RR4J [Record Replay 4 Java]

RR4J is a tool that records java execution and later allows developers to replay locally. The tool solves one of the challenges developers faces when developing an application i.e sporadic bugs which are not replicable locally or system-dependent bugs which are hard to reproduce and given a micro-service architecture being incorporated everywhere, It adds an additional overhead of setting up all the services to replicate the issue and then fix it.

Build Requirements

1. C++ 11
2. Java 8
3. make
4. maven

Building Record Agent

1. Navigate to recordagent folder
2. Run make all command

Building Replay Agent

1. Navigate to replayagent folder
2. Run make all command

Building Java code

1. Run mvn clean install

Record Agent Configuration

Once the build is completed you need to configure the record agent, this can be done through config.json file and the file should be in the same place the agent is. Below is a sample configuration.

{
	"logFileName": "record.log",
	"logFileMaxSize": 100000,
	"maxLogFiles": 2,
	"recordDumpLocation":"",
	"delayedStart":5000,
	"recordCallerSnapshot": true,
	"blacklistPackages":["java/lang","sun/reflect","java/util", "java/io"],
	"rules": [
		{
			"method": "com/example/Test::MethodHasBug(IILjava/lang/String;)Ljava/lang/String;",
			"exception": "java.lang.NullPointerException",
			"additivity": false,
			"exitOnException": true,
			"threadsToMonitor" :1,
			"depth":2,
			"history":10,
			"threadsToRecord":10
		}
	]
}

Configurations	Usage
logFileName	Name of the log file which will be generated by record agent.
logFileMaxSize	Maximum log file size in bytes
maxLogFiles	Maximum log files
recordDumpLocation	Record dump location, if empty dump will be generated in the same location agent is present.
delayedStart	Delay start the record value in milliseconds
recordCallerSnapshot	True if you want to record 'this' object also, This is useful when method execution updating instance variables.
blacklistPackages	List of packages you want to blacklist
rules	Limited feature, currently only one rule is executed.
method	Method name you want to record. Type : className::methodName(args)returnType Note '.' (dot) should be replaced with '/' (slash). More info refer : https://docs.oracle.com/javase/8/docs/technotes/guides/jni/spec/types.html
exception	Exception if you are looking for, useful when the issue is intermittent. Exception should be given in fully qualified java name, ex: java.lang.NullPointerException
additivity	Not Implemented
exitOnException	True if you want to exit immediately after the exception is caught. Else false.
threadsToMonitor	Default is 1 Don't change the value
depth	Maximum depth you want to record. Example : methodA() --> Calls --↓ methodB() --> Calls --↓ methodC() methodA() is at depth 1 and methodB() is at depth 2, methodC() is not recorded its return value will be impersonated. This is useful if you don't bother about a particular function at certain depth. Ex: DB call, Network Call. You are only interested in the return value.
history	1. '0' only records one iteration which satisfies the rule 2. positive value, ex 10, 20, etc. Records the number of iteration provided. 3. A negative value, ex -10. This is like a sliding window where new records are added and previous records are discarded. This is useful when you are looking for an exception. exitOnException must be set to true else the recording process will never be completed.
threadsToRecord	A Total number of threads to record in a multi-threaded environment. Suppose if the system has 10 threads and the value provided is 3, Only first 3 threads that execute the method will be allowed now and also in the future.

Example

Recording the execution

Consider a simple java code, where I want to record a methodA(long time) execution. I will configure my record agent to record 3 iterations with below config values.

"method": "Dummy::methodA(J)I",
"exception": "",
"additivity": false,
"exitOnException": false,
"threadsToMonitor" :1,
"depth":2,
"history":3,
"threadsToRecord":1

class Dummy
{
	int methodA(long time)
	{
		String sTime = String.valueOf(time);
		int val = methodB(sTime);
		return val-1;
		
	}
	
	int methodB(String s)
	{
		int x = methodC().length();
		return s.length()-x;
	}
	
	String methodC()
	{
		String x = "hello world";
		return x;
	}
	
}

public class TestClass
{

	public static void main(String[] args) 
	{
		Dummy d = new Dummy();
		for(int i=0; i<30; i++)
		{
			try {Thread.sleep(1000);} catch(Exception ex) {}
			d.methodA(System.currentTimeMillis());
		}
	}

}

Once done, Start a program with the below command.

export LD_LIBRARY_PATH=<path/to/record/agent/so/file>
java -cp recordreplay-0.0.1-SNAPSHOT-jar-with-dependencies.jar:**TestClass class file path** -agentlib:recordagent TestClass

Post this you will see a dump file generated with the name recordDump-.dmp which can be passed as input during replay mode.

Replaying the execution

Run the following command to start the tool in replay mode.

export LD_LIBRARY_PATH=<path/to/replay/agent/so/file>
java -agentlib:replayagent -cp recordreplay-0.0.1-SNAPSHOT-jar-with-dependencies.jar:**TestClass class file path** com.rr4j.replay.Replay --dumpFile recordDump.dmp

You will see an interactor, you can also add -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8000 to start the debugger and attach an IDE.

To list all records use '-a' command

You can add a breakpoint in IDE and use '-rs recordIndex' to replay the particular record.

Future scope

1. Record thread state to provide a detailed breakdown, mimic race conditions.
2. Lamda function support.
3. Dynamic class support generated on the fly by ASM and other libraries.
4. Support controlling execution flow, recording only the particular path through line numbers.