add feature for region self to use ec2metadata service for configuration

implements solution for issue #26

cmd/sync/aws.go

@@ -7,6 +7,7 @@ import (
 
 	"github.com/aws/aws-sdk-go/aws"
 	"github.com/aws/aws-sdk-go/aws/session"
+	"github.com/aws/aws-sdk-go/aws/ec2metadata"
 	"github.com/aws/aws-sdk-go/service/autoscaling"
 	"github.com/aws/aws-sdk-go/service/autoscaling/autoscalingiface"
 	"github.com/aws/aws-sdk-go/service/ec2"
@@ -29,6 +30,24 @@ func NewAWSClient(data []byte) (*AWSClient, error) {
 		return nil, fmt.Errorf("error validating config: %v", err)
 	}
 
+	if cfg.Region == "self" {
+		metaSession, err := session.NewSession()
+		if err != nil {
+			return nil, err
+		}
+
+		metaClient := ec2metadata.New(metaSession)
+		if metaClient.Available() != true {
+			return nil, fmt.Errorf("ec2metadata service is unavailable")
+		}
+
+		region, err := metaClient.Region()
+		if err != nil {
+			return nil, fmt.Errorf("unable to retreive region from ec2metadata: %v", err)
+		}
+		cfg.Region = region
+	}
+
 	awsClient.config = cfg
 
 	err = awsClient.configure()

examples/aws.md

@@ -31,7 +31,7 @@ upstreams:
 * The `api_endpoint` key defines the NGINX Plus API endpoint.
 * The `sync_interval_in_seconds` key defines the synchronization interval: nginx-asg-sync checks for scaling updates every 5 seconds.
 * The `cloud_provider` key defines a cloud provider that will be used. The default is `AWS`. This means the key can be empty if using AWS. Possible values are: `AWS`, `Azure`.
-* The `region` key defines the AWS region where we deploy NGINX Plus and the Auto Scaling groups.
+* The `region` key defines the AWS region where we deploy NGINX Plus and the Auto Scaling groups. Setting `region` to `self` will use the EC2 Metadata service to retreive the region of the current instance.
 * The `upstreams` key defines the list of upstream groups. For each upstream group we specify:
   * `name` – The name we specified for the upstream block in the NGINX Plus configuration.
   * `autoscaling_group` – The name of the corresponding Auto Scaling group.