Replicas and load balancing
A scale group is several copies of the same lab — replicas — serving one domain. Every request to that domain is handed to the next replica in turn (round-robin), so the load is spread and one replica going down does not take the site with it.
By the end of this page you will have a group running and know which options to turn on for your kind of app.
When to use one
| Use a scale group when | Use a single lab or an extra copy when |
|---|---|
| A public web app needs to handle more traffic than one lab can | You are developing, or only you use it |
| You want the site to stay up while one replica restarts | The app keeps its state in memory or local files and cannot be split |
| You want to try a new version on a slice of real traffic first | You want separate machines for separate projects |
- A lab whose template allows copies. The Scale button is on the lab's Deployments tab only when it does.
- At least one domain. The group is reached through it.
- An app that listens on a port inside the lab.
1. Open the wizard
Open the lab, go to Deployments and click Scale. The wizard has six steps; Next checks each one before moving on.
2. Name the group and point at your app
| Field | What to enter |
|---|---|
| Scale group | A name for the group — lowercase letters, digits and dashes, up to 32 characters |
| App port | The port your app listens on inside the lab, for example 8080 |
3. Choose what replicas see
| Option | What it means |
|---|---|
| Shared account home (default) | Replicas see the same files as all your other labs. Edits show up everywhere. |
| Isolated group storage | One private home shared by this group only. It lasts until the last replica is removed. |
With isolated storage you choose how it starts: Fresh empty home or Clone my home. Cloning is cheap where your storage supports it, and greyed out where it does not — the same rule as for extra copies.
Every replica in a group reads and writes the same files. That is right for stateless or read-mostly apps. If your app writes files that another replica might overwrite, keep that data in a database instead.
4. Decide how many replicas
Fixed count runs exactly the number you choose, from 2 to 16.
Autoscale adds and removes replicas for you:
| Setting | Meaning |
|---|---|
| Min replicas / Max replicas | The range the group stays within, up to 16 |
| CPU | Add replicas when the group is busier than this percentage |
| Memory | Add replicas above this percentage |
Pick at least one of CPU or memory. The step also tells you how many replicas of this size your plan can afford.
If your plan's compute budget is zero, the Autoscale option is disabled and you can only run a fixed count.
How autoscaling behaves:
- It checks the group about once a minute.
- It grows a few replicas at a time and shrinks one at a time, so traffic does not swing wildly.
- The memory signal only ever adds replicas; it never removes them.
- Every replica still counts against your copy limit, so the group cannot grow past it whatever Max replicas says.
5. Traffic and safety
| Option | What it does | Turn it on when |
|---|---|---|
| Domains for this group | The shared URL. Pick at least one. | Always |
| Sticky sessions (on by default) | Keeps each visitor on the same replica | Your app has logins or sessions held in memory |
| Compress responses (gzip/br) | Compresses pages on the way out | Text-heavy sites |
| Health-check path | A path such as /health that must answer successfully; replicas that fail it are taken out of rotation until they recover | Always with autoscale, and recommended otherwise |
| Restrict to IPs / CIDRs | Only these addresses can reach the site, comma-separated, e.g. 203.0.113.0/24 | Staging sites, internal tools |
| Basic-auth user / password | Visitors must enter this user name and password | A quick lock on a preview site |
6. Review and create
Check the summary and click Create scale group. The starting replicas are deployed one after another — for autoscale, that is the Min replicas number — and the policy is saved. You land on the lab's Deployments tab with the new group card.
The group card
Groups appear on the lab's Deployments tab and under Instances → My Instances → Scale groups.
| On the card | Meaning |
|---|---|
| Scale group: name | The group |
| X/Y running | Replicas up, out of the replicas in the group |
| Domain badges | The shared URL — click to open it |
| auto 2–4 · cpu 70% | Autoscaling on, with its range and signal |
| fixed 3 | A fixed count, autoscaling off |
The card also lists the group's last few scaling events, and has three buttons:
| Button | Use it to |
|---|---|
| Scaling | Change the policy — fixed or autoscale, the range and the signals |
| Canary | Try a new version on part of the traffic (below) |
| Add replica | Add one or more replicas to the group by hand |
Each replica is an ordinary lab copy underneath, with its own row, dashboard and Activity tab.
Canary releases
A canary is one extra replica on the same URL that takes a controlled share of traffic, so you can try a change before every visitor sees it.
Click Canary on the group card and choose a mode:
| Mode | What happens |
|---|---|
| Weighted | A percentage of real traffic (1–99%, set with the slider) goes to the canary |
| Mirror | The canary receives a copy of all traffic, but its responses are thrown away — visitors are never affected |
Click Deploy canary. To go back to 100% on the main replicas, stop the canary.
Mirror mode is the safe way to see whether a new version crashes under real requests. Once it holds up, redeploy it as a weighted canary at a small percentage.
Limits
| Limit | Value |
|---|---|
| Replicas per group | 2 to 16 at creation; up to 16 with autoscale |
| Copy limit | Every replica counts, running or stopped — see Running extra labs |
| Autoscale | Paid plans only |
| Domains | At least one per group |
Pausing a replica
If the group has no health check, a paused replica stays in rotation and its share of requests hangs. The dashboard warns you before pausing it. Add a health-check path, or stop the replica instead of pausing it.
Do it with an AI assistant
Scale groups are set up in the dashboard only; an assistant connected over MCP cannot create or change one. It can list the replicas with list_labs and check each one with lab_status and lab_stats.
Next
- Running extra labs — single independent copies
- Domains — add the domain the group serves
- Sharing and workgroups — let others operate your replicas