Declarative SEMP

This section provides information about Solace PubSub+ Declarative SEMP.

PubSub+ Declarative SEMP enables you to configure a PubSub+ event broker (both appliance and software) by declaring the state of multiple configuration objects and then synchronizing that state to the broker.

This feature is in BETA with limited scalability and no production support. Future upgrades may not be backwards compatible.


PubSub+ Declarative SEMP enables a declarative API for the configuration of the PubSub+ event broker for services. Declarative API configuration relies on a target configuration without knowledge of the current configuration. The declarative configuration system then determines how to convert from the current state to the target state.

More specifically, Declarative SEMP provides for a declarative API based on the concept of a module, which is a data structure containing configuration information, and a plug-in that takes the module and a CRUD instruction and maps them to the SEMPv2 API in a declarative fashion. Typical instructions to be taken by the Declarative SEMP interface include:

  • Generate—Provides the current configuration of objects described in the input module

  • Plan—Identifies the series of commands to be applied to convert the current configuration to the target configuration module provided

  • Apply—Apply takes a plan or target configuration module and executes it to convert the current configuration to the target configuration

  • Destroy—Generates a plan to remove configuration elements in the target configuration file module

Illustration depicting the concepts described in the surrounding text.

While a declarative SEMP API could be used in a variety of applications, this feature is initially limited to service configuration, which is defined as the configuration elements required to add, remove, or change consumers and/or producers.

Use Cases

Good examples of uses for declarative configuration include golden configurations, configuration management systems, and service additions from teams other than the messaging infrastructure team.

PubSub+ Declarative SEMP supports the following use cases:

  • Application Team Self Serve Infrastructure as Code with Environment Separation
  • Configuration Management

Application Team Self Serve Infrastructure as Code with Environment Separation

In this case, suppose you have deployed an event mesh and your application teams have begun to adopt the use of the event mesh en masse. Your messaging infrastructure team cannot keep up with incoming requests to configure consumers and producers on the event mesh infrastructure. You need Solace to provide the ability for your application teams to write IaC to add or remove consumers and/or producers and other non-environmental components within certain constraints to the broker environment your team has already created. You also need to be able to copy services from one environment to the next. Finally, you need to be able to provide templates (for example, different queue types) for your application team’s IaC to leverage in their modules.

Configuration Management

In this case let's also suppose you have deployed an event mesh and your application teams have begun to adopt the use of the event mesh en masse. However, this time you need to be able to pull down the current configuration of the services (non-infrastructure components) on your event mesh and hold them in a set of configuration files at version control for your event mesh. These version control files can be utilized to revert your event mesh services to their prior configuration in the same environments in the event of configuration corruption or a failed attempt to reconfigure your event mesh services. The version control configurations can also be manually modified to apply these services to the other environments.


PubSub+ Declarative SEMP has the following dependencies:

  • Requires Terraform. Terraform is a command-line program that applies a declarative configuration to create or update as necessary a set of resources via pluggable providers (SEMP objects correspond to resources in Terraform). Providers are automatically downloaded as needed from the public Terraform registry or from private registries as specified in the configuration. The PubSub+ event broker providers are available in the standard Terraform public registry. There are two providers, one for the appliance and one for the software event broker. For more information about Terraform, see

  • In order to properly configure event broker objects for a particular release, you must use the appropriate event broker provider and SEMPv2 versions. For more information, see Version Compatibility .


PubSub+ Declarative SEMP has the following limitations:

  • Terraform apply will not be atomic. If interrupted by a user, failure, reboot, or switchover the configuration changes may be partly applied and there will be no attempt to rollback the changes.

  • Terraform must be the authoritative source of configuration. If there is any overlap between Terraform controlled configuration and either pre-existing configuration or modifications from other management interfaces, the behaviour will be undefined.

  • Apply operations may impact performance (especially large changes).

  • Application of configuration may not be hitless. In other words, brief service interruptions may occur during an apply. These can include a queue missing a published message, or clients being briefly disconnected.

  • Inter object references must be correctly encoded in Terraform configuration to have the apply work. It may not be possible to detect these problems until during the apply operation.

  • Declarative SEMP does not support OpenTofu.

  • Declarative SEMP is supported only on configuration elements specific to a Message VPN. In other words, the elements that you can configure under a Message VPN via PubSub+ Broker Manager.

Getting Started with PubSub+ Declarative SEMP

For a description of how to use the Solace event broker providers to execute operations against PubSub+ event broker configuration as described in Hashi Corp Language, see the documentation included with each provider:

The PubSub+ event broker provider also offers the ability to generate a configuration file from a preconfigured broker. For more information, see the software event broker or appliance provider documentation.

Terraform uses the snake case naming scheme, while SEMP uses camel case. It is also universal practice that resources and datasource be prefixed with the preferred local name of the provider. SEMP names will be converted appropriately for those two requirements but otherwise used as is. For example, this means that solacebroker_msg_vpn is the Message VPN resource name and max_subscription_count is the attribute for the maximum subscription count, since MsgVpn is the SEMP API object name and maxSubscriptionCount is the name of the SEMP attribute.

Version Compatibility

This section lists the releases of the PubSub+ event broker with the corresponding required PubSub+ provider and SEMPv2 versions.

PubSub+ Event Broker Version PubSub+ Event Broker Provider Version SEMPv2 Version
10.6.1 0.9.1+ 2.39
10.4.1 0.9.0 2.36

If you use an older Solace event broker provider with a newer event broker, you will not have the option to configure the new resources that might come with the new event broker load. Similarly, for an older event broker with a newer Solace provider, you might have the ability to create resources that the event broker doesn’t support. In that case your Terraform configuration will fail since SEMP doesn’t have the ability to configure your request.