## GoVPP

This set of packages provide the API for communication with VPP from Go. It consists of the following packages:

- [govpp](govpp.go): provides the entry point to govpp functionality
- [api](api/api.go): API for communication with govpp core using Go channels (without the need of importing the govpp core package itself),
- [core](core/): main functionality of the govpp, sends and receives the messages to/from VPP using the adapter, marshalls/unmarshalls them and forwards them between the client Go channels and the VPP,
- [adapter](adapter/): adapter between govpp core and the VPP, responsible for sending and receiving binary-encoded data via shared memory,
- [binapi_generator](binapi_generator/generator.go): Generator of Go structs out of the VPP binary API definitions in JSON format,
- [examples](examples/): example VPP management application that exercises the govpp API on real-world use-cases.

The design with separated govpp API package ([api](api/api.go)) and the govpp core package ([core](core/)) enables 
plugin-based infrastructure, where one entity acts as a master responsible for talking with VPP (e.g. Agent 
Core on the schema below) and multiple entities act as clients that are using the master for the communication with 
VPP (Agent Plugins on the schema below). The clients can be built into standalone shared libraries without the need 
of linking the govpp core and all its dependencies into them.

```
                                       +--------------+
    +--------------+                   |              |
    |              |                   | Agent Plugin |
    |              |                   |              |
    |  Agent Core  |                   +--------------+
    |              |            +------+  govpp API   |
    |              |            |      +--------------+
    +--------------+     Go     |
    |              |  channels  |      +--------------+
    |  govpp core  +------------+      |              |
    |              |            |      | Agent Plugin |
    +------+-------+            |      |              |
           |                    |      +--------------+
binary API |                    +------+  govpp API   |
  (shmem)  |                           +--------------+
           |
    +------+-------+
    |              |
    |      VPP     |    
    |              |
    +--------------+
```


## Example usage
Generating Go bindings from the JSON files located in the `/usr/share/vpp/api/` directory 
into the Go packages that will be created inside of the `bin_api` directory:
```
go run binapi_generator/generator.go --input-dir=/usr/share/vpp/api/ --output-dir=bin_api
```

Usage of the generated bindings:
```go
func main() {
	conn, _ := govpp.Connect()
	defer conn.Disconnect()

	ch, _ := conn.NewAPIChannel()
	defer ch.Close()
  
	req := &acl.ACLAddReplace{
		ACLIndex: ^uint32(0),
		Tag:      []byte("access list 1"),
		R: []acl.ACLRule{
			{
				IsPermit:       1,
				SrcIPAddr:      net.ParseIP("10.0.0.0").To4(),
				SrcIPPrefixLen: 8,
				DstIPAddr:      net.ParseIP("192.168.1.0").To4(),
				DstIPPrefixLen: 24,
				Proto:          6,
			},
			{
				IsPermit:       1,
				SrcIPAddr:      net.ParseIP("8.8.8.8").To4(),
				SrcIPPrefixLen: 32,
				DstIPAddr:      net.ParseIP("172.16.0.0").To4(),
				DstIPPrefixLen: 16,
				Proto:          6,
			},
		},
	}
	reply := &acl.ACLAddReplaceReply{}

	err := ch.SendRequest(req).ReceiveReply(reply)
}
```

The example above uses simple wrapper API over underlying go channels, see [example_client](examples/example_client.go) 
for more examples, including the example on how to use the Go channels directly.


## Build & Installation Procedure
Govpp uses `vppapiclient` library from VPP codebase to communicate with VPP. To build govpp, vpp-dev package must be installed,
either [from packages](https://wiki.fd.io/view/VPP/Installing_VPP_binaries_from_packages) or 
[from sources](https://wiki.fd.io/view/VPP/Build,_install,_and_test_images#Build_A_VPP_Package).

To build & install `vpp-dev` from sources:
```
git clone https://gerrit.fd.io/r/vpp
cd vpp
make install-dep
make bootstrap
make pkg-deb
cd build-root
sudo dpkg -i vpp*.deb
```

To build & install all govpp binaries into your `$GOPATH`:
```
cd govpp
go build ./...
go install ./...
```


## Building Go bindings from VPP binary APIs
Once you have `binapi_generator` installed in your `$GOPATH`, you can use it to generate Go bindings from
VPP APis in JSON format. The JSON input can be specified as a single file (`input-file` argument), or
as a directory that will be scanned for all `.json` files (`input-dir`). The generated Go bindings will
be placed into `output-dir` (by default current working directory), where each Go package will be placed into 
a separated directory, e.g.:

```
binapi_generator --input-file=examples/bin_api/acl.api.json --output-dir=examples/bin_api
binapi_generator --input-dir=examples/bin_api --output-dir=examples/bin_api
```

In Go, [go generate](https://blog.golang.org/generate) tool can be leveraged to ease the code generation
process. It allows to specify generator instructions in any one of the regular (non-generated) `.go` files
that are dependent on generated code using special comments, e.g. the one from [example_client](examples/example_client.go):
```go
// go:generate binapi_generator --input-dir=bin_api --output-dir=bin_api
```

The comment must start at the beginning of the line and have no spaces between the `//` and the `go:generate`. 
After that marker, the rest of the line specifies a command for `go generate` to run. 

The `go generate` tool automatically traverses the code base, looks for the special comments in Go code and 
invokes the code generation, e.g.:
```
go generate ./...
```
Invokes all `go:generate` rules in all Go packages recursively.