Associate an external id with a task, so that it can be cancelled later.

src/Elasticsearch.Net/Configuration/RequestConfiguration.cs

@@ -23,6 +23,12 @@ public interface IRequestConfiguration
 		/// </summary>
 		string ContentType { get; set; }
 
+		/// <summary>
+		/// Associate an Id with this user-initiated task, such that it can be located in the cluster task list.
+		/// Valid only for Elasticsearch 6.2.0+
+		/// </summary>
+		string OpaqueId { get; set; }
+
 		/// <summary>
 		/// Force a different Accept header on the request
 		/// </summary>
@@ -95,6 +101,7 @@ public class RequestConfiguration : IRequestConfiguration
 		public TimeSpan? RequestTimeout { get; set; }
 		public TimeSpan? PingTimeout { get; set; }
 		public string ContentType { get; set; }
+		public string OpaqueId { get; set; }
 		public string Accept { get; set; }
 		public int? MaxRetries { get; set; }
 		public Uri ForceNode { get; set; }
@@ -121,6 +128,7 @@ public class RequestConfigurationDescriptor : IRequestConfiguration
 		TimeSpan? IRequestConfiguration.RequestTimeout { get; set; }
 		TimeSpan? IRequestConfiguration.PingTimeout { get; set; }
 		string IRequestConfiguration.ContentType { get; set; }
+		string IRequestConfiguration.OpaqueId { get; set; }
 		string IRequestConfiguration.Accept { get; set; }
 
 		int? IRequestConfiguration.MaxRetries { get; set; }
@@ -152,6 +160,7 @@ public RequestConfigurationDescriptor(IRequestConfiguration config)
 			Self.RunAs = config?.RunAs;
 			Self.ClientCertificates = config?.ClientCertificates;
 			Self.ThrowExceptions = config?.ThrowExceptions;
+			Self.OpaqueId = config?.OpaqueId;
 		}
 
 		/// <summary>
@@ -170,6 +179,16 @@ public RequestConfigurationDescriptor RequestTimeout(TimeSpan requestTimeout)
 			return this;
 		}
 
+		/// <summary>
+		/// Associate an Id with this user-initiated task, such that it can be located in the cluster task list.
+		/// Valid only for Elasticsearch 6.2.0+
+		/// </summary>
+		public RequestConfigurationDescriptor OpaqueId(string opaqueId)
+		{
+			Self.OpaqueId = opaqueId;
+			return this;
+		}
+
 		public RequestConfigurationDescriptor PingTimeout(TimeSpan pingTimeout)
 		{
 			Self.PingTimeout = pingTimeout;

src/Elasticsearch.Net/Transport/Pipeline/RequestData.cs

@@ -12,6 +12,7 @@ public class RequestData
 	{
 		public const string MimeType = "application/json";
 		public const string RunAsSecurityHeader = "es-security-runas-user";
+		public const string OpaqueIdHeader = "X-Opaque-Id";
 
 		public Uri Uri => this.Node != null ? new Uri(this.Node.Uri, this.PathAndQuery) : null;
 
@@ -77,6 +78,10 @@ private RequestData(
 			this.RequestMimeType = local?.ContentType ?? MimeType;
 			this.Accept = local?.Accept ?? MimeType;
 			this.Headers = global.Headers != null ? new NameValueCollection(global.Headers) : new NameValueCollection();
+
+			if (!string.IsNullOrEmpty(local?.OpaqueId))
+				this.Headers.Add(OpaqueIdHeader, local.OpaqueId);
+
 			this.RunAs = local?.RunAs;
 			this.SkipDeserializationForStatusCodes = global?.SkipDeserializationForStatusCodes;
 			this.ThrowExceptions = local?.ThrowExceptions ?? global.ThrowExceptions;

src/Nest/Cluster/TaskManagement/GetTask/TaskInfo.cs

@@ -36,5 +36,11 @@ public class TaskInfo
 
 		[JsonProperty("cancellable")]
 		public bool Cancellable { get; internal set; }
+
+		[JsonProperty("headers")]
+		public IReadOnlyDictionary<string, string> Headers { get; internal set; } = EmptyReadOnly<string, string>.Dictionary;
+
+		[JsonProperty("children")]
+		public IReadOnlyCollection<TaskInfo> Children { get; internal set; } = EmptyReadOnly<TaskInfo>.Collection;
 	}
 }

src/Nest/Cluster/TaskManagement/ListTasks/ListTasksResponse.cs

@@ -73,6 +73,12 @@ public class TaskState
 
 		[JsonProperty("parent_task_id")]
 		public TaskId ParentTaskId { get; internal set; }
+
+		[JsonProperty("cancellable")]
+		public bool Cancellable { get; internal set; }
+
+		[JsonProperty("headers")]
+		public IReadOnlyDictionary<string, string> Headers { get; internal set; } = EmptyReadOnly<string, string>.Dictionary;
 	}
 
 	public class TaskStatus

src/Tests/Search/Search/SearchApiTests.cs

@@ -351,4 +351,57 @@ protected override void ExpectResponse(ISearchResponse<Project> response)
 			response.ShouldBeValid();
 		}
 	}
+
+	public class OpaqueIdApiTests : ApiIntegrationTestBase<ReadOnlyCluster, ISearchResponse<Project>, ISearchRequest, SearchDescriptor<Project>, SearchRequest<Project>>
+	{
+		private const string OpaqueId = "123456";
+
+		public OpaqueIdApiTests(ReadOnlyCluster cluster, EndpointUsage usage) : base(cluster, usage) { }
+
+		protected override object ExpectJson => new { };
+
+		protected override int ExpectStatusCode => 200;
+		protected override bool ExpectIsValid => true;
+		protected override HttpMethod HttpMethod => HttpMethod.POST;
+		protected override string UrlPath => $"/project/doc/_search?scroll=10m";
+
+		protected override Func<SearchDescriptor<Project>, ISearchRequest> Fluent => s => s
+			.RequestConfiguration(r => r.OpaqueId(OpaqueId))
+			.Query(q => q)
+			.Scroll("10m"); // Create a scroll in order to keep the task around.
+
+		protected override SearchRequest<Project> Initializer => new SearchRequest<Project>()
+		{
+			RequestConfiguration = new RequestConfiguration
+			{
+				OpaqueId = OpaqueId
+			}
+		};
+
+		protected override LazyResponses ClientUsage() => Calls(
+			fluent: (c, f) => c.Search(f),
+			fluentAsync: (c, f) => c.SearchAsync(f),
+			request: (c, r) => c.Search<Project>(r),
+			requestAsync: (c, r) => c.SearchAsync<Project>(r)
+		);
+
+		protected override void OnAfterCall(IElasticClient client)
+		{
+			var tasks = client.ListTasks(d => d.RequestConfiguration(r => r.OpaqueId(OpaqueId)));
+			tasks.Should().NotBeNull();
+			foreach (var node in tasks.Nodes)
+			{
+				foreach (var task in node.Value.Tasks)
+				{
+					task.Value.Headers[RequestData.OpaqueIdHeader].Should().Be(OpaqueId);
+				}
+			}
+			base.OnAfterCall(client);
+		}
+
+		protected override void ExpectResponse(ISearchResponse<Project> response)
+		{
+			response.ShouldBeValid();
+		}
+	}
 }