Setting up an Xcache reverse proxy

This page describes how to set up an experimental HTTP reverse proxy layer for CernVM-FS based on Xcache.


This is not a replacement for a general site forward proxy. Forwarding needs to be defined separately in the Xcache configuration for each destination Stratum 1, and the client CVMFS_SERVER_URL configuration has to point to a separate forwarder URL for each server. This document is for the convenience of people who want to experiment with this configuration.


  • A machine (labeled Machine A) to serve the contents of the CernVM-FS repository. Should have CernVM-FS server tools installed, as well as XRootD.

  • A second machine (labeled Machine B) to use as a reverse proxy. Only XRootD is needed on this machine.

  • A CernVM-FS client to mount the repository, for testing.


XRootD is a high-performance, scalable file distribution solution. It has a plugin-based architecture and can be configured to suit various use cases. In the Xcache configuration, an XRootD daemon functions as a reverse proxy, serving the contents of a data repository over HTTP.

The following diagram shows how Xcache can be deployed as a cache layer between a CernVM-FS repository and client machines:


Machine A contains a CernVM-FS repository, served by default over HTTP. An Xcache instance is running on a second machine. By default, Xcache can only ingest files from another XRootD instance - we start an instance of XRootD on the same machine as the CernVM-FS repository, configured to export the repository using the XRootD protocol. The following configuration can be used for this instance of XRootD, replacing <CVMFS_REPOSITORY_NAME> with the actual name of the repository:

oss.localroot /srv
all.export /cvmfs/<CVMFS_REPOSITORY_NAME> r/o

all.adminpath /var/spool/xrootd
all.pidpath   /var/run/xrootd

xrd.trace all

The Xcache instance running on the second machine can be pointed to the XRootD daemon started on the first one (<CVMFS_REPOSITORY_NAME> should be replaced with the actual repository name and MACHINE_A_HOSTNAME with the actual host name of the first machine):

all.adminpath /var/spool/xrootd
all.pidpath   /var/run/xrootd

oss.localroot  /data/namespace

all.export /cvmfs/<CVMFS_REPOSITORY_NAME> meta /data/xrdcinfos data /data/datafiles

xrd.protocol http:3000 /usr/lib64/
xrd.trace all

ofs.osslib   /usr/lib64/
pss.cachelib /usr/lib64/
pss.config streams 32
pss.origin = <MACHINE_A_HOSTNAME>:1094

pfc.ram 4g
pfc.diskusage 0.5 0.6
pfc.spaces data meta
pfc.blocksize 1M
pfc.prefetch 0
pfc.trace info

With this configuration, Xcache re-exports the contents of the repository over HTTP, on port 3000. Interested CernVM-FS clients can be configured to use the Xcache instance by modifying the CVMFS_SERVER_URL variable:


Cache invalidation

A current limitation of Xcache is that cached files are never invalidated. In the context of CernVM-FS, this means that newly published root catalogs are not picked up automatically. An Xcache plugin is being developed to address this limitation.

Ingestion over HTTP

A new XRootD client plugin is being developed to allow the Xcache instance to ingest files over HTTP:


This set up is non-intrusive, as the machine serving the CernVM-FS repository no longer needs to be modified in any way. Xcache could thus be deployed as a reverse proxy layer for existing CernVM-FS stratum servers.