Producing and consuming messages from a Kafka client

  • Release version: Yokohama
  • Updated August 13, 2025
  • 2 minutes to read
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    Summary of Producing and consuming messages from a Kafka client

    This guide explains how ServiceNow customers can produce and consume messages with Hermes using Kafka clients. Hermes uses a pair of Kafka clusters for failover, requiring specific configurations for producers and consumers to ensure reliable message exchange.

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    Key Requirements and Configuration

    • Topic Creation: Begin by creating a topic in the Hermes Kafka cluster before exchanging messages.
    • Processes Needed:
      • 1 producer process to send messages to Hermes.
      • 2 consumer processes to receive messages from Hermes, one connected to each Kafka cluster for failover.
    • Distinct Consumer Clients: Configure two separate consumer clients with different bootstrap addresses and port ranges (4100-4150 and 4200-4250). Each client can have multiple consumer group IDs.
    • Port Ranges: Work with your network administrator to open these port ranges to enable communication:
      • Producer: 4000-4050
      • Consumer 1: 4100-4150
      • Consumer 2: 4200-4250
    • Connectivity Testing: Use the OpenSSL sclient command to verify ports are accessible through your firewall.

    Bootstrap Address Configuration

    Producers and consumers connect to Hermes Kafka clusters using specific bootstrap addresses mapped to their port ranges:

    • Producers: Use ports 4000 to 4050, for example: <instancename>.service-now.com:4000 through :4003.
    • Consumers: Two clients needed:
      • Consumer A: ports 4100-4150
      • Consumer B: ports 4200-4250

    Each client’s bootstrap address should list multiple ports within the assigned range for load balancing and failover.

    Using the Command-Line Interface (CLI)

    • Listing topics may return results from one or both Kafka clusters depending on synchronization timing.
    • Topics created for failover have cluster identification prefixes.
    • The CLI shows internal Hermes topics along with user-created topics.
    • Topics cannot be deleted from both clusters simultaneously via CLI; delete topic records directly within your ServiceNow instance to remove from both clusters.

    Learn how to produce and consume messages to Hermes from a Kafka client.

    To begin exchanging messages from a Kafka client, start by creating a topic in the Hermes Kafka cluster. After you successfully create the topic, you must create a total of three processes to communicate with Hermes.

    • One process is required to produce messages to Hermes.
    • Two processes are required to consume messages from Hermes. Two processes are required because Hermes uses a pair of Kafka clusters for failover purposes. If one cluster goes down, data is produced to the other Hermes Kafka cluster.
      Important:
      You must configure two distinct consumer bootstrap addresses, one for each consumer client.

    One consumer process uses ports in the 410x range and the other uses ports in the 420x range. This means you must define two distinct clients, one for each port range. Within each client configuration, you can define consumer group IDs.

    Required port ranges

    Work with your network administrator to ensure that the following port ranges are open before you begin exchanging messages using Hermes:
    • Producer: 4000-4050
    • Consumer1: 4100-4150
    • Consumer2: 4200-4250

    To confirm that the ports are accessible through the firewall, run the following command:

    openssl s_client -connect <instance_name>.service-now.com:<port> -servername <instance_name>.service-now.com
    • Change the service-now.com domain if your instance uses a different domain name.
    • Complete the test using ports from each of the three port ranges. For example: test 4000-4003, 4100-4103, and 4200-4203.

    If the command fails internally but succeeds from a client outside the network, consult your network administrator to gain access through the network firewall.

    Bootstrap addresses

    Use the following port mappings to connect producers and consumers to the Kafka cluster bootstrap addresses. All Application Delivery Controllers used by Hermes follow this same convention.

    Important:
    The following Hermes bootstrap address examples use ports 4000-4003, 4100-4103, and 4200-4203. However, you must open the following port ranges to ensure connectivity as additional clusters are added to the Hermes Messaging Service.
    • Producer: 4000-4050
    • Consumer1: 4100-4150
    • Consumer2: 4200-4250

    Producer clients use ports ranging from 4000 to 4050. For example:

    Producers = "<instance_name>.service-now.com:4000,<instance_name>.service-now.com:4001,<instance_name>.service-now.com:4002,<instance_name>.service-now.com:4003"

    Because Hermes uses a pair of Kafka clusters, you must configure two consumer clients with separate consumer bootstrap addresses.

    Important:
    You must configure two distinct consumer bootstrap addresses, one for each consumer client.
    • The first consumer client uses ports ranging from 4100 to 4150. For example:
      Consumer A = "<instance_name>.service-now.com:4100,<instance_name>.service-now.com:4101,<instance_name>.service-now.com:4102,<instance_name>.service-now.com:4103"
    • The second consumer client uses ports ranging from 4200 to 4250. For example:
      Consumer B = "<instance_name>.service-now.com:4200,<instance_name>.service-now.com:4201,<instance_name>.service-now.com:4202,<instance_name>.service-now.com:4203"

    When you configure producer and consumer properties for your own Kafka client, use this string pattern.

    Using the command-line interface (CLI)

    When using the list command to view a list of topics, a list of current topics are returned from one or both clusters. Topics might be returned from only one cluster, depending on when the last synchronization occurred. Topics created for failover purposes are differentiated by a three-letter cluster identification prefix.

    When accessing the Hermes clusters using the CLI, internal topics appear in addition to any topics you've created. For details, see KB1705399.

    You can't delete a topic from both Kafka clusters using a single command in the command-line interface (CLI). However, you can delete the topic from both clusters by deleting the topic record in your instance. See Delete a topic in Hermes.