HikariCP _{Ultimate JDBC Connection Pool  ^{We came, we saw, we kicked its ass}}

TL;DR

There is nothing faster. There is nothing more reliable. There is nothing more correct. HikariCP is an essentially zero-overhead Production-ready connection pool.

Using a stub-JDBC implementation to isolate and measure the overhead of HikariCP, 60+ Million JDBC operations were performed in 12ms on a commodity PC. 460x faster that the next fastest connection pool.

Performance

Let's look at some performance numbers. HikariCP was only compared to BoneCP because, really, DBCP and C3P0 are old and slow. We would have run the BoneCP benchmarks but their [methodolgy is flawed] (https://github.com/brettwooldridge/HikariCP/wiki/Benchmarking) so we wrote our own.

MixedBench

This is the so called "Mixed" benchmark, and it executes a representative array of JDBC operations in a realistic mix. We think median is the number to pay attention to, rather than average (which can get skewed). Median meaning 50% of the iterations were slower, 50% were faster. 500 threads were started, and the underlying connection pool contained 200 connections. Measurements taken in nanoseconds and converted to milliseconds.

Pool	Med (ms)	Avg (ms)	Max (ms)
BoneCP	5533	3756	8189
HikariCP	12	11	32

A breakdown of the mix operations is:

Operation	Invocations
DataSource.getConnection()	1000
PreparedStatement.prepareStatement()	200,000
PreparedStatement.setInt()	30,000,000
PreparedStatement.addBatch()	10,000,000
PreparedStatement.executeBatch()	100,000
PreparedStatement.executeQuery()	100,000
PreparedStatement.close()	200,000
ResultSet.next()	10,000,000
ResultSet.getInt()	10,000,000
ResultSet.close()	100,000
Connection.close()	1000

The JVM JIT was "warmed up" with a single run through, then 4 runs were made from which the run with the lowest median time was chosen.

The benchmark was run using a stub (nop) implementation of an underlying DataSource, Connection, PreparedStatement, and ResultSet, so the driver was taken completely out of the equation so that the performance and overhead of the pools themselves could be measured. Care was taken to ensure that the JIT does not eliminate or "optimize away" the stub code.

The test was performed on an Intel Core i7 (3770) 3.4GHz iMac, MacOS X 10.8, 32GB RAM. The JVM benchmark was run with: -server -XX:+UseParallelGC -Xms256m -Xss256k -Dthreads=500 -DpoolMax=200. The benchmark is available in the src/test/java folder in the package com.zaxxer.hikari.performance in a main class called Benchmark.

In Summary

500 threads ran 60,702,000 JDBC operations each, HikariCP did this in a median of 12ms per thread.

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(In)correctness

Sometimes "correctness" is objective, and sometimes it is subjective. One example of objective incorrectness in BoneCP is ResultSet handling. Every connection pool needs to wrap the underlying Connection, Statement, CallableStatement, and PreparedStatement, and ResultSet classes. However, BoneCP does not wrap ResultSet.

ResultSet must be wrapped, because ResultSet.getStatement() must return the wrapped Statement that generated it, not the underlying Statement. Hibernate 4.3 for one relies on this semantic.

If BoneCP were to wrap ResultSet, which comprises 20,100,000 of the 60,702,000 operations in MixedBench, its performance numbers would be poorer. Take note that HikariCP does properly wrap ResultSet and still achives the numbers above.

One example of subjective incorrectness is that BoneCP does not test a Connection immediately before dispatching it from the pool. In our opinion, this one "flaw" (or "feature") alone renders BoneCP unsuitable for Production use. The number one responsibility of a connection pool is to not give out possibly bad connections. Of course there are no guarantees, and the connection could drop in the few tens of microseconds between the test and its use in your code, but it is much more reliable than testing once a minute or only when a SQLException has already occurred.

BoneCP may claim that testing a connection on dispatch from the pool negatively impacts performance. However, not doing so negatively impacts reliability. Addtionatlly, HikariCP supports the JDBC4 Connection.isValid() API, which for many drivers provides a fast non-query based aliveness test. Regardless, it will always test a connection just microseconds before handing it to you. Add to that the fact that the ratio of getConnection() calls to other wrapped JDBC calls is extremely small you you'll find that at an application level there is very little performance impact.

A particularly silly "benchmark" on the BoneCP site starts 500 threads each performing 100 ds.getConnection() / connection.close() calls with 0ms delay between. Who does that? The typical "mix" is dozens or hundreds of JDBC operations between obtaining the connection and closing it (hence the "MixBench") above. But ok, we can run this "benchmark" too; times in Microseconds and measure the per-thread times across all 500 threads.

Pool	Med (μs)	Avg (μs)	Max (μs)
BoneCP	19467	8762	30851
HikariCP	74	62	112

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Knobs

Where are all the knobs? HikariCP has plenty of "knobs" as you can see in the configuration section below, but comparatively less than some other pools. This is a design philosophy. Configuring a connection pool, even for a large production environment, is not rocket science.

The HikariCP design semantic is minimalist. You probably need to configure the idle timeout for connections in the pool, but do you really need to configure how often the pool is swept to retire them? You might think you do, but if you do you're probably doing something wrong.

We're not going to (overly) question the design decisions of other pools, but we will say that some other pools seem to implement a lot of "gimmicks" that proportedly improve performance. HikariCP achieves high-performance even in pools beyond realistic deployment sizes. Either these "gimmicks" are a case of premature optimization, poor design, or lack of understanding of how to leaverage what the JVM JIT can do for you to full effect.

Missing Knobs

In keeping with the simple is better or less is more design philosophy, some knobs and features are intentionally left out. Here are two, and the rationale.

Statement Cache
Most major database JDBC drivers already have a PreparedStatement cache that can be configured (Oracle, MySQL, PostgreSQL, Derby, etc). A statement cache in the pool would add unneeded weight and no additional functionality.

JDBC drivers have a special relationship with the remote database in that they are directly connected and can share internal state that is synchronized with the backend in a way that an external cache cannot.

It is simply unnecessary with modern database drivers to implement this at the pool level.

Log Statement Text / Slow Query Logging
Like Statement caching, most major database vendors support statement logging through properties of their own driver. This includes Oracle, MySQL, Derby, MSSQL, and others. We consider this a "development-time" feature. For those few databases that do not support it, jdbcdslog-exp is a good option. It also provides some nice additional stuff like timing, logging slow queries only, and PreparedStatement bound parameter logging. Great stuff during development, and even pre-Production.

Trust us, you don't want this feature -- even disabled -- in a production connection pool. If we can figure out how to do it without impacting performance we might implement it, but we consider even checking a additional boolean as inducing too much overhead into your queries and results.

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Configuration (Knobs, baby!)

The following is the various properties that can be configured in the pool, their behavior, and their defaults. HikariCP uses milliseconds for all time values, be careful.

Rather than coming out of the box with almost nothing configured, HikariCP comes with sane defaults that let a great many deployments run without any additional tweaking (except for the DataSource and connection URL).

acquireIncrement
This property controls the maximum number of connections that are acquired at one time, with the exception of pool initialization. Default: 5

acquireRetries
This is a per-connection attempt retry count used during new connection creation (acquisition). If a connection creation attempt fails there will be a wait of acquireRetryDelay milliseconds followed by another attempt, up to the number of retries configured by this property. Default: 3

acquireRetryDelay
This property controls the number of milliseconds to delay between attempts to acquire a connection to the database. If acquireRetries is 0, this property has no effect. Default: 750

connectionTestQuery
This is for "legacy" databases that do not support the JDBC4 Connection.isValid() API. This is the query that will be executed just before a connection is given to you from the pool to validate that the connection to the database is still alive. It is database dependent and should be a query that takes very little processing by the database (eg. "VALUES 1"). See the jdbc4ConnectionTest property for a more efficent alive test. One of either this property or jdbc4ConnectionTest must be specified. Default: none

connectionTimeout
This property controls the maximum number of milliseconds that a client (that's you) will wait for a connection from the pool. If this time is exceeded without a connection becoming available, an SQLException will be thrown. Default: 5000

connectionUrl
The is the JDBC connection URL string specific to your database. Default: none

dataSourceClassName
This is the name of the DataSource class provided by the JDBC driver. Consult the documentation for your specific JDBC driver to get this class name. Note XA data sources are not supported. XA requires a real transaction manager like bitronix. Default: none

idleTimeout
This property controls the maximum amount of time (in milliseconds) that a connection is allowed to sit idle in the pool. Whether a connection is retired as idle or not is subject to a maximum variation of +30 seconds, and average variation of +15 seconds. A connection will never be retired as idle before this timeout. A value of 0 means that idle connections are never removed from the pool. Default: 600000 (10 minutes)

jdbc4ConnectionTest
This property is a boolean value that determines whether the JDBC4 Connection.isValid() method is used to check that a connection is still alive. This value is mutually exlusive with the connectionTestQuery property, and this method of testing connection validity should be preferred if supported by the JDBC driver. Default: true

leakDetectionThreshold
This property controls the amount of time that a connection can be out of the pool before a message is logged indicating a possible connection leak. A value of 0 means leak detection is disabled. While the default is 0, and other connection pool implementations state that leak detection is "not for production" as it imposes a high overhead, at least in the case of HikariCP the imposed overhead is only 5μs (microseconds) split between getConnection() and close(). Maybe other pools are doing it wrong, but feel free to use leak detection under HikariCP in production environments if you wish. Default: 0

maxLifetime
This property controls the maximum lifetime of a connection in the pool. When a connection reaches this timeout, even if recently used, it will be retired from the pool. An in-use connection will never be retired, only when it is idle will it be removed. We strongly recommend setting this value, and using something reasonable like 30 minutes or 1 hour. A value of 0 indicates no maximum lifetime (infinite lifetime), subject of course to the idleTimeout setting. Default: 1800000 (30 minutes)

maximumPoolSize
This property controls the maximum size that the pool is allowed to reach, including both idle and in-use connections. Basically this value will determine the maximum number of actual connections to the database backend. A reasonable value for this is best determined by your execution environment. When the pool reaches this size, and no idle connections are available, calls to getConnection() will block for up to connectionTimeout milliseconds before timing out. Default: 60

minimumPoolSize
This property controls the minimum number of connections that HikariCP tries to maintain in the pool, including both idle and in-use connections. If the connections dip below this value, HikariCP will make a best effort to restore them quickly and efficiently. A reasonable value for this is best determined by your execution environment. Default: 10

poolName
This property represents a user-defined name for the connection pool and appears mainly in a JMX management console to identify pools and pool configurations. Default: auto-generated

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JMX Management

The following properties are configurable in real-time as the pool is running via a JMX management console such as JConsole:

• acquireIncrement
• acquireRetries
• acquireRetryDelay
• connectionTimeout
• idleTimeout
• leakDetectionThreshold
• maxLifetime
• minimumPoolSize
• maximumPoolSize

Requirements

• Java 6 and above
• Javassist library
• slf4j library