What causes "HDFS Writer not supported" in apache/druid?

Use openOutputStream() and wrap it in an OutputStreamWriter/PrintWriter if character writing is needed.

apache/druid · error · UOE

HDFS Writer not supported

Error message

HDFS Writer not supported

What it means

The HDFS FileObject's openWriter() throws UOE("HDFS Writer not supported"): character/Writer-based output to HDFS is deliberately not implemented in this FileObject adapter. Writes must go through openOutputStream(), which returns a real OutputStream backed by FileSystem.create. The exception is thrown unconditionally whenever openWriter is invoked.

Source

Thrown at extensions-core/hdfs-storage/src/main/java/org/apache/druid/storage/hdfs/HdfsDataSegmentPuller.java:152

        return fs.create(path, overwrite);
      }

      @Override
      public Reader openReader(boolean ignoreEncodingErrors)
      {
        throw new UOE("HDFS Reader not supported");
      }

      @Override
      public CharSequence getCharContent(boolean ignoreEncodingErrors)
      {
        throw new UOE("HDFS CharSequence not supported");
      }

      @Override
      public Writer openWriter()
      {
        throw new UOE("HDFS Writer not supported");
      }

      @Override
      public long getLastModified()
      {
        try {
          final FileSystem fs = path.getFileSystem(config);
          return fs.getFileStatus(path).getModificationTime();
        }
        catch (IOException ex) {
          throw new HdfsIOException(ex);
        }
      }

      @Override
      public boolean delete()
      {
        try {

View on GitHub (pinned to 9b90983fd2)

Solutions

Use openOutputStream() and wrap it in an OutputStreamWriter/PrintWriter if character writing is needed.
Keep byte-level writes for HDFS targets; reserve Writer-based APIs for local or HTTP pullers.
Branch on URI scheme before writing and route hdfs:// to the OutputStream API.

Example fix

// before
try (Writer w = fileObject.openWriter()) { w.write(json); }
// after
try (Writer w = new OutputStreamWriter(fileObject.openOutputStream(), StandardCharsets.UTF_8)) { w.write(json); }

Defensive patterns

Strategy: type-guard

Validate before calling

if ("hdfs".equalsIgnoreCase(uri.getScheme())) { writeViaOutputStream(uri, data); }

Type guard

boolean supportsWriter(FileObject fo) { return !isHdfsFileObject(fo); }

Try / catch

try (Writer w = fo.openWriter()) { w.write(s); }
catch (UOE e) { try (Writer w = new OutputStreamWriter(fo.openOutputStream(), UTF_8)) { w.write(s); } }

Prevention

Use openOutputStream (optionally wrapped in OutputStreamWriter) for all HDFS writes.
Pass the desired overwrite flag to buildFileObject so stream creation behaves as expected.

When it happens

Trigger: Invoking openWriter() on the FileObject from HdfsDataSegmentPuller.buildFileObject(uri, config, overwrite) — e.g. code that writes text artifacts (logs, descriptors) via the generic FileObject Writer API against hdfs:// URIs.

Common situations: Porting local-disk writing code to HDFS deep storage; utility libraries that take a FileObject and prefer openWriter for character output.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/1cc0b463f65e2392. Report an issue: GitHub.