com.espertech.esper.epl.variable
Class VariableServiceImpl

java.lang.Object
  extended by com.espertech.esper.epl.variable.VariableServiceImpl
All Implemented Interfaces:
VariableService

public class VariableServiceImpl
extends Object
implements VariableService

Variables service for reading and writing variables, and for setting a version number for the current thread to consider variables for.

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:

  1. lock-less read of the current and prior version, locked reads for older versions
  2. atomicity by keeping multiple versions for each variable and a threadlocal that receives the current version each call
  3. one write lock for all variables (required to coordinate with single global version number), however writes are very fast (entry to collection plus increment an int) and therefore blocking should not be an issue

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.


Field Summary
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.
protected static int ROLLOVER_WRITER_BOUNDARY
          Sets the boundary above which a write transaction rolls over all variable's version lists.
 
Constructor Summary
protected VariableServiceImpl(int startVersion, long millisecondLifetimeOldVersions, TimeProvider timeProvider, VariableStateHandler optionalStateHandler)
          Ctor.
  VariableServiceImpl(long millisecondLifetimeOldVersions, TimeProvider timeProvider, VariableStateHandler optionalStateHandler)
          Ctor.
 
Method Summary
 void checkAndWrite(int variableNumber, Object newValue)
          Check type of the value supplied and writes the new variable value.
 void commit()
          Commits the variable outstanding changes.
 void createNewVariable(String variableName, Class type, Object value, StatementExtensionSvcContext extensionServicesContext)
          Creates a new variable.
 VariableReader getReader(String variableName)
          Returns a reader that provides access to variable values.
 ReadWriteLock getReadWriteLock()
          Lock for use in atomic writes to the variable space.
 Map<String,VariableReader> getVariables()
          Returns a map of variable name and reader, for thread-safe iteration.
 void registerCallback(int variableNumber, VariableChangeCallback variableChangeCallback)
          Registers a callback invoked when the variable is written with a new value.
 void rollback()
          Rolls back the variable outstanding changes.
 void setLocalVersion()
          Sets the variable version that subsequent reads consider.
 String toString()
           
 void write(int variableNumber, Object newValue)
          Writes a new variable value.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait
 

Field Detail

ROLLOVER_READER_BOUNDARY

protected static final int ROLLOVER_READER_BOUNDARY
Sets the boundary above which a reader considers the high-version list of variable values. For use in roll-over when the current version number overflows the ROLLOVER_WRITER_BOUNDARY.

See Also:
Constant Field Values

ROLLOVER_WRITER_BOUNDARY

protected static final int ROLLOVER_WRITER_BOUNDARY
Sets the boundary above which a write transaction rolls over all variable's version lists.

See Also:
Constant Field Values

HIGH_WATERMARK_VERSIONS

protected static final 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.

See Also:
Constant Field Values
Constructor Detail

VariableServiceImpl

public VariableServiceImpl(long millisecondLifetimeOldVersions,
                           TimeProvider timeProvider,
                           VariableStateHandler optionalStateHandler)
Ctor.

Parameters:
millisecondLifetimeOldVersions - number of milliseconds a version may hang around before expiry
timeProvider - provides the current time
optionalStateHandler - a optional plug-in that may store variable state and retrieve state upon creation

VariableServiceImpl

protected VariableServiceImpl(int startVersion,
                              long millisecondLifetimeOldVersions,
                              TimeProvider timeProvider,
                              VariableStateHandler optionalStateHandler)
Ctor.

Parameters:
startVersion - the first version number to start from
millisecondLifetimeOldVersions - number of milliseconds a version may hang around before expiry
timeProvider - provides the current time
optionalStateHandler - a optional plug-in that may store variable state and retrieve state upon creation
Method Detail

setLocalVersion

public void setLocalVersion()
Description copied from interface: VariableService
Sets the variable version that subsequent reads consider.

Specified by:
setLocalVersion in interface VariableService

registerCallback

public void registerCallback(int variableNumber,
                             VariableChangeCallback variableChangeCallback)
Description copied from interface: VariableService
Registers a callback invoked when the variable is written with a new value.

Specified by:
registerCallback in interface VariableService
Parameters:
variableNumber - the variable index number
variableChangeCallback - a callback

createNewVariable

public void createNewVariable(String variableName,
                              Class type,
                              Object value,
                              StatementExtensionSvcContext extensionServicesContext)
                       throws VariableExistsException,
                              VariableTypeException
Description copied from interface: VariableService
Creates a new variable.

Specified by:
createNewVariable in interface VariableService
Parameters:
variableName - name of the variable
type - variable type
value - initialization value; String values are allowed and parsed according to type
extensionServicesContext - is extensions for implementing resilience attributes of variables
Throws:
VariableExistsException - if the variable name is already in use
VariableTypeException - if the variable type cannot be recognized

getReader

public VariableReader getReader(String variableName)
Description copied from interface: VariableService
Returns a reader that provides access to variable values. The reader considers the version currently set via setLocalVersion.

Specified by:
getReader in interface VariableService
Parameters:
variableName - the variable that the reader should read
Returns:
reader

write

public void write(int variableNumber,
                  Object newValue)
Description copied from interface: VariableService
Writes a new variable value.

Must be followed by either a commit or rollback.

Specified by:
write in interface VariableService
Parameters:
variableNumber - the index number of the variable to write (from VariableReader)
newValue - the new value

getReadWriteLock

public ReadWriteLock getReadWriteLock()
Description copied from interface: VariableService
Lock for use in atomic writes to the variable space.

Specified by:
getReadWriteLock in interface VariableService
Returns:
read write lock for external coordinated write

commit

public void commit()
Description copied from interface: VariableService
Commits the variable outstanding changes.

Specified by:
commit in interface VariableService

rollback

public void rollback()
Description copied from interface: VariableService
Rolls back the variable outstanding changes.

Specified by:
rollback in interface VariableService

checkAndWrite

public void checkAndWrite(int variableNumber,
                          Object newValue)
                   throws VariableValueException
Description copied from interface: VariableService
Check type of the value supplied and writes the new variable value.

Must be followed by either a commit or rollback.

Specified by:
checkAndWrite in interface VariableService
Parameters:
variableNumber - the index number of the variable to write (from VariableReader)
newValue - the new value
Throws:
VariableValueException

toString

public String toString()
Overrides:
toString in class Object

getVariables

public Map<String,VariableReader> getVariables()
Description copied from interface: VariableService
Returns a map of variable name and reader, for thread-safe iteration.

Specified by:
getVariables in interface VariableService
Returns:
variable names and readers

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