> ## 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.

# Placement in Swarm

> Managing service placement in Docker Swarm to optimize workload distribution across nodes with different resource profiles.

Managing service placement in Docker Swarm ensures that each workload runs on the best-suited node. By default, Swarm distributes tasks evenly across any node with available resources. In real-world clusters, however, you often have nodes optimized for different workloads:

| Node Name | Label                   | Profile                                |
| --------- | ----------------------- | -------------------------------------- |
| worker1   | `type=CPU-optimized`    | CPU-optimized (batch processing)       |
| worker2   | `type=memory-optimized` | Memory-optimized (real-time analytics) |
| worker3   | `type=GP`               | General-purpose (web servers)          |

And services such as:

| Service Name       | Resource Profile |
| ------------------ | ---------------- |
| batch-processing   | CPU-intensive    |
| realtime-analytics | Memory-intensive |
| web                | General-purpose  |

Without explicit placement constraints, Swarm might schedule a CPU-heavy task on a memory-optimized node or vice versa, causing contention and inefficiency.

## 1. Labeling Nodes

Assign key-value labels to each node to reflect its resource profile:

```bash theme={null}
docker node update --label-add type=CPU-optimized    worker1
docker node update --label-add type=memory-optimized worker2
docker node update --label-add type=GP               worker3
```

<Callout icon="lightbulb" color="#1CB2FE">
  After labeling, confirm with:

  ```bash theme={null}
  docker node inspect --format '{{ .Description.Hostname }}: {{ .Spec.Labels }}' worker1
  ```
</Callout>

Below is a diagram illustrating the labeled nodes and associated workloads:

<Frame>
  ![The image illustrates a system architecture with three worker nodes labeled as CPU-optimized, memory-optimized, and general-purpose, alongside components for web servers, batch processing, and real-time analytics.](https://kodekloud.com/kk-media/image/upload/v1752873928/notes-assets/images/Docker-Certified-Associate-Exam-Course-Placement-in-Swarm/system-architecture-worker-nodes.jpg)
</Frame>

## 2. Applying Placement Constraints

Use the `--constraint` flag with `docker service create` to bind services to nodes based on labels or built-in properties.

### 2.1 Match a Label

Schedule CPU-intensive and memory-intensive services on their respective optimized nodes:

```bash theme={null}
docker service create \
  --name batch-processing \
  --constraint 'node.labels.type==CPU-optimized' \
  batch-processing

docker service create \
  --name realtime-analytics \
  --constraint 'node.labels.type==memory-optimized' \
  realtime-analytics
```

### 2.2 Exclude a Label

Prevent the web service from running on memory-optimized nodes:

```bash theme={null}
docker service create \
  --name web \
  --constraint 'node.labels.type!=memory-optimized' \
  web
```

This ensures `web` is placed on either the CPU-optimized or general-purpose node.

<Callout icon="triangle-alert" color="#FF6B6B">
  Constraint expressions are case-sensitive and must be enclosed in quotes.\
  For more options, see the [Service Create reference][service_create].
</Callout>

### 2.3 Using Built-in Node Properties

You can also constrain by node role. For example, to run a service only on worker nodes (excluding managers):

```bash theme={null}
docker service create \
  --name worker-only-service \
  --constraint 'node.role==worker' \
  alpine:latest ping docker.com
```

## 3. Summary of Constraints

| Service Name        | Constraint                           |
| ------------------- | ------------------------------------ |
| batch-processing    | `node.labels.type==CPU-optimized`    |
| realtime-analytics  | `node.labels.type==memory-optimized` |
| web                 | `node.labels.type!=memory-optimized` |
| worker-only-service | `node.role==worker`                  |

By combining node labels with placement constraints, you guarantee that each workload in your Swarm cluster runs on the most suitable hardware, improving both performance and resource utilization.

## Links and References

* [Docker Swarm Overview](https://docs.docker.com/engine/swarm/)
* [Service Create reference][service_create]
* [Docker Engine CLI](https://docs.docker.com/engine/reference/commandline/)

[service_create]: https://docs.docker.com/engine/reference/commandline/service_create/

<CardGroup>
  <Card title="Watch Video" icon="video" cta="Learn more" href="https://learn.kodekloud.com/user/courses/docker-certified-associate-exam-course/module/16b8b1e1-1e1f-4e11-976f-8d5c1223c53d/lesson/2fd9461d-bd7c-42f2-a5b3-af8ef2c484ee" />
</CardGroup>