Pulsar Java client

You can use Pulsar Java client to create Java producer, consumer, and readers of messages and to perform administrative tasks. The current version of the Java client is 2.6.0.

All the methods in producer, consumer, and reader of a Java client are thread-safe.

Javadoc for the Pulsar client is divided into two domains by package as follows.

Package	Description	Maven Artifact
org.apache.pulsar.client.api	The producer and consumer API	org.apache.pulsar:pulsar-client:2.6.0
org.apache.pulsar.client.admin	The Java admin API	org.apache.pulsar:pulsar-client-admin:2.6.0
org.apache.pulsar.client.all	Includes both pulsar-client and pulsar-client-admin Both pulsar-client and pulsar-client-admin are shaded packages and they shade dependencies independently. Consequently, the applications using both pulsar-client and pulsar-client-admin have redundant shaded classes. It would be troublesome if you introduce new dependencies but forget to update shading rules. In this case, you can use pulsar-client-all, which shades dependencies only one time and reduces the size of dependencies.	org.apache.pulsar:pulsar-client-all:2.6.0

This document focuses only on the client API for producing and consuming messages on Pulsar topics. For how to use the Java admin client, see Pulsar admin interface.

Installation

The latest version of the Pulsar Java client library is available via Maven Central. To use the latest version, add the pulsar-client library to your build configuration.

Maven

If you use Maven, add the following information to the pom.xml file.

<!-- in your <properties> block -->
<pulsar.version>2.6.0</pulsar.version>

<!-- in your <dependencies> block -->
<dependency>
  <groupId>org.apache.pulsar</groupId>
  <artifactId>pulsar-client</artifactId>
  <version>${pulsar.version}</version>
</dependency>

Gradle

If you use Gradle, add the following information to the build.gradle file.

def pulsarVersion = '2.6.0'

dependencies {
    compile group: 'org.apache.pulsar', name: 'pulsar-client', version: pulsarVersion
}

Connection URLs

To connect to Pulsar using client libraries, you need to specify a Pulsar protocol URL.

You can assign Pulsar protocol URLs to specific clusters and use the pulsar scheme. The default port is 6650. The following is an example of localhost.

pulsar://localhost:6650

If you have multiple brokers, the URL is as follows.

pulsar://localhost:6550,localhost:6651,localhost:6652

A URL for a production Pulsar cluster is as follows.

pulsar://pulsar.us-west.example.com:6650

If you use TLS authentication, the URL is as follows.

pulsar+ssl://pulsar.us-west.example.com:6651

Client

You can instantiate a PulsarClient object using just a URL for the target Pulsar cluster like this:

PulsarClient client = PulsarClient.builder()
        .serviceUrl("pulsar://localhost:6650")
        .build();

If you have multiple brokers, you can initiate a PulsarClient like this:

PulsarClient client = PulsarClient.builder()
        .serviceUrl("pulsar://localhost:6650,localhost:6651,localhost:6652")
        .build();

Default broker URLs for standalone clusters

If you run a cluster in standalone mode, the broker is available at the pulsar://localhost:6650 URL by default.

If you create a client, you can use the loadConf configuration. The following parameters are available in loadConf.

Type	Name	Description	Default
String	serviceUrl	Service URL provider for Pulsar service	None
String	authPluginClassName	Name of the authentication plugin	None
String	authParams	String represents parameters for the authentication plugin Example key1:val1,key2:val2	None
long	operationTimeoutMs	Operation timeout	30000
long	statsIntervalSeconds	Interval between each stats info Stats is activated with positive statsInterval Set statsIntervalSeconds to 1 second at least	60
int	numIoThreads	The number of threads used for handling connections to brokers	1
int	numListenerThreads	The number of threads used for handling message listeners	1
boolean	useTcpNoDelay	Whether to use TCP no-delay flag on the connection to disable Nagle algorithm	true
boolean	useTls	Whether to use TLS encryption on the connection	false
string	tlsTrustCertsFilePath	Path to the trusted TLS certificate file	None
boolean	tlsAllowInsecureConnection	Whether the Pulsar client accepts untrusted TLS certificate from broker	false
boolean	tlsHostnameVerificationEnable	Whether to enable TLS hostname verification	false
int	concurrentLookupRequest	The number of concurrent lookup requests allowed to send on each broker connection to prevent overload on broker	5000
int	maxLookupRequest	The maximum number of lookup requests allowed on each broker connection to prevent overload on broker	50000
int	maxNumberOfRejectedRequestPerConnection	The maximum number of rejected requests of a broker in a certain time frame (30 seconds) after the current connection is closed and the client creates a new connection to connect to a different broker	50
int	keepAliveIntervalSeconds	Seconds of keeping alive interval for each client broker connection	30
int	connectionTimeoutMs	Duration of waiting for a connection to a broker to be established If the duration passes without a response from a broker, the connection attempt is dropped	10000
int	requestTimeoutMs	Maximum duration for completing a request	60000
int	defaultBackoffIntervalNanos	Default duration for a backoff interval	TimeUnit.MILLISECONDS.toNanos(100);
long	maxBackoffIntervalNanos	Maximum duration for a backoff interval	TimeUnit.SECONDS.toNanos(30)

Check out the Javadoc for the PulsarClient class for a full list of configurable parameters.

In addition to client-level configuration, you can also apply producer and consumer specific configuration as described in sections below.

Producer

In Pulsar, producers write messages to topics. Once you've instantiated a PulsarClient object (as in the section above), you can create a Producer for a specific Pulsar topic.

Producer<byte[]> producer = client.newProducer()
        .topic("my-topic")
        .create();

// You can then send messages to the broker and topic you specified:
producer.send("My message".getBytes());

By default, producers produce messages that consist of byte arrays. You can produce different types by specifying a message schema.

Producer<String> stringProducer = client.newProducer(Schema.STRING)
        .topic("my-topic")
        .create();
stringProducer.send("My message");

Make sure that you close your producers, consumers, and clients when you do not need them.

producer.close();
consumer.close();
client.close();

Close operations can also be asynchronous:

producer.closeAsync()
   .thenRun(() -> System.out.println("Producer closed"))
   .exceptionally((ex) -> {
       System.err.println("Failed to close producer: " + ex);
       return null;
   });

Configure producer

If you instantiate a Producer object by specifying only a topic name as the example above, use the default configuration for producer.

If you create a producer, you can use the loadConf configuration. The following parameters are available in loadConf.

Type	Name	Description	Default
String	topicName	Topic name	null
String	producerName	Producer name	null
long	sendTimeoutMs	Message send timeout in ms. If a message is not acknowledged by a server before the sendTimeout expires, an error occurs.	30000
boolean	blockIfQueueFull	If it is set to true, when the outgoing message queue is full, the Send and SendAsync methods of producer block, rather than failing and throwing errors. If it is set to false, when the outgoing message queue is full, the Send and SendAsync methods of producer fail and ProducerQueueIsFullError exceptions occur. The MaxPendingMessages parameter determines the size of the outgoing message queue.	false
int	maxPendingMessages	The maximum size of a queue holding pending messages. For example, a message waiting to receive an acknowledgment from a broker. By default, when the queue is full, all calls to the Send and SendAsync methods fail unless you set BlockIfQueueFull to true.	1000
int	maxPendingMessagesAcrossPartitions	The maximum number of pending messages across partitions. Use the setting to lower the max pending messages for each partition (#setMaxPendingMessages(int)) if the total number exceeds the configured value.	50000
MessageRoutingMode	messageRoutingMode	Message routing logic for producers on partitioned topics. Apply the logic only when setting no key on messages. Available options are as follows: pulsar.RoundRobinDistribution: round robin pulsar.UseSinglePartition: publish all messages to a single partition pulsar.CustomPartition: a custom partitioning scheme	pulsar.RoundRobinDistribution
HashingScheme	hashingScheme	Hashing function determining the partition where you publish a particular message (partitioned topics only). Available options are as follows: pulsar.JavaStringHash: the equivalent of String.hashCode() in Java pulsar.Murmur3_32Hash: applies the Murmur3 hashing function pulsar.BoostHash: applies the hashing function from C++'s Boost library	HashingScheme.JavaStringHash
ProducerCryptoFailureAction	cryptoFailureAction	Producer should take action when encryption fails. FAIL: if encryption fails, unencrypted messages fail to send. SEND: if encryption fails, unencrypted messages are sent.	ProducerCryptoFailureAction.FAIL
long	batchingMaxPublishDelayMicros	Batching time period of sending messages.	TimeUnit.MILLISECONDS.toMicros(1)
int	batchingMaxMessages	The maximum number of messages permitted in a batch.	1000
boolean	batchingEnabled	Enable batching of messages.	true
CompressionType	compressionType	Message data compression type used by a producer. Available options: LZ4 ZLIB ZSTD SNAPPY	No compression

You can configure parameters if you do not want to use the default configuration.

For a full list, see the Javadoc for the ProducerBuilder class. The following is an example.

Producer<byte[]> producer = client.newProducer()
    .topic("my-topic")
    .batchingMaxPublishDelay(10, TimeUnit.MILLISECONDS)
    .sendTimeout(10, TimeUnit.SECONDS)
    .blockIfQueueFull(true)
    .create();

Message routing

When using partitioned topics, you can specify the routing mode whenever you publish messages using a producer. For more information on specifying a routing mode using the Java client, see the Partitioned Topics cookbook.

Async send

You can publish messages asynchronously using the Java client. With async send, the producer puts the message in a blocking queue and returns it immediately. Then the client library sends the message to the broker in the background. If the queue is full (max size configurable), the producer is blocked or fails immediately when calling the API, depending on arguments passed to the producer.

The following is an example.

producer.sendAsync("my-async-message".getBytes()).thenAccept(msgId -> {
    System.out.printf("Message with ID %s successfully sent", msgId);
});

As you can see from the example above, async send operations return a MessageId wrapped in a CompletableFuture.

Configure messages

In addition to a value, you can set additional items on a given message:

producer.newMessage()
    .key("my-message-key")
    .value("my-async-message".getBytes())
    .property("my-key", "my-value")
    .property("my-other-key", "my-other-value")
    .send();

You can terminate the builder chain with sendAsync() and get a future return.

Consumer

In Pulsar, consumers subscribe to topics and handle messages that producers publish to those topics. You can instantiate a new consumer by first instantiating a PulsarClient object and passing it a URL for a Pulsar broker (as above).

Once you've instantiated a PulsarClient object, you can create a Consumer by specifying a topic and a subscription.

Consumer consumer = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscribe();

The subscribe method will auto subscribe the consumer to the specified topic and subscription. One way to make the consumer listen on the topic is to set up a while loop. In this example loop, the consumer listens for messages, prints the contents of any received message, and then acknowledges that the message has been processed. If the processing logic fails, you can use negative acknowledgement to redeliver the message later.

while (true) {
  // Wait for a message
  Message msg = consumer.receive();

  try {
      // Do something with the message
      System.out.printf("Message received: %s", new String(msg.getData()));

      // Acknowledge the message
      consumer.acknowledge(msg);
  } catch (Exception e) {
      // Message failed to process, redeliver later
      consumer.negativeAcknowledge(msg);
  }
}

Configure consumer

If you instantiate a Consumer object by specifying only a topic and subscription name as in the example above, the consumer uses the default configuration.

When you create a consumer, you can use the loadConf configuration. The following parameters are available in loadConf.

Type	Name	Description	Default
Set<String>	topicNames	Topic name	Sets.newTreeSet()
Pattern	topicsPattern	Topic pattern	None
String	subscriptionName	Subscription name	None
SubscriptionType	subscriptionType	Subscription type Four subscription types are available: Exclusive Failover Shared Key_Shared	SubscriptionType.Exclusive
int	receiverQueueSize	Size of a consumer's receiver queue. For example, the number of messages accumulated by a consumer before an application calls Receive. A value higher than the default value increases consumer throughput, though at the expense of more memory utilization.	1000
long	acknowledgementsGroupTimeMicros	Group a consumer acknowledgment for a specified time. By default, a consumer uses 100ms grouping time to send out acknowledgments to a broker. Setting a group time of 0 sends out acknowledgments immediately. A longer ack group time is more efficient at the expense of a slight increase in message re-deliveries after a failure.	TimeUnit.MILLISECONDS.toMicros(100)
long	negativeAckRedeliveryDelayMicros	Delay to wait before redelivering messages that failed to be processed. When an application uses Consumer#negativeAcknowledge(Message), failed messages are redelivered after a fixed timeout.	TimeUnit.MINUTES.toMicros(1)
int	maxTotalReceiverQueueSizeAcrossPartitions	The max total receiver queue size across partitions. This setting reduces the receiver queue size for individual partitions if the total receiver queue size exceeds this value.	50000
String	consumerName	Consumer name	null
long	ackTimeoutMillis	Timeout of unacked messages	0
long	tickDurationMillis	Granularity of the ack-timeout redelivery. Using an higher tickDurationMillis reduces the memory overhead to track messages when setting ack-timeout to a bigger value (for example, 1 hour).	1000
int	priorityLevel	Priority level for a consumer to which a broker gives more priority while dispatching messages in Shared subscription type. The broker follows descending priorities. For example, 0=max-priority, 1, 2,... In shared subscription type, the broker first dispatches messages to the max priority level consumers if they have permits. Otherwise, the broker considers next priority level consumers. Example 1 If a subscription has consumerA with priorityLevel 0 and consumerB with priorityLevel 1, then the broker only dispatches messages to consumerA until it runs out permits and then starts dispatching messages to consumerB. Example 2 Consumer Priority, Level, Permits C1, 0, 2 C2, 0, 1 C3, 0, 1 C4, 1, 2 C5, 1, 1 Order in which a broker dispatches messages to consumers is: C1, C2, C3, C1, C4, C5, C4.	0
ConsumerCryptoFailureAction	cryptoFailureAction	Consumer should take action when it receives a message that can not be decrypted. FAIL: this is the default option to fail messages until crypto succeeds. DISCARD:silently acknowledge and not deliver message to an application. CONSUME: deliver encrypted messages to applications. It is the application's responsibility to decrypt the message. The decompression of message fails. If messages contain batch messages, a client is not be able to retrieve individual messages in batch. Delivered encrypted message contains EncryptionContext which contains encryption and compression information in it using which application can decrypt consumed message payload.	ConsumerCryptoFailureAction.FAIL
SortedMap<String, String>	properties	A name or value property of this consumer. properties is application defined metadata attached to a consumer. When getting a topic stats, associate this metadata with the consumer stats for easier identification.	new TreeMap()
boolean	readCompacted	If enabling readCompacted, a consumer reads messages from a compacted topic rather than reading a full message backlog of a topic. A consumer only sees the latest value for each key in the compacted topic, up until reaching the point in the topic message when compacting backlog. Beyond that point, send messages as normal. Only enabling readCompacted on subscriptions to persistent topics, which have a single active consumer (like failure or exclusive subscriptions). Attempting to enable it on subscriptions to non-persistent topics or on shared subscriptions leads to a subscription call throwing a PulsarClientException.	false
SubscriptionInitialPosition	subscriptionInitialPosition	Initial position at which to set cursor when subscribing to a topic at first time.	SubscriptionInitialPosition.Latest
int	patternAutoDiscoveryPeriod	Topic auto discovery period when using a pattern for topic's consumer. The default and minimum value is 1 minute.	1
RegexSubscriptionMode	regexSubscriptionMode	When subscribing to a topic using a regular expression, you can pick a certain type of topics. PersistentOnly: only subscribe to persistent topics. NonPersistentOnly: only subscribe to non-persistent topics. AllTopics: subscribe to both persistent and non-persistent topics.	RegexSubscriptionMode.PersistentOnly
DeadLetterPolicy	deadLetterPolicy	Dead letter policy for consumers. By default, some messages are probably redelivered many times, even to the extent that it never stops. By using the dead letter mechanism, messages have the max redelivery count. When exceeding the maximum number of redeliveries, messages are sent to the Dead Letter Topic and acknowledged automatically. You can enable the dead letter mechanism by setting deadLetterPolicy. Example client.newConsumer() .deadLetterPolicy(DeadLetterPolicy.builder().maxRedeliverCount(10).build()) .subscribe(); Default dead letter topic name is \{TopicName\}-\{Subscription\}-DLQ. To set a custom dead letter topic name: client.newConsumer() .deadLetterPolicy(DeadLetterPolicy.builder().maxRedeliverCount(10) .deadLetterTopic("your-topic-name").build()) .subscribe(); When specifying the dead letter policy while not specifying ackTimeoutMillis, you can set the ack timeout to 30000 millisecond.	None
boolean	autoUpdatePartitions	If autoUpdatePartitions is enabled, a consumer subscribes to partition increasement automatically. Note: this is only for partitioned consumers.	true
boolean	replicateSubscriptionState	If replicateSubscriptionState is enabled, a subscription state is replicated to geo-replicated clusters.	false

You can configure parameters if you do not want to use the default configuration. For a full list, see the Javadoc for the ConsumerBuilder class.

The following is an example.

Consumer consumer = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .ackTimeout(10, TimeUnit.SECONDS)
        .subscriptionType(SubscriptionType.Exclusive)
        .subscribe();

Async receive

The receive method receives messages synchronously (the consumer process is blocked until a message is available). You can also use async receive, which returns a CompletableFuture object immediately once a new message is available.

The following is an example.

CompletableFuture<Message> asyncMessage = consumer.receiveAsync();

Async receive operations return a Message wrapped inside of a CompletableFuture.

Batch receive

Use batchReceive to receive multiple messages for each call.

The following is an example.

Messages messages = consumer.batchReceive();
for (Object message : messages) {
  // do something
}
consumer.acknowledge(messages)

note

Batch receive policy limits the number and bytes of messages in a single batch. You can specify a timeout to wait for enough messages. The batch receive is completed if any of the following condition is met: enough number of messages, bytes of messages, wait timeout.

Consumer consumer = client.newConsumer()
.topic("my-topic")
.subscriptionName("my-subscription")
.batchReceivePolicy(BatchReceivePolicy.builder()
.maxNumMessages(100)
.maxNumBytes(1024 * 1024)
.timeout(200, TimeUnit.MILLISECONDS)
.build())
.subscribe();

The default batch receive policy is:

BatchReceivePolicy.builder()
.maxNumMessage(-1)
.maxNumBytes(10 * 1024 * 1024)
.timeout(100, TimeUnit.MILLISECONDS)
.build();

Multi-topic subscriptions

In addition to subscribing a consumer to a single Pulsar topic, you can also subscribe to multiple topics simultaneously using multi-topic subscriptions. To use multi-topic subscriptions you can supply either a regular expression (regex) or a List of topics. If you select topics via regex, all topics must be within the same Pulsar namespace.

The followings are some examples.

import org.apache.pulsar.client.api.Consumer;
import org.apache.pulsar.client.api.PulsarClient;

import java.util.Arrays;
import java.util.List;
import java.util.regex.Pattern;

ConsumerBuilder consumerBuilder = pulsarClient.newConsumer()
        .subscriptionName(subscription);

// Subscribe to all topics in a namespace
Pattern allTopicsInNamespace = Pattern.compile("public/default/.*");
Consumer allTopicsConsumer = consumerBuilder
        .topicsPattern(allTopicsInNamespace)
        .subscribe();

// Subscribe to a subsets of topics in a namespace, based on regex
Pattern someTopicsInNamespace = Pattern.compile("public/default/foo.*");
Consumer allTopicsConsumer = consumerBuilder
        .topicsPattern(someTopicsInNamespace)
        .subscribe();

In the above example, the consumer subscribes to the persistent topics that can match the topic name pattern. If you want the consumer subscribes to all persistent and non-persistent topics that can match the topic name pattern, set subscriptionTopicsMode to RegexSubscriptionMode.AllTopics.

Pattern pattern = Pattern.compile("public/default/.*");
pulsarClient.newConsumer()
        .subscriptionName("my-sub")
        .topicsPattern(pattern)
        .subscriptionTopicsMode(RegexSubscriptionMode.AllTopics)
        .subscribe();

note

By default, the subscriptionTopicsMode of the consumer is PersistentOnly. Available options of subscriptionTopicsMode are PersistentOnly, NonPersistentOnly, and AllTopics.

You can also subscribe to an explicit list of topics (across namespaces if you wish):

List<String> topics = Arrays.asList(
        "topic-1",
        "topic-2",
        "topic-3"
);

Consumer multiTopicConsumer = consumerBuilder
        .topics(topics)
        .subscribe();

// Alternatively:
Consumer multiTopicConsumer = consumerBuilder
        .topic(
            "topic-1",
            "topic-2",
            "topic-3"
        )
        .subscribe();

You can also subscribe to multiple topics asynchronously using the subscribeAsync method rather than the synchronous subscribe method. The following is an example.

Pattern allTopicsInNamespace = Pattern.compile("persistent://public/default.*");
consumerBuilder
        .topics(topics)
        .subscribeAsync()
        .thenAccept(this::receiveMessageFromConsumer);

private void receiveMessageFromConsumer(Object consumer) {
    ((Consumer)consumer).receiveAsync().thenAccept(message -> {
                // Do something with the received message
                receiveMessageFromConsumer(consumer);
            });
}

Subscription types

Pulsar has various subscription types to match different scenarios. A topic can have multiple subscriptions with different subscription types. However, a subscription can only have one subscription type at a time.

A subscription is identical with the subscription name; a subscription name can specify only one subscription type at a time. To change the subscription type, you should first stop all consumers of this subscription.

Different subscription types have different message distribution modes. This section describes the differences of subscription types and how to use them.

In order to better describe their differences, assuming you have a topic named "my-topic", and the producer has published 10 messages.

Producer<String> producer = client.newProducer(Schema.STRING)
        .topic("my-topic")
        .enableBatching(false)
        .create();
// 3 messages with "key-1", 3 messages with "key-2", 2 messages with "key-3" and 2 messages with "key-4"
producer.newMessage().key("key-1").value("message-1-1").send();
producer.newMessage().key("key-1").value("message-1-2").send();
producer.newMessage().key("key-1").value("message-1-3").send();
producer.newMessage().key("key-2").value("message-2-1").send();
producer.newMessage().key("key-2").value("message-2-2").send();
producer.newMessage().key("key-2").value("message-2-3").send();
producer.newMessage().key("key-3").value("message-3-1").send();
producer.newMessage().key("key-3").value("message-3-2").send();
producer.newMessage().key("key-4").value("message-4-1").send();
producer.newMessage().key("key-4").value("message-4-2").send();

Exclusive

Create a new consumer and subscribe with the Exclusive subscription type.

Consumer consumer = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscriptionType(SubscriptionType.Exclusive)
        .subscribe()

Only the first consumer is allowed to the subscription, other consumers receive an error. The first consumer receives all 10 messages, and the consuming order is the same as the producing order.

note

If topic is a partitioned topic, the first consumer subscribes to all partitioned topics, other consumers are not assigned with partitions and receive an error.

Failover

Create new consumers and subscribe with theFailover subscription type.

Consumer consumer1 = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscriptionType(SubscriptionType.Failover)
        .subscribe()
Consumer consumer2 = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscriptionType(SubscriptionType.Failover)
        .subscribe()
//conumser1 is the active consumer, consumer2 is the standby consumer.
//consumer1 receives 5 messages and then crashes, consumer2 takes over as an  active consumer.

Multiple consumers can attach to the same subscription, yet only the first consumer is active, and others are standby. When the active consumer is disconnected, messages will be dispatched to one of standby consumers, and the standby consumer then becomes active consumer.

If the first active consumer is disconnected after receiving 5 messages, the standby consumer becomes active consumer. Consumer1 will receive:

("key-1", "message-1-1")
("key-1", "message-1-2")
("key-1", "message-1-3")
("key-2", "message-2-1")
("key-2", "message-2-2")

consumer2 will receive:

("key-2", "message-2-3")
("key-3", "message-3-1")
("key-3", "message-3-2")
("key-4", "message-4-1")
("key-4", "message-4-2")

note

If a topic is a partitioned topic, each partition has only one active consumer, messages of one partition are distributed to only one consumer, and messages of multiple partitions are distributed to multiple consumers.

Shared

Create new consumers and subscribe with Shared subscription type.

Consumer consumer1 = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscriptionType(SubscriptionType.Shared)
        .subscribe()

Consumer consumer2 = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscriptionType(SubscriptionType.Shared)
        .subscribe()
//Both consumer1 and consumer2 are active consumers.

In shared subscription type, multiple consumers can attach to the same subscription and messages are delivered in a round robin distribution across consumers.

If a broker dispatches only one message at a time, consumer1 receives the following information.

("key-1", "message-1-1")
("key-1", "message-1-3")
("key-2", "message-2-2")
("key-3", "message-3-1")
("key-4", "message-4-1")

consumer2 receives the following information.

("key-1", "message-1-2")
("key-2", "message-2-1")
("key-2", "message-2-3")
("key-3", "message-3-2")
("key-4", "message-4-2")

Shared subscription is different from Exclusive and Failover subscription types. Shared subscription has better flexibility, but cannot provide order guarantee.

Key_shared

This is a new subscription type since 2.4.0 release. Create new consumers and subscribe with Key_Shared subscription type.

Consumer consumer1 = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscriptionType(SubscriptionType.Key_Shared)
        .subscribe()

Consumer consumer2 = client.newConsumer()
        .topic("my-topic")
        .subscriptionName("my-subscription")
        .subscriptionType(SubscriptionType.Key_Shared)
        .subscribe()
//Both consumer1 and consumer2 are active consumers.

Key_Shared subscription is like Shared subscription, all consumers can attach to the same subscription. But it is different from Key_Shared subscription, messages with the same key are delivered to only one consumer in order. The possible distribution of messages between different consumers (by default we do not know in advance which keys will be assigned to a consumer, but a key will only be assigned to a consumer at the same time).

consumer1 receives the following information.

("key-1", "message-1-1")
("key-1", "message-1-2")
("key-1", "message-1-3")
("key-3", "message-3-1")
("key-3", "message-3-2")

consumer2 receives the following information.

("key-2", "message-2-1")
("key-2", "message-2-2")
("key-2", "message-2-3")
("key-4", "message-4-1")
("key-4", "message-4-2")

If batching is enabled at the producer side, messages with different keys are added to a batch by default. The broker will dispatch the batch to the consumer, so the default batch mechanism may break the Key_Shared subscription guaranteed message distribution semantics. The producer needs to use the KeyBasedBatcher.

Producer producer = client.newProducer()
        .topic("my-topic")
        .batcherBuilder(BatcherBuilder.KEY_BASED)
        .create();

Or the producer can disable batching.

Producer producer = client.newProducer()
        .topic("my-topic")
        .enableBatching(false)
        .create();

note

If the message key is not specified, messages without key are dispatched to one consumer in order by default.

Reader

With the reader interface, Pulsar clients can "manually position" themselves within a topic and reading all messages from a specified message onward. The Pulsar API for Java enables you to create Reader objects by specifying a topic and a MessageId.

The following is an example.

byte[] msgIdBytes = // Some message ID byte array
MessageId id = MessageId.fromByteArray(msgIdBytes);
Reader reader = pulsarClient.newReader()
        .topic(topic)
        .startMessageId(id)
        .create();

while (true) {
    Message message = reader.readNext();
    // Process message
}

In the example above, a Reader object is instantiated for a specific topic and message (by ID); the reader iterates over each message in the topic after the message is identified by msgIdBytes (how that value is obtained depends on the application).

The code sample above shows pointing the Reader object to a specific message (by ID), but you can also use MessageId.earliest to point to the earliest available message on the topic of MessageId.latest to point to the most recent available message.

When you create a reader, you can use the loadConf configuration. The following parameters are available in loadConf.

Type	Name	Description	Default
String	topicName	Topic name.	None
int	receiverQueueSize	Size of a consumer's receiver queue. For example, the number of messages that can be accumulated by a consumer before an application calls Receive. A value higher than the default value increases consumer throughput, though at the expense of more memory utilization.	1000
ReaderListener<T>	readerListener	A listener that is called for message received.	None
String	readerName	Reader name.	null
String	subscriptionRolePrefix	Prefix of subscription role.	null
CryptoKeyReader	cryptoKeyReader	Interface that abstracts the access to a key store.	null
ConsumerCryptoFailureAction	cryptoFailureAction	Consumer should take action when it receives a message that can not be decrypted. FAIL: this is the default option to fail messages until crypto succeeds. DISCARD: silently acknowledge and not deliver message to an application. CONSUME: deliver encrypted messages to applications. It is the application's responsibility to decrypt the message. The message decompression fails. If messages contain batch messages, a client is not be able to retrieve individual messages in batch. Delivered encrypted message contains EncryptionContext which contains encryption and compression information in it using which application can decrypt consumed message payload.	ConsumerCryptoFailureAction.FAIL
boolean	readCompacted	If enabling readCompacted, a consumer reads messages from a compacted topic rather than a full message backlog of a topic. A consumer only sees the latest value for each key in the compacted topic, up until reaching the point in the topic message when compacting backlog. Beyond that point, send messages as normal. readCompacted can only be enabled on subscriptions to persistent topics, which have a single active consumer (for example, failure or exclusive subscriptions). Attempting to enable it on subscriptions to non-persistent topics or on shared subscriptions leads to a subscription call throwing a PulsarClientException.	false
boolean	resetIncludeHead	If set to true, the first message to be returned is the one specified by messageId. If set to false, the first message to be returned is the one next to the message specified by messageId.	false

Sticky key range reader

In sticky key range reader, broker will only dispatch messages which hash of the message key contains by the specified key hash range. Multiple key hash ranges can be specified on a reader.

The following is an example to create a sticky key range reader.

pulsarClient.newReader()
        .topic(topic)
        .startMessageId(MessageId.earliest)
        .keyHashRange(Range.of(0, 10000), Range.of(20001, 30000))
        .create();

Total hash range size is 65536, so the max end of the range should be less than or equal to 65535.

Schema

In Pulsar, all message data consists of byte arrays "under the hood." Message schemas enable you to use other types of data when constructing and handling messages (from simple types like strings to more complex, application-specific types). If you construct, say, a producer without specifying a schema, then the producer can only produce messages of type byte[]. The following is an example.

Producer<byte[]> producer = client.newProducer()
        .topic(topic)
        .create();

The producer above is equivalent to a Producer<byte[]> (in fact, you should always explicitly specify the type). If you'd like to use a producer for a different type of data, you'll need to specify a schema that informs Pulsar which data type will be transmitted over the topic.

Schema example

Let's say that you have a SensorReading class that you'd like to transmit over a Pulsar topic:

public class SensorReading {
    public float temperature;

    public SensorReading(float temperature) {
        this.temperature = temperature;
    }

    // A no-arg constructor is required
    public SensorReading() {
    }

    public float getTemperature() {
        return temperature;
    }

    public void setTemperature(float temperature) {
        this.temperature = temperature;
    }
}

You could then create a Producer<SensorReading> (or Consumer<SensorReading>) like this:

Producer<SensorReading> producer = client.newProducer(JSONSchema.of(SensorReading.class))
        .topic("sensor-readings")
        .create();

The following schema formats are currently available for Java:

• No schema or the byte array schema (which can be applied using Schema.BYTES):

  
  Producer<byte[]> bytesProducer = client.newProducer(Schema.BYTES)
      .topic("some-raw-bytes-topic")
      .create();
  

  Or, equivalently:

  
  Producer<byte[]> bytesProducer = client.newProducer()
      .topic("some-raw-bytes-topic")
      .create();
  

• String for normal UTF-8-encoded string data. Apply the schema using Schema.STRING:

  
  Producer<String> stringProducer = client.newProducer(Schema.STRING)
      .topic("some-string-topic")
      .create();
  

• Create JSON schemas for POJOs using Schema.JSON. The following is an example.

  
  Producer<MyPojo> pojoProducer = client.newProducer(Schema.JSON(MyPojo.class))
      .topic("some-pojo-topic")
      .create();
  

• Generate Protobuf schemas using Schema.PROTOBUF. The following example shows how to create the Protobuf schema and use it to instantiate a new producer:

  
  Producer<MyProtobuf> protobufProducer = client.newProducer(Schema.PROTOBUF(MyProtobuf.class))
      .topic("some-protobuf-topic")
      .create();
  

• Define Avro schemas with Schema.AVRO. The following code snippet demonstrates how to create and use Avro schema.

  
  Producer<MyAvro> avroProducer = client.newProducer(Schema.AVRO(MyAvro.class))
      .topic("some-avro-topic")
      .create();
  

Authentication

Pulsar currently supports multiple authentication schemes: TLS, Athenz, Kerberos, and JSON Web Token (JWT). You can use the Pulsar Java client with all of them.

TLS Authentication

To use TLS, enableTls method is deprecated and you need to use "pulsar+ssl://" in serviceUrl to enable, point your Pulsar client to a TLS cert path, and provide paths to cert and key files.

The following is an example.

Map<String, String> authParams = new HashMap();
authParams.put("tlsCertFile", "/path/to/client-cert.pem");
authParams.put("tlsKeyFile", "/path/to/client-key.pem");

Authentication tlsAuth = AuthenticationFactory
        .create(AuthenticationTls.class.getName(), authParams);

PulsarClient client = PulsarClient.builder()
        .serviceUrl("pulsar+ssl://my-broker.com:6651")
        .tlsTrustCertsFilePath("/path/to/cacert.pem")
        .authentication(tlsAuth)
        .build();

Athenz

To use Athenz as an authentication provider, you need to use TLS and provide values for four parameters in a hash:

• tenantDomain
• tenantService
• providerDomain
• privateKey

You can also set an optional keyId. The following is an example.

Map<String, String> authParams = new HashMap();
authParams.put("tenantDomain", "shopping"); // Tenant domain name
authParams.put("tenantService", "some_app"); // Tenant service name
authParams.put("providerDomain", "pulsar"); // Provider domain name
authParams.put("privateKey", "file:///path/to/private.pem"); // Tenant private key path
authParams.put("keyId", "v1"); // Key id for the tenant private key (optional, default: "0")

Authentication athenzAuth = AuthenticationFactory
        .create(AuthenticationAthenz.class.getName(), authParams);

PulsarClient client = PulsarClient.builder()
        .serviceUrl("pulsar+ssl://my-broker.com:6651")
        .tlsTrustCertsFilePath("/path/to/cacert.pem")
        .authentication(athenzAuth)
        .build();

Supported pattern formats

The privateKey parameter supports the following three pattern formats:

• file:///path/to/file
• file:/path/to/file
• data:application/x-pem-file;base64,<base64-encoded value>