Amazon Neptune Autoscaling - NeptuneInstanceDescriber

NeptuneInstanceDescriber.cs

public interface INeptuneInstanceDescriber : IDisposable
{
    Task<(DBInstance Writer, ImmutableArray<DBInstance> Readers)> GetInstances(string clusterIdentifier);
}

// provides the current cluster topology (i.e. the instances that exist and their availability)
// see https://docs.aws.amazon.com/neptune/latest/userguide/api-instances.html#DBInstance and related pages
public class NeptuneInstanceDescriber : INeptuneInstanceDescriber
{
    private readonly IAmazonNeptune neptune;

    public NeptuneInstanceDescriber(IAmazonNeptune neptune)
    {
        this.neptune = neptune;
    }
    
    public async Task<(DBInstance Writer, ImmutableArray<DBInstance> Readers)> GetInstances(string clusterIdentifier)
    {
        var describeClusterRequest = new DescribeDBClustersRequest { DBClusterIdentifier = clusterIdentifier };
        var cluster = (await neptune.DescribeDBClustersAsync(describeClusterRequest)).DBClusters.Single();

        var writerMember = cluster.DBClusterMembers.Single(m => m.IsClusterWriter);
        var readerMembers = cluster.DBClusterMembers.Where(m => !m.IsClusterWriter).ToImmutableArray();

        var describeInstancesRequest = new DescribeDBInstancesRequest
        {
            Filters = new List<Filter> {new() {Name = "db-cluster-id", Values = new List<string> {clusterIdentifier}}}
        };
        var instances = (await neptune.DescribeDBInstancesAsync(describeInstancesRequest)).DBInstances;

        var writer = instances.Single(i => i.DBInstanceIdentifier == writerMember.DBInstanceIdentifier);
        var readers = instances
            .Where(i => readerMembers.Any(r => r.DBInstanceIdentifier == i.DBInstanceIdentifier))
            .ToImmutableArray();
            
        return (writer, readers);
    }

    public void Dispose()
    {
        neptune.Dispose();
    }
}