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Using IMetadataInfo::GetModuleContextContainer to store configuration data

If you write an IIS7 module, you would typically want to control its behavior based on some configuration data. IIS7 makes it really easy for developers to extend configuration schema so that their customers can put configuration data for their modules with other IIS configuration in applicationHost.config and web.config. Native module developers can then use AhAdmin APIs to read this configuration data. Reading configuration data for each request can be expensive. Developers would typically think of improving the performance by reading the configuration data once and then keeping the data in memory which can be used for other requests. To do this right, you need to worry about following things.
  • Keep unique data only for the paths where configuration changes and not for all possible configuration paths.
  • Detect configuration changes so that module always behave as per the latest configuration.
  • Only delete the configuration data of paths which are affected by the change and not throw everything.
  • Make sure this configuration store is accessed in thread safe manner.
Instead of implementing this from scratch, you can use IMetadata interface available in httpserv.h. Call IHttpContext::GetMetadata() to get IMetadata object which is different for each unique configuration path and then use the IMetadataInfo::GetModuleContextContainer()->SetModuleContext() to store any data you like for your module. Configuration data is a perfect example of kind of data you would like to store for a unique configuration path. Stored data can be retrieved using IMetadataInfo::GetModuleContextContainer()->GetModuleContext(). In addition to making sure that a unique metadata object is available only for paths where configuration changes, IIS will also take care of deleting proper stored contexts when a change notification arrives. Here is how doing this will look like in code.

//
// Globals
//
HTTP_MODULE_ID g_pModuleContext = NULL;
IHttpServer *  g_pGlobalInfo    = NULL;

HRESULT
WINAPI
RegisterModule(
    DWORD                          dwServerVersion,
    IHttpModuleRegistrationInfo *  pModuleInfo,
    IHttpServer *                  pGlobalInfo
)
{
    ...

    g_pGlobalInfo    = pGlobalInfo;
    g_pModuleContext = pModuleInfo->GetId( );

    ...
}

class MODULE_CONFIG : public IHttpStoredContext
{
public:

    //
    // Always call this method to get configuration data
    //
    static
    HRESULT
    GetModuleConfig(
        IHttpContext *   pContext,
        MODULE_CONFIG ** ppModuleConfig
    );

    HRESULT
    Initialize(
        IHttpContext * pContext
    );

    // virtual
    VOID
    CleanupStoredContext(
        VOID
    )
    {
        delete this;
    }

    //
    // Stored Configuration Data
    //
};

//static
HRESULT
MODULE_CONFIG::GetModuleConfig(
    IHttpContext *   pContext,
    MODULE_CONFIG ** ppModuleConfig
)
{
    HRESULT                       hr                 = S_OK;
    MODULE_CONFIG *               pModuleConfig      = NULL;
    IHttpModuleContextContainer * pMetadataContainer = NULL;

    pMetadataContainer = pContext->GetMetadata()->GetModuleContextContainer();
    pModuleConfig = (MODULE_CONFIG *)pMetadataContainer->GetModuleContext( g_pModuleContext );

    if ( pModuleConfig != NULL )
    {
        //
        // We found stored data for this module for the metadata
        // object which is different for unique configuration path
        //
        *ppModuleConfig = pModuleConfig;
        return S_OK;
    }

    //
    // If we reach here, that means this is first request or first
    // request after a configuration change IIS core will throw stored context
    // if a change notification arrives for this metadata path
    //
    pModuleConfig = new MODULE_CONFIG();
    if ( pModuleConfig == NULL )
    {
        return E_OUTOFMEMORY;
    }

    //
    // Read module configuration data and store in MODULE_CONFIG
    //
    hr = pModuleConfig->Initialize( pContext );
    if ( FAILED( hr ) )
    {
        pModuleConfig->CleanupStoredContext();
        pModuleConfig = NULL;

        return hr;
    }

    //
    // Store MODULE_CONFIG data as metadata stored context
    //
    hr = pMetadataContainer->SetModuleContext( pModuleConfig,
                                               g_pModuleContext );
    if ( FAILED( hr ) )
    {
        pModuleConfig->CleanupStoredContext();
        pModuleConfig = NULL;

        //
        // It is possible that some other thread stored context before this thread
        // could do. Check returned hr and return context stored by other thread
        //
        if ( hr == HRESULT_FROM_WIN32( ERROR_ALREADY_ASSIGNED ) )
        {
            *ppModuleConfig = (MODULE_CONFIG *)pMetadataContainer->GetModuleContext( g_pModuleContext );
            return S_OK;
        }
    }

    *ppModuleConfig = pModuleConfig;
    return hr;
}

HRESULT
MODULE_CONFIG::Initialize(
    IHttpContext * pContext
)
{
    HRESULT                hr               = S_OK;
    IAppHostAdminManager * pAdminManager    = NULL;
    IAppHostElement *      pAppHostElement  = NULL;
    BSTR                   bstrSectionName  = NULL;
    BSTR                   bstrConfigPath   = NULL;
    ...

    pAdminManager = g_pGlobalInfo->GetAdminManager();
    ...

    //
    // Read configuration data at metapath of IMetadataInfo object
    //
    bstrConfigPath = SysAllocString( pContext->GetMetadata()->GetMetaPath() );
    ...

    hr = pAdminManager->GetAdminSection( bstrSectionName,
                                         bstrConfigPath,
                                         &pAppHostElement);
    ...
}

Hope this helps.
Kanwal

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