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Hadoop Record I/O
Hadoop record I/O contains classes and a record description language
translator for simplifying serialization and deserialization of records in a
language-neutral manner.
Introduction
Software systems of any significant complexity require mechanisms for data
interchange with the outside world. These interchanges typically involve the
marshaling and unmarshaling of logical units of data to and from data streams
(files, network connections, memory buffers etc.). Applications usually have
some code for serializing and deserializing the data types that they manipulate
embedded in them. The work of serialization has several features that make
automatic code generation for it worthwhile. Given a particular output encoding
(binary, XML, etc.), serialization of primitive types and simple compositions
of primitives (structs, vectors etc.) is a very mechanical task. Manually
written serialization code can be susceptible to bugs especially when records
have a large number of fields or a record definition changes between software
versions. Lastly, it can be very useful for applications written in different
programming languages to be able to share and interchange data. This can be
made a lot easier by describing the data records manipulated by these
applications in a language agnostic manner and using the descriptions to derive
implementations of serialization in multiple target languages.
This document describes Hadoop Record I/O, a mechanism that is aimed
at
enabling the specification of simple serializable data types (records)
enabling the generation of code in multiple target languages for
marshaling and unmarshaling such types
providing target language specific support that will enable application
programmers to incorporate generated code into their applications
The goals of Hadoop Record I/O are similar to those of mechanisms such as XDR,
ASN.1, PADS and ICE. While these systems all include a DDL that enables
the specification of most record types, they differ widely in what else they
focus on. The focus in Hadoop Record I/O is on data marshaling and
multi-lingual support. We take a translator-based approach to serialization.
Hadoop users have to describe their data in a simple data description
language. The Hadoop DDL translator rcc generates code that users
can invoke in order to read/write their data from/to simple stream
abstractions. Next we list explicitly some of the goals and non-goals of
Hadoop Record I/O.
Goals
Support for commonly used primitive types. Hadoop should include as
primitives commonly used builtin types from programming languages we intend to
support.
Support for common data compositions (including recursive compositions).
Hadoop should support widely used composite types such as structs and
vectors.
Code generation in multiple target languages. Hadoop should be capable of
generating serialization code in multiple target languages and should be
easily extensible to new target languages. The initial target languages are
C++ and Java.
Support for generated target languages. Hadooop should include support
in the form of headers, libraries, packages for supported target languages
that enable easy inclusion and use of generated code in applications.
Support for multiple output encodings. Candidates include
packed binary, comma-separated text, XML etc.
Support for specifying record types in a backwards/forwards compatible
manner. This will probably be in the form of support for optional fields in
records. This version of the document does not include a description of the
planned mechanism, we intend to include it in the next iteration.
Non-Goals
Serializing existing arbitrary C++ classes.
Serializing complex data structures such as trees, linked lists etc.
Built-in indexing schemes, compression, or check-sums.
Dynamic construction of objects from an XML schema.
The remainder of this document describes the features of Hadoop record I/O
in more detail. Section 2 describes the data types supported by the system.
Section 3 lays out the DDL syntax with some examples of simple records.
Section 4 describes the process of code generation with rcc. Section 5
describes target language mappings and support for Hadoop types. We include a
fairly complete description of C++ mappings with intent to include Java and
others in upcoming iterations of this document. The last section talks about
supported output encodings.
Data Types and Streams
This section describes the primitive and composite types supported by Hadoop.
We aim to support a set of types that can be used to simply and efficiently
express a wide range of record types in different programming languages.
Primitive Types
For the most part, the primitive types of Hadoop map directly to primitive
types in high level programming languages. Special cases are the
ustring (a Unicode string) and buffer types, which we believe
find wide use and which are usually implemented in library code and not
available as language built-ins. Hadoop also supplies these via library code
when a target language built-in is not present and there is no widely
adopted "standard" implementation. The complete list of primitive types is:
byte: An 8-bit unsigned integer.
boolean: A boolean value.
int: A 32-bit signed integer.
long: A 64-bit signed integer.
float: A single precision floating point number as described by
IEEE-754.
double: A double precision floating point number as described by
IEEE-754.
ustring: A string consisting of Unicode characters.
buffer: An arbitrary sequence of bytes.
Composite Types
Hadoop supports a small set of composite types that enable the description
of simple aggregate types and containers. A composite type is serialized
by sequentially serializing it constituent elements. The supported
composite types are:
record: An aggregate type like a C-struct. This is a list of
typed fields that are together considered a single unit of data. A record
is serialized by sequentially serializing its constituent fields. In addition
to serialization a record has comparison operations (equality and less-than)
implemented for it, these are defined as memberwise comparisons.
vector: A sequence of entries of the same data type, primitive
or composite.
map: An associative container mapping instances of a key type to
instances of a value type. The key and value types may themselves be primitive
or composite types.
Streams
Hadoop generates code for serializing and deserializing record types to
abstract streams. For each target language Hadoop defines very simple input
and output stream interfaces. Application writers can usually develop
concrete implementations of these by putting a one method wrapper around
an existing stream implementation.
DDL Syntax and Examples
We now describe the syntax of the Hadoop data description language. This is
followed by a few examples of DDL usage.
Hadoop DDL Syntax
recfile = *include module *record
include = "include" path
path = (relative-path / absolute-path)
module = "module" module-name
module-name = name *("." name)
record := "class" name "{" 1*(field) "}"
field := type name ";"
name := ALPHA (ALPHA / DIGIT / "_" )*
type := (ptype / ctype)
ptype := ("byte" / "boolean" / "int" |
"long" / "float" / "double"
"ustring" / "buffer")
ctype := (("vector" "<" type ">") /
("map" "<" type "," type ">" ) ) / name)
A DDL file describes one or more record types. It begins with zero or
more include declarations, a single mandatory module declaration
followed by zero or more class declarations. The semantics of each of
these declarations are described below:
include: An include declaration specifies a DDL file to be
referenced when generating code for types in the current DDL file. Record types
in the current compilation unit may refer to types in all included files.
File inclusion is recursive. An include does not trigger code
generation for the referenced file.
module: Every Hadoop DDL file must have a single module
declaration that follows the list of includes and precedes all record
declarations. A module declaration identifies a scope within which
the names of all types in the current file are visible. Module names are
mapped to C++ namespaces, Java packages etc. in generated code.
class: Records types are specified through class
declarations. A class declaration is like a Java class declaration.
It specifies a named record type and a list of fields that constitute records
of the type. Usage is illustrated in the following examples.
Examples
A simple DDL file links.jr with just one record declaration.
module links {
class Link {
ustring URL;
boolean isRelative;
ustring anchorText;
};
}
A DDL file outlinks.jr which includes another
include "links.jr"
module outlinks {
class OutLinks {
ustring baseURL;
vector outLinks;
};
}
Code Generation
The Hadoop translator is written in Java. Invocation is done by executing a
wrapper shell script named named rcc. It takes a list of
record description files as a mandatory argument and an
optional language argument (the default is Java) --language or
-l. Thus a typical invocation would look like:
$ rcc -l C++ ...
Target Language Mappings and Support
For all target languages, the unit of code generation is a record type.
For each record type, Hadoop generates code for serialization and
deserialization, record comparison and access to record members.
C++
Support for including Hadoop generated C++ code in applications comes in the
form of a header file recordio.hh which needs to be included in source
that uses Hadoop types and a library librecordio.a which applications need
to be linked with. The header declares the Hadoop C++ namespace which defines
appropriate types for the various primitives, the basic interfaces for
records and streams and enumerates the supported serialization encodings.
Declarations of these interfaces and a description of their semantics follow:
RecFormat: An enumeration of the serialization encodings supported
by this implementation of Hadoop.
InStream: A simple abstraction for an input stream. This has a
single public read method that reads n bytes from the stream into
the buffer buf. Has the same semantics as a blocking read system
call. Returns the number of bytes read or -1 if an error occurs.
OutStream: A simple abstraction for an output stream. This has a
single write method that writes n bytes to the stream from the
buffer buf. Has the same semantics as a blocking write system
call. Returns the number of bytes written or -1 if an error occurs.
RecordReader: A RecordReader reads records one at a time from
an underlying stream in a specified record format. The reader is instantiated
with a stream and a serialization format. It has a read method that
takes an instance of a record and deserializes the record from the stream.
RecordWriter: A RecordWriter writes records one at a
time to an underlying stream in a specified record format. The writer is
instantiated with a stream and a serialization format. It has a
write method that takes an instance of a record and serializes the
record to the stream.
Record: The base class for all generated record types. This has two
public methods type and signature that return the typename and the
type signature of the record.
Two files are generated for each record file (note: not for each record). If a
record file is named "name.jr", the generated files are
"name.jr.cc" and "name.jr.hh" containing serialization
implementations and record type declarations respectively.
For each record in the DDL file, the generated header file will contain a
class definition corresponding to the record type, method definitions for the
generated type will be present in the '.cc' file. The generated class will
inherit from the abstract class hadoop::Record. The DDL files
module declaration determines the namespace the record belongs to.
Each '.' delimited token in the module declaration results in the
creation of a namespace. For instance, the declaration module docs.links
results in the creation of a docs namespace and a nested
docs::links namespace. In the preceding examples, the Link class
is placed in the links namespace. The header file corresponding to
the links.jr file will contain:
namespace links {
class Link : public hadoop::Record {
// ....
};
};
Each field within the record will cause the generation of a private member
declaration of the appropriate type in the class declaration, and one or more
acccessor methods. The generated class will implement the serialize and
deserialize methods defined in hadoop::Record+. It will also
implement the inspection methods type and signature from
hadoop::Record. A default constructor and virtual destructor will also
be generated. Serialization code will read/write records into streams that
implement the hadoop::InStream and the hadoop::OutStream interfaces.
For each member of a record an accessor method is generated that returns
either the member or a reference to the member. For members that are returned
by value, a setter method is also generated. This is true for primitive
data members of the types byte, int, long, boolean, float and
double. For example, for a int field called MyField the folowing
code is generated.
For a ustring or buffer or composite field. The generated code
only contains accessors that return a reference to the field. A const
and a non-const accessor are generated. For example:
Code generation for Java is similar to that for C++. A Java class is generated
for each record type with private members corresponding to the fields. Getters
and setters for fields are also generated. Some differences arise in the
way comparison is expressed and in the mapping of modules to packages and
classes to files. For equality testing, an equals method is generated
for each record type. As per Java requirements a hashCode method is also
generated. For comparison a compareTo method is generated for each
record type. This has the semantics as defined by the Java Comparable
interface, that is, the method returns a negative integer, zero, or a positive
integer as the invoked object is less than, equal to, or greater than the
comparison parameter.
A .java file is generated per record type as opposed to per DDL
file as in C++. The module declaration translates to a Java
package declaration. The module name maps to an identical Java package
name. In addition to this mapping, the DDL compiler creates the appropriate
directory hierarchy for the package and places the generated .java
files in the correct directories.
Mapping Summary
DDL Type C++ Type Java Type
boolean bool boolean
byte int8_t byte
int int32_t int
long int64_t long
float float float
double double double
ustring std::string Text
buffer std::string java.io.ByteArrayOutputStream
class type class type class type
vector std::vector java.util.ArrayList
map std::map java.util.TreeMap
Data encodings
This section describes the format of the data encodings supported by Hadoop.
Currently, three data encodings are supported, namely binary, CSV and XML.
Binary Serialization Format
The binary data encoding format is fairly dense. Serialization of composite
types is simply defined as a concatenation of serializations of the constituent
elements (lengths are included in vectors and maps).
Composite types are serialized as follows:
class: Sequence of serialized members.
vector: The number of elements serialized as an int. Followed by a
sequence of serialized elements.
map: The number of key value pairs serialized as an int. Followed
by a sequence of serialized (key,value) pairs.
Serialization of primitives is more interesting, with a zero compression
optimization for integral types and normalization to UTF-8 for strings.
Primitive types are serialized as follows:
byte: Represented by 1 byte, as is.
boolean: Represented by 1-byte (0 or 1)
int/long: Integers and longs are serialized zero compressed.
Represented as 1-byte if -120 <= value < 128. Otherwise, serialized as a
sequence of 2-5 bytes for ints, 2-9 bytes for longs. The first byte represents
the number of trailing bytes, N, as the negative number (-120-N). For example,
the number 1024 (0x400) is represented by the byte sequence 'x86 x04 x00'.
This doesn't help much for 4-byte integers but does a reasonably good job with
longs without bit twiddling.
float/double: Serialized in IEEE 754 single and double precision
format in network byte order. This is the format used by Java.
ustring: Serialized as 4-byte zero compressed length followed by
data encoded as UTF-8. Strings are normalized to UTF-8 regardless of native
language representation.
buffer: Serialized as a 4-byte zero compressed length followed by the
raw bytes in the buffer.
CSV Serialization Format
The CSV serialization format has a lot more structure than the "standard"
Excel CSV format, but we believe the additional structure is useful because
it makes parsing a lot easier without detracting too much from legibility
the delimiters around composites make it obvious when one is reading a
sequence of Hadoop records
Serialization formats for the various types are detailed in the grammar that
follows. The notable feature of the formats is the use of delimiters for
indicating the certain field types.
A string field begins with a single quote (').
A buffer field begins with a sharp (#).
A class, vector or map begins with 's{', 'v{' or 'm{' respectively and
ends with '}'.
The CSV format can be described by the following grammar:
The XML serialization format is the same used by Apache XML-RPC
(http://ws.apache.org/xmlrpc/types.html). This is an extension of the original
XML-RPC format and adds some additional data types. All record I/O types are
not directly expressible in this format, and access to a DDL is required in
order to convert these to valid types. All types primitive or composite are
represented by <value> elements. The particular XML-RPC type is
indicated by a nested element in the <value> element. The encoding for
records is always UTF-8. Primitive types are serialized as follows:
byte: XML tag <ex:i1>. Values: 1-byte unsigned
integers represented in US-ASCII
boolean: XML tag <boolean>. Values: "0" or "1"
int: XML tags <i4> or <int>. Values: 4-byte
signed integers represented in US-ASCII.
long: XML tag <ex:i8>. Values: 8-byte signed integers
represented in US-ASCII.
float: XML tag <ex:float>. Values: Single precision
floating point numbers represented in US-ASCII.
double: XML tag <double>. Values: Double precision
floating point numbers represented in US-ASCII.
ustring: XML tag <;string>. Values: String values
represented as UTF-8. XML does not permit all Unicode characters in literal
data. In particular, NULLs and control chars are not allowed. Additionally,
XML processors are required to replace carriage returns with line feeds and to
replace CRLF sequences with line feeds. Programming languages that we work
with do not impose these restrictions on string types. To work around these
restrictions, disallowed characters and CRs are percent escaped in strings.
The '%' character is also percent escaped.
buffer: XML tag <string&>. Values: Arbitrary binary
data. Represented as hexBinary, each byte is replaced by its 2-byte
hexadecimal representation.
Composite types are serialized as follows:
class: XML tag <struct>. A struct is a sequence of
<member> elements. Each <member> element has a <name>
element and a <value> element. The <name> is a string that must
match /[a-zA-Z][a-zA-Z0-9_]*/. The value of the member is represented
by a <value> element.
vector: XML tag <array<. An <array> contains a
single <data> element. The <data> element is a sequence of
<value> elements each of which represents an element of the vector.
map: XML tag <array>. Same as vector.
For example:
class {
int MY_INT; // value 5
vector MY_VEC; // values 0.1, -0.89, 2.45e4
buffer MY_BUF; // value '\00\n\tabc%'
}
