Ehcache is a cache library introduced in October 2003 with the key goal of improving performance by reducing the load on underlying resources. Ehcache is not just for general-purpose caching, however, but also for caching Hibernate (second-level cache), data access objects, security credentials, web pages. It can also be used for SOAP and RESTful server caching, application persistence, and distributed caching.
cache: Wiktionary defines a cache as “a store of things that will be required in future, and can be retrieved rapidly.” That is the nub of it. In computer science terms, a cache is a collection of temporary data which either duplicates data located elsewhere or is the result of a computation. Once in the cache, the data can be repeatedly accessed inexpensively.
cache-hit: When a data element is requested of the cache and the element exists for the given key, it is referrred to as a cache hit (or simply ‘hit’).
cache-miss: When a data element is requested of the cache and the element does not exist for the given key, it is referred to as a cache miss (or simply ‘miss’).
system-of-record: The core premise of caching assumes that there is a source of truth for the data. This is often referred to as a system-of-record (SOR). The cache acts as a local copy of data retrieved from or stored to the system-of-record. This is often a traditional database, although it may be a specialized file system or some other reliable long-term storage. For the purposes of using Ehcache, the SOR is assumed to be a database.
SOR: See system-of-record.
Why caching works
Locality of Reference
While Ehcache concerns itself with Java objects, caching is used throughout computing, from CPU caches to the DNS system. Why? Because many computer systems exhibit “locality of reference”. Data that is near other data or has just been used is more likely to be used again.
The Long Tail
Chris Anderson, of Wired Magazine, coined the term “The Long Tail” to refer to Ecommerce systems. The idea that a small number of items may make up the bulk of sales, a small number of blogs might get the most hits and so on. While there is a small list of popular items, there is a long tail of less popular ones.
The Long Tail
The Long Tail is itself a vernacular term for a Power Law probability distribution. They don’t just appear in ecommerce, but throughout nature. One form of a Power Law distribution is the Pareto distribution, commonly know as the 80:20 rule. This phenomenon is useful for caching. If 20% of objects are used 80% of the time and a way can be found to reduce the cost of obtaining that 20%, then the system performance will improve.
Will an Application Benefit from Caching?
The short answer is that it often does, due to the effects noted above.
The medium answer is that it often depends on whether it is CPU bound or I/O bound. If an application is I/O bound then then the time taken to complete a computation depends principally on the rate at which data can be obtained. If it is CPU bound, then the time taken principally depends on the speed of the CPU and main memory.
While the focus for caching is on improving performance, it it also worth realizing that it reduces load. The time it takes something to complete is usually related to the expense of it. So, caching often reduces load on scarce resources.
Speeding up CPU-bound Applications
CPU bound applications are often sped up by:
- improving algorithm performance
- parallelizing the computations across multiple CPUs (SMP) or multiple machines (Clusters).
- upgrading the CPU speed.
The role of caching, if there is one, is to temporarily store computations that may be reused again. An example from Ehcache would be large web pages that have a high rendering cost. Another caching of authentication status, where authentication requires cryptographic transforms.
Speeding up I/O-bound Applications
Many applications are I/O bound, either by disk or network operations. In the case of databases they can be limited by both.
There is no Moore’s law for hard disks. A 10,000 RPM disk was fast 10 years ago and is still fast. Hard disks are speeding up by using their own caching of blocks into memory.
Network operations can be bound by a number of factors:
- time to set up and tear down connections
- latency, or the minimum round trip time
- throughput limits
- marshalling and unmarhshalling overhead
The caching of data can often help a lot with I/O bound applications. Some examples of Ehcache uses are:
- Data Access Object caching for Hibernate
- Web page caching, for pages generated from databases.
Increased Application Scalability
The flip side of increased performance is increased scalability. Say you have a database which can do 100 expensive queries per second. After that it backs up and if connections are added to it it slowly dies.
In this case, caching may be able to reduce the workload required. If caching can cause 90 of that 100 to be cache hits and not even get to the database, then the database can scale 10 times higher than otherwise.
How much will an application speed up with Caching?
The short answer
The short answer is that it depends on a multitude of factors being:
- how many times a cached piece of data can and is reused by the application
- the proportion of the response time that is alleviated by caching
In applications that are I/O bound, which is most business applications, most of the response time is getting data from a database. Therefore the speed up mostly depends on how much reuse a piece of data gets.
In a system where each piece of data is used just once, it is zero. In a system where data is reused a lot, the speed up is large.
The long answer, unfortunately, is complicated and mathematical. It is considered next.
Applying Amdahl’s Law
Amdahl’s law, after Gene Amdahl, is used to find the system speed up from a speed up in part of the system.
1 / ((1 - Proportion Sped Up) + Proportion Sped Up / Speed up)
The following examples show how to apply Amdahl’s law to common situations. In the interests of simplicity, we assume:
- a single server
- a system with a single thing in it, which when cached, gets 100% cache hits and lives forever.
Persistent Object Relational Caching
A Hibernate Session.load() for a single object is about 1000 times faster from cache than from a database.
A typical Hibernate query will return a list of IDs from the database, and then attempt to load each. If Session.iterate() is used Hibernate goes back to the database to load each object.
Imagine a scenario where we execute a query against the database which returns a hundred IDs and then load each one. The query takes 20% of the time and the roundtrip loading takes the rest (80%). The database query itself is 75% of the time that the operation takes. The proportion being sped up is thus 60% (75% * 80%).
The expected system speedup is thus:
1 / ((1 - .6) + .6 / 1000) = 1 / (.4 + .0006) = 2.5 times system speedup
Web Page Caching
An observed speed up from caching a web page is 1000 times. Ehcache can retrieve a page from its SimplePageCachingFilter in a few ms.
Because the web page is the end result of a computation, it has a proportion of 100%.
The expected system speedup is thus:
1 / ((1 - 1) + 1 / 1000) = 1 / (0 + .0001) = 1000 times system speedup
Web Page Fragment Caching
Caching the entire page is a big win. Sometimes the liveness requirements vary in different parts of the page. Here the SimplePageFragmentCachingFilter can be used.
Let’s say we have a 1000 fold improvement on a page fragment that taking 40% of the page render time.
The expected system speedup is thus:
1 / ((1 - .4) + .4 / 1000) = 1 / (.6 + .0004) = 1.6 times system speedup
In real life cache entrie do not live forever. Some examples that come close are “static” web pages or fragments of same, like page footers, and in the database realm, reference data, such as the currencies in the world.
Factors which affect the efficiency of a cache are:
- liveness—how live the data needs to be. The less live the more it can be cached
- proportion of data cached—what proportion of the data can fit into the resource limits of the machine. For 32 bit Java systems, there was a hard limit of 2GB of address space. While now relaxed, garbage collection issues make it harder to go a lot large. Various eviction algorithms are used to evict excess entries.
- Shape of the usage distribution—If only 300 out of 3000 entries can be cached, but the Pareto distribution applies, it may be that 80% of the time, those 300 will be the ones requested. This drives up the average request lifespan.
- Read/Write ratio—The proportion of times data is read compared with how often it is written. Things such as the number of rooms left in a hotel will be written to quite a lot. However the details of a room sold are immutable once created so have a maximum write of 1 with a potentially large number of reads.
Ehcache keeps these statistics for each Cache and each element, so they can be measured directly rather than estimated.
Also in real life, we generally do not find a single server? Assume a round robin load balancer where each hit goes to the next server. The cache has one entry which has a variable lifespan of requests, say caused by a time to live. The following table shows how that lifespan can affect hits and misses.
Server 1 Server 2 Server 3 Server 4 M M M M H H H H H H H H H H H H H H H H ... ... ... ...
The cache hit ratios for the system as a whole are as follows:
Entry Lifespan Hit Ratio Hit Ratio Hit Ratio Hit Ratio in Hits 1 Server 2 Servers 3 Servers 4 Servers 2 1/2 0/2 0/2 0/2 4 3/4 2/4 1/4 0/4 10 9/10 8/10 7/10 6/10 20 19/20 18/20 17/20 16/10 50 49/50 48/50 47/20 46/50
The efficiency of a cluster of standalone caches is generally:
(Lifespan in requests - Number of Standalone Caches) / Lifespan in requests
Where the lifespan is large relative to the number of standalone caches, cache efficiency is not much affected. However when the lifespan is short, cache efficiency is dramatically affected. (To solve this problem, Ehcache supports distributed caching, where an entry put in a local cache is also propagated to other servers in the cluster.)
A cache version of Amdahl’s law
From the above we now have:
1 / ((1 - Proportion Sped Up * effective cache efficiency) + (Proportion Sped Up * effective cache efficiency)/ Speed up) effective cache efficiency = (cache efficiency) * (cluster efficiency)
Web Page example
Applying this to the earlier web page cache example where we have cache efficiency of 35% and average request lifespan of 10 requests and two servers:
cache efficiency = .35 cluster efficiency = .(10 - 1) / 10 = .9 effective cache efficiency = .35 * .9 = .315 1 / ((1 - 1 * .315) + 1 * .315 / 1000) = 1 / (.685 + .000315) = 1.45 times system speedup
What if, instead the cache efficiency is 70%; a doubling of efficiency. We keep to two servers.
cache efficiency = .70 cluster efficiency = .(10 - 1) / 10 = .9 effective cache efficiency = .70 * .9 = .63 1 / ((1 - 1 * .63) + 1 * .63 / 1000) = 1 / (.37 + .00063) = 2.69 times system speedup
What if, instead the cache efficiency is 90%. We keep to two servers.
cache efficiency = .90 cluster efficiency = .(10 - 1) / 10 = .9 effective cache efficiency = .9 * .9 = .81 1 / ((1 - 1 * .81) + 1 * .81 / 1000) = 1 / (.19 + .00081) = 5.24 times system speedup
Why is the reduction so dramatic? Because Amdahl’s law is most sensitive to the proportion of the system that is sped up.