Add exporter ForceFlush

src/OpenTelemetry/BaseExportProcessor.cs

@@ -59,6 +59,12 @@ internal override void SetParentProvider(BaseProvider parentProvider)
 
         protected abstract void OnExport(T data);
 
+        /// <inheritdoc />
+        protected override bool OnForceFlush(int timeoutMilliseconds)
+        {
+            return this.exporter.ForceFlush(timeoutMilliseconds);
+        }
+
         /// <inheritdoc />
         protected override bool OnShutdown(int timeoutMilliseconds)
         {

src/OpenTelemetry/BaseExporter.cs

@@ -57,6 +57,38 @@ public abstract class BaseExporter<T> : IDisposable
         /// <returns>Result of the export operation.</returns>
         public abstract ExportResult Export(in Batch<T> batch);
 
+        /// <summary>
+        /// Flushes the exporter, blocks the current thread until flush
+        /// completed, shutdown signaled or timed out.
+        /// </summary>
+        /// <param name="timeoutMilliseconds">
+        /// The number (non-negative) of milliseconds to wait, or
+        /// <c>Timeout.Infinite</c> to wait indefinitely.
+        /// </param>
+        /// <returns>
+        /// Returns <c>true</c> when flush succeeded; otherwise, <c>false</c>.
+        /// </returns>
+        /// <exception cref="ArgumentOutOfRangeException">
+        /// Thrown when the <c>timeoutMilliseconds</c> is smaller than -1.
+        /// </exception>
+        /// <remarks>
+        /// This function guarantees thread-safety.
+        /// </remarks>
+        public bool ForceFlush(int timeoutMilliseconds = Timeout.Infinite)
+        {
+            Guard.InvalidTimeout(timeoutMilliseconds, nameof(timeoutMilliseconds));
+
+            try
+            {
+                return this.OnForceFlush(timeoutMilliseconds);
+            }
+            catch (Exception ex)
+            {
+                OpenTelemetrySdkEventSource.Log.SpanProcessorException(nameof(this.ForceFlush), ex);
+                return false;
+            }
+        }
+
         /// <summary>
         /// Attempts to shutdown the exporter, blocks the current thread until
         /// shutdown completed or timed out.
@@ -102,6 +134,27 @@ public void Dispose()
             GC.SuppressFinalize(this);
         }
 
+        /// <summary>
+        /// Called by <c>ForceFlush</c>. This function should block the current
+        /// thread until flush completed, shutdown signaled or timed out.
+        /// </summary>
+        /// <param name="timeoutMilliseconds">
+        /// The number (non-negative) of milliseconds to wait, or
+        /// <c>Timeout.Infinite</c> to wait indefinitely.
+        /// </param>
+        /// <returns>
+        /// Returns <c>true</c> when flush succeeded; otherwise, <c>false</c>.
+        /// </returns>
+        /// <remarks>
+        /// This function is called synchronously on the thread which called
+        /// <c>ForceFlush</c>. This function should be thread-safe, and should
+        /// not throw exceptions.
+        /// </remarks>
+        protected virtual bool OnForceFlush(int timeoutMilliseconds)
+        {
+            return true;
+        }
+
         /// <summary>
         /// Called by <c>Shutdown</c>. This function should block the current
         /// thread until shutdown completed or timed out.

src/OpenTelemetry/CHANGELOG.md

@@ -6,6 +6,9 @@
   `FormatException` if it fails to parse any of the supported environment
   variables.
 
+* Added `BaseExporter.ForceFlush`.
+  ([#2525](https://github.com/open-telemetry/opentelemetry-dotnet/pull/2525))
+
 ## 1.2.0-beta1
 
 Released 2021-Oct-08