DataGrid WP3 MAR-2002 Timestamps

We would like the information system to associate each measurement with the time it was made We would like to handle fresh and archive data in a uniform manner All information must carry a timestamp

Timestamps – what? ASCII timestamp Formatted string E.g T12:45:50.36Z Binary timestamp Local binary representation Linux – two ints/longs

ASCII Format A number of efforts currently underway IETF currently has an RFC for defining date & time format for internet based on the standard representation of dates and times ISO8601 GGF also has a standard model, more or less same; But includes meta-info such as precision & accuracy information

ASCII Format IETF format Yyyy-mm-ddThh:mm:ss.FfffZ T & Z (specifies UTC) are compulsory GGF format (grd-perf-15-1) As above but with the inclusion of accuracy resolution information Yyyy-mm-ddThh:mm:ss.FfffZxxxryya Number before ‘r’ is the resolution, ‘a’ is accuracy eg T12:46: Z r.00001a

Binary Format New/Current Unix struct timeval is implemented using two long integers First is 64-bit signed second value Last is 64-bit signed micro-second Not enough precision for future applications with fractional part. Resolution can be different for platforms and programming languages

Timestamps How is time defined & referenced ? Based on the rotation of the earth on its axis – Universal Time (UT/UT0/UT1/UT2) TAI (Temps Atomique International ) (a.k.a Atomic time) S.I. Second defined as cycles of the caesium resonance UTC Time (Coordinated Universal Time) Same S.I. SECOND as TAI, but adjusted to include steps (leap seconds) to keep within.9s of UT1

Timestamps How is time referenced ? In relation to the time-definition’s epoch Each definition has a different epoch TAI UTC (sort of) GPS

Timestamps – UT1,UTC & TAI But UTC = TAI – n x (1s) and UTC kept within 0.9s of UT1 (irregular) Integer leapseconds since 1972 TAI - UTC rebased to 10s IERS (Paris) determines when to introduce +/- leapsecond, very irregular Introduced as an extra second at end of a month (currently only June and December used)

Leapseconds -problem Problem converting between ASCII and binary (UTC) timestamps Must possess times at which leapsecond was introduced All sites/nodes must view the same information “ A second is represented by an integer from 0 to 60; the values 60 and 61 occur only for leap seconds and even then only in Java implementations that actually track leap seconds correctly” – JDK documentation

Leapseconds, an example Normal (common) year = (365 x 86400) seconds 1998 lasted longer (due to aggregate deceleration of the earth due by tidal forces) A leapsecond was introduced by IERS 1-December :59:58 1-December :59:59 1-December :59:60 (+1s) 1-December :00:00

Problem- Timestamp Conversion ab c a, produces information Ia with timestamp ta Ia(ta) c, also consumes information Ia(ta) b consumes information Ia(ta) Sites a & c are corrected for leap seconds, b is not 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.

Solution All sites obtain leapsecond information from the same information producer Use R-GMA Only need one node to obtain data from USNO ftp site. This node will publish information via a DBProducerServlet Timestamp class will use this information to convert between formats

Solution (Part A) SQL-DB DB Producer Leapsecond prod DB Producer Servlet U.S.N.O. Consumer Servlet Timestamp obj Application (Local Cache) Leap Second data

Solution (Part B) ASCII to binary conversion ASCII t a -> Binary t b Find corresponding leapsecond value, L for t a Corrected binary value = t b + L

Solution (Part B) Binary to ASCII ->conversion Binary t a -> ASCII t b Find corresponding leapsecond value, L for t a Corrected binary value = t b - L Easy to reverse engineer the ASCII fields from the binary second amount

Solution (Part B) ASCII --> binary --> ASCII 450MHz P2 + Sun JDK cache file size adapts to no.leapseconds since 1972 [22 so far] ~20mS startup [to read from cache file] when timestamp object is instantiated 225 sec for 10 6 conversions tested multiple +ve and -ve leapseconds Blackdown order-of-mag. slower - why ?

Solution (Part B) Must decide upon Binary and ASCII second fraction format. UNIX binary fraction is given as the elapsed micro-seconds (resolution not good enough for future applications) GGF were proposing a binary fraction of a second value (regressed to ASCII only) IETF only have a decimal fraction of variable length

The Earth Was Created 4 Billion Years Ago… Must decide on an epoch for binary timestamps NTP = 1/1/1900TAI = 1/1/1958 UNIX= 1/1/1970UTC= 1/1/1972 GPS = 1/1/1980Y2K = 1/1/2000 Most sensible TAI, UTC or Y2K Easy to convert from others UNIX -> non-integer leapsecond value

To do… Decide upon Epoch Minimum binary fraction-of-second format Accuracy/resolution info Write CRON scripts to obtain leapsecond info from USNO to populate cache files to randomize the timing of these actions