proxy

type ConditionalReverseProxy struct {
	// Director must be a function which
	// modifies the request into a new request to be
	// sent using Transport. It must return true if
	// the request should be sent, and false if the
	// request should be aborted and a new response
	// should be generated.
	// Director must not access the provided
	// Request after returning.
	Director func(*http.Request) bool

	// The transport used to perform proxy requests.
	// If nil, http.DefaultTransport is used.
	Transport http.RoundTripper

	// FlushInterval specifies the flush interval
	// to flush to the client while copying the
	// response body.
	// If zero, no periodic flushing is done.
	// A negative value means to flush immediately
	// after each write to the client.
	// The FlushInterval is ignored when ReverseProxy
	// recognizes a response as a streaming response;
	// for such responses, writes are flushed to the client
	// immediately.
	FlushInterval time.Duration

	// ErrorLog specifies an optional logger for errors
	// that occur when attempting to proxy the request.
	// If nil, logging goes to os.Stderr via the log package's
	// standard logger.
	ErrorLog *log.Logger

	// BufferPool optionally specifies a buffer pool to
	// get byte slices for use by io.CopyBuffer when
	// copying HTTP response bodies.
	BufferPool httputil.BufferPool

	// CreateResponse is an optional function that creates a
	// Response without sending a request to the backend.
	// It is called if Director returns false.
	//
	// CreateResponse should not modify the request, except for
	// consuming and closing the Request's Body. CreateResponse may
	// read fields of the request in a separate goroutine. Callers
	// should not mutate or reuse the request until the Response's
	// Body has been closed.
	//
	// CreateResponse must always close the body, including on errors,
	// but depending on the implementation may do so in a separate
	// goroutine even after CreateResponse returns. This means that
	// callers wanting to reuse the body for subsequent requests
	// must arrange to wait for the Close call before doing so.
	//
	// The Request's URL and Header fields must be initialized.
	//
	// If CreateResponse is not set, the default implementation
	// will return a 403 response.
	//
	// If CreateResponse returns an error, ErrorHandler is called
	// with its error value. If ErrorHandler is nil, its default
	// implementation is used.
	CreateResponse func(http.ResponseWriter, *http.Request) error

	// ModifyResponse is an optional function that modifies the
	// Response from the backend. It is called if the backend
	// returns a response at all, with any HTTP status code.
	// If the backend is unreachable, the optional ErrorHandler is
	// called without any call to ModifyResponse.
	//
	// If ModifyResponse returns an error, ErrorHandler is called
	// with its error value. If ErrorHandler is nil, its default
	// implementation is used.
	ModifyResponse func(*http.Response) error

	// ErrorHandler is an optional function that handles errors
	// reaching the backend or errors from ModifyResponse.
	//
	// If nil, the default is to log the provided error and return
	// a 502 Status Bad Gateway response.
	ErrorHandler func(http.ResponseWriter, *http.Request, error)
}