What causes "Failed to construct server URL." in apache/druid?

Fix the offending host string to be scheme://hostname:port (valid URL authority)

apache/druid · error · IllegalArgumentException

Failed to construct server URL.

Error message

Failed to construct server URL.

What it means

When creating a DruidServerHolder, the server's host string is converted into a URL for the queryable-server syncer. A MalformedURLException from that conversion is wrapped in IAE with this message, meaning the configured host string is not a valid URL/authority format.

Source

Thrown at server/src/main/java/org/apache/druid/client/HttpServerInventoryView.java:562

    {
      this.druidServer = druidServer;

      try {
        HostAndPort hostAndPort = HostAndPort.fromString(druidServer.getHost());
        this.syncer = new ChangeRequestHttpSyncer<>(
            smileMapper,
            httpClient,
            inventorySyncExecutor,
            new URL(druidServer.getScheme(), hostAndPort.getHost(), hostAndPort.getPort(), "/"),
            "/druid-internal/v1/segments",
            SEGMENT_LIST_RESP_TYPE_REF,
            config.getServerTimeout(),
            config.getServerUnstabilityTimeout(),
            createSyncListener()
        );
      }
      catch (MalformedURLException ex) {
        throw new IAE(ex, "Failed to construct server URL.");
      }
    }

    void start()
    {
      syncer.start();
    }

    void stop()
    {
      syncer.stop();
      stopped.set(true);
    }

    boolean isStopped()
    {
      return stopped.get();
    }

View on GitHub (pinned to 9b90983fd2)

Solutions

Fix the offending host string to be scheme://hostname:port (valid URL authority)
Validate host configuration at startup before it reaches the inventory view
Check the external discovery source for malformed entries
Log/inspect the wrapped MalformedURLException cause for the exact parse failure

Example fix

// before
config.getHost() == "http:/broker1:8080" // MalformedURLException
// after
config.getHost() == "http://broker1:8080"

Defensive patterns

Strategy: validation

Validate before calling

private static void validateHost(String host) {
  try { new java.net.URL(host); } catch (MalformedURLException e) { throw new IllegalArgumentException("bad host: " + host, e); }
}

Prevention

Validate host strings as URLs at config load time
Use scheme://hostname:port format consistently
Check discovery service output format

When it happens

Trigger: druid.client.httpServerInventory entries or discovery payloads containing hosts with invalid characters, missing scheme/hostname, or malformed host:port values.

Common situations: Typo'd host config (e.g. 'http:/host:8080' single slash), whitespace or control characters in host, port values with invalid characters, discovery service returning malformed host strings.

Understand the failure class

Background: "Invalid URL" errors: why new URL(), URI.parse, and reqwest::Url reject your string — missing scheme, whitespace, and bad path format — this error's family across 39 libraries.

Related errors

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/3caacfdefa63e0c3. Report an issue: GitHub.