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 * [Introduction](README.md)
     - [Contribute to the documentation](contribute_docs.md)
-
+* [Developer HowTos](developer.md)
+    - [xproto](xproto.md)
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+# xproto
+
+xproto is a variant of Google Protobufs that incorporates XOS’ data modeling features. Its goal is to encode XOS data models and facilitate the generation of code that depends on those data models.
+
+In general, the goal of XOS data modeling is to abstract technology-independent information out of service implementations. An example of this is the database layer of XOS, which is implemented purely in xproto, and converted into Django data models at build time, but could also target a different technology, such as NoSQL.
+
+In this chapter, we explain how to write xproto specs and how to use the xproto toolset to generate code. It is assumed that you are in a working CORD development environment. Please consult [this document](https://github.com/opencord/platform-install#creating-a-development-environment-on-you-machine) for instructions on how to bring one up. You do not need to bring up a full pod to use xproto, the frontend environment described in the linked document is sufficient.
+
+
+## Generating code from an existing xproto file
+
+Drop an xproto file in your working directory. You can copy and paste the following content into a file named slice.xproto.
+
+```protobuf
+message Slice (PlCoreBase){
+     required string name = 1 [max_length = 80, content_type = "stripped", blank = False, help_text = "The Name of the Slice", null = False, db_index = False];
+     required bool enabled = 2 [help_text = "Status for this Slice", default = True, null = False, db_index = False, blank = True];
+     required bool omf_friendly = 3 [default = False, null = False, db_index = False, blank = True];
+     required string description = 4 [help_text = "High level description of the slice and expected activities", max_length = 1024, null = False, db_index = False, blank = True];
+     required string slice_url = 5 [db_index = False, max_length = 512, null = False, content_type = "url", blank = True];
+     required manytoone site->Site:slices = 6 [help_text = "The Site this Slice belongs to", null = False, db_index = True, blank = False];
+     required int32 max_instances = 7 [default = 10, null = False, db_index = False, blank = False];
+     optional manytoone service->Service:slices = 8 [db_index = True, null = True, blank = True];
+     optional string network = 9 [blank = True, max_length = 256, null = True, db_index = False, choices = "((None, 'Default'), ('host', 'Host'), ('bridged', 'Bridged'), ('noauto', 'No Automatic Networks'))"];
+     optional string exposed_ports = 10 [db_index = False, max_length = 256, null = True, blank = True];
+     optional manytoone serviceClass->ServiceClass:slices = 11 [db_index = True, null = True, blank = True];
+     optional manytoone creator->User:slices = 12 [db_index = True, null = True, blank = True];
+     optional manytoone default_flavor->Flavor:slices = 13 [db_index = True, null = True, blank = True];
+     optional manytoone default_image->Image:slices = 14 [db_index = True, null = True, blank = True];
+     optional manytoone default_node->Node:slices = 15 [db_index = True, null = True, blank = True];
+     optional string mount_data_sets = 16 [default = "GenBank", max_length = 256, content_type = "stripped", blank = True, null = True, db_index = False];
+     required string default_isolation = 17 [default = "vm", choices = "(('vm', 'Virtual Machine'), ('container', 'Container'), ('container_vm', 'Container In VM'))", max_length = 30, blank = False, null = False, db_index = False];
+     required manytomany tags->Tag = 18 [db_index = False, null = False, blank = True];
+}
+```
+
+Now copy the file `Makefile` from the directory `orchestration/xos/genx/tool` in the CORD source code to the current directory, and run `make PREFIX=&lt;location of genx directory&gt;` This should generate a file called `slice.py` in your current directory. If there were multiple files, then it would generate python Django models for each of them.
+
+The tool that processes xproto files and generates code is called xosgen. You can print its syntax by running `xosgen --help`.
+
+```usage: xosgen [-h] [--rev] --target TARGET [--output OUTPUT] 
+              [--kvpairs KV]
+              <input file> [<input file> ...]```
+
+xosgen takes as input a set of xproto files and a target. The target is a jinja2 template that specifies the format of the code to be generated. xosgen converts xproto into an intermediate representation (IR) and feeds it into the target. For example, to generate Django templates from a set of xproto files, you can run the following command:
+
+```xosgen --target targets/django-split.xtarget *.xproto```
+
+
+
+## Writing an xproto file 
+
+xproto is based on Google Protobufs. This means that any protobuf file also qualifies as xproto. We currently use the Protobuf v2 syntax. For example, the file below specifies a model that describes container images:
+
+```protobuf
+message Image {
+     required string name = 1 [db_index = False, max_length = 256, null = False, content_type = "stripped", blank = False];
+     required string kind = 2 [default = "vm", choices = "(('vm', 'Virtual Machine'), ('container', 'Container'))", max_length = 30, blank = False, null = False, db_index = False];
+     required string disk_format = 3 [db_index = False, max_length = 256, null = False, content_type = "stripped", blank = False];
+     required string container_format = 4 [db_index = False, max_length = 256, null = False, content_type = "stripped", blank = False];
+     optional string path = 5 [max_length = 256, content_type = "stripped", blank = True, help_text = "Path to image on local disk", null = True, db_index = False];
+     optional string tag = 6 [max_length = 256, content_type = "stripped", blank = True, help_text = "For Docker Images, tag of image", null = True, db_index = False];
+}
+```
+
+xproto contains several extensions, encoded as Protobuf options, which the xproto toolchain recognizes at the top level. That is, these options are not declared explicitly in Protobuf syntax.
+
+
+## xproto extensions to Google Protobufs
+
+**Inheritance**
+
+- xproto
+  ```protobuf
+  message EC2Instance (Instance, EC2Object) {
+        // EC2Instance inherits the fields of  Instance
+  }
+  ```
+
+- protobuf
+  ```protobuf
+  message EC2Instance  {
+        option bases = "Instance,EC2Object"
+  }
+  ```
+
+Inheritance instructs the xproto processor that a model inherits the fields of a set of base models. Note that these base model fields are not copied into the derived model automatically. However, the fields can be accessed in an xproto target.
+
+**Links**
+
+- xproto
+  ```protobuf
+  message Instance {
+        required manytoone slice:Slice->instances = 1;
+  }
+  ```
+
+- protobuf
+  ```protobuf
+  message Instance {
+        required int32 slice = 1 [model="Slice", link="manytoone", src_port="slice", dst_port="instances"];
+  }
+  ```
+
+Links are references to one model from another. A link specifies the type of the reference (manytoone, manytomany, onetomany, or onetoone), name of the field that contains the reference (_slice_ in the above example), its type (Slice), the name of the field in the peer model that points back to the current model, and a “through” field, specifying a model declared separately as an xproto message, that stores properties of the link.
+
+The example below illustrates a manytomany link from Image to Deployment, which goes through the model “ImageDeployments”
+
+- xproto
+
+  ```protobuf
+  required manytomany deployments->Deployment/ImageDeployments:images = 7 [help_text = "Select which images should be instantiated on this deployment", null = False, db_index = False, blank = True];
+  ```
+
+
+- Protobuf
+
+  ```protobuf
+  required int32 deployments = 7 [help_text = "Select which images should be instantiated on this deployment", null = False, db_index = False, blank = True, model="Deployment", through="ImageDeployments", dst_port="images", link="manytomany"];
+  ```
+
+
+**Model options**
+
+```protobuf
+option name = "Name of service"
+option verbose_name = "Verbose name of service";
+option app_name = "Name of app containing service";
+```
+
+The above options declare information about models. They can be declared for models individually, or at the top level in the xproto definition, in which case they are inherited by all of the models in that definition.
+
+**Field options**
+
+The field options supported by the xproto processor are listed below.
+
+```protobuf
+option null = True/False
+```
+
+The null option specifies whether a field has to be set or not.
+
+```protobuf
+option help_text = “Descriptive text goes here”;
+```
+
+Help text describes a field.
+
+```protobuf
+option default = “Default value of field”;
+```
+
+The default value of the field.
+
+```protobuf
+option max_length = 128;
+```
+
+The maximum length of a field whose type is string.
+
+```protobuf
+option blank = False;
+```
+
+Whether a field can be empty.
+
+```protobuf
+option choices = "(('vm', 'Virtual Machine'), ('container', 'Container'))"
+```
+
+The set of valid values for a field. Each inner tuple specifies equivalence classes. E.g. vm is equivalent to Virtual Machine.
+
+```protobuf
+option db_index = True
+```
+
+Whether the field is an index field. Used by database targets.
+
+
+## The xproto toolchain: an overview 
+
+The figure below illustrates the processing of an xproto file. The xosgen tool converts the xproto file into an intermediate representation and passes it to a target, which in turn generates the output code. The target has access to a library of auxiliary functions implemented in Python. The target itself is written as a jinja2 template.
+
+![xproto toolchain](toolchain.png)
+
+## The IR 
+
+The IR is a representation of a parsed xproto file in the form of nested Python dictionaries. Here is a description of its structure.
+
+```protobuf
+"proto": {
+    "messages": [
+         {"name": "foo", fields: [{...}], links: [{...}], rlinks: [{...}], options: [{...}]}
+    ]
+},
+"context": {
+    "command line option 1": "value - see the --kv option of xosgen"
+},
+"options": {
+    "top level option 1": "value of option 1"
+}
+```
+
+
+## Writing a target 
+
+A target is a template written in jinja2 that takes the IR described in the previous section as input and generates some code, such as Python, Protobufs, unit tests etc. We will take up a few examples. The example below generates a GraphViz dot file from a set of xproto representations:
+
+```python
+digraph {
+{% for m in proto.messages %}
+  {%- for l in m.links %}
+  {{ m.name }} -> {{ l.peer }};
+  {%- endfor %}
+{% endfor %}
+}```
+
+This target loops through all of the messages in a proto definition and through the links in each message. For each link, it formats and outputs an edge in a graph in Graphviz’ dot notation.
+
+```
+{{ proto }}
+```
+
+This target simply prints the IR for an xproto definition.
+
+```python
+{% for m in proto.messages -%}
+{% for r in m.rlinks %}
+    def enumerate_{{ xos_singularize(r) }}_ids:
+        return map(lambda x:x['id'], {{ xos_pluralize(r) }})
+{% endfor %}
+{% endfor -%}
+```
+
+The target above outputs a Python function that enumerates the ids of the objects from which the current object is linked.
+
+## Library functions
+
+xproto targets can use a set of library functions implemented in Python. These can be found in the file lib.py in the genx/tool directory. These functions are listed below
+
+- `xproto_unquote(string)` Unquotes a string. For example, `"This is a help string"` is converted into `This is a help string.`    
+- `xproto_singularize(field)` Converts an english plural into its singular. It is extracted from the `singular` option for a field if such an option is specified, otherwise, it performs the conversion automatically using the library `pattern.en`.
+- `xproto_pluralize(field)` The reverse of xproto_singularize.
+