Slow Light 
In 1999, Danish physicist Lene Vestergaard Hau led a combined team from Harvard University and the Rowland Institute for Science which succeeded in slowing a beam of light to about 17 meters per second.
Slow Light degrades response times of object methods and synchronous remote calls as concurrency increases.
We've used it primarily to test monitoring and fault tolerance of integration points under scalability tests. We love using it to test Hystrix integrations and configurations.
Introduction and Motivation
Slow Light consists of two sibling tools, Slow Light Embedded and Slow Light Proxy Server. Both tools interpose themselves between a caller and an API to degrade responses, either with response times or error creation.
Why do this? We've found that certain points in system architectures will inevitably experience degradation and failure. For instance, an application may leverage a remote address validation service. That address validation service, at some point in the future, will probably have an outage or scaling issues or just the network will fail. Slow Light provides us with the ability to examine the client application's behavior under a variety of scenarios where a remote service is having issues. Typically, this is done during performance testing; it often exposes situations where calls results can be cached better or logic can be made fault tolerant. We've even seen situations where degraded responses with a third party caused out-of-memory errors in the JVM because objects could not be released to garbage collection frequently enough under load.
When Slow Light is combined with specialized Fault Tolerance tools such as Hystrix, Slow Light becomes similar to testing smoke alarms and automatic sprinkler systems. Slow Light creates the smoke which triggers fault tolerance code and sets of alarms.
Slow Light Architectures
Slow Light Embedded
Slow Light Embedded runs inside a Java Process and wraps service interfaces with a Java Proxy that incorporates a ThreadPool and specialized InvocationHandler; the handler monitors concurrency on the ThreadPool and degrades Proxy responses according to configuration and concurrency rules.
Slow Light Embedded requires altering code within your application or IoC configuration. This is a fairly simple process when using Spring, Guice, PicoContainer, or other IoC injectors that support an AOP model. When not using one of these tools, Slow Light Embedded is usually done at service instantiation - often using existing factories. Many people provide toggling that enables or disables Slow Light Embedded (for an example of feature toggle please see https://github.com/tacitknowledge/flip).
with Hystrix With a Fault Tolerance tool like Hystrix, which also proxies/wraps Service APIs, we insert Hystrix around Slow Light. Slow Light itself wraps
Slow Light Proxy Server
Slow Light Proxy Server is a standalone JVM application that proxies remote, synchronous TCP/IP calls. It does not require altering code in client applications as it runs external to the process. Slow Light Proxy Server uses Netty and some special ChannelHandler implementations to slow, delay, discard, forward, and generally play mischevious games with remote calls.
with Hystrix
When using Slow Light Proxy Server with a Fault Tolerance tool like Hystrix, you can achieve the same smoke and smoke alarm behavior as Slow Light Embedded.
A disadvantage of Slow Light Proxy Server is that it can't create faults and degradation with system resources like file I/O. If you need to simulate failures in non-network resources, use Slow Light Embedded.
Use it!
A Sample from Slow Light Embedded
In general you just need to do these things:
//This object needs to have an interface to proxy, but can be real app code, a real service, or a stub
// supporting concrete classes without interfaces is a future TODO
Object targetToWrapInProxy;
.
.
.
//set up how the service should degrade
DegradationStrategy degradationStrategy
= new DefaultDegradationStrategy(serviceDemandTime,
serviceTimeout,
passRate,
new Class<Exception>[]{MyException.class}
);
//Set up a NamedThreadFactory. optional but highly suggested to make debugging and monitoring easier.
//The below code would generate threads with names similar to the below
// Thread[threadfactory-<factoryname>-groupnumber-1-thread-1],5,parentgroup-main-groupname-<factoryname>1]
//Spring users may wish to use CustomizableThreadFactory, which supports naming
ThreadFactory threadFactory = new NamedThreadFactory("<factoryname>");
//set up a fixed thread pool
ThreadPoolExecutor executorService = (ThreadPoolExecutor) Executors.newFixedThreadPool(capacity,threadFactory);
//set up InvocationHandler with execution pool and strategy
final DegradationHandler handler = new DegradationHandler(conversation,
executorService, degradationStrategy);
Object wrappedProxy = Proxy.newProxyInstance(targetToWrapInProxy.getClass().getClassLoader(),
targetToWrapInProxy.getClass().getInterfaces(),
handler);
//now replace calls to target with the wrapped proxy. In general, pretty simple to integrate with IoC frameworks,
// AOP stuff, or JNDIA Sample from Slow Light Proxy Server
This configuration proxies calls between port 10011 and google.com:80:
- Simple Proxy (first 2 min)
- Proxy with specified (1KB/s) timed delays (after 2 min)
- Discard incoming packets and keep connection open (after 5 min)
{
"id" : "solrServer",
"type" : "proxy",
"localPort" : "9012",
"params" : {
"host" : "localhost",
"port" : "8983"
},
"handlers" : [
{
"name" : "delayHandler",
"type" : "com.tacitknowledge.slowlight.proxyserver.handler.DelayChannelHandler",
"params" : {"maxDataSize" : "0", "delay" : "0", "timeFrame" : "5"},
"behaviorFunctions" : [
{
"paramName" : "delay",
"type" : "com.tacitknowledge.slowlight.proxyserver.handler.behavior.LinearBehavior",
"ranges" : {"120000" : "130000"},
"params" : {
"value" : "1000"
}
},
{
"paramName" : "maxDataSize",
"type" : "com.tacitknowledge.slowlight.proxyserver.handler.behavior.LinearBehavior",
"ranges" : {"120000" : "130000"},
"params" : {
"value" : "1024"
}
}
]
},
{
"name" : "discardHandler",
"type" : "com.tacitknowledge.slowlight.proxyserver.handler.DiscardChannelHandler",
"params" : {"enabled" : "false", "timeFrame" : "5"},
"behaviorFunctions" : [
{
"paramName" : "enabled",
"type" : "com.tacitknowledge.slowlight.proxyserver.handler.behavior.LinearBehavior",
"ranges" : {"300000" : ""},
"params" : {
"value" : "true"
}
}
]
}
]
}For detailed information on how to configure and use Slow Light please use the following links:
Where do I get Slow Light?
Slow Light is open source and is hosted at Github.
Notes
The default implementations are thread safe. You can re-use a single proxy across threads, assuming the target object is also safe.
Extension Points
Feel free to write your own DegradationStrategy and pop it into the DegradationHandler
Future Feature Notes
stable releases
Current Slowlight release is available in the public maven repository as com.tacitknowledge:slowlight:2.0.0
Release Notes
Slow Light development progresses against the development branch, with merges into master at releases.