private long findLatestFileNameGeneration(ImmutableMap<String, BlobMetaData> blobs) {
    long generation = -1;
    for (String name : blobs.keySet()) {
      if (!name.startsWith("__")) {
        continue;
      }
      if (name.contains(".part")) {
        name = name.substring(0, name.indexOf(".part"));
      }

      try {
        long currentGen = Long.parseLong(name.substring(2) /*__*/, Character.MAX_RADIX);
        if (currentGen > generation) {
          generation = currentGen;
        }
      } catch (NumberFormatException e) {
        logger.warn("file [{}] does not conform to the '__' schema");
      }
    }
    return generation;
  }
  private void doSnapshot(final Snapshot snapshot) throws IndexShardGatewaySnapshotFailedException {
    ImmutableMap<String, BlobMetaData> blobs;
    try {
      blobs = blobContainer.listBlobs();
    } catch (IOException e) {
      throw new IndexShardGatewaySnapshotFailedException(shardId, "failed to list blobs", e);
    }

    long generation = findLatestFileNameGeneration(blobs);
    CommitPoints commitPoints = buildCommitPoints(blobs);

    currentSnapshotStatus.index().startTime(System.currentTimeMillis());
    currentSnapshotStatus.updateStage(SnapshotStatus.Stage.INDEX);

    final SnapshotIndexCommit snapshotIndexCommit = snapshot.indexCommit();
    final Translog.Snapshot translogSnapshot = snapshot.translogSnapshot();

    final CountDownLatch indexLatch = new CountDownLatch(snapshotIndexCommit.getFiles().length);
    final CopyOnWriteArrayList<Throwable> failures = new CopyOnWriteArrayList<Throwable>();
    final List<CommitPoint.FileInfo> indexCommitPointFiles = Lists.newArrayList();

    int indexNumberOfFiles = 0;
    long indexTotalFilesSize = 0;
    for (final String fileName : snapshotIndexCommit.getFiles()) {
      StoreFileMetaData md;
      try {
        md = store.metaData(fileName);
      } catch (IOException e) {
        throw new IndexShardGatewaySnapshotFailedException(
            shardId, "Failed to get store file metadata", e);
      }

      boolean snapshotRequired = false;
      if (snapshot.indexChanged() && fileName.equals(snapshotIndexCommit.getSegmentsFileName())) {
        snapshotRequired = true; // we want to always snapshot the segment file if the index changed
      }

      CommitPoint.FileInfo fileInfo = commitPoints.findPhysicalIndexFile(fileName);
      if (fileInfo == null
          || !fileInfo.isSame(md)
          || !commitPointFileExistsInBlobs(fileInfo, blobs)) {
        // commit point file does not exists in any commit point, or has different length, or does
        // not fully exists in the listed blobs
        snapshotRequired = true;
      }

      if (snapshotRequired) {
        indexNumberOfFiles++;
        indexTotalFilesSize += md.length();
        // create a new FileInfo
        try {
          CommitPoint.FileInfo snapshotFileInfo =
              new CommitPoint.FileInfo(
                  fileNameFromGeneration(++generation), fileName, md.length(), md.checksum());
          indexCommitPointFiles.add(snapshotFileInfo);
          snapshotFile(snapshotIndexCommit.getDirectory(), snapshotFileInfo, indexLatch, failures);
        } catch (IOException e) {
          failures.add(e);
          indexLatch.countDown();
        }
      } else {
        indexCommitPointFiles.add(fileInfo);
        indexLatch.countDown();
      }
    }
    currentSnapshotStatus.index().files(indexNumberOfFiles, indexTotalFilesSize);

    try {
      indexLatch.await();
    } catch (InterruptedException e) {
      failures.add(e);
    }
    if (!failures.isEmpty()) {
      throw new IndexShardGatewaySnapshotFailedException(
          shardId(), "Failed to perform snapshot (index files)", failures.get(failures.size() - 1));
    }

    currentSnapshotStatus
        .index()
        .time(System.currentTimeMillis() - currentSnapshotStatus.index().startTime());

    currentSnapshotStatus.updateStage(SnapshotStatus.Stage.TRANSLOG);
    currentSnapshotStatus.translog().startTime(System.currentTimeMillis());

    // Note, we assume the snapshot is always started from "base 0". We need to seek forward if we
    // want to lastTranslogPosition if we want the delta
    List<CommitPoint.FileInfo> translogCommitPointFiles = Lists.newArrayList();
    int expectedNumberOfOperations = 0;
    boolean snapshotRequired = false;
    if (snapshot.newTranslogCreated()) {
      if (translogSnapshot.lengthInBytes() > 0) {
        snapshotRequired = true;
        expectedNumberOfOperations = translogSnapshot.estimatedTotalOperations();
      }
    } else {
      // if we have a commit point, check that we have all the files listed in it in the blob store
      if (!commitPoints.commits().isEmpty()) {
        CommitPoint commitPoint = commitPoints.commits().get(0);
        boolean allTranslogFilesExists = true;
        for (CommitPoint.FileInfo fileInfo : commitPoint.translogFiles()) {
          if (!commitPointFileExistsInBlobs(fileInfo, blobs)) {
            allTranslogFilesExists = false;
            break;
          }
        }
        // if everything exists, we can seek forward in case there are new operations, otherwise, we
        // copy over all again...
        if (allTranslogFilesExists) {
          translogCommitPointFiles.addAll(commitPoint.translogFiles());
          if (snapshot.sameTranslogNewOperations()) {
            translogSnapshot.seekForward(snapshot.lastTranslogLength());
            if (translogSnapshot.lengthInBytes() > 0) {
              snapshotRequired = true;
              expectedNumberOfOperations =
                  translogSnapshot.estimatedTotalOperations()
                      - snapshot.lastTotalTranslogOperations();
            }
          } // else (no operations, nothing to snapshot)
        } else {
          // a full translog snapshot is required
          if (translogSnapshot.lengthInBytes() > 0) {
            expectedNumberOfOperations = translogSnapshot.estimatedTotalOperations();
            snapshotRequired = true;
          }
        }
      } else {
        // no commit point, snapshot all the translog
        if (translogSnapshot.lengthInBytes() > 0) {
          expectedNumberOfOperations = translogSnapshot.estimatedTotalOperations();
          snapshotRequired = true;
        }
      }
    }
    currentSnapshotStatus.translog().expectedNumberOfOperations(expectedNumberOfOperations);

    if (snapshotRequired) {
      CommitPoint.FileInfo addedTranslogFileInfo =
          new CommitPoint.FileInfo(
              fileNameFromGeneration(++generation),
              "translog-" + translogSnapshot.translogId(),
              translogSnapshot.lengthInBytes(),
              null /* no need for checksum in translog */);
      translogCommitPointFiles.add(addedTranslogFileInfo);
      try {
        snapshotTranslog(translogSnapshot, addedTranslogFileInfo);
      } catch (Exception e) {
        throw new IndexShardGatewaySnapshotFailedException(
            shardId, "Failed to snapshot translog", e);
      }
    }
    currentSnapshotStatus
        .translog()
        .time(System.currentTimeMillis() - currentSnapshotStatus.translog().startTime());

    // now create and write the commit point
    currentSnapshotStatus.updateStage(SnapshotStatus.Stage.FINALIZE);
    long version = 0;
    if (!commitPoints.commits().isEmpty()) {
      version = commitPoints.commits().iterator().next().version() + 1;
    }
    String commitPointName = "commit-" + Long.toString(version, Character.MAX_RADIX);
    CommitPoint commitPoint =
        new CommitPoint(
            version,
            commitPointName,
            CommitPoint.Type.GENERATED,
            indexCommitPointFiles,
            translogCommitPointFiles);
    try {
      byte[] commitPointData = CommitPoints.toXContent(commitPoint);
      blobContainer.writeBlob(
          commitPointName, new FastByteArrayInputStream(commitPointData), commitPointData.length);
    } catch (Exception e) {
      throw new IndexShardGatewaySnapshotFailedException(
          shardId, "Failed to write commit point", e);
    }

    // delete all files that are not referenced by any commit point
    // build a new CommitPoint, that includes this one and all the saved ones
    List<CommitPoint> newCommitPointsList = Lists.newArrayList();
    newCommitPointsList.add(commitPoint);
    for (CommitPoint point : commitPoints) {
      if (point.type() == CommitPoint.Type.SAVED) {
        newCommitPointsList.add(point);
      }
    }
    CommitPoints newCommitPoints = new CommitPoints(newCommitPointsList);
    // first, go over and delete all the commit points
    for (String blobName : blobs.keySet()) {
      if (!blobName.startsWith("commit-")) {
        continue;
      }
      long checkedVersion =
          Long.parseLong(blobName.substring("commit-".length()), Character.MAX_RADIX);
      if (!newCommitPoints.hasVersion(checkedVersion)) {
        try {
          blobContainer.deleteBlob(blobName);
        } catch (IOException e) {
          // ignore
        }
      }
    }
    // now go over all the blobs, and if they don't exists in a commit point, delete them
    for (String blobName : blobs.keySet()) {
      String name = blobName;
      if (!name.startsWith("__")) {
        continue;
      }
      if (blobName.contains(".part")) {
        name = blobName.substring(0, blobName.indexOf(".part"));
      }
      if (newCommitPoints.findNameFile(name) == null) {
        try {
          blobContainer.deleteBlob(blobName);
        } catch (IOException e) {
          // ignore, will delete it laters
        }
      }
    }
  }