com.bigdata.rdf.sail.webapp.lbs

Class AbstractLBSPolicy

java.lang.Object

com.bigdata.rdf.sail.webapp.lbs.AbstractLBSPolicy

All Implemented Interfaces:

QuorumListener, IHALoadBalancerPolicy, IHAPolicyLifeCycle, Serializable

Direct Known Subclasses:

AbstractHostLBSPolicy, NOPLBSPolicy, RoundRobinLBSPolicy

public abstract class AbstractLBSPolicy
extends Object
implements IHALoadBalancerPolicy, QuorumListener, Serializable

Abstract base class establishes a listener for quorum events, tracks the services that are members of the quorum, and caches metadata about those services (especially the requestURL at which they will respond).

Author:

Bryan Thompson

See Also:

Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	AbstractLBSPolicy.InitParams

Field Summary

Fields

Modifier and Type	Field and Description
protected AtomicReference<WeakReference<IHAJournal>>	journalRef A WeakReference to the HAJournal avoids pinning the HAJournal.
protected AtomicReference<UUID>	serviceIDRef The UUID of the HAJournalServer.
protected AtomicReference<ServiceScore[]>	serviceTableRef This is the table of known services.

Constructor Summary

Constructors

Constructor and Description
AbstractLBSPolicy()

Method Summary

Methods

Modifier and Type	Method and Description
protected boolean	conditionallyForwardReadRequest(HALoadBalancerServlet servlet, javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) Hook provides the IHALoadBalancerPolicy with an opportunity to forward a read-request to the local service rather than proxying the request to a service selected by the load balancer (a local forward has less overhead than proxying to either the local host or a remote service, which makes it more efficient under some circumstances to handle the read-request on the service where it was originally received).
protected void	conditionallyUpdateServiceTable() Conditionally update the serviceTableRef iff it does not exist or is empty.
void	destroy() Destroy the policy (stop any asynchronous processing, release any resources).
protected IHAJournal	getJournal() Return the cached reference to the HAJournal.
String	getLeaderURI(javax.servlet.http.HttpServletRequest request) Return the Request-URI to which a non-idempotent request will be proxied.
protected ServiceScore	getLocalServiceScore() Return the ServiceScore for the HAGlue service running on this host within this webapp.
protected ServiceScore	getServiceScoreForHostname(String hostname) Return the first service found for the indicated host.
void	init(javax.servlet.ServletConfig servletConfig, IIndexManager indexManager) Initialize the policy.
void	notify(QuorumEvent e) Notify the client of a quorum event.
boolean	service(boolean isLeaderRequest, HALoadBalancerServlet servlet, javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) Invoked for each request.
String	toString() Note: implementation is non-blocking!
protected void	toString(StringBuilder sb) Extension hook for toString() - implementation MUST NOT block.
protected void	updateServiceTable() Update the per-service table.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait

Methods inherited from interface com.bigdata.rdf.sail.webapp.lbs.IHALoadBalancerPolicy

getReaderURI

Field Detail

journalRef

protected final AtomicReference<WeakReference<IHAJournal>> journalRef

A WeakReference to the HAJournal avoids pinning the HAJournal.

serviceIDRef

protected final AtomicReference<UUID> serviceIDRef

The UUID of the HAJournalServer.

serviceTableRef

protected final AtomicReference<ServiceScore[]> serviceTableRef

This is the table of known services. We can scan the table for a service UUID and then forward a request to the pre-computed requestURL associated with that UUID. If the requestURL is null then we do not know how to reach that service and can not proxy the request.

Constructor Detail

AbstractLBSPolicy

public AbstractLBSPolicy()

Method Detail

toString

public String toString()

Note: implementation is non-blocking!

Overrides:

toString in class Object

toString

protected void toString(StringBuilder sb)

Extension hook for toString() - implementation MUST NOT block.

Parameters:

sb - Buffer where you can write additional state.

getJournal

protected IHAJournal getJournal()

Return the cached reference to the HAJournal.

Returns:

The reference or null iff the reference has been cleared or has not yet been set.

destroy

public void destroy()

Description copied from interface: IHAPolicyLifeCycle

Destroy the policy (stop any asynchronous processing, release any resources).

Specified by:

destroy in interface IHAPolicyLifeCycle

init

public void init(javax.servlet.ServletConfig servletConfig,
        IIndexManager indexManager)
          throws javax.servlet.ServletException

Description copied from interface: IHAPolicyLifeCycle

Initialize the policy.

Specified by:

init in interface IHAPolicyLifeCycle

Throws:

javax.servlet.ServletException

service

public boolean service(boolean isLeaderRequest,
              HALoadBalancerServlet servlet,
              javax.servlet.http.HttpServletRequest request,
              javax.servlet.http.HttpServletResponse response)
                throws javax.servlet.ServletException,
                       IOException

Description copied from interface: IHALoadBalancerPolicy

Invoked for each request. If the response is not committed, then it will be handled by the HALoadBalancerServlet.

Specified by:

service in interface IHALoadBalancerPolicy

Parameters:

isLeaderRequest - true iff this request must be directed to the leaeder and false iff this request may be load balanced over the joined services. UPDATEs MUST be handled by the leader. Read requests can be handled by any service that is joined with the met quorum.

servlet - The HALoadBalancerServlet. This is exposed in order to allow the IHALoadBalancerPolicy to perform a local forward of the request.

request - The request.

response - The response.

Returns:

true iff the request was handled.

Throws:

javax.servlet.ServletException

IOException

conditionallyForwardReadRequest

protected boolean conditionallyForwardReadRequest(HALoadBalancerServlet servlet,
                                      javax.servlet.http.HttpServletRequest request,
                                      javax.servlet.http.HttpServletResponse response)
                                           throws IOException

Hook provides the IHALoadBalancerPolicy with an opportunity to forward a read-request to the local service rather than proxying the request to a service selected by the load balancer (a local forward has less overhead than proxying to either the local host or a remote service, which makes it more efficient under some circumstances to handle the read-request on the service where it was originally received).

Throws:

IOException

getLeaderURI

public final String getLeaderURI(javax.servlet.http.HttpServletRequest request)

Return the Request-URI to which a non-idempotent request will be proxied.

This implementation rewrites the requestURL such that the request will be proxied to the quorum leader.

Specified by:

getLeaderURI in interface IHALoadBalancerPolicy

Parameters:

request - The request.

Returns:

The proxyTo Request-URI -or- null if we could not find a service to which we could proxy this request.

getLocalServiceScore

protected ServiceScore getLocalServiceScore()

Return the ServiceScore for the HAGlue service running on this host within this webapp.

getServiceScoreForHostname

protected ServiceScore getServiceScoreForHostname(String hostname)

Return the first service found for the indicated host.

Parameters:

hostname - The hostname.

Returns:

The first service for that host.

notify

public void notify(QuorumEvent e)

Notify the client of a quorum event.

The listener MUST NOT take any event that could block. In particular, it MUST NOT wait on another thread that will access the Quorum as that will cause a deadlock around the internal lock maintained by the Quorum.

The services table is updated if a services joins or leaves the quorum. FIXME The QuorumListener is unregistered by AbstractQuorum.terminate(). This happens any time the HAJournalServer goes into the error state. When this occurs, we stop getting QuorumEvents and the policy stops being responsive (it can not proxy the request to a service that is still up because it does not know what services are up, or maybe it just can not learn if services go down).

We probably need to either NOT clear the quorum listener and/or add an event type that is sent when Quorum.terminate() is called and/or use our own listener (independent of the HAJournalServer, which would require us to use an HAClient).

This should be robust even when the HAQuorumService is not running. We do not want to be unable to proxy to another service just because this one is going through an error state. Would it make more sense to have a 2nd Quorum object for this purpose - one that is not started and stopped by the HAJournalServer?

Specified by:

notify in interface QuorumListener

See Also:

(HAJournal start() delay)

conditionallyUpdateServiceTable

protected void conditionallyUpdateServiceTable()

Conditionally update the serviceTableRef iff it does not exist or is empty.

updateServiceTable

protected void updateServiceTable()

Update the per-service table.

See Also:

serviceTableRef