public class VariableServiceImpl extends Object implements VariableService
Consider a statement as follows: select * from MyEvent as A where A.val > var1 and A.val2 > var1 and A.val3 > var2
Upon statement execution we need to guarantee that the same atomic value for all variables is applied for all variable reads (by expressions typically) within the statement.
Designed to support:
As an alternative to a version-based design, a read-lock for the variable space could also be used, with the following disadvantages: The write lock may just not be granted unless fair locks are used which are more expensive; And a read-lock is more expensive to acquire for multiple CPUs; A thread-local is still need to deal with "set var1=3, var2=var1+1" assignments where the new uncommitted value must be visible in the local evaluation.
Every new write to a variable creates a new version. Thus when reading variables, readers can ignore newer versions and a read lock is not required in most circumstances.
This algorithm works as follows:
A thread processing an event into the engine via sendEvent() calls the "setLocalVersion" method once before processing a statement that has variables. This places into a threadlocal variable the current version number, say version 570.
A statement that reads a variable has an ExprVariableNode that has a VariableReader handle
obtained during validation (example).
The VariableReader takes the version from the threadlocal (570) and compares the version number with the
version numbers held for the variable.
If the current version is same or lower (520, as old or older) then the threadlocal version,
then use the current value.
If the current version is higher (571, newer) then the threadlocal version, then go to the prior value.
Use the prior value until a version is found that as old or older then the threadlocal version.
If no version can be found that is old enough, output a warning and return the newest version. This should not happen, unless a thread is executing for very long within a single statement such that lifetime-old-version time speriod passed before the thread asks for variable values.
As version numbers are counted up they may reach a boundary. Any write transaction after the boundary is reached performs a roll-over. In a roll-over, all variables version lists are newly created and any existing threads that read versions go against a (old) high-collection, while new threads reading the reset version go against a new low-collection.
The class also allows an optional state handler to be plugged in to handle persistence for variable state. The state handler gets invoked when a variable changes value, and when a variable gets created to obtain the current value from persistence, if any.
| Modifier and Type | Field and Description |
|---|---|
protected static int |
HIGH_WATERMARK_VERSIONS
Applicable for each variable if more then the number of versions accumulated, check
timestamps to determine if a version can be expired.
|
protected static int |
ROLLOVER_READER_BOUNDARY
Sets the boundary above which a reader considers the high-version list of variable values.
|
| Modifier | Constructor and Description |
|---|---|
protected |
VariableServiceImpl(int startVersion,
long millisecondLifetimeOldVersions,
TimeProvider timeProvider,
EventAdapterService eventAdapterService,
VariableStateHandler optionalStateHandler)
Ctor.
|
|
VariableServiceImpl(long millisecondLifetimeOldVersions,
TimeProvider timeProvider,
EventAdapterService eventAdapterService,
VariableStateHandler optionalStateHandler)
Ctor.
|
| Modifier and Type | Method and Description |
|---|---|
void |
allocateVariableState(String variableName,
int agentInstanceId,
StatementExtensionSvcContext extensionServicesContext,
boolean isRecoveringResilient) |
void |
checkAndWrite(String variableName,
int agentInstanceId,
Object newValue)
Check type of the value supplied and writes the new variable value.
|
void |
commit()
Commits the variable outstanding changes.
|
void |
createNewVariable(String optionalContextName,
String variableName,
String variableType,
boolean constant,
boolean array,
boolean arrayOfPrimitive,
Object value,
EngineImportService engineImportService) |
void |
deallocateVariableState(String variableName,
int agentInstanceId) |
void |
destroy() |
VariableReader |
getReader(String variableName,
int agentInstanceIdAccessor)
Returns a reader that provides access to variable values.
|
ConcurrentHashMap<Integer,VariableReader> |
getReadersPerCP(String variableName) |
ReadWriteLock |
getReadWriteLock()
Lock for use in atomic writes to the variable space.
|
VariableMetaData |
getVariableMetaData(String variableName) |
Map<String,VariableReader> |
getVariableReadersNonCP()
Returns a map of variable name and reader, for thread-safe iteration.
|
String |
isContextVariable(String variableName) |
void |
registerCallback(String variableName,
int agentInstanceId,
VariableChangeCallback variableChangeCallback)
Registers a callback invoked when the variable is written with a new value.
|
void |
removeVariableIfFound(String name)
Removes a variable.
|
void |
rollback()
Rolls back the variable outstanding changes.
|
void |
setLocalVersion()
Sets the variable version that subsequent reads consider.
|
String |
toString() |
void |
unregisterCallback(String variableName,
int agentInstanceId,
VariableChangeCallback variableChangeCallback)
Removes a callback.
|
void |
write(int variableNumber,
int agentInstanceId,
Object newValue)
Writes a new variable value.
|
protected static final int ROLLOVER_READER_BOUNDARY
protected static final int HIGH_WATERMARK_VERSIONS
public VariableServiceImpl(long millisecondLifetimeOldVersions,
TimeProvider timeProvider,
EventAdapterService eventAdapterService,
VariableStateHandler optionalStateHandler)
millisecondLifetimeOldVersions - number of milliseconds a version may hang around before expirytimeProvider - provides the current timeoptionalStateHandler - a optional plug-in that may store variable state and retrieve state upon creationeventAdapterService - event adaptersprotected VariableServiceImpl(int startVersion,
long millisecondLifetimeOldVersions,
TimeProvider timeProvider,
EventAdapterService eventAdapterService,
VariableStateHandler optionalStateHandler)
startVersion - the first version number to start frommillisecondLifetimeOldVersions - number of milliseconds a version may hang around before expirytimeProvider - provides the current timeoptionalStateHandler - a optional plug-in that may store variable state and retrieve state upon creationeventAdapterService - for finding event typespublic void destroy()
destroy in interface VariableServicepublic void removeVariableIfFound(String name)
VariableServiceremoveVariableIfFound in interface VariableServicename - to removepublic void setLocalVersion()
VariableServicesetLocalVersion in interface VariableServicepublic void registerCallback(String variableName, int agentInstanceId, VariableChangeCallback variableChangeCallback)
VariableServiceregisterCallback in interface VariableServicevariableName - variable nameagentInstanceId - agent instance idvariableChangeCallback - a callbackpublic void unregisterCallback(String variableName, int agentInstanceId, VariableChangeCallback variableChangeCallback)
VariableServiceunregisterCallback in interface VariableServicevariableName - variable nameagentInstanceId - agent instance idvariableChangeCallback - a callbackpublic void createNewVariable(String optionalContextName, String variableName, String variableType, boolean constant, boolean array, boolean arrayOfPrimitive, Object value, EngineImportService engineImportService) throws VariableExistsException, VariableTypeException
createNewVariable in interface VariableServiceVariableExistsExceptionVariableTypeExceptionpublic void allocateVariableState(String variableName, int agentInstanceId, StatementExtensionSvcContext extensionServicesContext, boolean isRecoveringResilient)
allocateVariableState in interface VariableServicepublic void deallocateVariableState(String variableName, int agentInstanceId)
deallocateVariableState in interface VariableServicepublic VariableMetaData getVariableMetaData(String variableName)
getVariableMetaData in interface VariableServicepublic VariableReader getReader(String variableName, int agentInstanceIdAccessor)
VariableServicegetReader in interface VariableServicevariableName - the variable that the reader should readagentInstanceIdAccessor - agent instance id of accessorpublic String isContextVariable(String variableName)
isContextVariable in interface VariableServicepublic void write(int variableNumber,
int agentInstanceId,
Object newValue)
VariableServiceMust be followed by either a commit or rollback.
write in interface VariableServicevariableNumber - the index number of the variable to write (from VariableReader)agentInstanceId - agent instance idnewValue - the new valuepublic ReadWriteLock getReadWriteLock()
VariableServicegetReadWriteLock in interface VariableServicepublic void commit()
VariableServicecommit in interface VariableServicepublic void rollback()
VariableServicerollback in interface VariableServicepublic void checkAndWrite(String variableName, int agentInstanceId, Object newValue) throws VariableValueException
VariableServiceMust be followed by either a commit or rollback.
checkAndWrite in interface VariableServicevariableName - variable nameagentInstanceId - agent instance idnewValue - the new valueVariableValueExceptionpublic Map<String,VariableReader> getVariableReadersNonCP()
VariableServicegetVariableReadersNonCP in interface VariableServicepublic ConcurrentHashMap<Integer,VariableReader> getReadersPerCP(String variableName)
getReadersPerCP in interface VariableService