Using mosquitto in a shell script using a pipe - shell

Question

I'm using mosquitto on an openWRT device to receive some data from a server and then send this same data to a local printer to print this data.
I'm using this script to receive the data
mosquitto_sub -h "${HOST}" -k 30 -c -p 8883 -t "${TOPIC}" -u "${USERNAME}" -P "${PASSWORD}" --id "${ID}" | /bin/sh /bin/printer_execute "${TOPIC}" "${PRINTER}" "${USERNAME}" "${PASSWORD}"
And the printer_execute code:
#!/bin/sh
TOPIC="${1}"
PRINTER="${2}"
USERNAME="${3}"
PASSWORD="${4}"
while read MSG
do
echo "input: ${MSG}"
echo "INPUT MSG: " "${MSG}" >> /root/log
RES=`curl -m 2 --header "Content-Type: text/xml;charset=UTF-8" --header "SOAPAction: ''" --header "If-Modified-Since: Thu, 01 Jan 1970 00:00:00 GMT" --data "${MSG}" "http://${PRINTER}/cgi-bin/epos/service.cgi?devid=local_printer&timeout=5000"`
mosquitto_pub -h ${HOST_PLACEHOLDER} -p 8883 -t "${TOPIC}-response" -m "${RES}" -u "${USERNAME}" -P "${PASSWORD}"
echo "RESULT CURL: " "${RES}" >> /root/log
done
This solution works with a relatively low messages per second, but when the volume is too high the printer_execute code stop working. I'm pretty new to shell scripting and I guess the problem could be caused by the pipe and while read pattern or by the while exit condition, but i'm not really sure.
Anyone has some idea or found a similar problem and know how to solve this?
EDIT:
In light of the answers i have tried to do this:
EDIT2: Sorry in the first edit i just added what i modified but the entire script is like that and the scope should be correct for the variables.
#!/bin/sh
TOPIC="${1}"
PRINTER="${2}"
USERNAME="${3}"
PASSWORD="${4}"
PrintOne(){
MSG="${1}"
RES=$(curl [params])
mosquitto_pub -h [host] -p 8883 -d -t "${TOPIC}-response" -m "${RES}" -u "${USERNAME}" -P "${PASSWORD}"
echo "RESULT CURL: " "${RES}" >> /root/log
}
while read msg ; do
PrintOne "$msg" &
done
With the printone and the appersand this take one message and stop working, without the & it's just like it was before.

Answer 1

You could try making a function to handle one message and calling that in the background (by appending an ampersand) so that you can respond quickly and in parallel - and that will allow you to take longer to handle each message... for a period. If your messages continually arrive faster than you can handle them, there will inevitably be a backlog.
Something like this:
#!/bin/bash
PrintOne(){
echo "Received $1"
curl ...
mosquitto_pub ...
echo $RESULT
}
while read msg ; do
PrintOne "$msg" &
done
If you want a little example. change the code to this and save it as go, and make it executable with chmod +x go
#!/bin/bash
PrintOne(){
echo "Received $1"
sleep 2
echo "Finished $1"
}
while read msg ; do
PrintOne "$msg" &
done
Now send it 10 lines:
seq 10 | ./go
Then remove the ampersand, and do exactly the same thing again and you will see the difference.
A more complete version of my answer is as follows:
#!/bin/bash
PrintOne(){
TOPIC="${1}"
PRINTER="${2}"
USERNAME="${3}"
PASSWORD="${4}"
curl ...
mosquitto_pub ...
echo $RESULT
}
while read msg ; do
PrintOne "${1}" "${2}" "${3}" "${4}" &
done