Re: [PR] Make segment download from Peer servers more robust by retrying both peer discovery and download. [pinot]

["tibrewalpratik17 (via GitHub)" <gi...@apache.org>]

tibrewalpratik17 commented on code in PR #12317:
URL: https://github.com/apache/pinot/pull/12317#discussion_r1475797609


##########
pinot-common/src/main/java/org/apache/pinot/common/utils/fetcher/BaseSegmentFetcher.java:
##########
@@ -109,6 +112,38 @@ public File fetchUntarSegmentToLocalStreamed(URI uri, File dest, long rateLimit,
     throw new UnsupportedOperationException();
   }
 
+  // Download segment to a local location with retries.
+  @Override
+  public boolean fetchSegmentToLocal(String segmentName, File dest, HelixManager helixManager, String downloadScheme)
+      throws Exception {
+    try {
+      int attempt =
+          RetryPolicies.exponentialBackoffRetryPolicy(_retryCount, _retryWaitMs, _retryDelayScaleFactor).attempt(() -> {
+            // First find servers hosting the segment in ONLINE state.
+            List<URI> peerSegmentURIs =
+                PeerServerSegmentFinder.getPeerServerURIs(segmentName, downloadScheme, helixManager);
+            // Shuffle the list of URIs.
+            Collections.shuffle(peerSegmentURIs);
+            // Next get through the list of URIs to fetch the segment until success.
+            for (URI uri : peerSegmentURIs) {
+              try {
+                fetchSegmentToLocalWithoutRetry(uri, dest);

Review Comment:
   This will increase this function count from present 3 to `3 * number of peer servers`. Is that desired?



##########
pinot-core/src/main/java/org/apache/pinot/core/data/manager/realtime/RealtimeTableDataManager.java:
##########
@@ -658,11 +656,10 @@ private void downloadSegmentFromPeer(String segmentName, String downloadScheme,
     try {
       tempRootDir = getTmpSegmentDataDir("tmp-" + segmentName + "." + System.currentTimeMillis());
       File segmentTarFile = new File(tempRootDir, segmentName + TarGzCompressionUtils.TAR_GZ_FILE_EXTENSION);
-      // First find servers hosting the segment in a ONLINE state.
-      List<URI> peerSegmentURIs = PeerServerSegmentFinder.getPeerServerURIs(segmentName, downloadScheme, _helixManager);
       // Next download the segment from a randomly chosen server using configured scheme.
-      SegmentFetcherFactory.getSegmentFetcher(downloadScheme).fetchSegmentToLocal(peerSegmentURIs, segmentTarFile);
-      _logger.info("Fetched segment {} from: {} to: {} of size: {}", segmentName, peerSegmentURIs, segmentTarFile,
+      SegmentFetcherFactory.getSegmentFetcher(downloadScheme)
+          .fetchSegmentToLocal(segmentName, segmentTarFile, _helixManager, downloadScheme);

Review Comment:
   Do you think it's a good idea to expose helixManager in SegmentFetcher logic? Can we keep the logic of updating peerSegmentURIs with retries here itself?  
   Segment fetcher should just fetch segment given a URI.



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