# crm\_simulate

`crm_simulate` is a command-line tool that is part of the crmsh suite used for Pacemaker cluster management. It allows administrators to simulate cluster configuration changes and resource transitions without actually applying those changes to the live cluster. This simulation capability helps in testing and validating the effects of configuration modifications, resource constraints, and ordering rules before they are committed, thereby reducing the risk of misconfiguration or unexpected cluster behavior.

## Purpose

* **Pre-Deployment Testing:**\
  Simulate how the cluster would respond to configuration changes or resource actions. This is useful for validating new resource definitions, constraint modifications, or failover scenarios in a safe environment.
* **Troubleshooting:**\
  Diagnose potential issues by simulating transitions. For example, you can simulate the impact of bringing a node into standby or forcing a resource to migrate to another node.
* **What-If Analysis:**\
  Understand the effects of different configurations, ordering constraints, or colocation rules on resource allocation and cluster behavior without impacting production services.

## How It Works

* **Input Configuration:**\
  `crm_simulate` uses the current cluster configuration (or a specified configuration file) to compute the expected transitions. It takes into account resource definitions, constraints, and node states as they are defined in the Cluster Information Base (CIB).
* **Simulated Transitions:**\
  The tool simulates events such as resource failures, recoveries, or manual actions like starting or stopping a resource. It then outputs the series of actions that the cluster manager (Pacemaker) would execute in response to those events.
* **Output:**\
  The simulation results include a step-by-step description of the proposed actions, such as resource stop/start sequences, fencing operations, or relocation of resources. This output is meant for analysis and does not affect the actual state of the cluster.

## Basic Syntax

```bash
crm_simulate [options]
```

* **Options:**\
  You can specify various options to adjust the simulation parameters, such as selecting a specific scenario or increasing the verbosity of the output. Common options include:
  * `-v` or `--verbose`: Produce detailed simulation output.
  * `-f <file>`: Use a specified CIB or configuration file for the simulation instead of the active configuration.
  * Other options may be available depending on your crmsh version.

## Example Usage

1.  **Simulate the Active Configuration:**

    To simulate what would happen with the current cluster configuration:

    ```bash
    crm_simulate -v
    ```

    This will output detailed steps of resource transitions and actions that would be taken under simulated conditions.
2.  **Simulate Using a Specific Configuration File:**

    To test changes from a new configuration file without applying them:

    ```bash
    crm_simulate -f /path/to/test-cib.xml -v
    ```

    This allows you to preview the impact of proposed changes before updating the live cluster.

## Best Practices

* **Review Output Thoroughly:**\
  Use the detailed output to understand each simulated action. Verify that resource constraints and ordering rules behave as expected.
* **Test in a Safe Environment:**\
  If possible, run simulations on a non-production cluster or with a test configuration file to prevent any accidental changes to production resources.
* **Integrate into Change Management:**\
  Incorporate simulation as part of your cluster configuration change process to catch potential issues before they impact service availability.

## Conclusion

`crm_simulate` is a valuable tool for administrators managing Pacemaker clusters. It provides a safe way to validate and troubleshoot configuration changes and resource transitions by simulating the cluster's response. By using `crm_simulate`, you can gain insights into how the cluster will behave under various scenarios, ultimately leading to more reliable and predictable cluster operations.
