diff --git a/server/src/test/java/org/opensearch/indices/replication/SegmentFileTransferHandlerTests.java b/server/src/test/java/org/opensearch/indices/replication/SegmentFileTransferHandlerTests.java
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+++ b/server/src/test/java/org/opensearch/indices/replication/SegmentFileTransferHandlerTests.java
@@ -0,0 +1,251 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.indices.replication;
+
+import org.apache.lucene.index.CorruptIndexException;
+import org.apache.lucene.index.IndexFileNames;
+import org.junit.Assert;
+import org.opensearch.Version;
+import org.opensearch.action.ActionListener;
+import org.opensearch.cluster.node.DiscoveryNode;
+import org.opensearch.common.bytes.BytesReference;
+import org.opensearch.common.util.CancellableThreads;
+import org.opensearch.core.internal.io.IOUtils;
+import org.opensearch.index.shard.IndexShard;
+import org.opensearch.index.shard.IndexShardTestCase;
+import org.opensearch.index.store.Store;
+import org.opensearch.index.store.StoreFileMetadata;
+import org.opensearch.indices.recovery.FileChunkWriter;
+import org.opensearch.indices.recovery.MultiChunkTransfer;
+
+import java.io.IOException;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.TimeUnit;
+import java.util.function.IntSupplier;
+
+import static java.util.Collections.emptyMap;
+import static java.util.Collections.emptySet;
+import static org.mockito.Mockito.*;
+
+public class SegmentFileTransferHandlerTests extends IndexShardTestCase {
+
+    private IndexShard shard;
+    private StoreFileMetadata[] filesToSend;
+    private final DiscoveryNode targetNode = new DiscoveryNode(
+        "foo",
+        buildNewFakeTransportAddress(),
+        emptyMap(),
+        emptySet(),
+        Version.CURRENT
+    );
+
+    final int fileChunkSizeInBytes = 5000;
+    final int maxConcurrentFileChunks = 1;
+    private CancellableThreads cancellableThreads;
+    final IntSupplier translogOps = () -> 0;
+
+    @Override
+    public void setUp() throws Exception {
+        super.setUp();
+        cancellableThreads = new CancellableThreads();
+        shard = spy(newStartedShard(true));
+        filesToSend = getFilestoSend(shard);
+        // we should only have a Segments_N file at this point.
+        assertEquals(1, filesToSend.length);
+    }
+
+    private StoreFileMetadata[] getFilestoSend(IndexShard shard) throws IOException {
+        final Store.MetadataSnapshot metadata = shard.store().getMetadata();
+        return metadata.asMap().values().toArray(StoreFileMetadata[]::new);
+    }
+
+    @Override
+    public void tearDown() throws Exception {
+        closeShards(shard);
+        super.tearDown();
+    }
+
+    public void testSendFiles_invokesChunkWriter() throws IOException, InterruptedException {
+        // use counDownLatch and countDown when our chunkWriter is invoked.
+        final CountDownLatch countDownLatch = new CountDownLatch(1);
+        final FileChunkWriter chunkWriter = spy(new FileChunkWriter() {
+            @Override
+            public void writeFileChunk(
+                StoreFileMetadata fileMetadata,
+                long position,
+                BytesReference content,
+                boolean lastChunk,
+                int totalTranslogOps,
+                ActionListener<Void> listener
+            ) {
+                assertTrue(filesToSend[0].isSame(fileMetadata));
+                assertTrue(lastChunk);
+                countDownLatch.countDown();
+            }
+        });
+
+        SegmentFileTransferHandler handler = new SegmentFileTransferHandler(
+            shard,
+            targetNode,
+            chunkWriter,
+            logger,
+            shard.getThreadPool(),
+            cancellableThreads,
+            fileChunkSizeInBytes,
+            maxConcurrentFileChunks
+        );
+        final MultiChunkTransfer<StoreFileMetadata, SegmentFileTransferHandler.FileChunk> transfer = handler.sendFiles(
+            shard.store(),
+            filesToSend,
+            translogOps,
+            mock(ActionListener.class)
+        );
+
+        // start the transfer
+        transfer.start();
+        countDownLatch.await(5, TimeUnit.SECONDS);
+        verify(chunkWriter, times(1)).writeFileChunk(any(), anyLong(), any(), anyBoolean(), anyInt(), any());
+        IOUtils.close(transfer);
+    }
+
+    public void testSendFiles_cancelThreads_beforeStart() throws IOException, InterruptedException {
+        final FileChunkWriter chunkWriter = spy(new FileChunkWriter() {
+            @Override
+            public void writeFileChunk(
+                StoreFileMetadata fileMetadata,
+                long position,
+                BytesReference content,
+                boolean lastChunk,
+                int totalTranslogOps,
+                ActionListener<Void> listener
+            ) {
+                Assert.fail();
+            }
+        });
+        SegmentFileTransferHandler handler = new SegmentFileTransferHandler(
+            shard,
+            targetNode,
+            chunkWriter,
+            logger,
+            shard.getThreadPool(),
+            cancellableThreads,
+            fileChunkSizeInBytes,
+            maxConcurrentFileChunks
+        );
+
+        final MultiChunkTransfer<StoreFileMetadata, SegmentFileTransferHandler.FileChunk> transfer = handler.sendFiles(
+            shard.store(),
+            filesToSend,
+            translogOps,
+            mock(ActionListener.class)
+        );
+
+        // start the transfer
+        cancellableThreads.cancel("test");
+        transfer.start();
+        verifyNoInteractions(chunkWriter);
+        IOUtils.close(transfer);
+    }
+
+    public void testSendFiles_cancelThreads_afterStart() throws IOException, InterruptedException {
+        // index a doc a flush so we have more than 1 file to send.
+        indexDoc(shard, "_doc", "test");
+        flushShard(shard, true);
+        filesToSend = getFilestoSend(shard);
+
+        // we should have 4 files to send now -
+        // [_0.cfe, _0.si, _0.cfs, segments_3]
+        assertEquals(4, filesToSend.length);
+
+        final CountDownLatch countDownLatch = new CountDownLatch(1);
+        FileChunkWriter chunkWriter = spy(new FileChunkWriter() {
+            @Override
+            public void writeFileChunk(
+                StoreFileMetadata fileMetadata,
+                long position,
+                BytesReference content,
+                boolean lastChunk,
+                int totalTranslogOps,
+                ActionListener<Void> listener
+            ) {
+                // cancel the threads at this point, we'll ensure this is not invoked more than once.
+                cancellableThreads.cancel("test");
+                listener.onResponse(null);
+            }
+        });
+        SegmentFileTransferHandler handler = new SegmentFileTransferHandler(
+            shard,
+            targetNode,
+            chunkWriter,
+            logger,
+            shard.getThreadPool(),
+            cancellableThreads,
+            fileChunkSizeInBytes,
+            maxConcurrentFileChunks
+        );
+
+        final MultiChunkTransfer<StoreFileMetadata, SegmentFileTransferHandler.FileChunk> transfer = handler.sendFiles(
+            shard.store(),
+            filesToSend,
+            translogOps,
+            new ActionListener<Void>() {
+                @Override
+                public void onResponse(Void unused) {
+                    // do nothing here, we will just resolve in test.
+                }
+
+                @Override
+                public void onFailure(Exception e) {
+                    assertEquals(CancellableThreads.ExecutionCancelledException.class, e.getClass());
+                    countDownLatch.countDown();
+                }
+            }
+        );
+
+        // start the transfer
+        transfer.start();
+        countDownLatch.await(30, TimeUnit.SECONDS);
+        verify(chunkWriter, times(1)).writeFileChunk(any(), anyLong(), any(), anyBoolean(), anyInt(), any());
+        IOUtils.close(transfer);
+    }
+
+    public void testSendFiles_CorruptIndexException() throws Exception {
+        final CancellableThreads cancellableThreads = new CancellableThreads();
+        SegmentFileTransferHandler handler = new SegmentFileTransferHandler(
+            shard,
+            targetNode,
+            mock(FileChunkWriter.class),
+            logger,
+            shard.getThreadPool(),
+            cancellableThreads,
+            fileChunkSizeInBytes,
+            maxConcurrentFileChunks
+        );
+        final StoreFileMetadata SEGMENTS_FILE = new StoreFileMetadata(
+            IndexFileNames.SEGMENTS,
+            1L,
+            "0",
+            org.apache.lucene.util.Version.LATEST
+        );
+
+        doNothing().when(shard).failShard(anyString(), any());
+        assertThrows(
+            CorruptIndexException.class,
+            () -> {
+                handler.handleErrorOnSendFiles(
+                    shard.store(),
+                    new CorruptIndexException("test", "test"),
+                    new StoreFileMetadata[] { SEGMENTS_FILE }
+                );
+            }
+        );
+
+        verify(shard, times(1)).failShard(any(), any());
+    }
+}