This slideshow explained about the database connection pooling and how to do it using c3p0.

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 What is a Connection?
 What is a Connection Pool?
 How Connection Pool Works?
 Connection Life Cycle
 Connection Poolers
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 Introduction to c3p0
 Using c3p0
 Configuring c3p0
 Benefits of Connection Pooling

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• In computer science, a database connection is the
means by which a database server and its client
software communicate with each other.
• The term is used whether or not the client and
the server are on different machines.
• Connections are built by supplying an underlying
driver or provider with a connection string.
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• Connection string is used to address a specific
database or server and to provide instance and user
authentication credentials.
• Connections are a key concept in data-centric
programming.
• Since some DBMSs require considerable time to connect,
connection pooling is used to improve performance.
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• Database connections are finite and expensive and
can take a long time to create relative to the
operations performed on them.
• It is very inefficient for an application to create
and close a database connection whenever it needs
to update a database.
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• The application server maintains a pool of ready to use
connections to a data store.
o Application client requests a connections using a data
source or a connection factory object.
o Connection is retrieved from the pool.
• Opening a connection only when needed, and closing it
as soon as the work is done, rather than holding a
connection open for the entire life of the
application.
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• In this manner, a relatively small number of connections can
service a large number of requests.
• Using too many connections may just cause thrashing rather than
get more useful work done.
• It is desirable to set some limit on the number of connections
in the pool.
• In case all the connections are in use, the operation can block
until a connection is returned to the pool, or an error may be
returned.
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• A managed connection object is always in one of
three states.
DoesNotExist, InFreePool or InUse
• After a connection is created, it can be in
either,
o InUse (if allocated to an application)
o InFreePool state
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• Between these three states are transitions.
 Examaple: If the application has called the data
source or connection factory getConnection() method:
FreePoolState -> InUse state
• After a connection is closed with the close() method, it is
either,
o Returned to the free pool
o Destroyed
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• Here are some of the connection poolers that we can
use to create a pool of connections,
o Apache DBCP - Java Database Connection Pooling library
o BoneCP - The JDBC Connection Pool
o MongoPool - Manages MongoDB Connections
o SQL Relay - Database Connection Pool library with API
available in all programming languages
o c3p0 - JDBC3 Connection and Statement Pooling
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• c3p0 is an easy-to-use library for augmenting traditional
(DriverManager-based) JDBC drivers, including DataSources
that implement Connection and Statement Pooling.
• In particular, c3p0 provides several useful services:
• A class which adapt traditional DriverManager-based JDBC
drivers to the newer javax.sql.DataSource scheme for
acquiring database Connections.
• Transparent pooling of Connection and PreparedStatements
behind DataSources.
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• There are three ways to create a c3p0 connection pool.
i. Using a ComboPooledDataSource.
ii. DataSources factory class.
iii.Build our own pool-backed DataSource by directly instantiating
PoolBackedDataSource and setting its ConectionPoolDataSource.

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Most user prefer using this method than the other methods.

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• We can use the static factory class com.mchange.v2.c3p0.DataSources
to build unpooled DataSources from traditional JDBC drivers, and to
build pooled DataSources from unpooled DataSources.

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We can programmatically set the configuration parameters and
override the default parameters using map like in the code above.

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• Here are some of the methods that we can use to query/get the
information of the current status of the data sources.
 public int getNumConnectionsDefaultUser()
 public int getNumIdleConnectionsDefaultUser()
 public int getNumBusyConnectionsDefaultUser()
 public int getNumConnections(String username, String password)
 public int getNumConnectionsAllUsers()
c3p0 maintains separate pools for Connections with
distinct authentications.

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o Using destroy method

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o Using close method
Unreferenced instances of PooledDataSource that are not closed by clients close()
themselves prior to garbage collection in their finalize() methods.

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• There are several ways to modify c3p0 properties: By changing
the property values associated with a particular DataSource in
your code, or you can configure c3p0 externally,
i. via a simple Java properties file
ii. via HOCON (typesafe-config) files (if and only if you
bundle the typesafe-config library with your application)
iii.via an XML configuration file
iv. via System properties

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• DataSources must be configured before they are used (during or
after the construction), c3p0 does support property
modifications midstream.
 However if you obtain a DataSource by instantiating a
ComboPooledDataSource, configure it by simply calling
appropriate setter methods offered by that class before
attempting a call to getConnection().

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 If you obtain a DataSource by using factory methods of the
utility class com.mchange.v2.c3p0.DataSources, and wish to
use a non-default configuration, you can supply a Map of
property names.

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• To override the library's built-in defaults, create a file called
c3p0.properties and place it at the "root" of your classpath or
classloader.
• The file must be available as a classloader resource under the name
/c3p0.properties, in the classloader that loaded c3p0's jar file.
• The format of c3p0.properties should be a normal Java Properties file
format, whose keys are c3p0 configurable properties.An example
c3p0.properties file is given in the next slide.

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• HOCON (Human-Optimized Config Object Notation) is a format for
human-readable data, and a superset of JSON.
• You must specify ordinary config params explicitly inside a c3p0
scope one way or another, even in a c3p0.conf file. "Dot notation"
can be used equivalently to scopes.

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• You can use the XML config file for all c3p0 configuration,
including configuration of defaults, named configurations, per-
user overrides, and configuration extensions.
• By default, c3p0 will look for an XML configuration file in its
classloader's resource path under the name "/c3p0-config.xml".

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• c3p0 properties can also be defined as System properties, using
the same "c3p0." prefix for properties specified in a
c3p0.properties file.

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• Under some circumstances, statement pooling can dramatically
improve application performance.
• Whether and how much statement pooling will help depends on how
much parsing, planning, and optimizing of queries your databases
does when the statements are prepared.

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• We can configure statement pooling in c3p0 via the following
configuration parameters:
• maxStatements-Defines the total number PreparedStatements a DataSource
will cache.
• maxStatementsPerConnection-Defines how many statements each pooled
Connection is allowed to own.
• statementCacheNumDeferredCloseThreads-If greater than zero, the Statement
pool will defer physically closing cached Statements until its parent
Connection is not in use by any client or internally by the pool itself.

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• c3p0 DataSources are designed (and configured by default) to
recover from temporary database outages, such as those which occur
during a database restart or brief loss of network connectivity.
• You can affect how c3p0 handles errors in acquiring Connections
via the configurable properties described in the next slide.

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• acquireRetryAttempts-How much attempts to get a connection, default is
infinite.
• acquireRetryDelay-Delay between retry attempts.
• breakAfterAcquireFailure-If 'true', DataSource will consider itself
broken after a failed round of Connection
attempts, and future client requests will fail
immediately.

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• Performance-The connection cost is paid for once and amortized
across all the consuming components.
• Diagnostics-If you have one sub-system responsible for connecting
to the database, it becomes easier to diagnose and analyze
database connection usage.
• Maintainability-Again, if you have one sub-system responsible for
handing out database connections, your code will be easier to
maintain than if each component connected to the database itself.

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