WP3 Budapest SEP2002

1
WP3 Budapest SEP-2002

• EDG Timestamping
• John Ryan, Brian Coghlan
• Trinity College Dublin

2
Contents

• EDG timestamping overview
• Decisions made for TB1.2
• Issues
• Future

3
EDG Timestamping

• Universal Time (UT/UT0/UT1/UT2)
• based on rotation of earth on its axis
• TAI (aka atomic time)
• based on caesium resonance
• Coordinated Universal Time is derived
• UTC TAI leapseconds
• UTC kept within 0.9s of UT1
• IERS (Paris) decides
• introduces /- leapseconds, very irregularly

4
EDG Timestamping

• The problem

When b utilises Ia(ta) it must first convert to
binary, this value is now incorrect, by as much
as 32 seconds, but at least 1
5
EDG Timestamping

• Solution all sites get leapsecond info, from
  same information source
• Uses R-GMA
• Obtains data from USNO ftp site.
• Publishes info via DBProducerServlet
• Timestamp class uses this info to correctly
  convert between formats

6
EDG Timestamping
7
Decisions Made for TB1.2

• Epoch
• ASCII UTC date string 1-JAN-1972
• binary TAI second count 1-JAN-1958
• Binary fraction-of-second format
• Count of pS
• Accuracy/resolution info
• Preserved in ASCII if defined in binary
• Preserved in binary if defined in ASCII

8
Issues CRON Scripts

• None in CVS tree yet
• Need scripts to
• obtain leapsecond info from USNO
• NOT DONE
• populate cache files
• NOT DONE
• randomize the timing of these actions
• NOT DONE
• Must be able to be set up by LCFG

9
Future Timestamp Formats

• TB1.2 ASCII format
• does not preserve record of use of leapseconds
• CAN use lots of space on disk
• 2002-01-16T124648.7567Z.00000001r.00001a
• 32 16 16 16 16 16 bits .756
• 112 bits before fractional secs
• excluding T, Z, r and a can be implied
• excluding accuracy and resolution
• excluding record of use of leapseconds

10
Future Timestamp Formats

• TB1.2 binary format
• Count of pS
• long2 if accuracy resolution NOT used
• long4 if accuracy resolution are used
• Fractional parts are fixed-format
• Issues
• 4 longs use lots of space on disk
• does not preserve record of use of leapseconds
• cannot distinguish between EDG UNIX time

11
Future Timestamp Formats

• We propose to
• add optional record of use of leapseconds
• last_leapsecond_update (L) field
• add to EDG ASCII and binary timestamps
• allows missed updates to be recognized
• add EDG packed binary timestamp format(s)
• particularly for secondary storage
• packed types use less space

12
Future Timestamp Formats

• EDG packed binary timestamp formats
• distinct EDG formats
• extensible typing using format type field
• automatic recognition of EDG formats
• include resolution accuracy fields
• include last_leapsecond_update field
• preserves record of use of leapseconds
• default packed format Long2 counting pS

13
Future Timestamp Formats

• Things to think about
• Leapseconds may be dispensed with
• OGSA may define their own timestamp

14
EDG Timestamping

• THE END