# Machine step
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A machine step is used to read or write data from/to other systems, or to modify the variables in a workflow. Machine steps are invisible to the operator except when something goes wrong (e.g. the ERP system responds with an error message). If a machine step goes wrong, the operator gets an error message and is transported back to the most recent user step.
Machine steps that potentially modify data in another system are called *non-reversible*. Incoming connections to non-reversible machine steps are adorned with an orthogonal line indicating that an operator will not be able to user the back button in the flow client once this step has been successfully executed.
Avoid consecutive non-reversible Machine steps, because each machine step is executed atomically (in its own transaction), any changes to underlying systems are committed as soon as the step is completed. Therefore, in case a transaction has already been committed, the Flow Client cannot go back to the previous user step without risking data inconsistency. If this happens, the operator is forced to exit the workflow right away.
In the example below (see picture), the operator would be forced to exit the workflow if machine step **B** executed successfully but machine step **C** failed.
#### Connectors:
Read more about connectors [here](https://help.novacuraflow.com/development/flow-studio/connectors).
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