This Processor polls Apache Kafka for data using KafkaConsumer API available with Kafka 2.0. When a message is received from Kafka, this Processor emits a FlowFile where the content of the FlowFile is the value of the Kafka message.
By default, this processor will subscribe to one or more Kafka topics in such a way that the topics to consume from are randomly assigned to the nodes in the NiFi cluster. Consider a scenario where a single Kafka topic has 8 partitions and the consuming NiFi cluster has 3 nodes. In this scenario, Node 1 may be assigned partitions 0, 1, and 2. Node 2 may be assigned partitions 3, 4, and 5. Node 3 will then be assigned partitions 6 and 7.
In this scenario, if Node 3 somehow fails or stops pulling data from Kafka, partitions 6 and 7 may then be reassigned to the other two nodes. For most use cases, this is desirable. It provides fault tolerance and allows the remaining nodes to pick up the slack. However, there are cases where this is undesirable.
One such case is when using NiFi to consume Change Data Capture (CDC) data from Kafka. Consider again the above scenario. Consider that Node 3 has pulled 1,000 messages from Kafka but has not yet delivered them to their final destination. NiFi is then stopped and restarted, and that takes 15 minutes to complete. In the meantime, Partitions 6 and 7 have been reassigned to the other nodes. Those nodes then proceeded to pull data from Kafka and deliver it to the desired destination. After 15 minutes, Node 3 rejoins the cluster and then continues to deliver its 1,000 messages that it has already pulled from Kafka to the destination system. Now, those records have been delivered out of order.
The solution for this, then, is to assign partitions statically instead of dynamically. In this way, we can assign Partitions 6 and 7 to Node 3 specifically. Then, if Node 3 is restarted, the other nodes will not pull data from Partitions 6 and 7. The data will remain queued in Kafka until Node 3 is restarted. By using this approach, we can ensure that the data that already was pulled can be processed (assuming First In First Out Prioritizers are used) before newer messages are handled.
In order to provide a static mapping of node to Kafka partition(s), one or more user-defined properties must be added using the naming scheme
partitions.<hostname> with the value being a comma-separated list of Kafka partitions to use. For example,
partitions.nifi-01=0, 3, 6, 9, partitions.nifi-02=1, 4, 7, 10, and partitions.nifi-03=2, 5, 8, 11.
The hostname that is used can be the fully qualified hostname, the "simple" hostname, or the IP address. There must be an entry for each node in
the cluster, or the Processor will become invalid. If it is desirable for a node to not have any partitions assigned to it, a Property may be
added for the hostname with an empty string as the value.
NiFi cannot readily validate that all Partitions have been assigned before the Processor is scheduled to run. However, it can validate that no partitions have been skipped. As such, if partitions 0, 1, and 3 are assigned but not partition 2, the Processor will not be valid. However, if partitions 0, 1, and 2 are assigned, the Processor will become valid, even if there are 4 partitions on the Topic. When the Processor is started, the Processor will immediately start to fail, logging errors, and avoid pulling any data until the Processor is updated to account for all partitions. Once running, if the number of partitions is changed, the Processor will continue to run but not pull data from the newly added partitions. Once stopped, it will begin to error until all partitions have been assigned. Additionally, if partitions that are assigned do not exist (e.g., partitions 0, 1, 2, 3, 4, 5, 6, and 7 are assigned, but the Topic has only 4 partitions), then the Processor will begin to log errors on startup and will not pull data.
In order to use a static mapping of Kafka partitions, the "Topic Name Format" must be set to "names" rather than "pattern." Additionally, all Topics that are to be consumed must have the same number of partitions. If multiple Topics are to be consumed and have a different number of partitions, multiple Processors must be used so that each Processor consumes only from Topics with the same number of partitions.
The Security Protocol property allows the user to specify the protocol for communicating with the Kafka broker. The following sections describe each of the protocols in further detail.
This option provides an unsecured connection to the broker, with no client authentication and no encryption. In order to use this option the broker must be configured with a listener of the form:
PLAINTEXT://host.name:port
This option provides an encrypted connection to the broker, with optional client authentication. In order to use this option the broker must be configured with a listener of the form:
SSL://host.name:port
In addition, the processor must have an SSL Context Service selected.
If the broker specifies ssl.client.auth=none, or does not specify ssl.client.auth, then the client will not be required to present a certificate. In this case, the SSL Context Service selected may specify only a truststore containing the public key of the certificate authority used to sign the broker's key.
If the broker specifies ssl.client.auth=required then the client will be required to present a certificate. In this case, the SSL Context Service must also specify a keystore containing a client key, in addition to a truststore as described above.
This option uses SASL with a PLAINTEXT transport layer to authenticate to the broker. In order to use this option the broker must be configured with a listener of the form:
SASL_PLAINTEXT://host.name:port
In addition, the Kerberos Service Name must be specified in the processor.
If the SASL mechanism is GSSAPI, then the client must provide a JAAS configuration to authenticate.
An example of the JAAS config file would be the following:
KafkaClient {
com.sun.security.auth.module.Krb5LoginModule required
useKeyTab=true
storeKey=true
keyTab="/path/to/nifi.keytab"
serviceName="kafka"
principal="nifi@YOURREALM.COM";
};
NOTE: The serviceName in the JAAS file must match the Kerberos Service Name in the processor.
The JAAS configuration can be provided by either of below ways
java.arg.16=-Djava.security.auth.login.config=/path/to/kafka_client_jaas.conf
sasl.jaas.config : com.sun.security.auth.module.Krb5LoginModule required
useKeyTab=true
storeKey=true
keyTab="/path/to/nifi.keytab"
serviceName="kafka"
principal="nifi@YOURREALM.COM";
Alternatively, the JAAS configuration when using GSSAPI can be provided by specifying the Kerberos Principal and Kerberos Keytab directly in the processor properties. This will dynamically create a JAAS configuration like above, and will take precedence over the java.security.auth.login.config system property.
If the SASL mechanism is PLAIN, then client must provide a JAAS configuration to authenticate, but the JAAS configuration must use Kafka's PlainLoginModule. An example of the JAAS config file would be the following:
KafkaClient {
org.apache.kafka.common.security.plain.PlainLoginModule required
username="nifi"
password="nifi-password";
};
The JAAS configuration can be provided by either of below ways
java.arg.16=-Djava.security.auth.login.config=/path/to/kafka_client_jaas.conf
sasl.jaas.config : org.apache.kafka.common.security.plain.PlainLoginModule required
username="nifi"
password="nifi-password";
NOTE: The dynamic properties of this processor are not secured and as a result the password entered when utilizing sasl.jaas.config will be stored in the flow.xml.gz file in plain-text, and will be saved to NiFi Registry if using versioned flows.
NOTE: It is not recommended to use a SASL mechanism of PLAIN with SASL_PLAINTEXT, as it would transmit the username and password unencrypted.
NOTE: The Kerberos Service Name is not required for SASL mechanism of PLAIN. However, processor warns saying this attribute has to be filled with non empty string. You can choose to fill any random string, such as "null".
NOTE: Using the PlainLoginModule will cause it be registered in the JVM's static list of Providers, making it visible to components in other NARs that may access the providers. There is currently a known issue where Kafka processors using the PlainLoginModule will cause HDFS processors with Keberos to no longer work.
If the SASL mechanism is SCRAM, then client must provide a JAAS configuration to authenticate, but the JAAS configuration must use Kafka's ScramLoginModule. Ensure that you add user defined attribute 'sasl.mechanism' and assign 'SCRAM-SHA-256' or 'SCRAM-SHA-512' based on kafka broker configurations. An example of the JAAS config file would be the following:
KafkaClient {
org.apache.kafka.common.security.scram.ScramLoginModule required
username="nifi"
password="nifi-password";
};
The JAAS configuration can be provided by either of below ways
java.arg.16=-Djava.security.auth.login.config=/path/to/kafka_client_jaas.conf
sasl.jaas.config : org.apache.kafka.common.security.scram.ScramLoginModule required
username="nifi"
password="nifi-password";
NOTE: The dynamic properties of this processor are not secured and as a result the password entered when utilizing sasl.jaas.config will be stored in the flow.xml.gz file in plain-text, and will be saved to NiFi Registry if using versioned flows.
NOTE: The Kerberos Service Name is not required for SASL mechanism of SCRAM-SHA-256 or SCRAM-SHA-512. However, processor warns saying this attribute has to be filled with non empty string. You can choose to fill any random string, such as "null".
This option uses SASL with an SSL/TLS transport layer to authenticate to the broker. In order to use this option the broker must be configured with a listener of the form:
SASL_SSL://host.name:port
See the SASL_PLAINTEXT section for a description of how to provide the proper JAAS configuration depending on the SASL mechanism (GSSAPI or PLAIN).
See the SSL section for a description of how to configure the SSL Context Service based on the ssl.client.auth property.