A key-value pair included in the HTTP header. It is a 2-tuple where the first entry is the key and the second is the value, i.e. (key, value). The metadata key is an ASCII str, and must be a valid HTTP header name. The metadata value can be either a valid HTTP ASCII str, or bytes. If bytes are provided, the key must end with ‘-bin’, i.e. ('binary-metadata-bin', b'\\x00\\xFF')


A sequence of metadatum.


A callable function that encodes an object into bytes. Applications are allowed to provide any customized serializer, so there isn’t a restriction for the input object (i.e. even None). On the server-side, the serializer is invoked with server handler’s return value; on the client-side, the serializer is invoked with outbound message objects.


A callable function that decodes bytes into an object. Same as serializer, the returned object doesn’t have restrictions (i.e. None allowed). The deserializer is invoked with inbound message bytes on both the server side and the client-side.


If an RPC is issued but the channel is in the TRANSIENT_FAILURE or SHUTDOWN states, the library cannot transmit the RPC at the moment. By default, the gRPC library will fail such RPCs immediately. This is known as “fail fast.” RPCs will not fail as a result of the channel being in other states (CONNECTING, READY, or IDLE).

When the wait_for_ready option is specified, the library will queue RPCs until the channel is READY. Any submitted RPCs may still fail before the READY state is reached for other reasons, e.g., the client channel has been shut down or the RPC’s deadline has been reached.


A list of key-value pairs to configure the underlying gRPC Core channel or server object. Channel arguments are meant for advanced usages and contain experimental API (some may not labeled as experimental). Full list of available channel arguments and documentation can be found under the “grpc_arg_keys” section of “grpc_types.h” header file ( For example, if you want to disable TCP port reuse, you may construct channel arguments like: options = (('grpc.so_reuseport', 0),).