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diff --git a/plans/usecases/pnf-sw-upgrade/simulators/pnfsim/pnf-sw-upgrade/subscriber.py b/plans/usecases/pnf-sw-upgrade/simulators/pnfsim/pnf-sw-upgrade/subscriber.py
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+++ b/plans/usecases/pnf-sw-upgrade/simulators/pnfsim/pnf-sw-upgrade/subscriber.py
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+#!/usr/bin/env python3
+
+# ============LICENSE_START=======================================================
+#  Copyright (C) 2020 Nordix Foundation.
+# ================================================================================
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+# SPDX-License-Identifier: Apache-2.0
+# ============LICENSE_END=========================================================
+
+__author__ = "Eliezio Oliveira <eliezio.oliveira@est.tech>"
+__copyright__ = "Copyright (C) 2020 Nordix Foundation"
+__license__ = "Apache 2.0"
+
+import time
+from concurrent.futures import ThreadPoolExecutor
+from threading import Timer
+
+import sysrepo as sr
+
+YANG_MODULE_NAME = 'pnf-sw-upgrade'
+
+#
+# ----- BEGIN Finite State Machine definitions -----
+#
+
+# Actions
+ACT_PRE_CHECK = 'PRE_CHECK'
+ACT_DOWNLOAD_NE_SW = 'DOWNLOAD_NE_SW'
+ACT_ACTIVATE_NE_SW = 'ACTIVATE_NE_SW'
+ACT_CANCEL = 'CANCEL'
+
+# States
+ST_CREATED = 'CREATED'
+ST_INITIALIZED = 'INITIALIZED'
+ST_DOWNLOAD_IN_PROGRESS = 'DOWNLOAD_IN_PROGRESS'
+ST_DOWNLOAD_COMPLETED = 'DOWNLOAD_COMPLETED'
+ST_ACTIVATION_IN_PROGRESS = 'ACTIVATION_IN_PROGRESS'
+ST_ACTIVATION_COMPLETED = 'ACTIVATION_COMPLETED'
+
+# Timeout used for timed transitions
+TO_DOWNLOAD = 7
+TO_ACTIVATION = 7
+
+
+def timestamper(sess, key_id):
+    xpath = xpath_of(key_id, 'state-change-time')
+    now = time.strftime("%Y-%m-%dT%H:%M:%SZ", time.gmtime())
+    state = sr.Val(now, sr.SR_STRING_T)
+    sess.set_item(xpath, state)
+
+
+def xpath_of(key_id, leaf_id):
+    selector = "[id='{0}']".format(key_id) if key_id else ''
+    return "/%s:software-upgrade/upgrade-package%s/%s" % (YANG_MODULE_NAME, selector, leaf_id)
+
+
+"""
+The finite state machine (FSM) is represented as a dictionary where the current state is the key, and its value is
+an object (also represented as a dictionary) with the following optional attributes:
+
+- on_enter: a function called when FSM enters this state;
+- transitions: a dictionary mapping every acceptable action to the target state;
+- timed_transition: a pair for a timed transition that will automatically occur after a given interval.
+"""
+STATE_MACHINE = {
+    ST_CREATED: {
+        'transitions': {ACT_PRE_CHECK: ST_INITIALIZED}
+    },
+    ST_INITIALIZED: {
+        'on_enter': timestamper,
+        'transitions': {ACT_DOWNLOAD_NE_SW: ST_DOWNLOAD_IN_PROGRESS}
+    },
+    ST_DOWNLOAD_IN_PROGRESS: {
+        'on_enter': timestamper,
+        'timed_transition': (TO_DOWNLOAD, ST_DOWNLOAD_COMPLETED),
+        'transitions': {ACT_CANCEL: ST_INITIALIZED}
+    },
+    ST_DOWNLOAD_COMPLETED: {
+        'on_enter': timestamper,
+        'transitions': {ACT_ACTIVATE_NE_SW: ST_ACTIVATION_IN_PROGRESS}
+    },
+    ST_ACTIVATION_IN_PROGRESS: {
+        'on_enter': timestamper,
+        'timed_transition': (TO_ACTIVATION, ST_ACTIVATION_COMPLETED),
+        'transitions': {ACT_CANCEL: ST_DOWNLOAD_COMPLETED}
+    },
+    ST_ACTIVATION_COMPLETED: {
+        'on_enter': timestamper,
+        'transitions': {ACT_ACTIVATE_NE_SW: ST_ACTIVATION_IN_PROGRESS}
+    }
+}
+
+#
+# ----- END Finite State Machine definitions -----
+#
+
+
+def main():
+    try:
+        conn = sr.Connection(YANG_MODULE_NAME)
+        sess = sr.Session(conn)
+        subscribe = sr.Subscribe(sess)
+
+        subscribe.module_change_subscribe(YANG_MODULE_NAME, module_change_cb, conn)
+
+        try:
+            print_current_config(sess, YANG_MODULE_NAME)
+        except Exception as e:
+            print(e)
+
+        sr.global_loop()
+
+        print("Application exit requested, exiting.")
+    except Exception as e:
+        print(e)
+
+
+# Function to be called for subscribed client of given session whenever configuration changes.
+def module_change_cb(sess, module_name, event, private_ctx):
+    try:
+        conn = private_ctx
+        change_path = xpath_of(None, 'action')
+        it = sess.get_changes_iter(change_path)
+        while True:
+            change = sess.get_change_next(it)
+            if change is None:
+                break
+            handle_change(conn, change.oper(), change.old_val(), change.new_val())
+    except Exception as e:
+        print(e)
+    return sr.SR_ERR_OK
+
+
+# Function to print current configuration state.
+# It does so by loading all the items of a session and printing them out.
+def print_current_config(session, module_name):
+    select_xpath = f"/{module_name}:*//*"
+
+    values = session.get_items(select_xpath)
+
+    if values is not None:
+        print("========== BEGIN CONFIG ==========")
+        for i in range(values.val_cnt()):
+            print(values.val(i).to_string(), end='')
+        print("=========== END CONFIG ===========")
+
+
+def handle_change(conn, op, old_val, new_val):
+    """
+    Handle individual changes on the model.
+    """
+    if op == sr.SR_OP_CREATED:
+        print("CREATED: %s" % new_val.to_string())
+        xpath = new_val.xpath()
+        last_node = xpath_ctx.last_node(xpath)
+        # Warning: 'key_value' modifies 'xpath'!
+        key_id = xpath_ctx.key_value(xpath, 'upgrade-package', 'id')
+        if key_id and last_node == 'action':
+            executor.submit(execute_action, conn, key_id, new_val.data().get_enum())
+    elif op == sr.SR_OP_DELETED:
+        print("DELETED: %s" % old_val.to_string())
+    elif op == sr.SR_OP_MODIFIED:
+        print("MODIFIED: %s to %s" % (old_val.to_string(), new_val.to_string()))
+    elif op == sr.SR_OP_MOVED:
+        print("MOVED: %s after %s" % (new_val.xpath(), old_val.xpath()))
+
+
+def execute_action(conn, key_id, action):
+    sess = sr.Session(conn)
+    try:
+        cur_state = sess.get_item(xpath_of(key_id, 'current-status')).data().get_enum()
+        next_state_str = STATE_MACHINE[cur_state]['transitions'].get(action, None)
+        if next_state_str:
+            handle_set_state(conn, key_id, next_state_str)
+        sess.delete_item(xpath_of(key_id, 'action'))
+        sess.commit()
+    finally:
+        sess.session_stop()
+
+
+def handle_set_state(conn, key_id, state_str):
+    sess = sr.Session(conn)
+    try:
+        state = sr.Val(state_str, sr.SR_ENUM_T)
+        sess.set_item(xpath_of(key_id, 'current-status'), state)
+        on_enter = STATE_MACHINE[state_str].get('on_enter', None)
+        if on_enter:
+            # noinspection PyCallingNonCallable
+            on_enter(sess, key_id)
+        sess.commit()
+        delay, next_state_str = STATE_MACHINE[state_str].get('timed_transition', [0, None])
+        if delay:
+            Timer(delay, handle_set_state, (conn, key_id, next_state_str)).start()
+    finally:
+        sess.session_stop()
+
+
+if __name__ == '__main__':
+    xpath_ctx = sr.Xpath_Ctx()
+    executor = ThreadPoolExecutor(max_workers=2)
+    main()