Dockerlith Application Container Entrypoint Script for Synchronizing Access to Shared NPM Dependencies

monolith_development_container_entrypoint

#!/usr/bin/env bash
# Application containers should not continue starting 
# if their dependencies are unverified, 
# so if any command in this script fails the container 
# will stop it’s initialization process and 
# an error message will be presented to the user. 
set -euo pipefail

# Set variables here
# The resource to have synchronized write access is the node_modules directory. 
# An empty, hidden file .npm_lock is created to use it’s file descriptor. 
# The file descriptor to be locked is number 200. 
# Finally, the .package_md5 file is used to store a hash string of the package.json file 
# so no call to npm install is made unless the package.json file has changed.
readonly NPM_DIR=/app/src/node_modules
readonly LOCK_FILE=$NPM_DIR/.npm_lock
readonly LOCK_FD=200
readonly MD5_FILE=$NPM_DIR/.package_md5

# Define functions here
# Here is a generic function that locks a file with a file descriptor, 
# using the flock unix command. 
# Docker containers share the host’s kernel, 
# so they can communicate with each other through these file descriptor locks.
lock() {
  # Create lock file.
  eval "exec $2>$1"

  # Get exclusive, waiting lock on the file descriptor of the lock file.
  flock --exclusive $2
}

# Releases the lock acquired with lock()
unlock() {
  echo "This container is releasing the lock on the npm libraries"
  flock --unlock $1
}

# Containers should only install npm libraries if the dependencies have changed 
# in package.json. Therefore, package.json has an MD5 hash saved to a shared file
# each time the dependencies are installed.
# If package.json changes due to new dependencies, 
# a different MD5 hash will be generated that differs from 
# the one that has been saved.
check_npm_libraries() {
  if [ ! -f $MD5_FILE ] || ! md5sum -c --status <<<"$(cat $MD5_FILE)"; then
    echo "INVALID"
  else
    echo "VALID"
  fi
}

# If the dependencies have changed, they are updated with a call to “npm install”. 
# After this, the hash of the package.json file the dependencies came from 
# is saved to the hash file. 
install_npm_libraries() {
  npm install grunt
  echo "Installing npm Libraries..."
  npm install --unsafe-perm --loglevel info
  write_md5_sum_to_md5_file
}

write_md5_sum_to_md5_file() {
  echo "package.json md5" $(md5sum ./package.json)
  md5sum ./package.json > $MD5_FILE
}

prepare_npm_volume() {
  #Try to get node_modules lock.
  lock $LOCK_FILE $LOCK_FD

  #Check status of npm libraries now that this container has the lock.
  echo "This container has the lock on the npm libraries."
  npm_library_status=$(check_npm_libraries)

  if [ "$npm_library_status" == "INVALID" ]; then
    install_npm_libraries
  else
    echo "Valid cached npm libraries detected, skipping npm install"
  fi

  # The lock must be explicitly released here, 
  # because the process is expected to continue after this script is done.
  unlock $LOCK_FD
}

#Execute commands here

prepare_npm_volume

echo "arguments received: $@"

exec "$@"