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IWMSCacheProxy::QueryCacheMissPolicy

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IWMSCacheProxy::QueryCacheMissPolicy

The server calls the QueryCacheMissPolicy method to query the cache plug-in for a cache-miss policy.

Syntax

  HRESULT QueryCacheMissPolicy(
  
  BSTR
  
  bstrOriginUrl
  ,
  
  IWMSContext*
  
  pUserContext
  ,
  
  IWMSCommandContext*
  
  pCommandContext
  ,
  
  IWMSContext*
  
  pPresentationContext
  ,
  IUnknown*  
  pCachePluginContext
  ,
  
  long
  
  lQueryType
  ,
  
  IWMSCacheProxyCallback*
  
  pCallback
  ,
  
  VARIANT
  
  varContext
  
  );

Parameters

bstrOriginUrl

[in] BSTR containing the origin URL.

pUserContext

[in] Pointer to an IWMSContext interface containing the user context.

pCommandContext

[in] Pointer to an IWMSCommandContext interface containing the command context.

pPresentationContext

[in] Pointer to an IWMSContext interface containing the presentation context.

pCachePluginContext

[in] Pointer to an IUnknown interface containing a cache plug-in context.

lQueryType

[in] Member of the WMS_CACHE_QUERY_TYPE_FLAGS enumeration type that indicates why the server called IWMSCacheProxy::QueryCache. This must be a bitwise OR of one or more of the following values.

Value Description
WMS_CACHE_QUERY_OPEN A client using a downstream proxy requested content.
WMS_CACHE_QUERY_GET_CONTENT_INFO A downstream proxy requested information about content cached on the remote computer.
WMS_CACHE_QUERY_CACHE_EVENT A cache event notice is being sent upstream. If WMS_CACHE_QUERY_LOCAL_EVENT is set, the cache event was generated by the local computer. Otherwise, it was sent by a downstream proxy server.
WMS_CACHE_QUERY_REVERSE_PROXY A downstream server is configured to be a reverse proxy server. If a cache proxy plug-in supports reverse proxy, it can use this flag to determine whether it must map client requests to an upstream server farm.
WMS_CACHE_QUERY_LOCAL_EVENT The local server is generating an event to send upstream.

pCallback

[in] Pointer to an IWMSCacheProxyCallback interface. The cache plug-in calls IWMSCacheProxyCallback::OnQueryCacheMissPolicy to respond to a call to QueryCacheMissPolicy.

varContext

[in] VARIANT containing a value defined by the server to identify which call to QueryCacheMissPolicy the plug-in is responding to when it calls IWMSCacheProxyCallback::OnQueryCacheMissPolicy. The plug-in must pass this value back unaltered.

Return Values

If the method succeeds, the plug-in must return S_OK. To report an error, the plug-in can return any HRESULT other than S_OK. If the plug-in uses the IWMSEventLog interface to log error information directly to the Windows Event Viewer, it is recommended that it return NS_E_PLUGIN_ERROR_REPORTED. Typically, the server attempts to make plug-in error information available to the server object model, the Windows Event Viewer, and the troubleshooting list in the details pane of the Windows Media Services MMC. However, if the plug-in uses the IWMSEventLog interface to send custom error information to the Windows Event Viewer, returning NS_E_PLUGIN_ERROR_REPORTED stops the server from also logging to the event viewer. For more information about retrieving plug-in error information, see Identifying Plug-in Errors.

Remarks

A proxy server must call QueryCache on the cache plug-in and receive a cache miss before it calls QueryCacheMissPolicy. A custom cache plug-in can use a WMS_CACHE_QUERY_TYPE_FLAGS enumeration value in the lQueryType parameter to direct implementation of a cache-miss policy.

Example Code

The following example illustrates one way that you can implement the QueryCacheMissPolicy method. How you choose to implement this method depends on the cache-miss policy that you want to create.

HRESULT STDMETHODCALLTYPE 

CCachePlugin::QueryCacheMissPolicy( BSTR bstrOriginUrl,
                                    IWMSContext *pUserContext,
                                    IWMSCommandContext *pCommandContext,
                                    IWMSContext *pPresentationContext,
                                    IUnknown *pCachePluginContext,
                                    long QueryType, 
                                    IWMSCacheProxyCallback *pCallback,
                                    VARIANT varContext
                                    )
{
    HRESULT hr = S_OK;

    // If the proxy is configured as a reverse proxy,
    // instruct the server to skip this plug-in.
    if( QueryType & WMS_CACHE_QUERY_REVERSE_PROXY )
    {
        hr = pCallback->OnQueryCacheMissPolicy( 
                            S_OK,
                            WMS_CACHE_QUERY_MISS_SKIP,
                            NULL,
                            NULL,
                            NULL,
                            varContext );
        return( hr );
    }

    // The downstream proxy has either raised an event or requested
    // information about content it has cached. Forward the request
    // to an upstream server.
    if( ( QueryType & WMS_CACHE_QUERY_CACHE_EVENT ) || 
        ( QueryType & WMS_CACHE_QUERY_GET_CONTENT_INFO ) )
    {
        hr = pCallback->OnQueryCacheMissPolicy( 
                S_OK,
                WMS_CACHE_QUERY_MISS_FORWARD_REQUEST,
                NULL,
                NULL,
                NULL,
                varContext );         
        return( hr );
    }

    // Call GetContentInformation().       
    hr = pCallback->GetContentInformation(
                       bstrOriginUrl,
                       pPresentationContext,
                       this,
                       (IWMSCacheProxyServerCallback *) this,
                       varContext
                       );
    return( S_OK );
}

Requirements

Header: streamcache.h.

Library: WMSServerTypeLib.dll.

Platform: Windows Server 2003, Enterprise Edition; Windows Server 2003, Datacenter Edition; Windows Server 2008.

See Also

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