com.bigdata.rdf.sail.webapp

Class HALoadBalancerServlet

java.lang.Object

javax.servlet.GenericServlet

javax.servlet.http.HttpServlet

org.eclipse.jetty.proxy.AbstractProxyServlet

org.eclipse.jetty.proxy.ProxyServlet

com.bigdata.rdf.sail.webapp.HALoadBalancerServlet

All Implemented Interfaces:

Serializable, javax.servlet.Servlet, javax.servlet.ServletConfig

public class HALoadBalancerServlet
extends org.eclipse.jetty.proxy.ProxyServlet

The HA Load Balancer servlet provides a transparent proxy for requests arriving its configured URL pattern (the "external" interface for the load balancer) to the root of the web application.

When successfully deployed, requests having prefix corresponding to the URL pattern for the load balancer (typically, "/bigdata/LBS/*") are automatically redirected to a joined service in the met quorum based on the configured load balancer policy.

The use of the load balancer is entirely optional. If the load balancer is not properly configured, then it will simply rewrite itself out of any request and the request will be handled by the host to which it was directed (no proxying).

Note: If the security rules permit, then clients MAY make requests directly against a specific service.

The load balancer policies are "HA aware." They will always redirect update requests to the quorum leader. Read requests will be directed to one of the services that is joined with the met quorum.

Author:

Bryan Thompson

See Also:

HA Load Balancer , Serialized Form

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static interface	HALoadBalancerServlet.InitParams

Nested classes/interfaces inherited from class org.eclipse.jetty.proxy.ProxyServlet

org.eclipse.jetty.proxy.ProxyServlet.ProxyInputStreamContentProvider, org.eclipse.jetty.proxy.ProxyServlet.ProxyResponseListener, org.eclipse.jetty.proxy.ProxyServlet.Transparent

Nested classes/interfaces inherited from class org.eclipse.jetty.proxy.AbstractProxyServlet

org.eclipse.jetty.proxy.AbstractProxyServlet.TransparentDelegate

Field Summary

Fields inherited from class org.eclipse.jetty.proxy.AbstractProxyServlet

_log, CLIENT_REQUEST_ATTRIBUTE, HOP_HEADERS

Constructor Summary

Constructors

Constructor and Description
HALoadBalancerServlet()

Method Summary

Methods

Modifier and Type	Method and Description
void	destroy()
protected URI	doRewriteURI(javax.servlet.http.HttpServletRequest request) Hook allows the servlet to rewrite the request.
void	forwardToLocalService(boolean isLeaderRequest, javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) Strip off the /LBS prefix from the requestURI and forward the request to the servlet at the resulting requestURI.
static String	getConfigParam(javax.servlet.ServletConfig servletConfig, Class<?> owningClass, String name, String def) Return the configured value of the named parameter.
IHALoadBalancerPolicy	getLBSPolicy() Return the current IHALoadBalancerPolicy.
void	init()
static <T> T	newInstance(javax.servlet.ServletConfig servletConfig, Class<?> owningClass, Class<? extends T> iface, String name, String def) Create an instance of some type based on the servlet init parameters.
protected void	onProxyResponseFailure(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response, org.eclipse.jetty.client.api.Response proxyResponse, Throwable failure)
protected void	onProxyRewriteFailed(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) Note: This offers an opportunity to handle a failure where we were unable to rewrite the request to some service, e.g., because the quorum is not met.
protected String	rewriteTarget(javax.servlet.http.HttpServletRequest request) Wrapper invokes doRewriteURI(HttpServletRequest) and handles any thrown exceptions.
protected void	service(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response)
void	setLBSPolicy(IHALoadBalancerPolicy newValue) Change the IHALoadBalancerPolicy associated with this instance of this servlet.
static void	setLBSPolicy(javax.servlet.ServletContext servletContext, IHALoadBalancerPolicy newValue) Set the current IHALoadBalancerPolicy for all HALoadBalancerServlet instances for the caller specified ServletContext.
void	setRewriter(IHARequestURIRewriter newValue) Change the IHARequestURIRewriter associated with this instance of this servlet.
String	toString()
static String	toString(javax.servlet.ServletContext servletContext) Return the IHALoadBalancerPolicys that are in force for the active HALoadBalancerServlet instances.

Methods inherited from class org.eclipse.jetty.proxy.ProxyServlet

newProxyResponseListener, onContinue, onResponseContent, proxyRequestContent

Methods inherited from class org.eclipse.jetty.proxy.AbstractProxyServlet

addProxyHeaders, addViaHeader, addXForwardedHeaders, copyRequestHeaders, createHttpClient, createLogger, expects100Continue, filterServerResponseHeader, findConnectionHeaders, getBlackListHosts, getHostHeader, getHttpClient, getRequestId, getTimeout, getViaHost, getWhiteListHosts, hasContent, newHttpClient, onClientRequestFailure, onProxyResponseSuccess, onServerResponseHeaders, sendProxyRequest, sendProxyResponseError, setTimeout, validateDestination

Methods inherited from class javax.servlet.http.HttpServlet

doDelete, doGet, doHead, doOptions, doPost, doPut, doTrace, getLastModified, service

Methods inherited from class javax.servlet.GenericServlet

getInitParameter, getInitParameterNames, getServletConfig, getServletContext, getServletInfo, getServletName, init, log, log

Methods inherited from class java.lang.Object

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

Constructor Detail

HALoadBalancerServlet

public HALoadBalancerServlet()

Method Detail

setLBSPolicy

public void setLBSPolicy(IHALoadBalancerPolicy newValue)

Change the IHALoadBalancerPolicy associated with this instance of this servlet. The new policy will be installed iff it can be initialized successfully. The old policy will be destroyed iff the new policy is successfully installed.

Parameters:

newValue - The new value (required).

getLBSPolicy

public IHALoadBalancerPolicy getLBSPolicy()

Return the current IHALoadBalancerPolicy.

setRewriter

public void setRewriter(IHARequestURIRewriter newValue)

Change the IHARequestURIRewriter associated with this instance of this servlet. The new policy will be installed iff it can be initialized successfully. The old policy will be destroyed iff the new policy is successfully installed.

Parameters:

newValue - The new value (required).

init

public void init()
          throws javax.servlet.ServletException

Extended to setup the as-configured IHALoadBalancerPolicy.

Note: If the deployment is does not support HA replication (e.g., either not HA or HA with replicationFactor:=1), then we still want to be able to forward to the local service.

Overrides:

init in class org.eclipse.jetty.proxy.AbstractProxyServlet

Throws:

javax.servlet.ServletException

See Also:

Cannot run queries in LBS mode with HA1 setup

setLBSPolicy

public static void setLBSPolicy(javax.servlet.ServletContext servletContext,
                IHALoadBalancerPolicy newValue)

Set the current IHALoadBalancerPolicy for all HALoadBalancerServlet instances for the caller specified ServletContext.

Parameters:

servletContext - The ServletContext.

newValue - The new IHALoadBalancerPolicy.

Throws:

IllegalArgumentException - if the new policy is null.

toString

public static String toString(javax.servlet.ServletContext servletContext)

Return the IHALoadBalancerPolicys that are in force for the active HALoadBalancerServlet instances.

Parameters:

servletContext - The ServletContext.

Returns:

The IHALoadBalancerPolicy[] -or- null if there are no HALoadBalancerServlets.

toString

public String toString()

Overrides:

toString in class Object

destroy

public void destroy()

Extended to destroy the as-configured IHALoadBalancerPolicy.

Specified by:

destroy in interface javax.servlet.Servlet

Overrides:

destroy in class org.eclipse.jetty.proxy.AbstractProxyServlet

getConfigParam

public static String getConfigParam(javax.servlet.ServletConfig servletConfig,
                    Class<?> owningClass,
                    String name,
                    String def)

Return the configured value of the named parameter. This method checks the environment variables first for a fully qualified value for the parameter using owningClass.name (or just name if owningClass is null). If no value is found for that variable, it checks the ServletContext for name (if the owningClass:=HALoadBalancerServlet) and otherwise for owningClass.name. If no value is found again, it returns the default value specified by the caller. This makes it possible to configure the behavior of the HALoadBalancerServlet using environment variables.

Parameters:

servletConfig - The ServletConfig.

owningClass - The class that declares the init-param (required). This serves as a namespace when searching the environment variables. This is also used to impose a namespace when searching web.xml when owningClass!=HALoadBalancerServlet.

name - The name of the servlet init-param.

def - The default value for the servlet init-param.

Returns:

newInstance

public static <T> T newInstance(javax.servlet.ServletConfig servletConfig,
                Class<?> owningClass,
                Class<? extends T> iface,
                String name,
                String def)
                     throws javax.servlet.ServletException

Create an instance of some type based on the servlet init parameters.

Note: The configuration parameter MAY also be specified as com.bigdata.rdf.sail.webapp.HALoadBalancerServlet.name.

Parameters:

servletConfig - The ServletConfig.

owningClass - The class that declares the init-param. This serves as a namespace when searching the environment variables.

iface - The interface that the type must implement.

name - The name of the servlet init parameter.

def - The default value for the servlet init parameter.

Returns:

The instance of the configured type.

Throws:

javax.servlet.ServletException - if anything goes wrong.

service

protected void service(javax.servlet.http.HttpServletRequest request,
           javax.servlet.http.HttpServletResponse response)
                throws javax.servlet.ServletException,
                       IOException

Overrides:

service in class org.eclipse.jetty.proxy.ProxyServlet

Throws:

javax.servlet.ServletException

IOException

forwardToLocalService

public void forwardToLocalService(boolean isLeaderRequest,
                         javax.servlet.http.HttpServletRequest request,
                         javax.servlet.http.HttpServletResponse response)
                           throws IOException

Strip off the /LBS prefix from the requestURI and forward the request to the servlet at the resulting requestURI. This forwarding effectively disables the LBS but still allows requests which target the LBS to succeed against the webapp on the same host.

Note: If the deployment is does not support HA replication (e.g., either not HA or HA with replicationFactor:=1), then we still want to be able to forward to the local service.

Parameters:

request - The request.

response - The response.

Throws:

IOException

javax.servlet.ServletException

See Also:

Cannot run queries in LBS mode with HA1 setup

rewriteTarget

protected final String rewriteTarget(javax.servlet.http.HttpServletRequest request)

Wrapper invokes doRewriteURI(HttpServletRequest) and handles any thrown exceptions.

Overrides:

rewriteTarget in class org.eclipse.jetty.proxy.AbstractProxyServlet

See Also:

doRewriteURI(HttpServletRequest)

doRewriteURI

protected URI doRewriteURI(javax.servlet.http.HttpServletRequest request)

Hook allows the servlet to rewrite the request. For update requests, rewrite the requestURL to the service that is the quorum leader. For read requests, rewrite the requestURL to the service having the least load.

Parameters:

request - The request.

Returns:

Return the URI if the request should be proxied to another service -or- null if the request should be locally forwarded.

onProxyRewriteFailed

protected void onProxyRewriteFailed(javax.servlet.http.HttpServletRequest request,
                        javax.servlet.http.HttpServletResponse response)

Note: This offers an opportunity to handle a failure where we were unable to rewrite the request to some service, e.g., because the quorum is not met. The implementation is overridden to forward the request to the local service. The local service will then generate an appropriate HTTP error response.

Overrides:

onProxyRewriteFailed in class org.eclipse.jetty.proxy.AbstractProxyServlet

onProxyResponseFailure

protected void onProxyResponseFailure(javax.servlet.http.HttpServletRequest request,
                          javax.servlet.http.HttpServletResponse response,
                          org.eclipse.jetty.client.api.Response proxyResponse,
                          Throwable failure)

Overridden to provide more information about the error. The implementation is derived from the jetty 9.1.4 implementation of the method in the base ProxyServlet class, but logs @ ERROR so we can see more about the underlying problem.

Overrides:

onProxyResponseFailure in class org.eclipse.jetty.proxy.AbstractProxyServlet

See Also:

HA LBS Gateway errors under heavy load TODO jetty 9.2 provides a fully asynchronous proxy servlet. We will wind up replacing our base class with that implementation soon, probably for the 1.3.2 release. Until then, this will provide additional diagnoistic information about the root causes when there is a gateway error (proxying fails). If we can find some patterns to these failures, then it would be useful to recharacterize more of them to encourage the client to retry the request. Those semantics are not really available for 502 (Bad Gateway). They are more a appropriate for both 503 (Service Unavailable - temporary overload), and 504 (Gateway Timeout). 503 might be the best choice if there is not an explicit timeout and the root cause does not clearly indicate a durable problem with the target host.