> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Demo Service in Swarm Replicated Global Parallelism Placements

> Learn to optimize Docker Swarm services by controlling update parallelism, scaling replicated services, running tasks globally, and enforcing placement constraints.

In this tutorial, you’ll learn how to optimize your Docker Swarm services by controlling update parallelism, scaling replicated services, running tasks globally, and enforcing placement constraints. Whether you’re rolling out updates or targeting specific nodes, these patterns will help you deploy reliably at scale.

We’ll cover:

* Controlling update parallelism
* Creating replicated services
* Running services in global mode
* Constraining services to specific nodes

***

## 1. Update Parallelism

By default, Swarm updates one task at a time. Adjusting the update parallelism speeds up rollouts or rollbacks across many tasks.

```bash theme={null}
docker service inspect secondservice --pretty
```

Example output:

```yaml theme={null}
UpdateConfig:
  Parallelism:         1
  On failure:          pause
  Monitoring Period:   5s
  Max failure ratio:   0
  Update order:        stop-first

RollbackConfig:
  Parallelism:         1
  On failure:          pause
  Monitoring Period:   5s
  Max failure ratio:   0
  Rollback order:      stop-first
```

To upgrade two tasks at a time, use `--update-parallelism`:

```bash theme={null}
docker service update \
  --image httpd:2 \
  --update-parallelism 2 \
  secondservice
```

Re-inspect to confirm the change:

```bash theme={null}
docker service inspect secondservice --pretty
```

You’ll see `UpdateConfig.Parallelism: 2` while the rollback parallelism remains unchanged.

<Callout icon="lightbulb" color="#1CB2FE">
  Increasing `--update-parallelism` accelerates rollouts but may spike resource usage. Tune based on your cluster capacity.
</Callout>

***

## 2. Replicated Service

A replicated service ensures a fixed number of identical tasks across the swarm.

```bash theme={null}
docker service create \
  --name replicatedtest \
  --replicas 8 \
  redis:latest
```

Progress output:

```text theme={null}
overall progress: 8 out of 8 tasks
1/8: running    [==============>              ]
…
8/8: running    [==============================]
verify: Service converged
```

Verify service and mode:

```bash theme={null}
docker service ls
docker service inspect replicatedtest --pretty | grep Mode
```

Output:

```text theme={null}
Mode: replicated
Replicas: 8
```

***

## 3. Global Service

Global mode deploys exactly one task per active node. When nodes join or leave, tasks are added or removed automatically.

```bash theme={null}
docker service create \
  --mode global \
  --name globaltest \
  redis:latest
```

You’ll see a task scheduled on each node:

```text theme={null}
overall progress: 6 out of 6 tasks
a1b2c3d4e5f6: running [==============================>]
…
z9y8x7w6v5u4: running [==============================>]
verify: Service converged
```

Confirm:

```bash theme={null}
docker service ls
docker node ls
```

***

## 4. Placement Constraints

Placement constraints let you target services to nodes with specific labels.

1. **Label the node** (replace `<NODE_ID>`):

   ```bash theme={null}
   docker node update --label-add env=dev <NODE_ID>
   ```

2. **Verify the label**:

   ```bash theme={null}
   docker node inspect <NODE_ID> --format '{{ .Spec.Labels }}'
   ```

3. **Create the constrained service**:

   ```bash theme={null}
   docker service create \
     --name placementtest \
     --constraint node.labels.env==dev \
     --replicas 3 \
     redis:latest
   ```

All three tasks will run only on the node labeled `env=dev`.

<Callout icon="triangle-alert" color="#FF6B6B">
  If no nodes match the constraint, the service remains pending. Always verify labels before deployment.
</Callout>

***

## Service Mode Comparison

| Service Mode | Description                         | Example Flag           |
| ------------ | ----------------------------------- | ---------------------- |
| Replicated   | Fixed number of tasks               | `--replicas 8`         |
| Global       | One task per active node            | `--mode global`        |
| UpdateConfig | Control rollout parallelism & order | `--update-parallelism` |

***

## Links and References

* [Docker Swarm Overview](https://docs.docker.com/engine/swarm/)
* [docker service update](https://docs.docker.com/engine/reference/commandline/service_update/)
* [Scheduling tasks in swarm mode](https://docs.docker.com/engine/swarm/how-swarm-mode-works/swarm-task/)

That concludes this demo on Docker Swarm update parallelism, service modes, and placement constraints.

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