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AppFabric Cache - more than one writers in the same time

Intr-un post anterior am discutat despre AppFabric Cache, care are la baza mecanismul de DataCache pentru Windows Cache Server. Intr-un mediu in care avem mai multe masini care scriu pe acelasi cache trebuie avuta grija destul de mare la urmatorul caz:
In acelasi timp 2 sau mai multe masini doresc sa scrie acelasi element in cache( aceiasi cheie).
Cuvantul cheie la aceasta problema este "IN ACELASI TIMP". In mod normal am avea urmatoarea implementare pentru a scrie un obiect in cache:
DataCacheFactory cache = new DataCacheFactory();
cache.Put(key,value);
Totul ar functiona fara nici o problema pana cand 2 sau mai multe instante ar incerca sa scrie in acelasi timp un element cu aceiasi key in cache. In acest caz se arunca o exceptie de tip DataCacheException, cu error codul setat DataCacheErrorCode.RetryLater.
Pentru a rezolva aceasta problema avem doua solutii, in functie de cat de probabil e sa apara un astfel de caz putem sa folosim una din solutii, sau o combinatie din cele doua.
Prima solutie ar fi sa me folosim de mecanismul de lock care exista pentru cache.
DataCacheLockHandle lockHandle;
var value = cache.GetAndLock(key, TimeSpan.FromSeconds(1), out lockHandle, true);
cache.PutAndUnlock(key, value, lockHandle);
Avantajul la aceasta solutie este ca inainte sa se faca scrierea se face un lock explicit pe obiectul din cache, dar trebuie avut grija ca fiecare aplicatie care foloseste cache-ul sa foloseasca metode care fac lock - in caz contrat obiectul poate sa fie accesat chiar daca s-a facut lock pe acesta. Un alt dezantaj la aceasta solutie este ca trebuie facute doua requesturi la server. Unul care face GET si altul care face PUT si totodata in cazul in care se intampla ceva intre GET si PUT obiectul din cache poate sa ramana blocat din cauza lock-ului pe care il facem( trebuie sa ne implementam un mecanism de fallback.
Urmatoarea solutie pe care o propun este mult mai primitiva, dar care conserva atat conexiunea la internet cat si durata cat timp obiectul este blocat.
int retrys=5;
while(true)
{
     try
     {
          cache.Put(key,value)
     }
     catch(DataCacheException ex)
     {
          if(ex.ErrorCode == DataCacheErrorCode.Retrylater)
          {
               if(retrys <= 0)
               {
                     throw;
               }
               retrys--;
               continue;
          }
          throw;
     }
}
In cazul in care acest caz nu o sa apara foarte des as alege a doua varianta. Este mult mai simpla si mai sigura. Cand folosim a doua varianta trebuie sa avem grija daca exista si alte aplicatii care acceseaza cache-ul nostru - toti consumatorii trebuie sa foloseasca mecanismul de lock, deoarece altfel nu ar avea nici o valoare modul in care scrim noi in cache.


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