Millau: an encoding format for efficient representation and exchange of XMLover the Web

Marc Girardot
Institut Eurécom
Sophia Antipolis, France
girardot@eurecom.fr

Neel Sundaresan
IBM Almaden Research Center
San Jose, California, USA
neel@almaden.ibm.com

Abstract

XML is poised to take the World Wide Web to the next level of innovation. XML data, large or small, with or without associated schema, will be exchanged between increasing number of applications running on diverse devices. Efficient storage and transportation of such data is an important issue. We have designed a system called Millau for efficient encoding and streaming of XML structures. In this paper we describe the Millau algorithms for compression of XML structures and data. Millau compression algorithms, in addition to separating structure and text for compression, take advantage of the associated schema (if available) in compressing the structure. Millau also defines a programming model corresponding to XML DOM and SAX for XML APIs for Millau streams of XML documents. Our experiments have shown significant performance gains of our algorithms and APIs. We describe some of these results in this paper. We also describe some applications of XML-based remote procedure calls and client-server applications based on Millau that take advantage of the compression and streaming technology defined by the system.

Keywords: Binary XML, compression, tokenization, streaming, proxy server, RPC


1. Introduction

As the World Wide Web transitions from just being a medium for browsing to a medium for commerce, XML (eXtensible Markup Language) [1] has emerged as the standard language for markup. Business to business applications over the Internet are increasingly adopting XML as the de facto standard for expressing messages, schema, and data. Consequently, XML is being increasingly used for Web based applications as an exchange wire format. On the other hand, with the popularity of the World Wide Web and increasing dependency on it to find information and to conduct business, the network bandwidth is being tested to its limit. One approach to address this bandwidth problem is to compress data on the network. Traditional data compression algorithms (e.g. Huffman coding [19] or LZ77 [18]) can achieve good compression rates on large text files but are less effective towards small sized files like the ones that may be typical in many eBusiness applications. Moreover, they cannot always treat data as a continuous stream. To be really efficient, they need to work on the entire file of a Web object. This is incompatible with the real time constraints of the Web. Finally, these compression systems do not retain this structural information in the data they exchange. Thus there is a need for compression and streaming system that works in the internet context with structured data. These requirements motivate our design for Millau.

The Wireless Application Protocol (WAP) [10] defines a format to reduce the transmission size of XML documents with no loss of functionality or semantic information. For example, it preserves the element structure of XML, allowing a browser to skip unknown elements or attributes. Millau extends this format to adapt it to business to business applications while improving on the compression algorithm itself. It separates structure compression from text compression. Further, it takes advantage of the schema and data types to enable better compression. To be compliant with the XML standards, it defines a parsing model based on both DOM and SAX at the same time taking advantage of the compression of the document. 

This paper is organized as follows. In section 2, we present the work related to compression and more precisely to XML compression. In section 3 we describe the Millau compression algorithm. We discuss the DOM and SAX support in Millau in section 4. In section 5 we study performance of the Millau system and discuss experimental results. In section 6 we briefly discuss some applications of our system. We describe a compression/decompression proxy server system for efficient data exchange and an XML RPC engine which takes advantage of the Millau compression model. In section 7 we draw conclusions on our work so far and describe work in progress and future research.


2. Related Work

A lot of work has already been done on lossless data compression [17]. Researchers have developed fast and powerful algorithms for data compression. Their principles are mostly based on Claude Shannon's Information Theory. A consequence of this theory is that a symbol that has a high probability has a low information content and will need fewer bits to encode. In order to compress data well, you need to select models that predict symbols with high probabilities. Huffman coding [19] achieves the minimum amount of redundancy possible in a fixed set of variable-length codes. It provides the best approximation for coding symbols when using fixed-width codes. Huffman coding uses a statistical modeling because it reads and encodes a single symbol at a time using the probability of that character's appearance. A dictionary-based compression scheme uses a different concept. It reads in input data and looks for groups of symbols that appear in a dictionary. If a string match is found, a pointer or index into the dictionary can be output instead of the code for the symbol. The longer the match, the better the compression  ratio. In LZ77 compression [18], for example, the dictionary consists of all the strings in a window into the previously read input stream. The deflate algorithm [6] uses a combination of the LZ77 compression and the Huffman coding. It is used in popular compression programs like GZIP [7] or ZLIB [5].

One drawback of these text compression algorithms is that they perform compression at the character level. If the algorithm is adaptive (as, for example, with LZ77), the algorithm slowly learns correlations between adjacent pairs of characters, then triples, quadruples and so on. The algorithm rarely has a chance to take advantage of longer range correlations before either the end of input is reached or the tables maintained by the algorithms are filled to capacity, specially with small files. To address this problem, R. Nigel Horspool and Gordon V. Cormack explore the use of words as basic units of the algorithm [21]. In most implementations of dictionary-based compression, the encoder operates online, incrementally inferring its dictionary of available phrases from previous parts of the message. An alternative approach proposed by N. Jasper Larsson and Alistair Moffat [22] is to infer a complete dictionary offline to optimize the choice of phrases so as to maximize compression performance.

The Wireless Application Protocol Forum [10] has proposed an encoding format for XML based on a table (the code space) that matches tokens to XML tags and attribute names [4]. It takes advantage both of the offline approach (the code space can be built offline) and of the word-based compression (tags and attribute names are usually the most frequent words in an XML document). Moreover, unlike the previous compression algorithms, it retains the structure of XML documents. But it does not compress at all the character data content nor the attribute values which are not defined in the DTD. Moreover, it does not suggest any strategy to build the code space in an efficient way. The Millau encoding format addresses both of these drawbacks: it is designed to compress character data and defines a strategy to build code space.


3. The Millau Compression Model

The Millau encoding format is an extension of the WAP Binary XML format. The WBXML (Wireless Application Protocol Binary XML) Content Format Specification [4] defines a compact binary representation of XML. This format is designed to reduce the transmission size of XML documents with no loss of functionality or semantic information. For example, WBXML preserves the element structure of XML, allowing a browser to skip unknown elements or attributes. More specifically, the WBXML content encodes the tag names and the attributes names and values with tokens (a token is a single byte).

In WBXML format, tokens are split into a set of overlapping "code spaces". The meaning of a particular token is dependent on the context in which it is used. There are two classifications of tokens: global tokens and application tokens. Global tokens are assigned a fixed set of codes in all contexts and are unambiguous in all situations. Global codes are used to encode inline data (e.g., strings, entities, opaque data, etc.) and to encode a variety of miscellaneous control functions. Application tokens have a context-dependent meaning and are split into two overlapping "code spaces", the "tag code space" and the "attribute code space":

In Millau format, an Attribute Start token is followed by a single Attribute Value token, string, entity or extension token. So there is no need to split the attribute token numeric range into two ranges ( less than 128 and 128 or greater) because each time the parser encounters and Attribute Start token followed by a non-reserved token, it knows that this non-reserved token is an Attribute Value token and that it can be followed only by and END token or another Attribute Start token. Thus instead two overlapping code spaces, we have three overlapping code spaces:

Notice that, in WBXML format, character data is not compressed. It is transmitted as strings inline, or as a reference in a string table which is transmitted at the beginning of the document. In Millau encoding format, character data can be transmitted on a separate stream. This allows to separate the content from the structure so that a browser can separately download the structure and the content or just a part of each. This further allows to compress the character data using traditional compression algorithms like deflate [6]. In the structure stream, character data is indicated by a special global token (STR or STR_ZIP) which indicates to the Millau parser (see section 4) that it must switch from the structure stream to the content stream if the user is interested in content and whether the content is compressed (STR) or uncompressed (STR_ZIP). Optionally, the length of the content is encoded as an integer in the structure stream right after the global token (STR_L or STR_ZIP_L). If the length is not indicated, the strings contained in the structure must terminate with a End Of String character or a null character.

We described how Millau encoding format efficiently represent character data but we must also take in consideration the fact that, in typical business to business communications, most of the attribute values are of primitive type like boolean, byte, integer or float. For example, in a set of typical business to business XML messages provided by the Open Application Group [14], 70% of the attribute values are of primitive type. These attribute values should not be transcoded in strings in a binary representation of an XML document. So in Millau, we use the extension codes to prefix primitives types like bytes, integers or floats. The following table reminds the meanings given to the global tokens by the WBXML Encoding Specification and also precises the meanings of the extension tokens which have been redefined for the needs of Millau (these tokens appear in bold in table 1).


Token Name Token Description
SWITCH_PAGE 0 Change the code page for the current token state. Followed by a single u_int8 indicating the new code page number.
END 1 Indicates the end of an attribute list or the end of an element.
ENTITY 2 A character entity. Followed by an integer encoding the character entity number.
STR_I 3 Inline string. Followed by a string.
LITERAL 4 An unknown tag or attribute name. Followed by an integer that encodes an offset into the string table.
FALSE 40 Encodes the boolean value false.
TRUE 41 Encodes the boolean value true.
FLOAT 42 Inline float. Token is followed by an integer representing the floating-point argument according to the IEEE 754 floating-point "single precision" bit layout.
PI 43 Processing instruction.
LITERAL_C 44 Unknown tag, with content.
STR_L 80 Indicates that uncompressed character data has been written to the content stream. Followed by an integer indicating the number of characters.
STR_ZIP_L 81 Indicates that compressed character data has been written to the content stream. Followed by an integer indicating the number of characters.
EXT_T_2 82 Inline integer. Token is followed by an integer.
STR_T 83 String table reference. Followed by an integer encoding a byte offset from the beginning of the string table.
LITERAL_A 84 Unknown tag, with attributes.
STR C0 Indicates that uncompressed character data has been written to the content stream.
STR_ZIP C1 Indicates that compressed character data has been written to the content stream.
BYTE C2 Inline byte. Followed by a single byte.
BINARY C3 Binary data. Followed by an integer indicating the number of bytes of binary data.
LITERAL_AC C4 Unknown tag, with content and attributes.
Table 1: Global WBXML and Millau tokens

The following is an example of a simple tokenized XML document. Here is the source document:

<?xml version="1.0" encoding=”UTF-8”?>
<!DOCTYPE Book [
    <!ELEMENT Book (Title, Chapter+, Picture+)>
    <!ATTLIST Book 
        Author CDATA #REQUIRED
        Genre (literature|science|history|cartoons) #REQUIRED >
    <!ELEMENT Title (#PCDATA)>
    <!ELEMENT Chapter (#PCDATA)>
    <!ATTLIST Chapter
         id ID #REQUIRED>
    <!ELEMENT Picture (#PCDATA)>
    <!ATTLIST Picture 
        Caption CDATA #REQUIRED>
]>
<Book Author="Anonymous" Genre=”literature”>
    <Title>Sample Book</Title>
    <Chapter Number="1">
        This is chapter 1. It is not very long or interesting.
    </Chapter>
    <Chapter Number="2">
        This is chapter 2. Although it is longer than chapter 1, it is not any more interesting.
    </Chapter>
    <Picture Caption="Nice picture">
        [base 64 encoded binary data]
    </Picture>
</Book>


Tokens for the tag code space, the attribute names code space, and the attribute value code space are defined in table 2.


Tag code space Attribute name code space Attribute value code space
Tag Name Token Attribute Name Token Attribute Value Token
Book 5 Author 5 literature 5
Title 6 Genre 6 science 6
Chapter 7 Number 7 history 7
Picture 8 Caption 8 cartoons 8
Table 2: Code space example

Tokenized form (numbers in hexadecimal) follows:

01 01 6A 00 C5 05 03 "Anonymous" 06 05 01 46 C1 C7 07 C2 01 01 C1 01 C7 07 C2 02 01 
C1 01 C8 08 03 "Nice picture" 01 C3 ... 01 01 


4. Millau API - Specification and Implementation

The Millau format is designed to represent XML documents in a compact way using tokens to represent tags and attributes instead of strings. We built parsers for documents encoded using this format implementing the two standard APIs DOM [3] and SAX [2]. DOM is the tree model API used to represent and process parsed XML document trees. The SAX API has an event-based streaming model typically used to process large XML documents without actually building a parse tree.

We provide two variants of SAX parsers. The first one produces traditional SAX events, as defined by the SAX API. This means that each time it encounters a tag token, it generates a startElement event passing the name of the tag. The name matching the tag token is found in the code spaces. We will describe later how the code spaces are built in our implementation. The second SAX parser, which we call the Millau BSAX (Binary SAX) parser, extends the SAX API by providing events which pass tokens instead of strings. This parser has been designed for applications that are able to handle tokens instead of strings. We show later how applications using tokens perform better than those using strings only.

We also provide two DOM-based parsers. The first one creates a conventional DOM tree from a Millau stream. The second one creates what we call a BDOM tree (Binary DOM tree). A BDOM tree is like a DOM tree but instead of storing node names it stores, for each node, a pair (page number, token) which uniquely identifies the node. Here follows a description of how each parser works.

4.1 The Millau SAX parser

A conventional SAX parser parses an XML stream and throws SAX events (e.g. characters, startElement, endElement)  that can be handled by a specific handler. Parameters can be passed through these events (e.g. the element name is passed through the startElement and endElement events). These events and their associated parameters are defined by the SAX API [2]. The Millau SAX parser has been designed to parse a Millau stream. It implements the SAX API. In the following paragraphs, we describe how it works.

Before reading tokens from the binary input stream, the Millau SAX parser creates a LIFO (last in - first out) stack in which it puts the names of the element that are opened and not yet closed. This is so that it can get the name of an element when it ends and send it to the handler. Then it reads tokens from the input stream until the stack is empty. When the stack is empty, it means that the root element has been closed. Table 3 specifies the action taken for each token type.


Token Action Taken
switch page read the next token which gives the current code page.
string inline read the inline string that follows and throw a character event.
extension read the following content according to its type, translates it into a string and throw a character event.
end token remove the last element of the tag names stack and throw an endElement event with the tag name which has been removed from the stack.
not a reserved token if the token is not a reserved token, then it is a tag token, so the parser looks for the corresponding tag name in the element code space (if not found, an exception is raised). It then calls a method which returns an attributes list. Eventually, it throws a startElement event with the tag name and its corresponding attribute list (if the element has attributes)
Table 3: Millau element tokens decision table

The getAttribute method tests the most significant bit of the tag token to know if this element has attribute. If the bit is 0, the element has no attribute and the method returns an empty list. If the bit is 1, the element has attributes and the method reads the attribute tokens from the input stream.

While the most significant bit of the next read tokens is 0, the parser knows that these tokens are not attribute value token. The tokens are processed, based upon their types, as described in table 4.


Token Action Taken
switch page read the next token which gives the current code page
not a reserved token if it is not a reserved token, then it is an attribute name token. So the parser looks for the corresponding name in the attribute name code space (if not found, an exception is raised). It then reads the attribute value.
end token end of the attribute list identified; return the attributes list.
Table 4: Millau attribute tokens decision table

The attribute value can be encoded as a token value, as an inline string (compressed or not) or as a primitive type like byte, integer, float, or boolean.

4.2 The Millau Binary SAX Parser

It is expected that parsing a compressed Millau stream using our SAX parser is faster than decompressing a compressed XML stream and then parsing it with a conventional SAX parser. But it could take more time than parsing a non compressed XML stream with a conventional SAX parser. We observed that the part of the processing which takes the most time with Millau SAX parser is the translation of the tokens in elements and attributes names. The reason for this it that, for each received token, the parser must search the code spaces for the corresponding strings. For example, if it receives an element token, it must search the corresponding element name in the element code space and this can take a lot of time, especially if there are many elements in the element code space. Skipping this translation step could make the encoded XML parsing faster. These tokens do not really need to be translated into strings at all. In fact, they can be directly processed by appropriate handlers which recognize the tokens. The design of such handlers and the efficiency aspects will be discussed later.

A Millau Binary SAX parser is like a SAX parser but instead of studying character based XML streams it operates on the binary encoded XML. Instead of passing tag names and attribute names and values to the handler, it passes encoding tokens without translating them into strings. More precisely, each time it throws a startElement event or an endElement event, it passes a pair (code page, element token) which uniquely identifies the element (See WBXML encoding specification [4] or section 2). For a startElement event, it also passes a Millau binary attribute list which is a variant of the XML SAX attribute list implementation. A Millau attribute list, instead of containing triples (attribute name, attribute type, attribute value) contains triples (attribute name uid, attribute type, attribute value uid) if the type of the attribute is "enumerated" or triples (attribute name uid, attribute type, attribute value) if the type of the attribute is "CDATA". A "uid" (unique identifier) is a pair (code page, token). It can uniquely identify an attribute name or an attribute value. Table 5 illustrates the differences between the two parsers:


Interface Millau conventional SAX parser Millau Binary SAX parser
Handler startElement(String name, AttributeList) startElement(int token, BAttributeList)
AttributeList getName(int i) returns the name getNameToken(int i) returns a token
AttributeList getValue(int i) returns the value getValueToken(int i) returns a token
getValue(int i) returns an Object
Table 5: Comparison between Millau conventional and binary SAX parsers

A Millau SAX handler must be able to recognize (code page, token) to trigger special processing adapted to the element or the attribute. It is faster than a conventional handler because, instead of comparing two strings (a time consuming operation), it just has to compare two pairs of bytes.

4.3 The Millau DOM parser

The DOM parser is able to build a DOM tree dynamically from a binary XML stream. The top-level architecture of the DOM parser is almost the same as the architecture of the SAX parser. Like the SAX parser, the DOM parser creates a LIFO stack to store the names of the opened elements. Then it reads tokens from the input stream until the stack is empty. It differs from the SAX parser in the processing which is done for each type of token. Table 6 gives the details:


Token Action Taken
switch page read the next token which gives the current code page.
string inline reads the inline string that follows and creates a text node and appends this text node to the last opened element (the first element of the LIFO stack).
extension read the content following the content according to its type, translates it into a string, creates a text node and appends it to the last opened element.
end token just remove the last element of the tag names stack.
not a reserved token if this is not a reserved token, then it is a tag token, so looks for the corresponding tag name in the element code space (if not found, an exception is raised). It then creates an element node. If the stack is empty, it means that this element is the root of the document, so it is appended to the document node. If the stack is not empty, the element is appended to the last opened element (the first in the LIFO stack). Eventually, the parser tests the last bit of the token, if it is 1, it invokes a method which gets the attributes for this element.
Table 6: Millau DOM parser decision table for element tokens

The getAttributes method reads the attribute tokens from the binary XML stream. While the most significant bit of the next read tokens is 0, the parser knows that these tokens are not attribute value tokens. Table 7 describes the action taken on different kinds of tokens:


Token Action Taken
switch page it reads the next token which gives the current code page.
end token this is the end of the attribute list. The method exits.
not a reserved token if it is not a reserved token, then it is an attribute name token. So the parser looks for the corresponding name in the attribute name code space (if not found, an exception is raised). It then reads the attribute value. Eventually, it adds this attribute to the current element.
Table 7: Millau DOM parser decision table for attribute tokens

The attribute value can be encoded as a token value or as an inline string (compressed or not).

4.4 The Millau Binary DOM parser

The Millau Binary DOM parser uses the Binary DOM (BDOM) API. Table 8 describes the action taken by the parser on different types of tokens:


Token Action Taken
switch page no change
string inline no change
extension the BDOM parser can create a primitive type node (boolean, byte, integer, float, binary data) defined by the BDOM API by invoking the methods createBooleanNode, createByteNode, createIntegerNode, createFloatNode or createBinaryData of the class BDocument. This node is then appended to the last opened element by invocation of the method appendChild.
end token no change
tag token the BDOM parser creates a BElement node by invoking the method createElement of the class BDocument with a short as parameter. The first byte (most significant) of this short is the code page of the tag and the second byte is the tag token.
Table 8: Millau Binary DOM parser decision table

4.5 The Millau Binary DOM API

The Binary DOM API implements all the interfaces of the DOM API as defined by the W3C [3]. The main advantage of a Binary DOM tree is that the tag and attribute names are stored not as strings but as tokens and is space-efficient. The correspondence between names and pairs (code page, token) is stored in the code spaces so that names can be normally retrieved for every element or attribute nodes. Attribute values can be stored as tokens, if available, as strings, or as primitive types. The primitive types currently supported by the BDOM API are boolean, byte, integer (4 bytes), and float. Element contents can also be stored as primitive types. For element contents, we have defined one more Binary Node, the Binary Data Node, which stores binary data without base 64 encoding, thus avoiding the 33% overload of the base 64 encoding. This is useful for binary files like images embedded in an XML document.

In addition to the methods of the DOM API, the BDOM API also provides methods for creating or retreiving elements or attributes by tokens instead of strings. This is useful for applications which have been designed to work with Millau format (see section 6). For example the class BElement (for Binary Element) which implements the DOM interface Element has also a method getTagToken() which returns a short where the first byte is the code page and the second page is the tag token. For convenience, the class BDocument which implements the DOM interface Document provides a method writeBinaryXML(OutputStream) which write the BDOM tree in Millau format to the OutputStream.

4.6 The Millau Code Spaces

The choice of the data structure to represent the code spaces is also important for good performance of the system. The translation time is mostly influenced by the time it takes to look up in the code spaces for a token or for its corresponding string. Depending of what the program needs to do, translating strings into tokens or tokens into strings, different data structure may be used. For example, to convert strings into tokens quickly, strings must be found quickly in a table. For this, it is better to use a hash table where the keys are the strings and the values are the corresponding tokens. But, if given a page number and an index in a code page the corresponding string must be found quickly, the best data structure is a two dimensional array indexed by page numbers and indexes in pages. If we need to be able to find a string from a token quickly or a token from a string, then we need to sort the table and then do a binary search to find a string corresponding to a token.

Next we describe the method to fill in the hash table for element code space. First, the page number variable is set to 0 and the index variable to 5 (the first four indexes are reserved for global tokens). For each element declaration, the system gets the element name, adds it in the hash table with the element name as the key and (56 x pageNumber + index) as the value. The system increments the index by 1. The size of a page for elements is 64 because the last two bits of the index are reserved so when the index reaches the value 64, the system increments the page number by 1 and resets the index to 5. When the page number reaches its maximum value 255, an exception is raised.

For each element declared, the system gets the corresponding attribute declaration from the previously built DOM tree. It adds the attribute name in the hash table with the attribute name as the key and (256 x pageNumber + index) as the value. If the attribute type is enumerated (enumerated attribute types are NOTATION or NAME_TOKEN_GROUP), then the system looks for the values of this enumerated attribute. For each value, it adds the attribute value in the hash table with the attribute value as the key and (256 x pageNumber + index) as the value. The system increments the index for the value by 1. The size of a page for attribute value is 128 so when the index reaches the value 128, the system increments the page number by 1 and resets the index to 5. When the page number reaches its maximum value 255, an exception is raised. If there are no values or when the values have been successfully added to the attribute value code space, the system increments the index for the name by 1. The size of a page for attribute name is 128 so when the index reaches the value 128, the system increments the page number by 1 and resets the index to 5. When the page number reaches its maximum value 255, an exception is raised.

Next, the method to fill in the 2-dimensional array for element code space is described. First, we set the page number variable to 0 and the index variable to 5 (the first four indexes are reserved for global tokens). For each element declaration, the system gets the element name, adds it in the elements array at position (page number, index). The system increments the index by 1. The size of a page for elements is 64 because the last two bits of the index are reserved so when the index reaches the value 64, the system increments the page number by 1 and resets the index to 5. When the page number reaches its maximum value 255, an exception is raised.

The attribute names code space and the attribute values code space can be merged into one so that each pair (attribute name, attribute value) is a single token instead of two tokens (name and value). The code space is filled as follows. For each element declared, the system gets the corresponding attribute declaration from the previously built DOM structure. If the attribute type is not enumerated (no specific value is declared for this attribute), then the system adds the attribute name in the attribute code space (hash table for the server, array for the client). If the attribute type is enumerated, then the system looks for the values of this enumerated attribute. For each value, it adds the pair (attribute name, attribute value) with a specific token in the attribute code space. When the server comes across an attribute with a value, it looks in the attribute code space for the couple (attribute name, attribute value). If it can find it, it sends this token. If it cannot find it, it looks for the attribute name in the attribute code space. If the name is found, the server sends the corresponding token for this name followed by a string inline token followed by the attribute value encoded in the charset specified at the beginning of the binary XML stream. If the name is not found, an exception is raised.

Attributes may be mandatory (#REQUIRED), optional (#IMPLIED), or can have fixed values (#FIXED). For mandatory or fixed attributes, it is not necessary to transmit tokens. To achieve this optimization the system can store in the element code space the names of the required or fixed attributes with the element name. For example, if attributes Author and Genre are required for element Book, the element code space stores the triplet (Book, Author, Genre) at the entry Book. This element code space is filled as follows. For each element declaration, the system gets the element name and the required and fixed attributes. It adds the element names and the required and fixed attribute names to the element code space. For the fixed attributes, it also adds their value. In the attribute code space, only the implied attributes will be stored with their corresponding values (if defined).

Notice that for applications which can work with tokens without translating them into strings, there is no need for code spaces. This saves memory and CPU. However, to facilitate the task of the developer of the application, the tokens can be stored as static variables with explicit names.


5. Experimental results

In this section we study the performance of the Millau system.

5.1 Theoretical compression rate

First we compute the theoretical size of Millau streams. Suppose:

The size W of the token encoded document can be computed as: W = 2N - M + 2A + K + T + S + 2P + H

The following explains the origin of each term in the above formula:

The first five parameters N, M, A, K, and T depend only on the document so we cannot change these parameters for a given document. The size of the text can be a large parameter. First of all, the number of unknown attribute values must be minimized. This can be done by encoding the attribute values given by the DTD for the attribute of the type enumerated. Another improvement is to pre-parse the document and look for the attribute values which are not in the DTD and appear more than once in the document. These attribute values can be put in the string table so that they will appear only one time in the binary XML stream.

The best solution to text compression is to separate the text from the document structure and compress it with an algorithm like GZIP [7]. The same can also be done for the unknown attribute values but it can cost in terms of processing time to have to switch to another stream for each attribute value. The size of the header depends mainly of the use of a string table : a string table contains the most frequently used strings in the document. They are then referenced in the document by an offset in the string table. The experiments show that the number of page switches can constitute a large overhead (a page switch is two bytes). This number can be further reduced by putting sibling elements on the same page. The DTD can be used to perform this optimization. (The number of page switches cannot be larger than 2(N + 2A) because there cannot be more page switches than the number of elements and attributes names and values.)

The following formula now gives a good estimate of the size X of the XML stream: X = (2N - M)n + A(a + v) + S where n is the mean length of an element name, a is the mean length of an attribute name, and v is the mean length of an attribute value.

The theoretical estimated compression rate is thus given by this formula :
C = W / X = (2N - M + 2A + K + T + S + 2P + H) / ((2N - M)n + A(a + v) + S)

We can compute a better estimate by using the exact size X of the XML stream if it is known. Usually, it is not an unknown parameter. We will use this value in the experimental computations. We observe that C is a function of the size of the markup. If elements and attributes are given long and explicit names in the original XML document, the compression rate can be very good.

5.2 Experimental compression rate

To measure the performance of our compression system, we have built a test package which considers the following parameters:

We ran our experiments with XML documents of varying sizes. We first discuss a typical set of results obtained with a 3 MB technical manual marked up in XML. This document is a valid XML document which means that it comes with a DTD which can be used to efficiently build the code spaces. Table 9 presents the results:

Size of the document (in bytes) 3 093 194
Size of the markup (in bytes) 2 038 952
Size of the character data (in bytes) 1 052 242
Number of elements 73 591
Number of empty elements 9 485
Number of attributes 25 611
Number of elements with attributes 22 529
Number of PCDATA elements 64 588
Table 9: Size of the markup for a technical documentation 

From the above table, we can compute a lower bound for the theoretical compression rate for the markup (PCDATA excluded) as defined previously. This is a lower bound and not an exact value because the number of page switches is not known a priori. Moreover, we assume that there are no attribute values encoded as strings (which is usually not true). In other words, we assume that every attribute value has a corresponding token. The lower bound of the size of the markup encoded in Millau format is thus given by: 2 * 73591 - 9485 + 2 * 25611 + 22529 + 64588 = 276036 bytes. Subsequently, the lower bound of the theoretical compression rate for the markup is given by: 276036 / 2038952 = 13.5 %.

Notice that the number of page switches cannot be larger than : 2 * (73591 + 2 * 25611) = 249626 bytes. So the upper bound for the Millau size is (if there are no attribute values encoded as strings): 276036 + 249626 = 525662 bytes. Subsequently, the higher bound of the theoretical compression rate for the markup is: 525662 / 2038952 = 25.7 %. This seems reasonably good for a worst case measure.

Table 10 shows the experimental compression rate achieved by Millau algorithm and GZIP algorithm (sizes are given in bytes):


  Initial document size Millau document size Millau compression GZIP document size GZIP compression
Whole document 3 093 194 593 795 19.2% 401 518 13.0%
Markup only 2 038 952 367 321 18.0% 175 044 8.6%
Data only 1 052 242 226 474 21.5% 226 474 21.5%
Table 10: Comparison of the compression rates acheived by Millau and GZIP algortihms

The figures compared are the compression rate of the markup using Millau encoding and using GZIP encoding (figures in bold in the chart). It can be seen that the compression rate achieved by Millau encoding is reasonable (18.0% is usually considered as a good compression rate for text). But we can also notice that the compression rate achieved by the GZIP algorithm for the same markup is very good (8.6%). Actually, this is not very surprising because the GZIP algorithm takes advantage of the redundancy of a document to compress it and the XML markup is highly redundant. There are a few tags which are repeated a large number of times in this large document. This is why the compression rate for GZIP outperforms the compression rate for Millau. But at the same time, GZIP does not retain structure of the document which is a disadvantage for fast documents processing.

To improve the compression rate of Millau, we can limit this number of page switches (about 30000) by reordering the code spaces so that opening tags which are close to each other in the document appear on the same code space page. We do not care about the closing tags because there are encoded with the reserved token END in Millau format. The code space optimization can be done from the DTD or from the XML document itself. To formalize the problem of optimizing from the XML document itself, we consider an ordered set T of n possible tags where n is bigger than the size p of a code space page. The document can be represented as a series of tags (ti) where each ti is element of T. We want to find a permutation of T that minimizes the number of page switches inside of this document. The first approach is to find the most frequent pair (ti, tj) (a pair is two different tags that appear next to each other in the XML document) and put them in the same page. A better approach consists in computing the mean distance between two tags for each pair (ti, tj) and then grouping tags in fixed-size clusters using a clustering algorithm. Each cluster here represents a code space page.

We are currently working on optmizing the code spaces from the DTD. From the DTD, we can estimate the probability that a tag follows another tag. The next step is to compute the probability that two tags will follow each other in less than p hops (where p is a page size). From these probabilities, we can compute mean distances between tags in order to apply a clustering algorithm as we do with the XML document itself.

An alternative to the code spaces approach is to encode the tags with variable length tokens. One or several bytes encode a tag according to its occurrence frequency. The 128 most frequent tags will be encoded with a single byte. The formats of these bytes is similar to the byte format of UTF-8 [23]. Table 11 shows the byte sequences:


Token length Byte sequence
1 byte 0xxxxxxx
2 bytes 110xxxxx 10xxxxxx
3 bytes 1110xxxx 10xxxxxx 10xxxxxx
4 bytes 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
5 bytes 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
6 bytes 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
Table 11: UTF- 8 byte sequences

5.3 Influence of the document size

In the previous section we computed the compression rate for a big XML document. This experiment has shown that, for this big document, GZIP was more efficient than Millau as regards the compression rate because GZIP takes advantage of the redundancy induced by the XML markup. It has also highlighted some theoretical problems that will need to be solved in further investigation. In this section we run experiments on a set of small XML files like the ones exchanged in business transactions and see if Millau can be more efficient with this kind of files than traditional compression algorithms like GZIP.

We have tested Millau compression algorithm on a set of 118 XML reference files from the Open Applications Group [14]. These sample files represent the typical files which are used in business transactions (e.g. sales orders, bills, payments). There sizes range from 1KB to 14KB. They all come with a specific DTD. We compressed them, first using GZIP algorithm [7] with a buffer size of 512 bytes, then using Millau on the XML stream. The Millau code spaces have been built from the DTDs provided with the XML sample files. On the diagram of figure 1, we represented the size of the compressed files with respect to the initial size. The red points are for GZIP while the blue ones are for Millau.


Figure 1: Size in KB of the compressed file (red: GZIP, blue: Millau) w.r.t. the initial size

First, we observe that the size of the Millau compressed file grows linearly with the file of the uncompressed file (the correlation coefficient between the Millau compressed size and the initial size is 0.9918 which is very close to 1). This means that the compression ratio is roughly constant (~ 20%) with respect to the document size. On the other hand, the compression rate of GZIP grows with the size of the initial document. This is because GZIP (like other lossless traditional compression algorithms) is designed to take advantage of the redundancy of character strings inside of a document. That is also why the compression of GZIP does not grow linearly (the correlation coefficient between the GZIP compressed size and the initial size is only 0.8995 to compare to 0.9918). Although it is true that the redundancy is high on large XML documents because of the limited number of allowed tags, it is also found that the occurrence frequency of repeated characters in a small XML document (like a sales order) might not be very high (because of the limited size of the document). So one can think that it is beneficial to map XML tags to tokens and that this mapping must be done offline from the DTD. The experience proves that this is beneficial for small documents (typically documents which size ranges from 0 to 5 KB -according to the crossing point between the two regressions lines --this is the case of most of the documents exchanged in business transactions-- here, only 20 of the 118 documents of our eBusiness test set are bigger than 5 KB).

5.4 Processing times

The third experiment measures the time to parse an XML stream (with and without compression) and compares that time to the time needed to parse a Millau stream. For the XML stream, we have used the SAX driver of the IBM XML parser [24]. For the Millau stream, we have obtained measurements with two kinds of parsers :

As a reminder, the Millau SAX parser implements the Parser interface defined by SAX so it must translate every token to its corresponding string. On the other hand, the Millau BSAX parser generates events with tokens instead of strings so it does not make any translation (see section 4).

For the measurements, the XML stream is created from a 3MB technical manual marked up in XML and served of a local disk. It is better to use a large file for timing experiments because the experiments have shown that the method currentTimeMillis of the Java class System was not able to measure time differences smaller than 10 milliseconds. SAX events are handled by basic handlers which do no processing at all. We measured the time to parse the stream from the creation of the stream to the endDocument event. Table 12 summarizes the results:

Time to parse the uncompressed XML stream 40 seconds
Time to parse the XML stream compressed with GZIP 40 seconds
Time to parse the Millau stream using a Millau SAX parser 8 seconds
Time to parse the Millau stream using a Millau BSAX parser 5 seconds
Table 12: Parsing time comparison using IBM SAX parser and Millau SAX parser

First, this indicates that the difference between parsing a compressed or uncompressed stream is negligible. Secondly, parsing a Millau stream is at least five times faster than parsing an ASCII XML stream. This result was initially somewhat surprising. However, this result can be explained by the fact that the operations performed during the parsing of a token stream are easier and less time consuming than the operations needed to parse a text XML stream: the parsing of a text stream involves string comparisons, a time consuming operations while the parsing of binary stream involves bytes comparison. Moreover, the fact that the content stream (which does not need any parsing) is separated from the structure stream makes the parsing more efficient. The processing speed is one of the greatest strengths of Millau and is the main reason why this model is particularly well adapted for business applications as well as streaming applications.

We can also observe that the time needed to translate tokens into strings by performing a look-up in the code space (3s to compare with 5s to parse the stream) is not a large overhead. This can be done if the application has not been designed to work with tokens. But for efficiency purposes it is preferable to work with tokens instead of strings as the time needed by the handler to compare two strings can be much larger than the time needed to compare two tokens.


6. Applications using Millau Streams

We have built two applications using the Millau APIs. The first application is a Millau compression Proxy Server and its companion Millau decompression Proxy Server. The second application is an implementation of the XML RPC using Millau as an exchange data format. For these two applications, we discuss the implementation design and why these are good applications of Millau encoding format.

6.1 Millau Compression Proxy Server

Studies have been made in prior research work in using HTTP proxy servers to compress data on the network [9] [11] [16]. These demonstrate that about 33% of the bandwidth can be saved easily by compressing the data exchanged on the network. In these studies, they have used conventional compression algorithms like ZLIB and GZIP to compress the text data. The fact that a significant portion of the Web objects electronic commerce have a small size makes these compression algorithms not efficient for this purpose. So it has been suggested that compression proxy server uses a static table that maps the frequently used HTML strings into tokens. However, they do not have a systematic scheme like ours to compress any XML document. Here, we investigate the architecture and the performances of an HTTP proxy server based on Millau Binary XML.

The system is composed of two proxy servers: a client proxy server and a server proxy server. The client proxy server is located "close" to the client, and the server proxy server is located "close" to the server. In an extreme situation the client proxy server may be merged with the client, and the server proxy server with the server. Also if there is seamless XML data flow between two locations in both directions there may be possibly more than one compression proxy server and decompression proxy server. We built our proxy servers using the WBI (Web Intelligence) proxy architecture system [15].

Before describing the architecture with more details, we introduce the architecture of WBI. WBI is a programmable HTTP request and HTTP response processor. WBI's data model is based on the request/response structure of HTTP version 1.0. Each request and each response consist of a structured part and a stream part. The structured part corresponds to the header and the stream part corresponds to the body. Millau mainly works on the stream part which is here supposed to be XML content. Notice that HTML content can be converted to XML thanks to an already existing WBI plugin so that Millau can then process this stream. WBI receives an HTTP request from a client, such as a Web browser, and produces an HTTP response that is returned to the client. The processing that happens in between is controlled by the modules programmed into WBI. A typical WBI transaction flow goes through three basic stages: request editors (RE), generators (G), and editors (E). A request editor receives a request and has the freedom to modify the request before passing it along. A generator receives a request and produces a corresponding response (i.e. a document). An editor receives a response and has the freedom to modify the response before passing it along. When all the steps are completed, the response is sent to the originating client.

The diagram of figure 2 shows the architecture of the Millau compression-decompression client proxy server and its associated compression-decompression server proxy server.


Figure 2: Workflow path of a Web request and its response in the Millau compression-decompression WBI proxy environment

Here, we describe the flow for a request-response transaction between a Client and a Server. The Client generates an XML request and expect an XML response from the Server. In the Client Proxy, the Request Editor (RE) compress the XML body of the HTTP request into a Millau stream using a Millau XML Tokenizer. The request is then handled by the Generator (G) which forward the request, now in Millau format, to the Server Proxy. In the Server Proxy, the Millau request is received by the Request Editor which decompresses the Millau stream using a Millau SAX Parser. The request, now in XML format, is handled by the Generator which forwards the request to the Server. The Server produces a response which is received by the Editor (E) of the Server Proxy. The Editor compresses the response in Millau format using an XML Tokenizer and send this encoded response to the originated client (the Client Proxy). The Editor of the Client Proxy decodes the response using a Millau SAX Parser and send it to the Client.

Notice that the Client Proxy is actually a simple WBI plugin that can be embedded in a LAN proxy cache machine or that can also be running directly on the client machine. An important issue is that the client proxy is "close" to the client machine. By "close" we mean that the connection between the client machine and the client proxy machine is fast compared to the connection between the client and the server (typically an Internet connection). For example, the client proxy machine must be on the same LAN than the client machine. In the same way, the server proxy must be "close" to the server machine.

To test the efficiency of this architecture, we compare the compression-decompression overhead with the download time saving that Millau compression realizes. We measured this compression-decompression overhead on a large set of small and big XML documents. The results show that the compression-decompression overhead is small compared to the download time that can be saved, specially on a low bandwidth connection like a modem connection. In the following table, we present mean results for small eBusiness documents and for large text documents (Shakespeare's plays encoded in XML by John Bosak [8]). The transmission time was computed for a 56Kbits/s modem line. Table 13 shows the results.


Document Type Mean size Transmission time Compression-decompression
Uncompressed small document 3647 bytes 521 ms N/A
Compressed small document 886 bytes 126 ms 98 ms
Uncompressed large document 213160 bytes 30451 ms N/A
Compressed large document 148269 bytes 21181 ms 1554 ms
Table 13: Millau compression-decompression time compared to transmission time on a 56Kbits/s modem line: the compression-decompression and transmission of a Millau stream is faster than the transmission of an uncompressed stream.

A typical eBusiness transaction is composed of a short request followed by a short answer. Without Millau, the transmission takes 1042 ms. With Millau, it takes 368 ms including the compression-decompression overhead giving a saving of 65% of the time. This can account for a significant portion on a large number of transactions. Note that the compression-decompression system has been implemented in Java. With an algorithm like GZIP which current implementation is in C, the compression-decompression time is smaller. We are working on a C implementation of the Millau compression-decompression algorithms which would allow a faster processing. 

6.2 Millau XML Remote Procedure Call

XML-RPC [12] is a very simple protocol for performing remote procedure calls over HTTP. It was designed by Userland Software, working with Microsoft. An XML-RPC message is an HTTP-POST request. The body of the request is in XML. A procedure executes on the server and the value it returns is also formatted in XML. Procedure parameters can be scalars, numbers, strings, dates, etc., and can also be complex record and list structures. In our implementation, the body of the request is in Millau Binary XML.

The Millau encoding format appears to be particularly well adapted to the XML RPC because the exchanged messages are usually very short and may not contain redundant tags. So, as demonstrated in the previous section, the traditional compression algorithms have usually poor performances in term of compression rate with these kind of messages. On the other hand, XML-RPC uses a limited set of tags (20 different tags) and no attributes. All the tags can hold on one code page so there is no switch page. Actually, because Millau RPC has been designed to work with tokens, it is not necessary to store the code page in memory. Moreover, most of the content is of primitive type and Millau RPC can thus take advantage of the possibility of transmitting primitive types without text encoding. All these features make that the processing of XML-RPC requests and answers is efficient.

The Millau XML-RPC system is composed of a client which generates Millau requests and gets the answers in Millau format from the XML-RPC server and of a server which gets the requests for the clients, invokes the corresponding method (if found) and sends the response in Millau format or an error message (for example if the method is not found). The HTTP requests are first handled by a Java Servlet which passes the Millau body to the server.

The init method of the servlet creates the server which does all the work. It also registers the handler object which methods can be called from the client side. The server provides a register method to register handler object with a specific name and a remove method to remove a specific handler object from its name. References to the handlers are stored in a hash table on the server side. Each time the servlet receives a request, it passes the input stream to the server. The server parses the request encoded in Millau Binary XML-RPC format using a Millau BDOM parser. We are currently working on a version using a Millau BSAX parser instead of BDOM for improved efficiency. The server tries to find the handler object and the method corresponding to the method name of the request. If it finds it, it calls this method and encodes the response in Millau Binary XML-RPC format using the previously described Millau BDOM API. It then calls the method writeBinaryXML of this API which generates a Millau stream that can be sent as the body of the response.

To create a Millau XML-RPC client, the user passes the URL of a valid Millau XML-RPC server. Then the client can open a persistent connection with the server. The user can then call the invoke method of the client, passing the name of the method as "handler.method" and a Vector of the parameters. Details on the type mapping between XML-RPC and Java can be found in [13]. From the method name and the parameters, the client will generate a Millau XML-RPC request using the Millau BDOM API. Then, it invokes the writeBinaryXML method on the BDOM tree to generate a Millau stream that can be sent to the Millau XML-RPC server through the previously opened socket. The client listens on the socket port to receive the response from the server. If it is a valid Millau XML-RPC response message, it is parsed and the result sent to the user; if it is an error message, the error is reported to the user. Notice that the error messages are also encoded in Millau XML-RPC format.

To evaluate the performance of this implementation, we made a benchmark which sends an array of 100 integers as a parameter and receives the same array as a return value. We compared the performances of our implementation with the Helma XML-RPC system [13]. The performance measure is given in table 14.


XML-RPC platform Mean number of calls per second
Millau Binary XML-RPC 27 calls per second
Helma XML-RPC 12 calls per second
Table 14: Performance comparison between Millau XML-RPC and Helma XML-RPC

Notice that HTTP may not be the best protocol to implement RPC because of the HTTP header which can be big compared to the Binary XML payload, especially if there are few parameters in the request. In case of very small requests, the performance improvement will not be significant because of the HTTP header overhead. A solution is to use persistent HTTP connections.


7. Conclusion

As large number of XML documents are exchanged and streamed over the Internet medium, techniques for compact and efficient representation and exchange for this data become essential. In this paper, we describe a system called Millau for efficient encoding and streaming of XML structures. While traditional data compression algorithms lose the structure of the documents, Millau keeps the XML hierarchical structure. Moreover, Millau enables the separation of the content from the structure in order to be able to compress the text or multimedia data separately from the XML structure. This allows achieving better compression rates. This further allows an application to do some processing on the structure without having to download large volume of data. Moreover, our experiments show that, though traditional data compression algorithms are able to perform high compression rate on large XML files, they are much less effective towards small sized XML documents like the ones exchanged in eCommerce transactions. Millau achieves better compression rates for such documents.

To allow manipulation of Millau encoded XML documents at a layer transparent from the application, we provide both a SAX API and a DOM API conform to the standards of the Web. Additionally, we provide for both APIs methods for allowing applications to work directly with tokens instead of processing strings. Our experiments have shown that the processing of Millau tokens where up to five times faster than the processing of XML tags. We also developed data structures to efficiently map tags to tokens and to store the tokens. We provide algorithms to build the code spaces from the associated XML DTDs in an efficient way. We are currently working on taking better advantage of the document structure to limit the number of page switches inside of a document and so to improve the compression rate.

To demonstrate the advantages of Millau encoding format, we built two applications on top of the previously mentioned APIs. The first one is a compression-decompression proxy server which takes advantage of the compact representation of XML that Millau provides to save Internet network bandwidth and also of the ease of processing. The second one, the Millau XML-RPC uses the methods which return token instead of strings for faster processing of parameters marshaled in XML. Moreover, it allows saving network bandwidth because of the compact format. These applications must be seen as a first step toward eBusiness transaction on the Internet.

Because it retains the structure of XML documents and because it is designed to be processed as a continuous stream, Millau encoding format can also be used for efficiently streaming structured multimedia content. For example, we built a solution for fragmenting a Millau document, associating a priority to each fragment, streaming each fragment independently, and rebuilding the whole document or parts of the document according to the user’s preferences or the browser capabilities. We applied this solution to the streaming of structured multimedia documents. We also designed a simple tool for the browsing and searching of XML structure and retrieval of multimedia content. This subject has been discussed in [20] and [25].


Acknowledgements

The authors would like to express their gratitude to Prof. Bernard Mérialdo for his help during the prior work done in [20]. We would also like to thank Anita Huang and Sami Rollins who reviewed this paper and helped improve the final version.


References

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Vitae

Marc Girardot is a research intern at the IBM Almaden Research Center, California, USA. He holds a DEA honors degree in Networking and Distributed Applications from University of Nice, France and a Telecommunications Engineering degree (major: Multimedia Communications) from Ecole Nationale Supérieure des Télécommunications, Paris, France. His areas of research interests and expertise include multimedia, virtual reality, information theory, and Internet technologies with focus on XML.

Dr. Neel Sundaresan is a research manager of the eMerging Internet Technologies department at the IBM Almaden Research Center. He has been with IBM since December 1995 and has pioneered several XML and internet related research projects. He was one of the chief architects of the Grand Central Station project at IBM Research for building XML-based search engines. He received his PhD in CS in 1995. He has done research and advanced technology work in the area of Compilers and Programming Languages, Parallel and Distributed Systems and Algorithms, Information Theory, Data Mining and Semi-structured Data, Speech Synthesis, Agent Systems, and Internet Tools and Technologies. He has over 30 research publications and has given several invited and refereed talks and tutorials at national and international conferences. He has been a member of the W3C standards effort.