openVXI的缓存实现机制

DocumentParser的FetchDocument方法的声明如下：

int DocumentParser::FetchDocument(const VXIchar * url,

const VXIMapHolder & properties,

VXIinetInterface * inet,

VXIcacheInterface * cache,

SimpleLogger & log,

VXMLDocument & document,

VXIMapHolder & docProperties,

bool isDefaults,

bool isRootApp,

VXIbyte **content,

VXIulong *size);

很明显声明用到了VXIcacheInterface，但是继续阅读代码，却没有发现使用了参数cache。就是参数中虽有VXIcacheInterface，但是函数中并没有用到。浏览一下函数FetchDocument，里面有这样的注释：

// (1) Load the VXML DTD for validation. This will override an externally

// specified DTD if the user provides a link.

// (2) Load the url into memory.

// (3) Pull the document from cache.

// (4) Not in cache; parse buffer into our VXML document representation

// (5) Write the compiled form out to the cache.

// (6) Parse was successful, process document. W e want only the top level node.

看一下// (3) Pull the document from cache.下面的代码，会发现是从一个叫DocumentStorageSingleton的变量里面。

DocumentStorageSingleton::Instance()->Retrieve(doc, buffer, bufSize, docURL.c_str())

看一下DocumentStorageSingleton里面有个成员变量DOC_STORAGE storage;其实是map类型的。注释所指的cache就是DocumentStorageSingleton。也就是说DocumentParser用到的cache的功能是由DocumentSt orageSingleton实现的。

看一下VXIcache.h开头的注释：

/* Abstract interface for accessing caching functionality, which

* permits writing arbitrary data into the cache with a client

* supplied key name, then retrieving that data from the cache one or

* more times by reading against that key name.

*

* Normally the cache implementation can choose to discard the data

* between the write and the read when necessary (due to running out

* of cache space, etc.), but it is also possible for clients to lock

* data in the cache to support built-in grammars and other data that

* is explicitly provisioned by system administrators and thus must

* not be removed unless by explicit system administrator command.

* The interface is a synchronous interface based on the ANSI/ISO C

* standard file I/O interface. The client of the interface may use this in

* an asynchronous manner by using non-blocking I/O operations,

* creating threads, or by invoking this from a separate server

* process.

*

* Typically the key name specified by clients will be the URL to the

* source form of the data that is being written back, such as the URL

* to the grammar text being used as the key name for the compiled

* grammar. In the case where the VXIinet and VXIcache implementations

* share the same underlying cache storage, it is thus necessary to

* use prefixes or some other namespace mechanism to avoid collisions

* with the cache entry for the original URL.

*

* However, the key names specified by clients may be very large in

* some cases. This is most common when writing back data generated

* from in-memory source text, such as when writing back the compiled

* grammar for a VoiceXML document in-line grammar. One possible

* VXIcache implementation approach is to use the MD5 algorithm as

* used in HTTP message headers (specified in RFC 1321 with a full

* implementation provided) to convert long key names to a 16 byte MD5

* value for indexing purposes, using Base64 encoding to convert the

* binary MD5 value to ASCII text if desired (as done in HTTP message

* headers).

*

* There is one cache interface per thread/line.

*/

大意是说可以放进去任意的数据，并可以用URL检索出来。URL如果太长，会用md5和base64处理。那c ache模块中的功能在那儿用到了那？在VXIclient.cpp的第1088行（左右）有下面的内容：

/* Create the cache resource. The cache resource will be used by

the recognizer and prompting components for caching of computed

data like compiled grammars and text-to-speech prompts. */

cacheResult = SBcacheCreateResource(newPlatform->VXIlog, &newPlatform->VXIcache);

注释的意思是cache是recognizer和prompting 组件用到的。在VXIprompt和VXIrec中，仅仅有VXIpromptResult VXIpromptCreateResource (

VXIlogInterface *log,

VXIinetInterface *inet,

VXIcacheInterface *cache,

VXIpromptInterface **prompt)

和

VXIREC_API VXIrecResult VXIrecCreateResource(VXIlogInterface *log,

VXIinetInterface *inet,

VXIcacheInterface *cache,

VXIpromptInterface *prompt,

VXItelInterface *tel,

VXIrecInterface **rec)

用到了，而且函数里面参数cache并没有用到，感觉要实现者自己写代码吧。代码没再继续往下看。