src/contrib/sqoop/doc/api-reference.txt

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Developer API Reference
-----------------------

This section is intended to specify the APIs available to application writers
integrating with Sqoop, and those modifying Sqoop. The next three subsections
are written from the following three perspectives: those using classes generated
by Sqoop, and its public library; those writing Sqoop extensions (i.e.,
additional ConnManager implementations that interact with more databases); and
those modifying Sqoop's internals. Each section describes the system in
successively greater depth.


The External API
~~~~~~~~~~~~~~~~

Sqoop auto-generates classes that represent the tables imported into HDFS. The
class contains member fields for each column of the imported table; an instance
of the class holds one row of the table. The generated classes implement the
serialization APIs used in Hadoop, namely the _Writable_ and _DBWritable_
interfaces.  They also contain other convenience methods: a +parse()+ method
that interprets delimited text fields, and a +toString()+ method that preserves
the user's chosen delimiters. The full set of methods guaranteed to exist in an
auto-generated class are specified in the interface
+org.apache.hadoop.sqoop.lib.SqoopRecord+.

Instances of _SqoopRecord_ may depend on Sqoop's public API. This is all classes
in the +org.apache.hadoop.sqoop.lib+ package. These are briefly described below.
Clients of Sqoop should not need to directly interact with any of these classes,
although classes generated by Sqoop will depend on them. Therefore, these APIs
are considered public and care will be taken when forward-evolving them.

* The +RecordParser+ class will parse a line of text into a list of fields,
  using controllable delimiters and quote characters.
* The static +FieldFormatter+ class provides a method which handles quoting and
  escaping of characters in a field which will be used in
  +SqoopRecord.toString()+ implementations.
* Marshaling data between _ResultSet_ and _PreparedStatement_ objects and
  _SqoopRecords_ is done via +JdbcWritableBridge+.
* +BigDecimalSerializer+ contains a pair of methods that facilitate
  serialization of +BigDecimal+ objects over the _Writable_ interface.

The Extension API
~~~~~~~~~~~~~~~~~

This section covers the API and primary classes used by extensions for Sqoop
which allow Sqoop to interface with more database vendors.

While Sqoop uses JDBC and +DBInputFormat+ (and +DataDrivenDBInputFormat+) to
read from databases, differences in the SQL supported by different vendors as
well as JDBC metadata necessitates vendor-specific codepaths for most databases.
Sqoop's solution to this problem is by introducing the ConnManager API
(+org.apache.hadoop.sqoop.manager.ConnMananger+).

+ConnManager+ is an abstract class defining all methods that interact with the
database itself. Most implementations of +ConnManager+ will extend the
+org.apache.hadoop.sqoop.manager.SqlManager+ abstract class, which uses standard
SQL to perform most actions. Subclasses are required to implement the
+getConnection()+ method which returns the actual JDBC connection to the
database. Subclasses are free to override all other methods as well. The
+SqlManager+ class itself exposes a protected API that allows developers to
selectively override behavior. For example, the +getColNamesQuery()+ method
allows the SQL query used by +getColNames()+ to be modified without needing to
rewrite the majority of +getColNames()+.

+ConnManager+ implementations receive a lot of their configuration data from a
Sqoop-specific class, +SqoopOptions+. While +SqoopOptions+ does not currently
contain many setter methods, clients should not assume +SqoopOptions+ are
immutable. More setter methods may be added in the future.  +SqoopOptions+ does
not directly store specific per-manager options. Instead, it contains a
reference to the +Configuration+ returned by +Tool.getConf()+ after parsing
command-line arguments with the +GenericOptionsParser+. This allows extension
arguments via "+-D any.specific.param=any.value+" without requiring any layering
of options parsing or modification of +SqoopOptions+.

All existing +ConnManager+ implementations are stateless. Thus, the system which
instantiates +ConnManagers+ may implement multiple instances of the same
+ConnMananger+ class over Sqoop's lifetime. If a caching layer is required, we
can add one later, but it is not currently available.

+ConnManagers+ are currently created by instances of the abstract class +ManagerFactory+ (See
MAPREDUCE-750). One +ManagerFactory+ implementation currently serves all of
Sqoop: +org.apache.hadoop.sqoop.manager.DefaultManagerFactory+.  Extensions
should not modify +DefaultManagerFactory+. Instead, an extension-specific
+ManagerFactory+ implementation should be provided with the new ConnManager.
+ManagerFactory+ has a single method of note, named +accept()+. This method will
determine whether it can instantiate a +ConnManager+ for the user's
+SqoopOptions+. If so, it returns the +ConnManager+ instance. Otherwise, it
returns +null+.

The +ManagerFactory+ implementations used are governed by the
+sqoop.connection.factories+ setting in sqoop-site.xml. Users of extension
libraries can install the 3rd-party library containing a new +ManagerFactory+
and +ConnManager+(s), and configure sqoop-site.xml to use the new
+ManagerFactory+.  The +DefaultManagerFactory+ principly discriminates between
databases by parsing the connect string stored in +SqoopOptions+.

Extension authors may make use of classes in the +org.apache.hadoop.sqoop.io+,
+mapred+, +mapreduce+, and +util+ packages to facilitate their implementations.
These packages and classes are described in more detail in the following
section.


Sqoop Internals
~~~~~~~~~~~~~~~

This section describes the internal architecture of Sqoop.

The Sqoop program is driven by the +org.apache.hadoop.sqoop.Sqoop+ main class.
A limited number of additional classes are in the same package; +SqoopOptions+
(described earlier) and +ConnFactory+ (which manipulates +ManagerFactory+
instances).

General program flow
^^^^^^^^^^^^^^^^^^^^

The general program flow is as follows:

+org.apache.hadoop.sqoop.Sqoop+ is the main class and implements _Tool_. A new
instance is launched with +ToolRunner+. It parses its arguments using the
+SqoopOptions+ class.  Within the +SqoopOptions+, an +ImportAction+ will be
chosen by the user. This may be import all tables, import one specific table,
execute a SQL statement, or others.

A +ConnManager+ is then instantiated based on the data in the +SqoopOptions+.
The +ConnFactory+ is used to get a +ConnManager+ from a +ManagerFactory+; the
mechanics of this were described in an earlier section.

Then in the +run()+ method, using a case statement, it determines which actions
the user needs performed based on the +ImportAction+ enum. Usually this involves
determining a list of tables to import, generating user code for them, and
running a MapReduce job per table to read the data.  The import itself does not
specifically need to be run via a MapReduce job; the +ConnManager.importTable()+
method is left to determine how best to run the import. Each of these actions is
controlled by the +ConnMananger+, except for the generating of code, which is
done by the +CompilationManager+ and +ClassWriter+. (Both in the
+org.apache.hadoop.sqoop.orm+ package.) Importing into Hive is also taken care
of via the +org.apache.hadoop.sqoop.hive.HiveImport+ class after the
+importTable()+ has completed. This is done without concern for the
+ConnManager+ implementation used.

A ConnManager's +importTable()+ method receives a single argument of type
+ImportJobContext+ which contains parameters to the method. This class may be
extended with additional parameters in the future, which optionally further
direct the import operation. Similarly, the +exportTable()+ method receives an
argument of type +ExportJobContext+. These classes contain the name of the table
to import/export, a reference to the +SqoopOptions+ object, and other related
data.

Subpackages
^^^^^^^^^^^

The following subpackages under +org.apache.hadoop.sqoop+ exist:

* +hive+ - Facilitates importing data to Hive.
* +io+ - Implementations of +java.io.*+ interfaces (namely, _OutputStream_ and
  _Writer_).
* +lib+ - The external public API (described earlier).
* +manager+ - The +ConnManager+ and +ManagerFactory+ interface and their
  implementations.
* +mapred+ - Classes interfacing with the old (pre-0.20) MapReduce API.
* +mapreduce+ - Classes interfacing with the new (0.20+) MapReduce API....
* +orm+ - Code auto-generation.
* +util+ - Miscellaneous utility classes.

The +io+ package contains _OutputStream_ and _BufferedWriter_ implementations
used by direct writers to HDFS. The +SplittableBufferedWriter+ allows a single
BufferedWriter to be opened to a client which will, under the hood, write to
multiple files in series as they reach a target threshold size. This allows
unsplittable compression libraries (e.g., gzip) to be used in conjunction with
Sqoop import while still allowing subsequent MapReduce jobs to use multiple
input splits per dataset.

Code in the +mapred+ package should be considered deprecated. The +mapreduce+
package contains +DataDrivenImportJob+, which uses the +DataDrivenDBInputFormat+
introduced in 0.21. The mapred package contains +ImportJob+, which uses the
older +DBInputFormat+. Most +ConnManager+ implementations use
+DataDrivenImportJob+; +DataDrivenDBInputFormat+ does not currently work with
Oracle in all circumstances, so it remains on the old code-path.

The +orm+ package contains code used for class generation. It depends on the
JDK's tools.jar which provides the com.sun.tools.javac package.

The +util+ package contains various utilities used throughout Sqoop:

* +ClassLoaderStack+ manages a stack of +ClassLoader+ instances used by the
  current thread. This is principly used to load auto-generated code into the
  current thread when running MapReduce in local (standalone) mode.
* +DirectImportUtils+ contains convenience methods used by direct HDFS
  importers.
* +Executor+ launches external processes and connects these to stream handlers
  generated by an AsyncSink (see more detail below).
* +ExportException+ is thrown by +ConnManagers+ when exports fail.
* +ImportException+ is thrown by +ConnManagers+ when imports fail.
* +JdbcUrl+ handles parsing of connect strings, which are URL-like but not
  specification-conforming. (In particular, JDBC connect strings may have
  +multi:part:scheme://+ components.)
* +PerfCounters+ are used to estimate transfer rates for display to the user.
* +ResultSetPrinter+ will pretty-print a _ResultSet_.

In several places, Sqoop reads the stdout from external processes. The most
straightforward cases are direct-mode imports as performed by the
+LocalMySQLManager+ and +DirectPostgresqlManager+. After a process is spawned by
+Runtime.exec()+, its stdout (+Process.getInputStream()+) and potentially stderr
(+Process.getErrorStream()+) must be handled. Failure to read enough data from
both of these streams will cause the external process to block before writing
more. Consequently, these must both be handled, and preferably asynchronously.

In Sqoop parlance, an "async sink" is a thread that takes an +InputStream+ and
reads it to completion. These are realized by +AsyncSink+ implementations. The
+org.apache.hadoop.sqoop.util.AsyncSink+ abstract class defines the operations
this factory must perform. +processStream()+ will spawn another thread to
immediately begin handling the data read from the +InputStream+ argument; it
must read this stream to completion. The +join()+ method allows external threads
to wait until this processing is complete.

Some "stock" +AsyncSink+ implementations are provided: the +LoggingAsyncSink+ will
repeat everything on the +InputStream+ as log4j INFO statements. The
+NullAsyncSink+ consumes all its input and does nothing.

The various +ConnManagers+ that make use of external processes have their own
+AsyncSink+ implementations as inner classes, which read from the database tools
and forward the data along to HDFS, possibly performing formatting conversions
in the meantime.