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Safe publication of array/collection/map contents written once


By : Minghao LI
Date : September 16 2020, 07:00 AM
it helps some times There is no “best way to perform safe publication”, as the decision for a way of publication depends on the actual use case involving publication.
So it’s not correct to say that Collections.unmodifiableMap(…) is not a safe publication. This method isn’t a publication at all.
code :
HashMap<String, List<Integer>> map = new HashMap<>();
List<Integer> l = new ArrayList<>();
l.add(42);
map.put("foo", l);

Thread t = new Thread() {
    @Override
    public void run() {
        System.out.println(map);
    }
};
t.start();
System.out.println(map);
t.join();
volatile Object[] array = new Object[size];
array[index] = value;
public E set(int index, E element) {
    synchronized (lock) {
        Object[] es = getArray();
        E oldValue = elementAt(es, index);

        if (oldValue != element) {
            es = es.clone(); // <- create a local copy
            es[index] = element; // <- modifies the copy that has not been published
        }
        // Ensure volatile write semantics even when oldvalue == element
        setArray(es); // <- publishes the copy
        return oldValue;
    }
}
public E set(int index, E element) {
    synchronized (lock) {
        Object[] es = getArray();
        E oldValue = elementAt(es, index);

        if (oldValue != element) {
            es = es.clone();
            es[index] = element;
            setArray(es);
        }
        return oldValue;
    }
}
        // Ensure volatile write semantics even when oldvalue == element
        setArray(es); // invoked even when oldvalue == element


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Safe publication of array of objects through a volatile reference


By : mdily
Date : March 29 2020, 07:55 AM
Any of those help Yes, that's a safe publication, assuming you also properly read the reference to array afterwards (e.g. through the same volatile variable, or under the same lock, or from the same collection). All writes to array and to MyObject in your code happens-before any read through the published reference, including the reads of array elements.
Counter-example: if you have published array early on, say, right after new MyObject[16], then happens-before between the writes in the loop and reads through the published reference would be absent.

How do I re-run my Meteor publication to refresh the contents of a collection on the client?


By : user3172279
Date : March 29 2020, 07:55 AM
With these it helps If you replace Math.random() with a reactive variable, that will cause your subscription to be reevaluated. For simplicity I'll use a session variable in this example.
Somewhere before the route runs (at the top of the file or in a before hook), initialize the variable:
code :
  Session.setDefault('randomValue', Math.random());
  Meteor.subscribe('question', Session.get('randomValue'));
  Session.set('randomValue', Math.random());

Safe publication of a not-threadsafe standard collection


By : Julie Mallam
Date : March 29 2020, 07:55 AM
may help you . JSR-133 Cookbook
http://gee.cs.oswego.edu/dl/jmm/cookbook.html

How to sort collection by array of values in publication? (array not in the collection items)


By : Pau Benet Prat
Date : March 29 2020, 07:55 AM
should help you out If you want to do it reactively, you can either store the ratings with the cats or map them later in Javascript. A third, non-reactive option would probably be to use the MongoDB aggregation pipeline with join's (added in 3.2), but I'm not familiar enough with that to provide an example at this time.
If you store the ratings with the cats, it becomes very simple:
code :
Meteor.publish('allCats', function (sortType) {
  /**
   * Assuming that Cats contains contents of the form:
   *  {
   *    "_id" : "rRq76LxsnPfmuh9DD",
   *    "register_id" : "20gnr3g",
   *    "name" : "Meow",
   *    "created_at": ...,
   *    "rating": 123,
   *    ...
   *  }
   **/
  if (sortType == 'rating') {
    return Cats.find({}, { sort: { rating: 1 } });
  }

  return Cats.find({}, { sort: { created_at: 1 } });
});
Meteor.publish('allCats', function() {
    return Cats.find({}, {sort: { created_at: 1 }});
});

Meteor.publish('allCatRatings', function() {
    return RatingLog.find();
});
Template.myTemplate.helpers({
  cats() {
    const sortType = Template.instance().sortType;
    if (sortType === 'rating') {
      return getCatsByRating();
    }
    return getCatsByCreationDate();
  }
});

function getCatsByCreationDate() {
  return Cats.find({}, {sort: { created_at: 1 }});
}

function getCatsByRating() {
  /**
   * Assuming that RatingLog contains contents of the form { _id:  catId, rating: 123 }
   **/
  const ratings = {};
  RatingLog.find().forEach(entry=>ratings[entry._id] = entry.rating);

  const defaultRating = 0;
  return Cats.find().fetch().sort(function (catA, catB) {
    const catARating = ratings[catA._id] || defaultRating;
    const catBRating = ratings[catB._id] || defaultRating;
    return catARating - catBRating;
  });
}

Java concurrency: safe publication of array


By : Bernard Rudi
Date : March 29 2020, 07:55 AM
To fix this issue Your first example is perfectly safe, because the tasks originate from the writer thread. As the docs say:
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