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Configuring Fallback Access

Fallback access allows CMSSW to retry opening a file from a different location if the primary source fails. This is present in all versions of CMSSW since 4.0.

Concretely, this is used to open a file in the regional Xrootd redirector if the site's storage system fails to find it. In CMS, this can happen by accident (the local site's copy is corrupted or deleted) or on purpose (if the glidein server is overflowing jobs to another site).

To configure fallback at a site, two files need to be edited in the site's SITECONF area:

• site-local-config.xml
• storage.xml

Editing site-local-config.xml

Locate the site-local-config.xml file; it should be in the SITECONF/local/JobConfig directory. In this XML file, you should find an <event-data> tag with one entry:

    <event-data>
      <catalog url="trivialcatalog_file://opt/osg/app/cmssoft/cms/SITECONF/local/PhEDEx/storage.xml?protocol=direct"/>
    </event-data>

This tells CMSSW the location of your storage.xml and the protocol ("direct", in this case) to use. Add a new using the xrootd protocol:

    <event-data>
      <catalog url="trivialcatalog_file://opt/osg/app/cmssoft/cms/SITECONF/local/PhEDEx/storage.xml?protocol=direct"/>
      <catalog url="trivialcatalog_file://opt/osg/app/cmssoft/cms/SITECONF/local/PhEDEx/storage.xml?protocol=xrootd"/>
    </event-data>

The protocol strings are arbitrary; if "xrootd" is already used at your site, "xrootd2" would do fine.

Additions to storage.xml

For US sites, add the following lines to PhEDEx/storage.xml:

  <!-- Xrootd fallback rules -->
  <lfn-to-pfn protocol="xrootd" destination-match=".*" path-match="/+store/(.*)" result="root://cmsxrootd.fnal.gov//store/$1"/>

If you used a different protocol name than "xrootd" above, change it here too.

For EU sites, use the following lines:

  <!-- Xrootd fallback rules -->
  <lfn-to-pfn protocol="xrootd" destination-match=".*" path-match="/+store/(.*)" result="root://xrootd-cms.infn.it//store/$1"/>

We use different endpoints for the different regions because the endpoints will prefer serving data from that region. This minimizes the transatlantic transfers.

Other fallback sites in EU

EU region has more redirectors which offer for countries geographically closer to the endpoint benefit from shorter transfer route hence define fallback as follows:

Italy

  <lfn-to-pfn protocol="xrootd" destination-match=".*" path-match="/+store/(.*)" result="root://xrootd-cms.infn.it:1194//store/$1"/>

France

  <lfn-to-pfn protocol="xrootd" destination-match=".*" path-match="/+store/(.*)" result="root://xrootd-cms.infn.it:1294//store/$1"/>

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