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# 3. Scenarios

The actions of a Record and Replay service are organized in scenarios.

A scenario is an instance of the scenario topic, a group of commands sharing the same `scenarioName`. Each service subscribes to the command topic and uses a content filter to only process commands with an `rnrId` matching the service name (or `*`).

It is possible to create an intricate nesting of scenarios by defining a scenario that includes control commands targeting other scenarios.

## 3.1. Different versions of the scenario topic

Starting with Vortex OpenSplice V6.5.2, the Record and Replay service provides two versions of the scenario topic: `rr_scenario` and `rr_scenario_v2`. The new `rr_scenario_v2` topic is contained in the `RnR_V2` IDL module, please see the RnR Topic API IDL specification in the Appendix for details of the changes between the original and version 2 of the data-model. Both versions will co-exist until a major version upgrade allows the merging of all features into a single module and topic. This will probably coincide with a migration of the data-model to Google Protocol Buffers (GPB) or a different extensible type scheme supported by Vortex OpenSplice at that time.

The original topic is still supported for backwards compatibility with applications developed for a previous version of OpenSplice or Record and Replay scenarios stored for re-use, i.e. in a persistent store. Also, because of the nature of the RnR service, the topic definitions of RnR may have been introduced in production environments that cannot easily be upgraded and/or restarted to replace the old topics with new ones. A topic mis-match would prevent a new version of RnR from starting on any node attached to the same domain. In those circumstances a new topic using new type-names is the only viable approach to support new RnR features. Transformations (partition, QoS) of data during replay are only available on the `rr_scenario_v2` topic because these new features require an extension of the `ADD_REPLAY` and `REMOVE_REPLAY` commands.

The next chapter describes all command-types, highlighting differences between the original and version 2.

Since all original features are also available using the `rr_scenario_v2` topic, it is not recommended to mix usage of the two topics in a single application. The service does support scenarios that use both original and V2 commands but order preservation cannot be guaranteed, since a scenario is no longer contained in a single instance but is in two instances on two different topics, necessitating two individual readers. In practice this can only introduce order reversal of commands when both `rr_scenario` and `rr_scenario_v2` commands arrive at the same time.

## 3.2. BuiltinScenario

Since commands are targeted at a service and a scenario, the service must start an initial scenario. If not, there wouldn’t be anything to address commands to.

During startup, the service starts this initial scenario, called the `BuiltinScenario`. This is a special scenario that is always running while the service is operational. It serves as the starting hook for any new scenarios. To run a new scenario, a start command must be published for the `BuiltinScenario`. Like any scenario, the `BuiltinScenario` can also process other commands like record and/or replay commands.

Note that the `BuiltinScenario` can not be stopped.

Since one can assume that the `BuiltinScenario` is always available and running, it is a safe choice to address config and control commands to the `BuiltinScenario`. In a dynamic and distributed environment, in which DDS is regularly used, this can be especially helpful when interacting with the service through scripts or perhaps when injecting commands stored in a persistent store.

## 3.3. Command durability

The command subscriber of the service is capable to read commands of any durability (`VOLATILE`, `TRANSIENT`, `PERSISTENT`). If commands are published with a transient and/or persistent durability, it is important to understand that these commands are managed by the middleware in addition to the service. Immediately after a scenario is started, any commands still managed by the middleware in transient or persistent stores are delivered to the service and processed as part of the scenario.

This is of special importance when ‘re-starting’ scenarios.
  Note that a scenario, strictly, is not restarted. It is removed and a new scenario with an identical name is created. If any of the commands of the original scenario were published (whether transient or persistent), these are delivered and processed again by the new scenario.

Since transient and persistent commands exist in the middleware and are not stored or processed by the service as long as the corresponding scenario isn’t started, the start command for a scenario does not have to be published before the scenario is defined, as one might assume. By changing the durability of different commands compromising a scenario, advanced use cases are possible using relatively simple scenarios.

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