perf(scripts): improve load balancing (#2165)

gunicorn does not perform a real load balancing.
We add a new option to the start.sh script that starts
multiple workers listening on different ports to let users
perform load balancing upstream if they prefer that option,
for example using nginx.

scripts/start.sh

@@ -1,14 +1,61 @@
 #!/bin/bash
+#
+# Starter script used in docker image.
+#
+# Usages are:
+#
+# 1. ./start.sh
+#    Starts antares web server with gunicorn.
+#    Number of workers is defined by GUNICORN_WORKERS env variable,
+#    or equal to 2*cpu + 1.
+#
+# 2. ./start.sh <module>
+#    Where module is for example "watcher".
+#    Starts one of the backend services (watcher, garbage collector, ...)
+#
+# 3. ./start.sh --no-gunicorn
+#    Starts multiple workers on different ports. You are expected to
+#    configure some load balancing upstream, in that case.
+#    Number of workers is defined by ANTARES_NB_WORKERS env variable,
+#    or equal to 2*cpu + 1.
 
 set -e
 
 CUR_DIR=$(cd "$(dirname "$0")" && pwd)
 BASE_DIR=$(dirname "$CUR_DIR")
 
+min() {
+    echo $(( $1 < $2 ? $1 : $2 ))
+}
+
+workers=$(min 30 ${ANTARES_NB_WORKERS:-$((2*$(nproc) + 1))}) # default (2*nproc + 1) and max is 30
+
+# Check for --no-gunicorn or --multiple-ports argument
+use_uvicorn=false
+for arg in "$@"
+do
+    if [[ $arg == "--no-gunicorn" || $arg == "--multiple-ports" ]]; then
+        use_uvicorn=true
+        break
+    fi
+done
+
 if [ -z "$1" ] ; then
   sh $CUR_DIR/pre-start.sh
   gunicorn --config $BASE_DIR/conf/gunicorn.py --worker-class=uvicorn.workers.UvicornWorker antarest.wsgi:app
+elif [ "$use_uvicorn" = true ]; then
+  sh $CUR_DIR/pre-start.sh
+  pids=() # Initialize empty array to store background process IDs
+  for ((i=0; i<workers; i++))
+  do
+    uvicorn antarest.wsgi:app --host 0.0.0.0 --port $((5000 + $i)) --log-level info --timeout-keep-alive 600 &
+    pids+=($!) # Store background process IDs
+  done
+  for pid in ${pids[*]};
+  do
+    wait $pid # Wait for each background process to finish
+  done
 else
   export PYTHONPATH=$BASE_DIR
   python3 $BASE_DIR/antarest/main.py -c $ANTAREST_CONF --module "$1"
-fi
+fi