Develop Reference

Handling additional data in Apache ServiceComb compensation methods

I'm currently looking at the implementations of saga pattern for distributed transactions and I found that Apache ServiceComp pack might be something that works for me.
However, I have found a problem that the limitation of compensating methods to have the same declaration as the methods they compensate may be a bottleneck.
From Apache's example:
#Compensable(compensationMethod = "cancel")
void order(CarBooking booking) {
booking.confirm();
bookings.put(booking.getId(), booking);
}
void cancel(CarBooking booking) {
Integer id = booking.getId();
if (bookings.containsKey(id)) {
bookings.get(id).cancel();
}
}
You can see that we have the same declaration for both methods.
But, what if I need additional information to compensate my transaction? For instance, I have a call to external system to update some flag to "true". When I need to compensate it, how do I make "cancel" method know what the original value of this flag was?
The things get more tricky when we update the whole object. How do I send the whole object before modification to the cancel transaction?
These limitation doesn't look quite promising. Do you know if there are approaches to fight with this limitation?

You can save localTxId and flag an in your application and use localTxId in the compensation method to get the flag
Map extmap = new HashMap();
#Autowired
OmegaContext omegaContext;
#Compensable(compensationMethod = "cancel")
void order(CarBooking booking) {
booking.confirm();
bookings.put(booking.getId(), booking);
//save flag
extmap.put(omegaContext.localTxId(),'Your flag')
}
void cancel(CarBooking booking) {
//get flag
extmap.get(omegaContext.localTxId());
Integer id = booking.getId();
if (bookings.containsKey(id)) {
bookings.get(id).cancel();
}
}

How To Put If Statement Inside Transaction Firebase Firestore

When the user clicks a button it's going to check if the Document in Firestore is exists or not, if it is, then just update (increase) some value, and if it isn't create a new document with the new value.
I could just do the normal documentRef.get(), and documentRef.set(), but when I tested it with 2 user press the button at the same time. Its just put the new value of one of them. and so I used transaction.
In this transaction, first I get the documentsnapshot, and I get the value.
and then I put if statement to check if the document is exist, the first statement which is document is exist is working fine, but when I deleted the document, the else statement didn't do anything.
Is it even possible to use if statement inside Firebase Firestore transaction?
Map<String, Object> newDocumentSet = new HashMap<>();
newDocumentSet.put("sold", 1);
newDocumentSet.put("price", myProductPrice);
mDb.runTransaction(new Transaction.Function<Void>() {
#Nullable
#Override
public Void apply(#NonNull Transaction transaction) throws FirebaseFirestoreException {
DocumentSnapshot myDocumentSnapshot = transaction.get(myDocumentRef);
long newAddSold = myDocumentSnapshot.getLong("sold") + 1;
long newAddPrice = myDocumentSnapshot.getLong("price") + myProductPrice;
if(myDocumentSnapshot.exists()){
transaction.update(myDocumentRef, "sold", newAddSold);
transaction.update(myDocumentRef, "price", newAddPrice);
}else {
transaction.set(myDocumentRef, newDocumentSet);
}
return null;
}
});
I don't know what's happening, it didn't show any error, please tell me if I made some mistake or there is another way of doing this.

or there is another way of doing this..
Yes there is, even a simpler one which in fact is more efficient than a transaction because it does not require a round trip between the client and Firebase servers. Assuming that the mDb points to the correct document, to increment the sold property by 1, please use the following line of code:
mDb.update("sold", FieldValue.increment(1));

All you need is to return a Map<String, dynamic> value from the runTransaction function and inside the run transaction make sure you return a map value like
if (a) {
return "value":true
} else {
return "value":false
}
Then in the return statement of your function assign mapDynamic["value"];. Then you will now which part of your code was executed. mapDynamic is just a variable name.

Unique ID number in static method through concurrency JAVA

I want to get unique ID from my domain object (table) ID each time a method is called. So that ID's do not repeat. I have a function that returns unique ID.
public static Long generateID (Short company)
throws Exception
{
IDDAO iDDAO = SpringApplicationContext.getBean (IDDAO.class);
ID iD = iDDAO.findByNaturalKey (new IDNatKey (company);
if (iD != null)
{
// Check if ID has reached limit, then reset the ID to the first ID
if (iD.getLatestIDno ().longValue () == iD.getLastIDno ().longValue ())
{
iD.setLatestIDno (iD.getFrstIDno ());
}
// Get next ID
iD.setLatestIDno (iD.getLatestIDno () + 1);
// update database with latest id
iDDAO.update (iD);
return iD.getLatestIDno ();
}
}
The issue is that if access the application from two machines and press button from UI to generate ID exactly at the same time, there are sometimes duplicate IDs returned from this method
e.g.
Long ID = TestClass.generateID (123);
This gives me duplicate sometimes.
I made the method like this
public static synchronized Long generateID (Short company)
throws Exception
so that only one thread can go in this function at a time, but the duplicate issue is still there.
I do not want to use database sequences as I do not want gaps in the ID sequences if the transaction rolls back, in that case sequence will be incremented still, which I do not want.Gaps at the middle are OK but not at end. E.g we have 1, 2 and 3 as IDs and 2 rolls back, that is OK. But if 3 rolls back, we should get 3 again when another user comes, in case of sequence, it will give 4
Please help me tell what I am doing incorrect ? static synchronized will still cause other threads to go inside this function at same time ? I have many other static (but not synchronized) functions in the class. Will this cause issue with them too if I make it static synchronized ?
Thanks
Aiden

You can use java.util.UUID. it will generate a universal uniqueId.

Keep 2 unique IDs:
a db-provided, internal transaction ID, created by an autoincrement every time a new transaction is built. Gaps may appear if transactions are rolled back.
a pretty, gap-less "ticket ID", assigned only once the transaction commits successfully.
Assign both from the DB - it is best to keep all shared state there, as the DB will guarantee ACID, while Java concurrency is far trickier to get right.

In that case I think you try the below :
synchronized(this){if (iD != null)
{
// Check if ID has reached limit, then reset the ID to the first ID
if (iD.getLatestIDno ().longValue () == iD.getLastIDno ().longValue ())
{
iD.setLatestIDno (iD.getFrstIDno ());
}
// Get next ID
iD.setLatestIDno (iD.getLatestIDno () + 1);
// update database with latest id
iDDAO.update (iD);
return iD.getLatestIDno ();
}}

Data commit issue in multithreading

I am new to Java and Hibernate.
I have implemented a functionality where I generate request nos. based on already saved request no. This is done by finding the maximum request no. and incrementing it by 1,and then again save i it to database.
However I am facing issues with multithreading. When two threads access my code at the same time both generate same request no. My code is already synchronized. Please suggest some solution.
synchronized (this.getClass()) {
System.out.println("start");
certRequest.setRequestNbr(generateRequestNumber(certInsuranceRequestAddRq.getAccountInfo().getAccountNumberId()));
reqId = Utils.getUniqueId();
certRequest.setRequestId(reqId);
ItemIdInfo itemIdInfo = new ItemIdInfo();
itemIdInfo.setInsurerId(certRequest.getRequestId());
certRequest.setItemIdInfo(itemIdInfo);
dao.insert(certRequest);
addAccountRel();
System.out.println("end");
}
Following is the output showing my synchronization:
start
end
start
end
Is it some Hibernate issue.
Does the use of transactional attribute in Spring affects the code commit in my Case?
I am using the following Transactional Attribute:
#Transactional(readOnly = false, propagation = Propagation.REQUIRED, rollbackFor = Exception.class)
EDIT: code for generateRequestNumber() shown in chat room.
public String generateRequestNumber(String accNumber) throws Exception {
String requestNumber = null;
if (accNumber != null) {
String SQL_QUERY = "select CERTREQUEST.requestNbr from CertRequest as CERTREQUEST, "
+ "CertActObjRel as certActObjRel where certActObjRel.certificateObjkeyId=CERTREQUEST.requestId "
+ " and certActObjRel.certObjTypeCd=:certObjTypeCd "
+ " and certActObjRel.certAccountId=:accNumber ";
String[] parameterNames = {"certObjTypeCd", "accNumber"};
Object[] parameterVaues = new Object[]
{
Constants.REQUEST_RELATION_CODE, accNumber
};
List<?> resultSet = dao.executeNamedQuery(SQL_QUERY,
parameterNames, parameterVaues);
// List<?> resultSet = dao.retrieveTableData(SQL_QUERY);
if (resultSet != null && resultSet.size() > 0) {
requestNumber = (String) resultSet.get(0);
}
int maxRequestNumber = -1;
if (requestNumber != null && requestNumber.length() > 0) {
maxRequestNumber = maxValue(resultSet.toArray());
requestNumber = Integer.toString(maxRequestNumber + 1);
} else {
requestNumber = Integer.toString(1);
}
System.out.println("inside function request number" + requestNumber);
return requestNumber;
}
return null;
}

Don't synchronize on the Class instance obtained via getClass(). It can have some strange side effects. See https://www.securecoding.cert.org/confluence/pages/viewpage.action?pageId=43647087
For example use:
synchronize(this) {
// synchronized code
}
or
private synchronized void myMethod() {
// synchronized code
}
To synchronize on the object instance.
Or do:
private static final Object lock = new Object();
private void myMethod() {
synchronize(lock) {
// synchronized code
}
}
Like #diwakar suggested. This uses a constant field to synchronize on to guarantee that this code is synchronizing on the same lock.
EDIT: Based on information from chat, you are using a SELECT to get the maximum requestNumber and increasing the value in your code. Then this value is set on the CertRequest which is then persisted in the database via a DAO. If this persist action is not committed (e.g. by making the method #Transactional or some other means) then another thread will still see the old requestNumber value. So you could solve this by making the code transactional (how depends on which frameworks you use etc.). But I agree with #VA31's answer which states that you should use a database sequence for this instead of incrementing the value in code. Instead of a sequence you could also consider using an auto-incement field in CertRequest, something like:
#GeneratedValue(strategy=GenerationType.AUTO)
private int requestNumber;
For getting the next value from a sequence you can look at this question.

You mentioned this information in your question.
I have implemented a functionality where I generate request nos. based on already saved request no. This is done by finding the maximum request no. and incrementing it by 1,and then again save i it to database.
On a first look, it seems the problem caused by multi appserver code. Threads are synchronised inside one JVM(appserver). If you are using more than one appserver then you have to do it differently using more robust approach by using server to server communication or by batch allocation of request no to each appserver.
But, if you are using only one appserver and multiple threads accessing the same code then you can put a lock on the instance of the class rather then the class itself.
synchronized(this) {
lastName = name;
nameCount++;
}
Or you can use the locks private to the class instance
private Object lock = new Object();
.
.
synchronized(lock) {
System.out.println("start");
certRequest.setRequestNbr(generateRequestNumber(certInsuranceRequestAddRq.getAccountInfo().getAccountNumberId()));
reqId = Utils.getUniqueId();
certRequest.setRequestId(reqId);
ItemIdInfo itemIdInfo = new ItemIdInfo();
itemIdInfo.setInsurerId(certRequest.getRequestId());
certRequest.setItemIdInfo(itemIdInfo);
dao.insert(certRequest);
addAccountRel();
System.out.println("end");
}
But make sure that your DB is updated by the new sequence no before the next thread is accessing it to get new one.

It is a good practice to generate "the request number (Unique Id)" by using the DATABASE SEQUENCE so that you don't need to synchronize your Service/DAO methods.

First thing:
Why are you getting the thread inside the method. I is not required here.
Also, one thing;
Can you try like this once:
final static Object lock = new Object();
synchronized (lock)
{
.....
}
what I feel is that object what you are calling is different so try this once.

Track visitors in play framework 2 application,caching and saving to DB

What I want to accomplish is a sane way of storing a list of the pages visited during each anonymous user session on my website. It is important that the sequence is correct and that one users navigation is not mixed up with another ones.
It is similar to this C# question: "track visitor info on every request", where they proposed to use a logfile for short-term storage. My site will only host ~2K users/day so I think I would be fine with having the cache in memory (atleast that seems easier).
I cant seem to figure out a solution that will work with the stateless Play framework since the solution requires saving data(to DB) only when an user has timed out and is not active anymore. My idea would rely on storing each request in memory and then call the database if a new request hasnt arrived in a certain time (user timeout). How would it be possible to do that?
Is there any other way you think might be better, perhaps storing all the requests from all users and then making a big write to db instead of one for each session?
EDIT:
I have now taken the easy way out, it would have been great to do what Salem mentioned but I cant figure out how to save a list to memory. My first plan was actually to use the Akka scheduler that would be altered (timer reset and a new post added) each time the user gets a new page but I simply dont know how to get in touch with a instance ion memory from a previous request so if someone could tell me that I would be very grateful.
For now I have solved my problem the "bad way" with requests to database for each page view. Code below in case someone also wanna be bad:
In the Action:
String uuid=session("uuid");
Logger.info("Old uuid:"+uuid);
String generatedId = UserTracker.updateTracker(uuid,id.toString()).toString();
if(uuid==null || !generatedId.equals(uuid)){
Logger.info("new UUID for user:"+generatedId);
session("uuid", generatedId);
}
UserTracker class:
#Entity
public class UserTracker extends Model {
#Id
#SequenceGenerator(name="ut_gen", sequenceName="user_tracker_seq", allocationSize=1)
#GeneratedValue(strategy = GenerationType.SEQUENCE, generator="ut_gen")
long session_id;
String navigations;
public static Long updateTracker(String session,String page){
UserTracker ut=null;
try{
Long sessionId=Long.parseLong(session);
ut = UserTracker.find.byId(sessionId);
if(ut!=null)
{
ut.addPage(page);
ut.update();
} else {
throw new NumberFormatException();
}
} catch (NumberFormatException e){
ut = new UserTracker(page);
ut.save();
}
return ut.session_id;
}
private void addPage(String page) {
// TODO Auto-generated method stub
if(navigations!=null && !navigations.isEmpty())
navigations+=","+page;
else
navigations=page;
}
public UserTracker(String page){
addPage(page);
save();
}
public static Finder<Long,UserTracker> find =
new Finder<Long,UserTracker>(Long.class,UserTracker.class);
}