Hessian 2.0 Web Services Protocol
hessian.txt

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Table of Contents

1.  Introduction
2.  Design Goals
3.  Hessian Grammar
4.  Messages and Envelopes
    4.1.  Call
        4.1.1.  Methods and Overloading
        4.1.2.  Arguments
        4.1.3.  Call examples
    4.2.  Envelope
        4.2.1.  Envelope examples
    4.3.  packet
    4.4.  Reply
        4.4.1.  Value
        4.4.2.  Faults
    4.5.  Versioning
5.  Service Location (URLs)
    5.1.  Object Naming (non-normative)
    5.2.  Object naming (non-normative) Example
6.  Bytecode map
§  Authors' Addresses
§  Intellectual Property and Copyright Statements

1.  Introduction

This document describes the portions of the Hessian 2.0 protocol concerning web services. The document is intended to be read by implementors of Hessian 2.0. Hessian 2.0 supports two types of network communication: remote procedure call (RPC) and message-based. These may also be viewed as synchronous and asynchronous communication.

RPC communication is based on "methods" invoked on a server. The method being invoked is specified in a Hessian 2.0 "call". Arguments to these methods are passed in the call and are serialized using the Hessian 2.0 serialization protocol. If the method was successfully invoked, the return value of the method is also serialized using Hessian 2.0 and sent to the client. If the method was not successfully invoked, a "fault" is returned to the client. RPC communication can use any underlying network protocol for transport such as HTTP or TCP.

Message-based communication is asynchronous and does not necessarily involve the use of methods, clients, or servers. Messages may or may not receive a response message. Messages simply contain other Hessian 2.0 objects. These may be simple types, aggregates like a list or map, or an "envelope". Envelopes may have headers that specify routing or other special processing information. They may also contain encrypted, signed, and/or compressed data. Thus using messages with envelopes can be useful in cases where end-to-end security is necessary. Message-based communication can also use any underlying network protocol such as HTTP or TCP and may be especially appropriate in queued message systems.

2.  Design Goals

Unlike older binary protocols, Hessian is both self-describing, compact, and portable across languages. The wire protocol for web services should be invisible to application writers, it should not require external schema or IDL.

The Hessian protocol has the following design goals:

• It must not require external IDL or schema definitions, i.e. the protocol should be invisible to application code.
• It must be language-independent.
• It must be simple so it can be effectively tested and implemented.
• It must be as fast as possible.
• It must be as compact as possible.
• It must support Unicode strings.
• It must support 8-bit binary data (i.e. without encoding or using attachments.)
• It must support encryption, compression, signature, and transaction context envelopes.

3.  Hessian Grammar

RPC/Messaging Grammar

top       ::= version content
          ::= call-1.0
          ::= reply-1.0

          # RPC-style call
call      ::= 'C' string int value*

call-1.0  ::= 'c' x01 x00 <hessian-1.0-call>

content   ::= call       # rpc call
          ::= fault      # rpc fault reply
          ::= reply      # rpc value reply
          ::= packet+    # streaming packet data
          ::= envelope+  # envelope wrapping content

envelope  ::= 'E' string env-chunk* 'Z'
env-chunk ::= int (string value)* packet int (string value)*

          # RPC fault
fault     ::= 'F' (value value)* 'Z'

          # message/streaming message
packet    ::= (x4f b1 b0 <data>)* packet
          ::= 'P' b1 b0 <data>
          ::= [x70 - x7f] <data>
          ::= [x80 - xff] <data>

          # RPC reply
reply     ::= 'R' value

reply-1.0 ::= 'r' x01 x00 <hessian-1.0-reply>

version   ::= 'H' x02 x00

4.  Messages and Envelopes

Hessian message syntax organizes serialized data for messaging and RPC applications. The envelope syntax enables compression, encryption, signatures, and any routing or context headers to wrap a Hessian message.

• Call ('C'): contains a Hessian RPC call, with a method name and arguments.
• Envelope ('E'): wraps a Hessian message for compression, encryption, etc. Envelopes can be nested.
• Hessian ('H'): introduces a Hessian stream and indicates its version.
• Packet ('P'): contains a sequence of serialized Hessian objects.
• Reply ('R'): contains a reply to a Hessian RPC call.
• Fault ('F'): contains a reply to a failed Hessian RPC call.
• Hessian 1.0 compatibility call ('c'): is a Hessian 1.0 call.
• Hessian 1.0 compatibility reply ('cr): is a Hessian 2.0 call.

4.1.  Call

Call Grammar

call ::= C string int value*

A Hessian call invokes a method on an object with an argument list. The object is specified by the container, e.g. for a HTTP request, it's the HTTP URL. The arguments are specified by Hessian serialization.

4.1.1.  Methods and Overloading

Method names must be unique. Two styles of overloading are supported: overloading by number of argumetns and overloading by argument types. Overloading is permitted by encoding the argument types in the method names. The types of the actual arguments must not be used to select the methods.

Method names beginning with _hessian_ are reserved.

Servers should accept calls with either the mangled method name or the unmangled method name. Clients should send the mangled method name.

4.1.1.1.  Overloading examples

add(int a, int b)  ->  add_int_int

add(double a, double b)  ->  add_double_double

add(shopping.Cart cart, shopping.Item item)  ->
  add_shopping.Cart_shopping.Item

4.1.2.  Arguments

Arguments immediately follow the method in positional order. Argument values use Hessian's serialization.

All arguments share references, i.e. the reference list starts with the first argument and continues for all other arguments. This lets two arguments share values.

4.1.2.1.  Arguments example

bean = new qa.Bean("foo", 13);

System.out.println(remote.eq(bean, bean));

---
H x02 x00
C
  x02 eq        # method name = "eq"
  x92           # two arguments
  M x07 qa.Bean # first argument
    x03 foo
    x9d
    Z
  Q x00         # second argument (ref to first)

The number and type of arguments are fixed by the remote method. Variable length arguments are forbidden. Implementations may take advantage of the expected type to improve performance.

4.1.3.  Call examples

obj.add2(2,3) call

H x02 x00         # Hessian 2.0
C                 # RPC call
  x04 add2        # method "add2"
  x92             # two arguments
  x92             # 2 - argument 1
  x93             # 3 - argument 2

obj.add2(2,3) reply

H x02 x00        # Hessian 2.0
R                # reply
  x95            # int 5

4.2.  Envelope

Envelope Grammar

envelope ::= E string env-chunk* Z

env-chunk ::= int (string value)* packet int (string value)*

A Hessian envelope wraps a Hessian message, adding headers and footers and possibly compressing or encrypting the wrapped message. The envelope type is identified by a method string, e.g. "com.caucho.hessian.io.Deflation" or "com.caucho.hessian.security.X509Encryption".

Some envelopes may chunk the data, providing multiple header/footer chunks. For example, a signature envelope might chunk a large streaming message to reduce the amount of buffering required to validate the signatures.

4.2.1.  Envelope examples

Identity Envelope

H x02 x00              # Hessian 2.0
E                      # envelope
  x06 Header           # "Header" adds headers, body is identity
  x90                  # no headers
  x87                  # final packet (7 bytes)
    R                  # RPC reply
    x05 hello          # "hello"
  x90                  # no footers
  Z                    # end of envelope

Chunked Identity Envelope

H x02 x00              # Hessian 2.0
E
  x06 Header           # "Header" envelope does nothing to the body
  x90                  # no headers
  x87                  # final packet (7 bytes)
    C                  # RPC call
    x05 hello          # hello()
    x91                # one arg
  x90                  # no footers

  x90                  # no headers
  x8d                  # final packet (13 bytes)
    x05 hello, world   # hello, world
  x90                  # no footers
  Z                    # end of envelope

Compression Envelope

H x02 x00
E
  x09 Deflation        # "Deflation" envelope compresses the body
  x90                  # no headers
  P x10 x00            # single chunk (4096)
    x78 x9c x4b...     # compressed message
  x90                  # no footers
  Z                    # end of envelope

4.3.  packet

packet Grammar

packet   ::= (x4f b1 b0 <data>) packet
         ::= 'P' b1 b0 <data>
         ::= [x70-x7f] b0 <data>
         ::= [x80-xff] <data>

A Hessian message contains a sequence of Hessian serialized objects. Messages can be used for multihop data transfer or simply for storing serialized data.

4.4.  Reply

Reply Grammar

reply       ::= R value
fault       ::= F map

4.4.1.  Value

A successful reply returns a single value and possibly some header information.

Integer 5 Envelope

H x02 x00
R
  x95

4.4.2.  Faults

Failed calls return a fault.

Each fault has a number of informative fields, expressed like <map> entries. The defined fields are code, message, and detail. code is one of a short list of strings defined below. message is a user-readable message. detail is an object representing the exception.

Remote Call throws FileNotFoundException

F
  H
  x04 code
  x10 ServiceException

  x07 message
  x0e File Not Found

  x06 detail
  M x1d java.io.FileNotFoundException
    Z
  Z

ProtocolException:

The Hessian request has some sort of syntactic error.

NoSuchObjectException:

The requested object does not exist.

NoSuchMethodException:

The requested method does not exist.

RequireHeaderException:

A required header was not understood by the server.

ServiceException:

The called method threw an exception.

4.5.  Versioning

The call and response tags include a major and minor byte. The current version is 2.0.

5.  Service Location (URLs)

Hessian services are identified by URLs. Typically, these will be HTTP URLs, although protocols would be possible as well.

5.1.  Object Naming (non-normative)

URLs are flexible enough to encode object instances as well as simple static service locations. The URL uniquely identifies the Hessian object. Thus, Hessian can support object-oriented services, e.g. naming services, entity beans, or session beans, specified by the URL without requiring extra method parameters or headers.

Object naming may use the query string convention that "?id=XXX" names the object "XXX" in the given service. This convention is recommented, but not required.

For example, a stock quote service might have a factory interface like http://foo.com/stock and object instances like http://foo.com?id=PEET. The factory interface would return valid object references through the factory methods.

5.2.  Object naming (non-normative) Example

As an example, the following format is used for EJB:

http://hostname/hessian

http://hostname/hessian

http://hostname/hessian

http://hostname/hessian identifies the EJB container. In Resin-EJB, this will refer to the EJB Servlet. "/hessian" is the servlet prefix (url-pattern.) HTTP is just used as an example; Hessian does not require the use of HTTP.

/ejb-name, the path info of the request, identifies the EJB name, specifically the home interface. EJB containers can contain several entity and session beans, each with its own EJB home. The ejb-name corresponds to the ejb-name in the deployment descriptor.

object-id identifies the specific object. For entity beans, the object-id encodes the primary key. For session beans, the object-id encodes a unique session identifier. Home interfaces have no ";ejbid=..." portion.

# Example Entity Home Identifier
http://localhost/hessian/my-entity-bean

# Example Entity Bean Identifier
http://localhost/hessian/my-entity-bean?ejbid=slytherin

# Example Session Home Identifier
http://localhost/hessian/my-session-bean

# Example Session Bean Identifier
http://localhost/hessian/my-session-bean?ejbid=M9Zs1Zm

6.  Bytecode map

Hessian is organized as a bytecode protocol. A Hessian reader is essentially a switch statement on the initial octet.

Bytecode Encoding

x00 - x42    # reserved
x43          # rpc call ('C')
x44          # reserved
x45          # envelope ('E')
x46          # fault ('F')
x47          # reserved
x48          # hessian version ('H')
x49 - x4f    # reserved
x4f          # packet chunk ('O')
x50          # packet end ('P')
x51          # reserved
x52          # rpc result ('R')
x53 - x59    # reserved
x5a          # terminator ('Z')
x5b - x5f    # reserved
x70 - x7f    # final packet (0 - 4096)
x80 - xff    # final packet for envelope (0 - 127)

Authors' Addresses

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