Ever waited for PR preview environments to spin up? Yeah, me too. Here's a pattern that changed the game for our team: pre-configured deployment slots with deterministic routing. PR preview environments deployment slots The Problem Traditional PR preview workflows go something like this: Open PR
CI/CD provisions a new environment
Wait... ⏳
Deploy code
Wait some more... ⏳
Finally get your preview URL Open PR CI/CD provisions a new environment Wait... ⏳ Deploy code Wait some more... ⏳ Finally get your preview URL The provisioning step is the killer. Whether you're using Kubernetes namespaces, cloud functions, or edge workers, creating resources takes time. The Solution: Pre-Configured Slots What if we flipped the script? Instead of creating environments on-demand, we pre-configure a fixed set of deployment slots: tokyo    🔗 https://tokyo.example.com
paris    🔗 https://paris.example.com
london   🔗 https://london.example.com
berlin   🔗 https://berlin.example.com
sydney   🔗 https://sydney.example.com
madrid   🔗 https://madrid.example.com
moscow   🔗 https://moscow.example.com
cairo    🔗 https://cairo.example.com
dubai    🔗 https://dubai.example.com
rome     🔗 https://rome.example.com tokyo    🔗 https://tokyo.example.com
paris    🔗 https://paris.example.com
london   🔗 https://london.example.com
berlin   🔗 https://berlin.example.com
sydney   🔗 https://sydney.example.com
madrid   🔗 https://madrid.example.com
moscow   🔗 https://moscow.example.com
cairo    🔗 https://cairo.example.com
dubai    🔗 https://dubai.example.com
rome     🔗 https://rome.example.com Then use a deterministic hash to map PR numbers to slots: - uses: kriasoft/pr-codename@v1
  id: pr

- run: wrangler deploy --env ${{ steps.pr.outputs.codename }} - uses: kriasoft/pr-codename@v1
  id: pr

- run: wrangler deploy --env ${{ steps.pr.outputs.codename }} PR #1234 always maps to tokyo. PR #1235 always maps to paris. No provisioning, no waiting. tokyo paris How It Works The magic happens in three parts: 1. Pre-Configure Your Slots First, set up your deployment slots. Here's a Cloudflare Workers example: # wrangler.toml
[env.tokyo]
name = "preview-tokyo"
route = "tokyo.example.com/*"

[env.paris]
name = "preview-paris"
route = "paris.example.com/*"

[env.london]
name = "preview-london"
route = "london.example.com/*"

# ... repeat for all slots # wrangler.toml
[env.tokyo]
name = "preview-tokyo"
route = "tokyo.example.com/*"

[env.paris]
name = "preview-paris"
route = "paris.example.com/*"

[env.london]
name = "preview-london"
route = "london.example.com/*"

# ... repeat for all slots 2. Deterministic Mapping The PR Codename Action uses a simple hash function to consistently map PR numbers to slot names: PR Codename Action const words = ["tokyo", "paris", "london", "berlin" /* ... */];
const index = prNumber % words.length;
return words[index]; const words = ["tokyo", "paris", "london", "berlin" /* ... */];
const index = prNumber % words.length;
return words[index]; The above is just an example, in reality it uses FNV-1a hashing algorithm. FNV-1a hashing algorithm 3. Deploy to the Slot Your GitHub Action workflow becomes dead simple: name: Deploy PR Preview

on:
  pull_request:
    types: [opened, synchronize]

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - uses: kriasoft/pr-codename@v1
        id: pr

      - name: Deploy to slot
        run: |
          npm ci
          npm run build
          wrangler deploy --env ${{ steps.pr.outputs.codename }}

      - name: Comment PR
        uses: actions/github-script@v7
        with:
          script: |
            github.rest.issues.createComment({
              issue_number: context.issue.number,
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: '🚀 Preview deployed to https://${{ steps.pr.outputs.codename }}.example.com'
            }) name: Deploy PR Preview

on:
  pull_request:
    types: [opened, synchronize]

jobs:
  deploy:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - uses: kriasoft/pr-codename@v1
        id: pr

      - name: Deploy to slot
        run: |
          npm ci
          npm run build
          wrangler deploy --env ${{ steps.pr.outputs.codename }}

      - name: Comment PR
        uses: actions/github-script@v7
        with:
          script: |
            github.rest.issues.createComment({
              issue_number: context.issue.number,
              owner: context.repo.owner,
              repo: context.repo.repo,
              body: '🚀 Preview deployed to https://${{ steps.pr.outputs.codename }}.example.com'
            }) The Benefits This pattern isn't just a neat trick; it fundamentally changes the rhythm of your development cycle. 🚀 Zero-Wait Deploys The biggest win. By eliminating the on-demand provisioning step, deployments start immediately. What used to be a 2-3 minute coffee break is now a 30-second task. Your developers stay in the flow, and your pipeline gets a whole lot faster. 🚀 Zero-Wait Deploys immediately 🔗 URLs You Can Actually Share Forget long, ugly, auto-generated URLs. With this pattern, PR #1234 always maps to https://tokyo.example.com. This URL is: 🔗 URLs You Can Actually Share https://tokyo.example.com Memorable: You can actually remember it.
Shareable: Perfect for dropping in a Slack channel, a Jira ticket, or even saying out loud during a Zoom call. No more "Hey, can you find that preview link for me?"
Bookmarkable: QA testers and product managers can bookmark slots for features they're tracking. Memorable: You can actually remember it. Memorable: Shareable: Perfect for dropping in a Slack channel, a Jira ticket, or even saying out loud during a Zoom call. No more "Hey, can you find that preview link for me?" Shareable: Bookmarkable: QA testers and product managers can bookmark slots for features they're tracking. Bookmarkable: 💰 No More Cloud Bill Surprises Dynamic environments are notorious for leaving behind orphaned resources that quietly drain your budget. With a fixed number of slots, your infrastructure costs become predictable. You know exactly what's running, and you never have to hunt down forgotten preview apps again. 💰 No More Cloud Bill Surprises 🧹 Cleanup? What Cleanup? When a PR is merged or closed, there's no complex teardown script to run. The slot simply becomes available for the next PR. You can even have a workflow that automatically deploys the main branch to the slot to keep it fresh. It's a self-cleaning system. 🧹 Cleanup? What Cleanup? main Real-World Considerations How Many Slots? We've found 10-15 slots work well for most teams. The math: 10 slots + 50 open PRs = each slot serves ~5 PRs
Only the latest deployment to each slot is accessible
Most teams only actively review a handful of PRs at once 10 slots + 50 open PRs = each slot serves ~5 PRs Only the latest deployment to each slot is accessible Most teams only actively review a handful of PRs at once Collision Handling Yes, PRs can map to the same slot. PR #1 and PR #11 both map to the same environment with 10 slots. This means newer deployments overwrite older ones—so if you're reviewing PR #1 and someone pushes PR #11, your preview disappears. This means newer deployments overwrite older ones In practice, this works for many teams because: Developers typically work on recent PRs
Old PR previews naturally expire
You can always trigger a redeploy to refresh Developers typically work on recent PRs Old PR previews naturally expire You can always trigger a redeploy to refresh When slots don't work well: Large teams, high PR velocity, or when multiple people need to review the same PR simultaneously. When slots don't work well: Database & Stateful Services The biggest challenge with any preview environment is handling databases and stateful services. With slots, you have a few options: Shared database: Fast and cheap, but schema migrations from one PR can break others
Database per slot: Better isolation, but requires seeding data for each slot
Database branching services: Tools like Neon offer instant database branches (premium option) Shared database: Fast and cheap, but schema migrations from one PR can break others Shared database: Database per slot: Better isolation, but requires seeding data for each slot Database per slot: Database branching services: Tools like Neon offer instant database branches (premium option) Database branching services: For simple stateless apps, this isn't an issue. For complex apps with databases, it's the main implementation challenge. Security Notes Use environment-specific secrets for each slot
Consider adding basic auth to preview domains
Implement automatic cleanup for stale deployments Use environment-specific secrets for each slot Consider adding basic auth to preview domains Implement automatic cleanup for stale deployments Beyond Basic Previews This pattern unlocks some cool possibilities: Persistent Test Environments: QA can bookmark specific slots for testing. Persistent Test Environments A/B Testing: Map feature flags to slots for instant switching. A/B Testing Geographic Testing: Actually deploy slots to different regions. Geographic Testing Try It Yourself Getting started is pretty straightforward: Install the action:
- uses: kriasoft/pr-codename@v1
  id: pr



Use the codename in your deploy:
deploy --env ${{ steps.pr.outputs.codename }}



Enjoy instant PR previews 🚀 Install the action:
- uses: kriasoft/pr-codename@v1
  id: pr Install the action: - uses: kriasoft/pr-codename@v1
  id: pr - uses: kriasoft/pr-codename@v1
  id: pr Use the codename in your deploy:
deploy --env ${{ steps.pr.outputs.codename }} Use the codename in your deploy: deploy --env ${{ steps.pr.outputs.codename }} deploy --env ${{ steps.pr.outputs.codename }} Enjoy instant PR previews 🚀 Enjoy instant PR previews 🚀 The full source is on GitHub if you want to customize the word list or hashing algorithm. GitHub Slots vs On-Demand: Quick Comparison Before you dive in, it's worth understanding how the pre-configured slots pattern stacks up against the traditional on-demand ephemeral environments. While this post focuses on slots, knowing the trade-offs helps you make the right choice for your team. Factor

Pre-Configured Slots

On-Demand Ephemeral



Setup Speed

⭐⭐⭐⭐⭐ Instant (pre-warmed)

⭐⭐⭐ Takes minutes (provisioning)



Cost Predictability

⭐⭐⭐⭐⭐ Fixed monthly cost

⭐⭐ Variable usage-based



Scalability

⭐⭐ Hard limit on concurrent PRs

⭐⭐⭐⭐⭐ Scales with team size



Isolation

⭐⭐ PRs can overwrite each other

⭐⭐⭐⭐⭐ Each PR gets own environment



Production Fidelity

⭐⭐⭐ Prone to environment drift

⭐⭐⭐⭐⭐ Clean slate every time



Maintenance

⭐⭐⭐⭐ Simple for basic apps

⭐⭐ Complex (build) / ⭐⭐⭐⭐ (buy)



Developer Experience

⭐⭐ Can be confusing/frustrating

⭐⭐⭐⭐⭐ Smooth parallel workflows Factor

Pre-Configured Slots

On-Demand Ephemeral



Setup Speed

⭐⭐⭐⭐⭐ Instant (pre-warmed)

⭐⭐⭐ Takes minutes (provisioning)



Cost Predictability

⭐⭐⭐⭐⭐ Fixed monthly cost

⭐⭐ Variable usage-based



Scalability

⭐⭐ Hard limit on concurrent PRs

⭐⭐⭐⭐⭐ Scales with team size



Isolation

⭐⭐ PRs can overwrite each other

⭐⭐⭐⭐⭐ Each PR gets own environment



Production Fidelity

⭐⭐⭐ Prone to environment drift

⭐⭐⭐⭐⭐ Clean slate every time



Maintenance

⭐⭐⭐⭐ Simple for basic apps

⭐⭐ Complex (build) / ⭐⭐⭐⭐ (buy)



Developer Experience

⭐⭐ Can be confusing/frustrating

⭐⭐⭐⭐⭐ Smooth parallel workflows Factor

Pre-Configured Slots

On-Demand Ephemeral Factor Factor Pre-Configured Slots Pre-Configured Slots On-Demand Ephemeral On-Demand Ephemeral Setup Speed

⭐⭐⭐⭐⭐ Instant (pre-warmed)

⭐⭐⭐ Takes minutes (provisioning) Setup Speed Setup Speed Setup Speed ⭐⭐⭐⭐⭐ Instant (pre-warmed) ⭐⭐⭐⭐⭐ Instant (pre-warmed) ⭐⭐⭐ Takes minutes (provisioning) ⭐⭐⭐ Takes minutes (provisioning) Cost Predictability

⭐⭐⭐⭐⭐ Fixed monthly cost

⭐⭐ Variable usage-based Cost Predictability Cost Predictability Cost Predictability ⭐⭐⭐⭐⭐ Fixed monthly cost ⭐⭐⭐⭐⭐ Fixed monthly cost ⭐⭐ Variable usage-based ⭐⭐ Variable usage-based Scalability

⭐⭐ Hard limit on concurrent PRs

⭐⭐⭐⭐⭐ Scales with team size Scalability Scalability Scalability ⭐⭐ Hard limit on concurrent PRs ⭐⭐ Hard limit on concurrent PRs ⭐⭐⭐⭐⭐ Scales with team size ⭐⭐⭐⭐⭐ Scales with team size Isolation

⭐⭐ PRs can overwrite each other

⭐⭐⭐⭐⭐ Each PR gets own environment Isolation Isolation Isolation ⭐⭐ PRs can overwrite each other ⭐⭐ PRs can overwrite each other ⭐⭐⭐⭐⭐ Each PR gets own environment ⭐⭐⭐⭐⭐ Each PR gets own environment Production Fidelity

⭐⭐⭐ Prone to environment drift

⭐⭐⭐⭐⭐ Clean slate every time Production Fidelity Production Fidelity Production Fidelity ⭐⭐⭐ Prone to environment drift ⭐⭐⭐ Prone to environment drift ⭐⭐⭐⭐⭐ Clean slate every time ⭐⭐⭐⭐⭐ Clean slate every time Maintenance

⭐⭐⭐⭐ Simple for basic apps

⭐⭐ Complex (build) / ⭐⭐⭐⭐ (buy) Maintenance Maintenance Maintenance ⭐⭐⭐⭐ Simple for basic apps ⭐⭐⭐⭐ Simple for basic apps ⭐⭐ Complex (build) / ⭐⭐⭐⭐ (buy) ⭐⭐ Complex (build) / ⭐⭐⭐⭐ (buy) Developer Experience

⭐⭐ Can be confusing/frustrating

⭐⭐⭐⭐⭐ Smooth parallel workflows Developer Experience Developer Experience Developer Experience ⭐⭐ Can be confusing/frustrating ⭐⭐ Can be confusing/frustrating ⭐⭐⭐⭐⭐ Smooth parallel workflows ⭐⭐⭐⭐⭐ Smooth parallel workflows Best for Slots: Small teams, simple apps, tight budgets Best for Slots: Best for On-Demand: Growing teams, complex apps, quality-focused Best for On-Demand: Nothing prevents you from mixing both approaches — use slots for rapid prototyping and on-demand for critical features. Nothing prevents you from mixing both approaches — use slots for rapid prototyping and on-demand for critical features. Wrapping Up Sometimes the best optimization is avoiding work altogether. By pre-configuring deployment slots and using deterministic routing, we eliminated the biggest bottleneck in our PR workflow. Give it a shot and let me know how it works for your team. Happy deploying! What patterns have you used for PR previews? Drop a comment below 👇 always curious to hear different approaches! What patterns have you used for PR previews? Drop a comment below 👇 always curious to hear different approaches!

Walkthroughs, tutorials, guides, and tips. This story will teach you how to do something new or how to do something better.

The code in this story is for educational purposes. The readers are solely responsible for whatever they build with it.

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