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## edu.stanford.smi.protege.util.transaction.cache
Interface Cache<S,V,R>

Type Parameters:
  `S` - the session type
  `V` - the variable type
  `R` - the result set type

All Known Implementing Classes:
  BasicCache, CompleteableCache, InvalidatableCache, ReadCommittedCache, ReadUncommittedCache, RepeatableReadCache

    public interface Cache<S,V,R>

This interface represents a simple cache mechanism for storing cached values either globally or on a per session basis. This cache mechanism is caching the values (of type R) of a variable (of type V). There are three issues that make the interpretation of this interface rather complex. The first issue is the easier one. When we do a cache lookup and get a non-null value, it is clear that the cache has the result that we want to use. But in some occasions it is important to also know that the cache is complete in the sense that if we get a null value it is also the right answer. To handle this we have created an interval between Cache.startCompleteCache and Cache.finishCompleteCache were all entries added form a complete list of possible values in the cache. We allow for the possibility that values will be invalidated between Cache.startCompleteCache and Cache.finishCompleteCache and even that they will be invalidated after and in that case we keep track of which items may be incomplete and which items are not. The second issue is a bruiser! Transactions!! Oh man!!! This adds a lot of complexity. This means that different sessions might see different values. A session in a transaction may see values that are not visible to other sessions. In addition, the behavior depends on the transaction isolation level. If the level is Read committed or higher, changes made by a session in a transaction are not visible to other sessions until the transaction is committed. This means that we need to distinguish between values read and values written and we need to keep track of who is doing the read or write. If the level is repeatable read or higher then the cache must not return a value for read even if the right value is known because the underlying mechanism needs to be informed of the read. Some usage cases follow. First, in many cases the cache will be a cache of frame values. In this case the caller will hold a weak hash map from Frames to a Cache from Slot-Facet-isTemplate objects to a list of values. Another example is the client who receives Cache updates from the server. The client will receive a cache update from the server only if either the update was made by the client itself or if the session executing the update is not in a transaction. Thus for this case the session type is a boolean indicating if the cache update came from the client itself.

Author:
  tredmond

Method Summary
`void` | `abortCompleteCache()`
`void` | `beginTransaction(S session)` The session has entered a transaction.
`void` | `commitTransaction(S session)` A transaction is committed.
`void` | `delete(S session)` This notifies the cache that the caller has made deleted the object that this cache represents.
`void` | `finishCompleteCache()` The startCompleteCache and the finishCompleteCache are part of a protocol.
`void` | `flush()` Try to avoid this.
`int` | `getTransactionNesting(S session)` Returns the transaction nesting for a session.
`boolean` | `isCacheComplete()`
`boolean` | `isDeleted()` This indicates that the cache has entered the deleted state.
`void` | `modifyCache(S session, V var)` This notifies the cache, that the caller has made a change to the value of the var.
`void` | `modifyCache(S session, V var, R value)` This notifies the cache that the caller has made a change to the value of var and that the caller knows the new value is value.
`CacheResult<R>` | `readCache(S session, V var)` If the variable var is cached then this returns the cached value.
`void` | `rollbackTransaction(S session)` A transaction is rolled back.
`void` | `startCompleteCache()` The startCompleteCache and the finishCompleteCache are part of a protocol.
`void` | `updateCache(S session, V var)` This call indicates that a read was scheduled but no useful value was retrieved.
`void` | `updateCache(S session, V var, R value)` The caller has received an update that the new value of var is value.

Method Detail

### readCache

    CacheResult<R> readCache(S session,
                             V var)

  If the variable var is cached then this returns the cached value. It can be null in which case the isValid() method should be called to determine if this is just a missing cache element or actually the correct value. This call is required to never return null so that the code sequence

          CacheResult result = cache.readCache(session, var);
          if (result.isValid()) {
             return result.getValue();
          else {
             go to the source to  find the right value
          }

  works and is the recommended approach.

  Parameters:
    `var` - a variable
  Returns:
    the value in the cache for the variable var. Never returns null.

### updateCache

    void updateCache(S session,
                     V var)

  This call indicates that a read was scheduled but no useful value was retrieved. This can happen between a startCompleteCache and a finishCompleteCache. The process filling the cache is supposed to read all possible variables that might have a value. But the process either doesn't want to read the var (too expensive) or was not able to. This allows the cache to understand that it does not have a value at this var.

  Parameters:
    `session` - the session making the change
    `var` -
    `value` -

### updateCache

    void updateCache(S session,
                     V var,
                     R value)

  The caller has received an update that the new value of var is value. This as viewed as a value read. The session is important for two reasons. First, if the transaction isolation level is repeatable read, a session must not use the cache on the first read during a transaction even if the correct value is known. The underlying mechanism must be invoked so that it can record that a read has occurred. Second the session is important to determine if the value being read might be visible only in the single

  Parameters:
    `session` - the session making the change
    `var` -
    `value` -

### modifyCache

    void modifyCache(S session,
                     V var)

  This notifies the cache, that the caller has made a change to the value of the var. It is important that the caller always calls one of the modifyCache calls when making changes to cached values.

  Parameters:
    `session` - the session making the change
    `var` - the variable being modified.

### modifyCache

    void modifyCache(S session,
                     V var,
                     R value)

  This notifies the cache that the caller has made a change to the value of var and that the caller knows the new value is value.

  Parameters:
    `session` - the session making the change
    `var` - the variable being changed
    `value` - the new value for the variable

### delete

    void delete(S session)

  This notifies the cache that the caller has made deleted the object that this cache represents. If this change is outside a transaction or gets committed, the cache enters the deleted state. Once in the deleted state all attempts to get a value return a value null value. Attempts to modify the cache after it has entered the deleted state will bring the cache back.

  Parameters:
    `session` -

### isDeleted

    boolean isDeleted()

  This indicates that the cache has entered the deleted state. Once in this state the cache cannot recover.

### flush

    void flush()

  Try to avoid this. If this is invoked and you are at READ_COMMITTED or above, the cache can repeatedly get flushed on commit and rollback transactions until everything is closed out.

### startCompleteCache

    void startCompleteCache()

  The startCompleteCache and the finishCompleteCache are part of a protocol. In the interval between the startCompleteCache and the finishCompleteCache, the values in the cache - with the exception of any entries that have been explicitly invalidated - are a complete list of all the real values.

### finishCompleteCache

    void finishCompleteCache()

  The startCompleteCache and the finishCompleteCache are part of a protocol. In the interval between the startCompleteCache and the finishCompleteCache, the values in the cache - with the exception of any entries that have been explicitly invalidated - are a complete list of all the real values.

### abortCompleteCache

    void abortCompleteCache()

### isCacheComplete

    boolean isCacheComplete()

### beginTransaction

    void beginTransaction(S session)

  The session has entered a transaction. Nesting is counted.

  Parameters:
    `session` -

### commitTransaction

    void commitTransaction(S session)

  A transaction is committed.

  Parameters:
    `session` -

### rollbackTransaction

    void rollbackTransaction(S session)

  A transaction is rolled back.

  Parameters:
    `session` -

### getTransactionNesting

    int getTransactionNesting(S session)

  Returns the transaction nesting for a session. This is really just a convenience method for the implementation.

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