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Node Matching

Node matching helps sovereign to decide which instances it should use as data when serving out envoy configuration to proxies.

For example imagine that you have 3 sources, each being an API with different data such as endpoints, routes, and listeners.
You may not want all of that data to be used when creating clusters, routes, and listeners for every envoy proxy that requests configuration from Sovereign.

If this isn't the case then you can turn node matching off entirely.

Default node matching behavior

By default, Sovereign expects each Instance to have a key service_clusters, which it will compare with a field named cluster which Envoy provides with each Discovery Request.

The following example shows an instance which has to two service clusters, group1 and group2

!!! example

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```json
[
    {
        "name": "instance1",
        "endpoints": [
            "server.local:8080"
        ],
        "service_clusters": [
            "group1",
            "group2"
        ]
    }
]
```

And an envoy proxy will make a discovery request which might look like:

!!! example

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```json
{
  "node": {
    "cluster": "group1",
    "metadata": {
      "instance_size": "small"
    },
    "build_version": "e5f864a82d4f27110359daa2fbdcb12d99e415b9/1.9.0/Clean/RELEASE"
  },
  "resource_names": [],
  "version_info": "0"
}
```

Since the proxy has a cluster with group1, Sovereign will include the above Instance when generating configuration.

Choosing which keys to compare

The node match key

Using the above discovery request example, we could select a different key to match on.

For example, if you wanted to provide different configuration based on the metadata, you could change the node match key as follows:

!!! example

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```yaml
matching: 
  node_key: metadata.instance_size
```

This means that sovereign will use instances with service_clusters that contain small.

The source match key

Say for example your source data doesn't contain a key named service_clusters - you can change the source match key in the same way as above.

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