Transmitting streams between processes

[dv]

Hello,

I want to decode media in one process and transmit it to another, which 
will playback the media.
Both processes will always run on the same host.
It is also important for the second process to be able to join in at any 
moment, and hear what is currently being decoded.
In other words, the first process does not have to keep decoded data 
cached. You could see the first process as something
like a "local web cast".

I have to use GStreamer 0.10 for the time being, since a port of the 
underlying application to 1.0 will not be done soon.

The problem I have is with transmitting the caps. The caps can change at 
any moment (for example, because now a stereo song
is playing, and the last song was a mono one). Here are some notes about 
solutions I've tried so far:

- gdppay works, but transmits the header only once. So, if a receiver 
exits and rejoins, it won't work.
- rtpgstpay is an interesting option, but I have to explicitely give the 
depayloader the caps of the media, which means
   I'd have to establish an out-of-band connection for transmitting caps 
like SDP, and that sounds like overkill if both processes are on
   the same host.
- The same goes for the TCP sinks and sources, they work, and can 
retransmit GDP headers, but I'd like to avoid the TCP overhead.
   Also, the TCP method requires the sender to be running before the 
receiver starts, and suffers from queuing problems
   (it often does not start to play right away).
- ALSA loopback is actually not GStreamer specific, but can be used for 
this. However, it does not allow to change the sample rate,
   number of channels etc. unless both sender and receiver disconnect 
from it.

Here is an example using shmsink and rtpgstpay:

sender:
gst-launch-0.10 videotestsrc pattern=ball ! rtpgstpay ! shmsink 
socket-path=/tmp/abc wait-for-connection=false

receiver:
gst-launch-0.10 shmsrc socket-path=/tmp/abc ! "application/x-rtp, 
media=(string)application, clock-rate=(int)90000, 
encoding-name=(string)X-GST, 
caps=(string)\"dmlkZW8veC1yYXcteXV2LCBmb3JtYXQ9KGZvdXJjYylZVVkyLCBjb2xvci1tYXRyaXg9KHN0cmluZylzZHR2LCBjaHJvbWEtc2l0ZT0oc3RyaW5nKW1wZWcyLCB3aWR0aD0oaW50KTMyMCwgaGVpZ2h0PShpbnQpMjQwLCBmcmFtZXJhdGU9KGZyYWN0aW9uKTMwLzE\\=\", 
payload=(int)96" ! rtpgstdepay ! autovideosink

this is almost perfect, it suffers from two problems: the sender must be 
running before the receiver starts, and the caps string must be set.

Does anyone have an idea? It seems like a common use case.