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+Thrift Remote Procedure Call
+============================
+
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+
+This document describes the high level message exchange between the Thrift RPC client and server.
+See [thrift-binary-protocol.md] and [thrift-compact-protocol.md] for a description of how the exchanges are encoded on
+the wire.
+
+In addition, this document compares the binary protocol with the compact protocol. Finally it describes the framed vs.
+unframed transport.
+
+The information here is _mostly_ based on the Java implementation in the Apache thrift library (version 0.9.1 and
+0.9.3). Other implementation however, should behave the same.
+
+For background on Thrift see the [Thrift whitepaper (pdf)](https://thrift.apache.org/static/files/thrift-20070401.pdf).
+
+# Contents
+
+* Thrift Message exchange for Remote Procedure Call
+  * Message
+  * Request struct
+  * Response struct
+* Protocol considerations
+  * Comparing binary and compact protocol
+  * Compatibility
+  * Framed vs unframed transport
+
+# Thrift Remote Procedure Call Message exchange
+
+Both the binary protocol and the compact protocol assume a transport layer that exposes a bi-directional byte stream,
+for example a TCP socket. Both use the following exchange:
+
+1. Client sends a `Message` (type `Call` or `Oneway`). The TMessage contains some metadata and the name of the method
+   to invoke.
+2. Client sends method arguments (a struct defined by the generate code).
+3. Server sends a `Message` (type `Reply` or `Exception`) to start the response.
+4. Server sends a struct containing the method result or exception.
+
+The pattern is a simple half duplex protocol where the parties alternate in sending a `Message` followed by a struct.
+What these are is described below.
+
+Although the standard Apache Thrift Java clients do not support pipelining (sending multiple requests without waiting
+for an response), the standard Apache Thrift Java servers do support it.
+
+## Message
+
+A *Message* contains:
+
+* _Name_, a string (can be empty).
+* _Message type_, a message types, one of `Call`, `Reply`, `Exception` and `Oneway`.
+* _Sequence id_, a signed int32 integer.
+
+The *sequence id* is a simple message id assigned by the client. The server will use the same sequence id in the
+message of the response. The client uses this number to detect out of order responses. Each client has an int32 field
+which is increased for each message. The sequence id simply wraps around when it overflows.
+
+The *name* indicates the service method name to invoke. The server copies the name in the response message.
+
+When the *multiplexed protocol* is used, the name contains the service name, a colon `:` and the method name. The
+multiplexed protocol is not compatible with other protocols.
+
+The *message type* indicates what kind of message is sent. Clients send requests with TMessages of type `Call` or
+`Oneway` (step 1 in the protocol exchange). Servers send responses with messages of type `Exception` or `Reply` (step
+3).
+
+Type `Reply` is used when the service method completes normally. That is, it returns a value or it throws one of the
+exceptions defined in the Thrift IDL file.
+
+Type `Exception` is used for other exceptions. That is: when the service method throws an exception that is not declared
+in the Thrift IDL file, or some other part of the Thrift stack throws an exception. For example when the server could
+not encode or decode a message or struct.
+
+In the Java implementation (0.9.3) there is different behavior for the synchronous and asynchronous server. In the async
+server all exceptions are send as a `TApplicationException` (see 'Response struct' below). In the synchronous Java
+implementation only (undeclared) exceptions that extend `TException` are send as a `TApplicationException`. Unchecked
+exceptions lead to an immediate close of the connection.
+
+Type `Oneway` is only used starting from Apache Thrift 0.9.3. Earlier versions do _not_ send TMessages of type `Oneway`,
+even for service methods defined with the `oneway` modifier.
+
+When client sends a request with type `Oneway`, the server must _not_ send a response (steps 3 and 4 are skipped). Note
+that the Thrift IDL enforces a return type of `void` and does not allow exceptions for oneway services.
+
+## Request struct
+
+The struct that follows the message of type `Call` or `Oneway` contains the arguments of the service method. The
+argument ids correspond to the field ids. The name of the struct is the name of the method with `_args` appended.
+For methods without arguments an struct is sent without fields.
+
+## Response struct
+
+The struct that follows the message of type `Reply` are structs in which exactly 1 of the following fields is encoded:
+
+* A field with name `success` and id `0`, used in case the method completed normally.
+* An exception field, name and id are as defined in the `throws` clause in the Thrift IDL's service method definition.
+
+When the message is of type `Exception` the struct is encoded as if it was declared by the following IDL:
+
+```
+exception TApplicationException {
+  1: string message,
+  2: i32 type
+}
+```
+
+The following exception types are defined in the java implementation (0.9.3):
+
+* _unknown_: 0, used in case the type from the peer is unknown.
+* _unknown method_: 1, used in case the method requested by the client is unknown by the server.
+* _invalid message type_: 2, no usage was found.
+* _wrong method name_: 3, no usage was found.
+* _bad sequence id_: 4, used internally by the client to indicate a wrong sequence id in the response.
+* _missing result_: 5, used internally by the client to indicate a response without any field (result nor exception).
+* _internal error_: 6, used when the server throws an exception that is not declared in the Thrift IDL file. 
+* _protocol error_: 7, used when something goes wrong during decoding. For example when a list is too long or a required
+ field is missing. 
+* _invalid transform_: 8, no usage was found.
+* _invalid protocol_: 9, no usage was found.
+* _unsupported client type_: 10, no usage was found.
+
+# Protocol considerations
+
+## Comparing binary and compact protocol
+
+The binary protocol is fairly simple and therefore easy to process. The compact protocol needs less bytes to send the
+same data at the cost of additional processing. As bandwidth is usually the bottleneck, the compact protocol is almost
+always slightly faster.
+
+## Compatibility
+
+A server could automatically determine whether a client talks the binary protocol or the compact protocol by
+investigating the first byte. If the value is `1000 0001` or `0000 0000` (assuming a name shorter then ±16 MB) it is the
+binary protocol. When the value is `1000 0010` it is talking the compact protocol.
+
+## Framed vs. unframed transport
+
+The first thrift binary wire format was unframed. This means that information is sent out in a single stream of bytes.
+With unframed transport the (generated) processors will read directly from the socket (though Apache Thrift does try to
+grab all available bytes from the socket in a buffer when it can).
+
+Later, Thrift introduced the framed transport.
+
+With framed transport the full request and response (the TMessage and the following struct) are first written to a
+buffer. Then when the struct is complete (transport method `flush` is hijacked for this), the length of the buffer is
+written to the socket first, followed by the buffered bytes. The combination is called a _frame_. On the receiver side
+the complete frame is first read in a buffer before the message is passed to a processor.
+
+The length prefix is a 4 byte signed int, send in network (big endian) order.
+The following must be true: `0` <= length <= `16384000` (16M).
+
+Framed transport was introduced to ease the implementation of async processors. An async processor is only invoked when
+all data is received. Unfortunately, framed transport is not ideal for large messages as the entire frame stays in
+memory until the message has been processed. In addition, the java implementation merges the incoming data to a single,
+growing byte array. Every time the byte array is full it needs to be copied to a new larger byte array.
+
+Framed and unframed transports are not compatible with each other.