Caching - the pitfall

At a current project we added some caches to our application to improve performance. However, we faced some issues I looked into and this article will demonstrate you should use caches with great precaution.

	<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
	<modelVersion>4.0.0</modelVersion>
	<groupId>com.pelssers</groupId>
	<artifactId>cachingdemo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<properties>
	<spring.version>3.1.2.RELEASE</spring.version>
	</properties>
	<dependencies>
	<dependency>
	<groupId>cglib</groupId>
	<artifactId>cglib-nodep</artifactId>
	<version>2.2</version>
	<scope>compile</scope>
	</dependency>
	<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-core</artifactId>
	<version>${spring.version}</version>
	<scope>compile</scope>
	</dependency>
	<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-beans</artifactId>
	<version>${spring.version}</version>
	<scope>compile</scope>
	</dependency>
	<dependency>
	<groupId>org.springframework</groupId>
	<artifactId>spring-context</artifactId>
	<version>${spring.version}</version>
	<scope>compile</scope>
	</dependency>
	<dependency>
	<groupId>net.sf.ehcache</groupId>
	<artifactId>ehcache-core</artifactId>
	<version>2.4.3</version>
	<scope>compile</scope>
	</dependency>
	<dependency>
	<groupId>com.google.guava</groupId>
	<artifactId>guava</artifactId>
	<version>14.0.1</version>
	</dependency>
	</dependencies>
	</project>

view raw gistfile1.xml hosted with ❤ by GitHub

	package com.pelssers.services;
	import java.util.List;
	public interface PricingService {
	List<Integer> getPriceHistory(Long productId);
	}

view raw gistfile1.java hosted with ❤ by GitHub

	package com.pelssers.services;
	import java.util.List;
	import org.springframework.cache.annotation.Cacheable;
	import com.google.common.collect.Lists;
	public class PricingServiceImpl implements PricingService {
	@Cacheable(value = "services.pricing", key = "#productId")
	public List<Integer> getPriceHistory(Long productId) {
	System.out.println("Fetching prices from DB");
	return Lists.newArrayList(2, 6, 8, 5, 9);
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

	package com.pelssers.configuration;
	import org.springframework.cache.CacheManager;
	import org.springframework.cache.annotation.EnableCaching;
	import org.springframework.cache.ehcache.EhCacheCacheManager;
	import org.springframework.cache.ehcache.EhCacheManagerFactoryBean;
	import org.springframework.context.annotation.Bean;
	import org.springframework.context.annotation.Configuration;
	import org.springframework.core.io.ClassPathResource;
	import com.pelssers.services.PricingService;
	import com.pelssers.services.PricingServiceImpl;
	@Configuration
	@EnableCaching
	public class DemoConfiguration {
	@Bean
	public PricingService pricingService() {
	return new PricingServiceImpl();
	}
	@Bean
	public EhCacheManagerFactoryBean ehCacheManagerFactoryBean() {
	EhCacheManagerFactoryBean ehCacheManagerFactoryBean = new EhCacheManagerFactoryBean();
	ehCacheManagerFactoryBean.setConfigLocation(new ClassPathResource("ehcache.xml"));
	return ehCacheManagerFactoryBean;
	}
	@Bean
	public CacheManager cacheManager() {
	EhCacheCacheManager cacheManager = new EhCacheCacheManager();
	cacheManager.setCacheManager(ehCacheManagerFactoryBean().getObject());
	return cacheManager;
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

	package com.pelssers.demo;
	import java.util.Collections;
	import java.util.List;
	import org.springframework.context.ApplicationContext;
	import org.springframework.context.annotation.AnnotationConfigApplicationContext;
	import org.springframework.stereotype.Component;
	import com.pelssers.configuration.DemoConfiguration;
	import com.pelssers.services.PricingService;
	@Component
	public class Main {
	/**
	* @param args
	*/
	public static void main(String[] args) {
	ApplicationContext context = new AnnotationConfigApplicationContext(DemoConfiguration.class);
	PricingService pricingService = context.getBean(PricingService.class);
	List<Integer> prices1 = pricingService.getPriceHistory(1L);
	System.out.println(prices1);
	Collections.reverse(prices1);
	List<Integer> prices2 = pricingService.getPriceHistory(1L);
	System.out.println(prices2);
	}
	}

view raw gistfile1.java hosted with ❤ by GitHub

	<ehcache>
	<diskStore path="java.io.tmpdir" />
	<defaultCache
	maxElementsInMemory="10000"
	eternal="false"
	timeToIdleSeconds="300"
	timeToLiveSeconds="600"
	overflowToDisk="true"
	maxElementsOnDisk="10000000"
	diskPersistent="true"
	diskExpiryThreadIntervalSeconds="120"
	memoryStoreEvictionPolicy="LRU"
	statistics="true" />
	<cache
	name="services.pricing"
	maxElementsInMemory="1000"
	eternal="false"
	overflowToDisk="false"
	timeToIdleSeconds="3600"
	timeToLiveSeconds="7200"
	memoryStoreEvictionPolicy="LFU"
	transactionalMode="off"
	statistics="true" />
	</ehcache>
	<!-- running Main will result in following output.
	Fetching prices from DB
	[2, 6, 8, 5, 9]
	[9, 5, 8, 6, 2]
	What do we see here... we only fetch the list of prices the first time.
	The second time we get back the cached value.
	BUT we actually modified the list of prices (even in our cache). Potential PITFALL !!
	-->

view raw gistfile1.xml hosted with ❤ by GitHub

	<ehcache>
	<diskStore path="java.io.tmpdir" />
	<defaultCache
	maxElementsInMemory="10000"
	eternal="false"
	timeToIdleSeconds="300"
	timeToLiveSeconds="600"
	overflowToDisk="true"
	maxElementsOnDisk="10000000"
	diskPersistent="true"
	diskExpiryThreadIntervalSeconds="120"
	memoryStoreEvictionPolicy="LRU"
	statistics="true" />
	<cache
	name="services.pricing"
	maxElementsInMemory="1000"
	eternal="false"
	overflowToDisk="false"
	timeToIdleSeconds="3600"
	timeToLiveSeconds="7200"
	memoryStoreEvictionPolicy="LFU"
	transactionalMode="off"
	statistics="true" copyOnWrite="true"
	copyOnRead="true" />
	</ehcache>
	<!-- running Main will result in following output.
	Fetching prices from DB
	[2, 6, 8, 5, 9]
	[2, 6, 8, 5, 9]
	What do we see here... we only fetch the list of prices the first time.
	The second time we get back the cached value. And even although we reversed
	the prices the first time it has NO effect on our cached value.
	This is in most situations what you want.
	-->

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