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# xDS Server Implementation
go-control-plane ships with a full [streaming implementation](https://github.com/envoyproxy/go-control-plane/blob/main/pkg/server/v3/server.go#L175) of the xDS protocol. Current support for the servers lists as follows:
- REST HTTP/1.1 *(This will soon be deprecated)*
- gRPC Bi-Di
- State of the World
- Incremental
## Getting Started
For a fully functional gRPC server, check out the provided example for what that looks like:
- https://github.com/envoyproxy/go-control-plane/blob/main/internal/example/server.go
### Callbacks
All go-control-plane xDS server implementations require `Callback` methods. Callbacks are executed at certain steps of the management server lifecycle. The interface to be implemented can be found [here](https://godoc.org/github.com/envoyproxy/go-control-plane/pkg/server/v2#Callbacks).
An example implemention of the Callback interface can be found below:
```go
import (
"context"
"log"
"sync"
core "github.com/envoyproxy/go-control-plane/envoy/config/core/v3"
discovery "github.com/envoyproxy/go-control-plane/envoy/service/discovery/v3"
)
type Callbacks struct {
Signal chan struct{}
Debug bool
Fetches int
Requests int
DeltaRequests int
DeltaResponses int
mu sync.Mutex
}
func (cb *Callbacks) Report() {
cb.mu.Lock()
defer cb.mu.Unlock()
log.Printf("server callbacks fetches=%d requests=%d\n", cb.Fetches, cb.Requests)
}
func (cb *Callbacks) OnStreamOpen(_ context.Context, id int64, typ string) error {
if cb.Debug {
log.Printf("stream %d open for %s\n", id, typ)
}
return nil
}
func (cb *Callbacks) OnStreamClosed(id int64, node *core.Node) {
if cb.Debug {
log.Printf("stream %d of node %s closed\n", id, node.Id)
}
}
func (cb *Callbacks) OnDeltaStreamOpen(_ context.Context, id int64, typ string) error {
if cb.Debug {
log.Printf("delta stream %d open for %s\n", id, typ)
}
return nil
}
func (cb *Callbacks) OnDeltaStreamClosed(id int64, node *core.Node) {
if cb.Debug {
log.Printf("delta stream %d of node %s closed\n", id, node.Id)
}
}
func (cb *Callbacks) OnStreamRequest(int64, *discovery.DiscoveryRequest) error {
cb.mu.Lock()
defer cb.mu.Unlock()
cb.Requests++
if cb.Signal != nil {
close(cb.Signal)
cb.Signal = nil
}
return nil
}
func (cb *Callbacks) OnStreamResponse(context.Context, int64, *discovery.DiscoveryRequest, *discovery.DiscoveryResponse) {
}
func (cb *Callbacks) OnStreamDeltaResponse(id int64, req *discovery.DeltaDiscoveryRequest, res *discovery.DeltaDiscoveryResponse) {
cb.mu.Lock()
defer cb.mu.Unlock()
cb.DeltaResponses++
}
func (cb *Callbacks) OnStreamDeltaRequest(id int64, req *discovery.DeltaDiscoveryRequest) error {
cb.mu.Lock()
defer cb.mu.Unlock()
cb.DeltaRequests++
if cb.Signal != nil {
close(cb.Signal)
cb.Signal = nil
}
return nil
}
func (cb *Callbacks) OnFetchRequest(_ context.Context, req *discovery.DiscoveryRequest) error {
cb.mu.Lock()
defer cb.mu.Unlock()
cb.Fetches++
if cb.Signal != nil {
close(cb.Signal)
cb.Signal = nil
}
return nil
}
func (cb *Callbacks) OnFetchResponse(*discovery.DiscoveryRequest, *discovery.DiscoveryResponse) {}
```
## Info
The internal go-control-plane gRPC server implementations take care of managing watches with the [Config Watcher](https://github.com/envoyproxy/go-control-plane/blob/main/pkg/cache/v3/cache.go#L45) when new xDS clients register themselves.
> *NOTE*: The server supports REST/JSON as well as gRPC bi-di streaming
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