Does the XML Catalog reliably redirect schema requests to a known‑good version after a failed upgrade, and what are its caching limits?

When an XML parser attempts to load a newer schema version and the load fails, the application may need to continue processing documents that still conform to the previous schema. The goal is to determine whether the parser reports the schema‑load error before it falls back to the old schema set, so that validation results are not ambiguous. Existing XML Catalog implementations can redirect schema location requests, but their caching behavior and handling of concurrent catalog updates are not fully specified, leaving uncertainty about when a fallback is triggered and whether stale schema references persist in components such as the JAXB context or XSD validator.

• Does the parser emit a schema‑load error event before activating any fallback schema set?
• How do XML Catalog caching policies affect the visibility of a redirected schema location during a failed upgrade?
• Can concurrent updates to the catalog cause a race condition where some threads see the old schema while others see the broken one?