# How to prevent multiple simultaneous deployments in Azure DevOps

CI/CD has completely transformed the way we work today. Instead of releasing the software once a month or somewhere in the future, we now release it multiple times a day. In our environment, each merge to the main branch triggers a deployment to production. Consequently, if we approve and merge several PRs simultaneously, we may encounter a race condition when two or more deployments attempt to deploy to the same environment.

The solution to this problem is an [Exclusive lock](https://learn.microsoft.com/en-us/azure/devops/pipelines/process/approvals?view=azure-devops&preserve-view=true&tabs=check-pass#exclusive-lock), which allows only one pipeline to proceed at a time. When multiple runs attempt to deploy, they may either wait (using a `sequential` lock) or be canceled, with only the latest one acquiring a lock (using a `runLatest`).

Now, let's examine the code snippet below:

```yaml
trigger:
  branches:
    include:
    - main

pool:
  vmImage: ubuntu-latest

lockBehavior: runLatest

stages:
- stage: 'deployment'
  jobs:
  - deployment: 'deployment'
    environment: production
    strategy:
      runOnce:
        deploy:
          steps:
          - script: sleep 30
```

The setting `lockBehavior: runLatest` indicates that we want to run only the latest deployment, canceling any other concurrent deployments. `sequential` is another possible lock behavior that allows all runs to deploy in a line.

`environment: production` specifies the environment to which we intend to deploy. It is crucial to define this parameter, as environments are considered protected resources in Azure DevOps, and we can apply an `Exclusive Lock` to them.

If you run the pipeline created from this YAML file multiple times, you probably won't notice any difference, as all deployments will run concurrently. To rectify this, we need to set an exclusive lock on our production environment. To do so:

* Navigate to `Environments`\->`production`\->`'Approvals and checks'` tab
    
* Click `Add check(+)` button
    
* Select `Exclusive lock`
    
* Click `Next` button
    
* Click `Create` button
    

Now the lock will take effect, and after running the pipeline multiple times, you will most likely observe that only the latest deployment is executed. If you need to deploy all runs concurrently, simply change `lockBehavior` to `sequential`.

Image credits: "[Photo of Train on Train Tracks](https://www.pexels.com/photo/photo-of-train-on-train-tracks-3617450/)" by [**Benjamin Suter**](https://www.pexels.com/@benjaminjsuter/)