Concurrency Model for Azure Managed Cache Service

The caching architecture allows any cache client to access any cached data if those clients have the appropriate network access and configuration settings. This presents a challenge to concurrency.

To help your application deal with concurrency issues, optimistic and pessimistic concurrency models are supported.

Optimistic Concurrency Model

In the optimistic concurrency model, updates to cached objects do not take locks. Instead, when the cache client gets an object from the cache, it also obtains and stores the current version of that object. When an update is required, the cache client sends the new value for the object along with the stored version object. The system only updates the object if the version sent matches the current version of the object in the cache. Every update to an object changes its version number, which prevents the update from overwriting someone else’s changes.

The example in this topic illustrates how optimistic concurrency maintains data consistency.

Example

In this example, two cache clients (cacheClientA and cacheClientB) try to update the same cached object, having the same key RadioInventory.

Time Zero: Both Clients Retrieve the Same Object

At time zero (T0), both cache clients instantiate a DataCacheItem class to capture the cached object that they intend to update, together with additional information associated with that cached object, such as version and tag information. This is illustrated in the following code example.

'cacheClientA pulls the FM radio inventory from cache
Dim strACSKey = "[Authentication token]"
Dim clientAFactorySecurity = New DataCacheSecurity(strAcsKey)

Dim clientAFactoryConfiguration = New DataCacheFactoryConfiguration()
'Set up desired parameters for Cache Factory
clientAFactoryConfiguration.SecurityProperties = clientAFactorySecurity

Dim clientACacheFactory As DataCacheFactory = New DataCacheFactory(clientAFactoryConfiguration)
Dim cacheClientA As DataCache = _
        clientACacheFactory.GetCache("catalog")
Dim radioInventoryA As DataCacheItem = _
        cacheClientA.GetCacheItem("RadioInventory")

'cacheClientB pulls the same FM radio inventory from cache
'clientBFactorySecurity and clientBFactoryConfiguration omitted for space
Dim clientBCacheFactory As DataCacheFactory = New DataCacheFactory(clientBFactoryConfiguration)
Dim cacheClientB As DataCache = _
       clientBCacheFactory.GetCache("catalog")
Dim radioInventoryB As DataCacheItem = _
        cacheClientB.GetCacheItem("RadioInventory")

//cacheClientA pulls the FM radio inventory from cache
string strACSKey = "[Authentication token]";
DataCacheSecurity clientAFactorySecurity = new DataCacheSecurity(strAcsKey);

DataCacheFactoryConfiguration clientAFactoryConfiguration = new DataCacheFactoryConfiguration();
// Set up desired parameters for Cache Factory
clientAFactoryConfiguration.SecurityProperties = clientAFactorySecurityDataCacheFactory;

clientACacheFactory = new DataCacheFactory(clientAFactoryConfiguration);
DataCache cacheClientA = clientACacheFactory.GetCache("catalog");
DataCacheItem radioInventoryA = 
    cacheClientA.GetCacheItem("RadioInventory");

//cacheClientB pulls the same FM radio inventory from cache
// clientBFactorySecurity and clientBFactoryConfiguration omitted for space
DataCacheFactory clientBCacheFactory = new DataCacheFactory();
DataCache cacheClientB = clientBCacheFactory.GetCache("catalog");
DataCacheItem radioInventoryB= 
    cacheClientB.GetCacheItem("RadioInventory");

Time One: The First Update Succeeds

At time one (T1), cacheClientA updates the cached object RadioInventory with a new value. When cacheClientA executes the Put method, the version associated with the RadioInventory cache item increments. At this time, cacheClientB has an out-of-date cache item. This is illustrated in the following example.

'at time T1, cacheClientA updates the FM radio inventory
Dim newRadioInventoryA As Integer = 155

cacheClientA.Put("RadioInventory", newRadioInventoryA, _
                 radioInventoryA.Version)

//at time T1, cacheClientA updates the FM radio inventory
int newRadioInventoryA = 155;

cacheClientA.Put("RadioInventory", newRadioInventoryA, 
    radioInventoryA.Version);

Time Two: The Second Update Fails

At time two (T2), cacheClientB tries to update the RadioInventory cached object by using what is now an out-of-date version number. To prevent the changes from cacheClientA from being overwritten, the cacheClientBPut method call fails. The cache client throws a DataCacheException object with the ErrorCode property set to CacheItemVersionMismatch. This is illustrated in the following code example.

'later, at time T2, cacheClientB tries to 
'update the FM radio inventory, receives DataCacheException with
'an error code equal to DataCacheErrorCode.CacheItemVersionMismatch.
Dim newRadioInventoryB As Integer = 130

cacheClientB.Put("RadioInventory", newRadioInventoryB, _
                 radioInventoryB.Version)

//later, at time T2, cacheClientB tries to 
//update the FM radio inventory, receives DataCacheException with
//an error code equal to DataCacheErrorCode.CacheItemVersionMismatch.
int newRadioInventoryB = 130;

cacheClientB.Put("RadioInventory", newRadioInventoryB,
    radioInventoryB.Version);

Pessimistic Concurrency Model

In the pessimistic concurrency model, the client explicitly locks objects to perform operations. Other operations that request locks are rejected (the system does not block requests) until the locks are released. When objects are locked, a lock handle is returned as an output parameter. The lock handle is required to unlock the object. In case the client application ends before freeing a locked object, time-outs are provided to release the locks. Locked objects are never expired, but they may expire immediately after they are unlocked if it is past their expiry time.

For more information about the methods used with the pessimistic concurrency model, see Concurrency Methods

See Also

Other Resources

Azure Managed Cache Service Features