> ## 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 Prepared Queries

> This guide covers leveraging Consul Prepared Queries for metadata-driven traffic routing, including creating, inspecting, and updating queries for seamless traffic shifts.

In this guide, we’ll cover how to leverage Consul Prepared Queries for metadata-driven traffic routing. You’ll learn to create, inspect, and update prepared queries to shift traffic seamlessly between service versions.

## Consul Cluster Overview

Start by verifying your cluster members on a server node:

```bash theme={null}
consul members
```

Sample output:

```text theme={null}
Node            Address               Status  Type    Build      Protocol  DC          Segment
consul-node-a   10.0.101.110:8301     alive   server  1.9.3+ent  2         us-east-1   <all>
consul-node-b   10.0.101.248:8301     alive   server  1.9.3+ent  2         us-east-1   <all>
web-server-01   10.0.101.177:8301     alive   client  1.9.3+ent  2         us-east-1   <default>
web-server-02   10.0.101.114:8301     alive   client  1.9.3+ent  2         us-east-1   <default>
```

For clarity, here’s the same data in a table:

| Node          | Address           | Status | Type   | Protocol | Datacenter |
| ------------- | ----------------- | ------ | ------ | -------- | ---------- |
| consul-node-a | 10.0.101.110:8301 | alive  | server | 2        | us-east-1  |
| consul-node-b | 10.0.101.248:8301 | alive  | server | 2        | us-east-1  |
| web-server-01 | 10.0.101.177:8301 | alive  | client | 2        | us-east-1  |
| web-server-02 | 10.0.101.114:8301 | alive  | client | 2        | us-east-1  |

Each client hosts an Apache-based e-commerce front end, registering the `front-end-eCommerce` service with version tags.

## Service Registration in the Consul UI

In the Consul web interface, you’ll see two instances of `front-end-eCommerce`. One is tagged `v7.05` and the other `v8`, both in the `production` environment.

<Frame>
  ![The image shows a web interface for managing services, specifically displaying two web servers under "front-end-eCommerce," with all service checks passing.](https://kodekloud.com/kk-media/image/upload/v1752877893/notes-assets/images/HashiCorp-Certified-Consul-Associate-Certification-Demo-Prepared-Queries/web-interface-managing-services-ecommerce.jpg)
</Frame>

| Server        | IP Address   | Version Tag |
| ------------- | ------------ | ----------- |
| web-server-02 | 10.0.101.114 | v7.05       |
| web-server-01 | 10.0.101.177 | v8          |

## Creating a Prepared Query

1. Save this JSON as `prepared-query.json`:

   ```json theme={null}
   {
     "Name": "eCommerce",
     "Service": {
       "Service": "front-end-eCommerce",
       "Tags": ["v7.05", "production"]
     }
   }
   ```

2. Register the query via Consul’s HTTP API:

   ```bash theme={null}
   curl --request POST --data @prepared-query.json http://10.0.101.110:8500/v1/query | jq
   ```

   Response:

   ```json theme={null}
   {
     "ID": "b34f3b89-68be-9285-8f3e-c05d5d09f7le"
   }
   ```

3. Inspect the full query definition:

   ```bash theme={null}
   curl http://10.0.101.110:8500/v1/query/b34f3b89-68be-9285-8f3e-c05d5d09f7le | jq
   ```

   ```json theme={null}
   {
     "ID": "b34f3b89-68be-9285-8f3e-c05d5d09f7le",
     "Name": "eCommerce",
     "Service": {
       "Service": "front-end-eCommerce",
       "Failover": { "NearestN": 0, "Datacenters": null }
     },
     "OnlyPassing": false,
     "Tags": ["v7.05", "production"]
     // ... other fields ...
   }
   ```

<Callout icon="lightbulb" color="#1CB2FE">
  Set `"OnlyPassing": true` in your query definition to ensure only healthy service instances are returned.
</Callout>

## Querying via DNS

Consul exposes prepared queries under the `*.query.consul` DNS domain. Run:

```bash theme={null}
dig @10.0.101.110 -p 8600 eCommerce.query.consul
```

You should see the IP of the `v7.05` instance:

```text theme={null}
;; ANSWER SECTION:
eCommerce.query.consul. 0 IN A 10.0.101.114
```

## Updating the Prepared Query to v8

When it’s time to shift traffic to version `v8`, update `prepared-query.json`:

```json theme={null}
{
  "Name": "eCommerce",
  "Service": {
    "Service": "front-end-eCommerce",
    "Tags": ["v8", "production"]
  }
}
```

Apply the update with a `PUT` request (replace the ID):

```bash theme={null}
curl --request PUT --data @prepared-query.json \
  http://10.0.101.110:8500/v1/query/b34f3b89-68be-9285-8f3e-c05d5d09f7le
```

Then verify with DNS again:

```bash theme={null}
dig @10.0.101.110 -p 8600 eCommerce.query.consul
```

```text theme={null}
;; ANSWER SECTION:
eCommerce.query.consul. 0 IN A 10.0.101.177
```

<Callout icon="triangle-alert" color="#FF6B6B">
  Always replace the query ID in your API URL when inspecting or updating prepared queries.
</Callout>

## Conclusion

Consul Prepared Queries enable you to route client requests based on service metadata without touching client configurations. By updating the query payload, you can perform zero-downtime version rollouts and A/B testing with ease.

## Links and References

* [Consul Prepared Queries](https://www.consul.io/docs/discovery/prepared-queries)
* [Consul DNS Interface](https://www.consul.io/docs/agent/dns)
* [Service Mesh with Consul Connect](https://www.consul.io/docs/connect)

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