Added Modeling concept expression

Change-Id: I91d543a920885ba84f59ef8e43424f646ff7bb28
Issue-ID: CCSDK-1453
Signed-off-by: Ezhilarasi <ezhrajam@in.ibm.com>

diff --git a/docs/microservices/expression.rst b/docs/microservices/expression.rst
new file mode 100644 (file)
index 0000000..38a7d62
--- /dev/null
+++ b/docs/microservices/expression.rst
@@ -0,0 +1,45 @@
+.. This work is licensed under a Creative Commons Attribution 4.0
+.. International License. http://creativecommons.org/licenses/by/4.0
+.. Copyright (C) 2019 IBM.
+
+Expression
+==========
+
+TOSCA provides for a set of functions to reference elements within the template or to retrieve runtime values.
+
+Below is a list of supported expressions
+
+get_input
+---------
+
+The get_input function is used to retrieve the values of properties declared within the inputs section of a TOSCA Service Template.
+
+http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454178
+
+get_property
+------------
+
+The get_property function is used to retrieve property values between modelable entities defined in the same service template.
+
+http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454178
+
+get_attribute
+-------------
+
+The get_attribute function is used to retrieve the values of named attributes declared by the referenced node or relationship template name.
+
+http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454179
+
+get_operation_output
+--------------------
+
+The get_operation_output function is used to retrieve the values of variables exposed / exported from an interface operation.
+
+http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454180
+
+get_artifact
+------------
+
+The get_artifact function is used to retrieve artifact location between modelable entities defined in the same service template.
+
+http://docs.oasis-open.org/tosca/TOSCA-Simple-Profile-YAML/v1.2/csd01/TOSCA-Simple-Profile-YAML-v1.2-csd01.html#_Toc494454182