backend

func (*APID) AppendInfo ¶

func (a *APID) AppendInfo(streaming bool, resp []byte) ([]byte, error)

AppendInfo is called to enhance response from the backend with additional data.

AppendInfo enhances upstream response with node metadata (target).

This method depends on grpc protobuf response structure, each response should look like:

  message SomeResponse {
    repeated SomeReply messages = 1; // please note field ID == 1
  }

  message SomeReply {
	   common.Metadata metadata = 1;
    <other fields go here ...>
  }

As 'SomeReply' is repeated in 'SomeResponse', if we concatenate protobuf representation of several 'SomeResponse' messages, we still get valid 'SomeResponse' representation but with more entries (feature of protobuf binary representation).

If we look at binary representation of any unary 'SomeResponse' message, it will always contain one protobuf field with field ID 1 (see above) and type 2 (embedded message SomeReply is encoded as string with length). So if we want to add fields to 'SomeReply', we can simply read field header, adjust length for new 'SomeReply' representation, and prepend new field header.

At the same time, we can add 'common.Metadata' structure to 'SomeReply' by simply appending or prepending 'common.Metadata' as a single field. This requires 'metadata' field to be not defined in original response. (This is due to the fact that protobuf message representation is concatenation of each field representation).

To build only single field (Metadata) we use helper message which contains exactly this field with same field ID as in every other 'SomeReply':

  message Empty {
    common.Metadata metadata = 1;
	}

As streaming replies are not wrapped into 'SomeResponse' with 'repeated', handling is simpler: we just need to append Empty with details.

So AppendInfo does the following: validates that response contains field ID 1 encoded as string, cuts field header, rest is representation of some reply. Marshal 'Empty' as protobuf, which builds 'common.Metadata' field, append it to original response message, build new header for new length of some response, and add back new field header.

func (*APID) BuildError ¶

func (a *APID) BuildError(streaming bool, err error) ([]byte, error)

BuildError is called to convert error from upstream into response field.

BuildError converts upstream error into message from upstream, so that multiple successful and failure responses might be returned.

This simply relies on the fact that any response contains 'Empty' message. So if 'Empty' is unmarshalled into any other reply message, all the fields are undefined but 'Metadata':

  message Empty {
   common.Metadata metadata = 1;
	}

 message EmptyResponse {
   repeated Empty messages = 1;
}

Streaming responses are not wrapped into Empty, so we simply marshall EmptyResponse message.

func (*APID) Close ¶

func (a *APID) Close()

Close connection.

func (*APID) GetConnection ¶

func (a *APID) GetConnection(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error)

GetConnection returns a grpc connection to the backend.

func (*APID) String ¶

func (a *APID) String() string

func (*APIDFactory) Flush ¶ added in v1.7.0

func (factory *APIDFactory) Flush()

Flush all cached backends.

This ensures that all connections are closed.

func (*APIDFactory) Get ¶

func (factory *APIDFactory) Get(target string) (proxy.Backend, error)

Get backend by target.

Get performs caching of backends.