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One of the important dimensions of developing multimodal applications is
being able to tell when things happen, in order to coordinate multiple input
gestures, for instance. Starting with version 4.0, we associate timestamps
with message traffic to support this need.
In the Communicator infrastructure, a timestamp is associated with a new
message when the Hub receives it. We do not create timestamps in the server,
because we can't depend on the servers having synchronized time to a fine
enough degree. While it's not completely accurate to perform the timestamping
when the Hub receives a message (rather than when the message is sent), we
do benefit from a forced serialization and a common time reference base. In
other words, we're betting that network latency, in those situations where
time synchronization is important, is less of an issue than non-synchronized
hosts. It's not clear that there's another bet to make; the best-known time
synchronization protocol, NTP, is only accurate to a granularity of seconds,
which is fine for most applications, but too coarse for multimodal synchronization
by at least two orders of magnitude.
When the Hub receives a new message, then, it creates a token and assigns
a 64-bit float as the current UTC time (i.e., number of seconds, with a granularity
of microseconds, since midnight, January 1, 1970). When the Hub a message
associated with that token to a server, it transmits the token's creation
time as part of the administrative information in the calling environment.
This time can be retrieved by the server via the environment.
double GalSS_EnvGetTokenTimestamp(GalSS_Environment
*env)
If env is an environment created by virtue of calling a dispatch
function, this function returns the creation time of the token which led
to the invocation of the dispatch function.
This functionality is also available to Hub programs; see the Builtin
dispatch function hub_token_timestamp.
Sometimes, the server may want to send a new message to the Hub, but preserve
the creation time of the environment it's writing through.
void GalSS_EnvInheritTokenTimestamp(GalSS_Environment
*env)
If env has a token creation time associated with it, this function
causes all new messages sent through env to inherit the creation time
of the originating token.
This functionality is also available when the Hub creates a new message;
see the Builtin dispatch function call_program.
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