[SNMP4J] Why is the USM a singleton ?

[Sjoerd van Doorn]

Hello all,
 
I'm working on an issue and I suspect the fact that the USM is a singleton, is part of the reasons I'm having problems.
Can someone explain why not have an instance of the USM for every MPv3 instance ?
 
My problem is having timeouts, usmStatsNotInTime, usmUnknownEngineId and MessageException (1404) every now and then.
I'm in a network with 200+ elements and I already have seen that the autoritiveEngineId of the elements is not unique, however I cannot have them changed for my purpose (I am aware that this is against RFC 3414)
The issues show when in parallel (multithreaded) querying appr. 15 devices.(I'll post a snipped at the end of my mail.
 
I suspect the internal administration of the USM is broken due to the fact that the engineID is non-unique and that this is causing my errors.
 
After analysing the code for a couple of days and going through the previous posts, I can see more people are having these kind of problems, however I could not find any solution.
I'm thinking that a modification of the USM from a singleton to a instance per MPv3 could solve the problem, but I can't realy oversee why it is designed as a singleton from the beginning. 
 
Here is my code (executed by 15 threads in parallel in synchronous mode.
 
private final Snmp4jAgent _agent;

private final String _requestType;

protected SnmpCommand(Snmp4jAgent agent, String requestType){
  _agent = agent;
_requestType = requestType;
initSecurityModels(createUSM());
}

protected void initSecurityModels( USM usm ){
SecurityModels.getInstance().addSecurityModel(usm);
}

protected USM createUSM(){
return new USM(SecurityProtocols.getInstance(),
new OctetString(MPv3.createLocalEngineID()),
0);
}

protected PDU createRequest(){
final PDU request = new ScopedPDU();
request.setType(PDU.getTypeFromString(_requestType));
request.setMaxRepetitions(15);
request.setNonRepeaters(0);
return request;
}

protected Target createTarget(){
final UserTarget target = new UserTarget();
target.setSecurityLevel(_agent.securityLevel);
target.setSecurityName(_agent.securityName);
target.setVersion(_agent.version);
target.setAddress(_agent.udpAddress);
target.setRetries(_agent.retries);
target.setTimeout(_agent.timeoutInSeconds * 1000);
return target;
}

protected Snmp createSnmp() throws IOException{
Snmp snmp = new Snmp(new DefaultUdpTransportMapping());
UsmUser user = new UsmUser(_agent.securityName,
_agent.authProtocol,
_agent.authPassphrase,
_agent.privProtocol,
_agent.privPassphrase);
snmp.getUSM().addUser(_agent.securityName, user);
return snmp;
}