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Scale applications based on Solace PubSub+ Event Broker Queues

Availability: 2.4+ Maintainer: Community


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Trigger Specification

This specification describes the solace-event-queue trigger that scales based on a Solace PubSub+ Event Broker queue.

- type: solace-event-queue
    solaceSempBaseURL:                  http://solace_broker:8080
    messageVpn:                         message-vpn
    queueName:                          queue_name
    messageCountTarget:                 '100'
    messageSpoolUsageTarget:            '100'       ### Megabytes (MB)
    messageReceiveRateTarget:           '50'        ### Messages/second over last 1 minute interval
    activationMessageCountTarget:       '10'
    activationMessageSpoolUsageTarget:  '5'         ### Megabytes (MB)
    activationMessageReceiveRateTarget: '1'         ### Messages/second over last 1 minute interval
    username:                           semp-user
    password:                           semp-pwd
    usernameFromEnv:                    ENV_VAR_USER
    passwordFromEnv:                    ENV_VAR_PWD

Parameter list:

  • solaceSempBaseURL - Solace SEMP Endpoint in format: <protocol>://<host-or-service>:<port>.
  • messageVpn - Message VPN hosted on the Solace broker.
  • queueName - Message Queue to be monitored.
  • messageCountTarget - The target number of messages manageable by a pod. The scaler will cause the replicas to increase if the queue message backlog is greater than the target value per active replica.
  • activationMessageCountTarget - Target message count oberved on a queue for activating the scaler (scaling from 0->1 or 1->0 replicas). Learn more about activation here. (Default: 0, Optional)
  • messageSpoolUsageTarget - Integer value expressed in Megabytes (MB). The target spool usage manageable by a pod. The scaler will cause the replicas to increase if the queue spool usage is greater than the target value per active replica.
  • activationMessageSpoolUsageTarget - Target message spool backlog (data stored in a queue expressed in Megabytes) for activating the scaler (scaling from 0->1 or 1->0 replicas). Learn more about activation here. (Default: 0, Optional)
  • messageReceiveRateTarget - Target number of messages/second managable by a replica.
  • activationMessageReceiveRateTarget - Target number of messages per second delivered to a queue for activating the scaler (scaling from 0->1 or 1->0 replicas). Learn more about activation here. (Default: 0, Optional)
  • username - User account with access to Solace SEMP RESTful endpoint.
  • password - Password for the user account.
  • usernameFromEnv - Environment variable set with SEMP user account.
  • passwordFromEnv - Environment variable set with password for the user account.

Parameter Requirements:

  • Parameters resolving the target queue are all required: solaceSempBaseURL, messageVpn, queueName
  • At least one of messageCountTarget, messageSpoolUsageTarget, or messageReceiveRateTarget is required. If one or more values are present, the metric value resulting in the highest desired replicas will be used. (Standard KEDA/HPA behavior)
  • The Solace PubSub+ Scaler polls the Solace SEMP REST API to monitor target queues. Currently, the scaler supports basic authentication. username and password are required for the solace-event-queue trigger to function. These values can be set in several different ways:
    • username and password may be set directly in the trigger metadata
    • Use TriggerAuthentication record. See Authentication Parameters below.
    • Alternatively, credentials may be passed from environment variables identified by usernameFromEnv and passwordFromEnv metadata fields. The values of these fields are the names of environment variables that must be available to the scaler at run-time.

Important Notes about Metric Configuration:

💡 Note: messageCountTarget provides good reactivity to changes in demand based on the queue undelivered backlog. The monitored queue value from SEMPv2 is collections.msgs.count

💡 Note: messageReceiveRateTarget provides the ability to achieve consumer stability under constant load. The value monitored from SEMPv2 is data.averageRxMsgRate, which is the average message delivery rate to a queue over a one minute period.

💡 Important: For best results, both messageCountTarget and messageReceiveRateTarget should be specified to configure a Solace Scaler. A combined approach capitalizes on the best characteristics of each metric.

💡 Note: Configured by itself, messageCountTarget will make consumer scaling reactive but may introduce flapping: the constant creation and destruction of replicas as the system tries to achieve a steady state.

For example, a Solace consumer app scaled by KEDA experiences an increase in the rate of message delivery to its source queue, resulting in a backlog (high message count). Based on the messageCountTarget, KEDA increases the number of replicas and the backlog is cleared. With the backlog reduced, KEDA scales-in the application, reducing the number of replicas. If the rate of message delivery remains high, the application may not be able to maintain the backlog with the lower number of replicas, causing KEDA to scale-out the workload again. The backlog is again cleared - and the pattern repeats. Using the messageReceiveRateTarget as an additional metric can be used to identify a suitable replica count to handle the inbound message rate while keeping the backlog clear and the application performant.

💡 Note: Configured by itself, messageReceiveRateTarget cannot scale consumers based on queue backlog.

💡 Activation Values: activationMessageCountTarget, activationMessageSpoolUsageTarget, and activationMessageReceiveRateTarget are assumed to be 0 (zero) if not specified.

Authentication Parameters

You can use TriggerAuthentication CRD to configure the username and password to connect to the management endpoint.

Username and Password based authentication:

  • username - Required. The username to use to connect to the Solace PubSub+ Event Broker’s SEMP endpoint.
  • password - Required. The password to use to connect to the Solace PubSub+ Event Broker’s SEMP endpoint.


The objects in the example below are declared in namespace=solace. It is not required to do so. If you do define a namespace for the configuration objects, then they should all be delcared in the same namespace.

apiVersion: v1
kind: Secret
  name:      solace-secret
  namespace: solace
    app: solace-consumer
type: Opaque
  SEMP_USER:         YWRtaW4=
kind: ScaledObject
  name:      solace-scaled-object
  namespace: solace
    apiVersion:    apps/v1
    kind:          Deployment
    name:          solace-consumer
  pollingInterval: 20
  cooldownPeriod:  60
  minReplicaCount:  0
  maxReplicaCount: 10
  - type: solace-event-queue
      ## Connection Details
      solaceSempBaseURL:        http://broker-pubsubplus.solace.svc.cluster.local:8080
      messageVpn:               test_vpn
      queueName:                SCALED_CONSUMER_QUEUE1
      ## Scaler Details, average values per replica
      messageCountTarget:       '50'
      messageSpoolUsageTarget:  '100000'
      messageReceiveRateTarget: '20'
      ## Activation - Scale from 0 replicas to active workload if one of the conditions is met
      activationMessageCountTarget:        '5'
      activationMessageSpoolUsageTarget:   '2'
      activationMessageReceiveRateTarget:  '5'
      name: solace-trigger-auth
kind: TriggerAuthentication
  name: solace-trigger-auth
  namespace: solace
    - parameter:   username
      name:        solace-secret
      key:         SEMP_USER
    - parameter:   password
      name:        solace-secret
      key:         SEMP_PASSWORD