debian-mirror-gitlab/workhorse-vendor/github.com/grpc-ecosystem/go-grpc-middleware/DOC.md

# grpc_middleware
`import "github.com/grpc-ecosystem/go-grpc-middleware"`

* [Overview](#pkg-overview)
* [Imported Packages](#pkg-imports)
* [Index](#pkg-index)

## <a name="pkg-overview">Overview</a>
`grpc_middleware` is a collection of gRPC middleware packages: interceptors, helpers and tools.

### Middleware
gRPC is a fantastic RPC middleware, which sees a lot of adoption in the Golang world. However, the
upstream gRPC codebase is relatively bare bones.

This package, and most of its child packages provides commonly needed middleware for gRPC:
client-side interceptors for retires, server-side interceptors for input validation and auth,
functions for chaining said interceptors, metadata convenience methods and more.

### Chaining
By default, gRPC doesn't allow one to have more than one interceptor either on the client nor on
the server side. `grpc_middleware` provides convenient chaining methods

Simple way of turning a multiple interceptors into a single interceptor. Here's an example for
server chaining:

	myServer := grpc.NewServer(
	    grpc.StreamInterceptor(grpc_middleware.ChainStreamServer(loggingStream, monitoringStream, authStream)),
	    grpc.UnaryInterceptor(grpc_middleware.ChainUnaryServer(loggingUnary, monitoringUnary, authUnary),
	)

These interceptors will be executed from left to right: logging, monitoring and auth.

Here's an example for client side chaining:

	clientConn, err = grpc.Dial(
	    address,
	        grpc.WithUnaryInterceptor(grpc_middleware.ChainUnaryClient(monitoringClientUnary, retryUnary)),
	        grpc.WithStreamInterceptor(grpc_middleware.ChainStreamClient(monitoringClientStream, retryStream)),
	)
	client = pb_testproto.NewTestServiceClient(clientConn)
	resp, err := client.PingEmpty(s.ctx, &myservice.Request{Msg: "hello"})

These interceptors will be executed from left to right: monitoring and then retry logic.

The retry interceptor will call every interceptor that follows it whenever when a retry happens.

### Writing Your Own
Implementing your own interceptor is pretty trivial: there are interfaces for that. But the interesting
bit exposing common data to handlers (and other middleware), similarly to HTTP Middleware design.
For example, you may want to pass the identity of the caller from the auth interceptor all the way
to the handling function.

For example, a client side interceptor example for auth looks like:

	func FakeAuthUnaryInterceptor(ctx context.Context, req interface{}, info *grpc.UnaryServerInfo, handler grpc.UnaryHandler) (interface{}, error) {
	   newCtx := context.WithValue(ctx, "user_id", "john@example.com")
	   return handler(newCtx, req)
	}

Unfortunately, it's not as easy for streaming RPCs. These have the `context.Context` embedded within
the `grpc.ServerStream` object. To pass values through context, a wrapper (`WrappedServerStream`) is
needed. For example:

	func FakeAuthStreamingInterceptor(srv interface{}, stream grpc.ServerStream, info *grpc.StreamServerInfo, handler grpc.StreamHandler) error {
	   newStream := grpc_middleware.WrapServerStream(stream)
	   newStream.WrappedContext = context.WithValue(ctx, "user_id", "john@example.com")
	   return handler(srv, stream)
	}

## <a name="pkg-imports">Imported Packages</a>

- [golang.org/x/net/context](https://godoc.org/golang.org/x/net/context)
- [google.golang.org/grpc](https://godoc.org/google.golang.org/grpc)

## <a name="pkg-index">Index</a>
* [func ChainStreamClient(interceptors ...grpc.StreamClientInterceptor) grpc.StreamClientInterceptor](#ChainStreamClient)
* [func ChainStreamServer(interceptors ...grpc.StreamServerInterceptor) grpc.StreamServerInterceptor](#ChainStreamServer)
* [func ChainUnaryClient(interceptors ...grpc.UnaryClientInterceptor) grpc.UnaryClientInterceptor](#ChainUnaryClient)
* [func ChainUnaryServer(interceptors ...grpc.UnaryServerInterceptor) grpc.UnaryServerInterceptor](#ChainUnaryServer)
* [func WithStreamServerChain(interceptors ...grpc.StreamServerInterceptor) grpc.ServerOption](#WithStreamServerChain)
* [func WithUnaryServerChain(interceptors ...grpc.UnaryServerInterceptor) grpc.ServerOption](#WithUnaryServerChain)
* [type WrappedServerStream](#WrappedServerStream)
  * [func WrapServerStream(stream grpc.ServerStream) \*WrappedServerStream](#WrapServerStream)
  * [func (w \*WrappedServerStream) Context() context.Context](#WrappedServerStream.Context)

#### <a name="pkg-files">Package files</a>
[chain.go](./chain.go) [doc.go](./doc.go) [wrappers.go](./wrappers.go) 

## <a name="ChainStreamClient">func</a> [ChainStreamClient](./chain.go#L136)
``` go
func ChainStreamClient(interceptors ...grpc.StreamClientInterceptor) grpc.StreamClientInterceptor
```
ChainStreamClient creates a single interceptor out of a chain of many interceptors.

Execution is done in left-to-right order, including passing of context.
For example ChainStreamClient(one, two, three) will execute one before two before three.

## <a name="ChainStreamServer">func</a> [ChainStreamServer](./chain.go#L58)
``` go
func ChainStreamServer(interceptors ...grpc.StreamServerInterceptor) grpc.StreamServerInterceptor
```
ChainStreamServer creates a single interceptor out of a chain of many interceptors.

Execution is done in left-to-right order, including passing of context.
For example ChainUnaryServer(one, two, three) will execute one before two before three.
If you want to pass context between interceptors, use WrapServerStream.

## <a name="ChainUnaryClient">func</a> [ChainUnaryClient](./chain.go#L97)
``` go
func ChainUnaryClient(interceptors ...grpc.UnaryClientInterceptor) grpc.UnaryClientInterceptor
```
ChainUnaryClient creates a single interceptor out of a chain of many interceptors.

Execution is done in left-to-right order, including passing of context.
For example ChainUnaryClient(one, two, three) will execute one before two before three.

## <a name="ChainUnaryServer">func</a> [ChainUnaryServer](./chain.go#L18)
``` go
func ChainUnaryServer(interceptors ...grpc.UnaryServerInterceptor) grpc.UnaryServerInterceptor
```
ChainUnaryServer creates a single interceptor out of a chain of many interceptors.

Execution is done in left-to-right order, including passing of context.
For example ChainUnaryServer(one, two, three) will execute one before two before three, and three
will see context changes of one and two.

## <a name="WithStreamServerChain">func</a> [WithStreamServerChain](./chain.go#L181)
``` go
func WithStreamServerChain(interceptors ...grpc.StreamServerInterceptor) grpc.ServerOption
```
WithStreamServerChain is a grpc.Server config option that accepts multiple stream interceptors.
Basically syntactic sugar.

## <a name="WithUnaryServerChain">func</a> [WithUnaryServerChain](./chain.go#L175)
``` go
func WithUnaryServerChain(interceptors ...grpc.UnaryServerInterceptor) grpc.ServerOption
```
Chain creates a single interceptor out of a chain of many interceptors.

WithUnaryServerChain is a grpc.Server config option that accepts multiple unary interceptors.
Basically syntactic sugar.

## <a name="WrappedServerStream">type</a> [WrappedServerStream](./wrappers.go#L12-L16)
``` go
type WrappedServerStream struct {
    grpc.ServerStream
    // WrappedContext is the wrapper's own Context. You can assign it.
    WrappedContext context.Context
}
```
WrappedServerStream is a thin wrapper around grpc.ServerStream that allows modifying context.

### <a name="WrapServerStream">func</a> [WrapServerStream](./wrappers.go#L24)
``` go
func WrapServerStream(stream grpc.ServerStream) *WrappedServerStream
```
WrapServerStream returns a ServerStream that has the ability to overwrite context.

### <a name="WrappedServerStream.Context">func</a> (\*WrappedServerStream) [Context](./wrappers.go#L19)
``` go
func (w *WrappedServerStream) Context() context.Context
```
Context returns the wrapper's WrappedContext, overwriting the nested grpc.ServerStream.Context()

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