused by: java.net.ConnectException: Connection refused: connect
    at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method) ~[na:1.8.0_161]
    at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:85) ~[na:1.8.0_161]
    at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350) ~[na:1.8.0_161]
    at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206) ~[na:1.8.0_161]
    at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188) ~[na:1.8.0_161]
    at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172) ~[na:1.8.0_161]
    at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) ~[na:1.8.0_161]
    at java.net.Socket.connect(Socket.java:589) ~[na:1.8.0_161]
    at com.mongodb.connection.SocketStreamHelper.initialize(SocketStreamHelper.java:59) ~[mongodb-driver-core-3.6.3.jar:na]
    at com.mongodb.connection.SocketStream.open(SocketStream.java:57) ~[mongodb-driver-core-3.6.3.jar:na]
    ... 3 common frames omitted
2018-05-02 12:03:07.507  INFO 9560 --- [           main] org.mongodb.driver.connection            : Opened connection [connectionId{localValue:7, serverValue:181}] to localhost:27001
2018-05-02 12:03:07.508  INFO 9560 --- [           main] org.mongodb.driver.connection            : Closed connection [connectionId{localValue:7, serverValue:181}] to localhost:27001 because the pool has been closed.
2018-05-02 12:03:07.511  INFO 9560 --- [           main] o.apache.catalina.core.StandardService   : Stopping service [Tomcat]
2

Upvotes: 1

Views: 2234

Answers (1)

Wan B.
Wan B.

Reputation: 18835

There are two things here:

at com.mongodb.connection.SocketStream.open(SocketStream.java:57) ~[mongodb-driver-core-3.6.3.jar:na]

There is a bug with MongoDB Java driver v3.6.3, where change streams cursor does not resume when there's an attempt to kill the existing cursor fails. This is described in JAVA-2821, and fixed in version 3.7.0 onwards.

collection.watch().forEach(printBlock);

The watch() method does't actually contact the server, instead you should use an iterator method. For example: 

MongoCursor<ChangeStreamDocument<Document>> cursor = collection.watch().iterator();

ChangeStreamDocument<Document> next = cursor.next();

while(cursor.hasNext()){
    next = cursor.next();
    System.out.println(next);
}

See also Spec: Resumable Error for the definition of error that is considered resumable.

Upvotes: 1

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