Appendix F   Dynamic Results Caching Functions


The functions described in this appendix allow you to write a results caching plugin for iPlanet Web Server. A results caching plugin, which is a Service SAF, caches data, a page, or part of a page in the web server address space, which the web server can refresh periodically on demand. An Init SAF initializes the callback function that performs the refresh.

A results caching plugin can generate a page for a request in three parts:

• A header, such as a page banner, which changes for every request

• A body, which changes less frequently

• A footer, which also changes for every request

Without this feature, a plugin would have to generate the whole page for every request (unless an IFRAME is used, where the header or footer is sent in the first response along with an IFRAME pointing to the body; in this case the browser must send another request for the IFRAME).

If the body of a page has not changed, the plugin needs to generate only the header and footer and to call the dr_net_write function (instead of net_write) with the following arguments:

• header

• footer

• handle to cache

• key to identify the cached object

The web server constructs the whole page by fetching the body from the cache. If the cache has expired, it calls the refresh function and sends the refreshed page back to the client.

An Init SAF that is visible to the plugin creates the handle to the cache. The Init SAF must pass the following parameters to the dr_cache_init function:

• RefreshFunctionPointer

• FreeFunctionPointer

• KeyComparatorFunctionPtr

• RefershInterval

The RefershInterval value must be a PrIntervalTime type. For more information, see the NSPR reference at:

http://www.mozilla.org/projects/nspr/reference/html/index.html

As an alternative, if the body is a file that is present in a directory within the web server system machine, the plugin can generate the header and footer and call the fc_net_write function along with the file name.

This appendix lists the most important functions a results caching plugin can use. For more information, see the following file:

server_root/plugins/include/drnsapi.h

dr_cache_destroy

The dr_cache_destroy function destroys and frees resources associated with a previously created and used cache handle. This handle can no longer be used in subsequent calls to any of the above functions unless another dr_cache_init is performed.

Syntax
void dr_cache_destroy(DrHdl *hdl);

Parameters
DrHdl *hdl is a pointer to a previously initialized handle to a cache (see dr_cache_init).

Returns
void

Example
dr_cache_destroy(&myHdl);

dr_cache_init

The dr_cache_init function creates a persistent handle to the cache, or NULL on failure. It is called by an Init SAF.

Syntax
PRInt32 dr_cache_init(DrHdl *hdl, RefreshFunc_t ref, FreeFunc_t fre, CompareFunc_t cmp, PRUint32 maxEntries, PRIntervalTime maxAge);

Returns
1 if successful.

0 if an error occurs.

Parameters
DrHdl hdl is a pointer to an unallocated handle.

RefreshFunc_t ref is a pointer to a cache refresh function. This can be NULL; see the DR_CHECK flag and DR_EXPIR return value for dr_net_write.

FreeFunc_t fre is a pointer to a function that frees an entry.

CompareFunc_t cmp is is a pointer to a key comparator function.

PRUint32 maxEntriesp is the maximum number of entries possible in the cache for a given hdl.

PRIntervalTime maxAgep is the maximum amount of time that an entry is valid. If 0, the cache never expires.

Example
if(!dr_cache_init(&hdl, (RefreshFunc_t)FnRefresh, (FreeFunc_t)FnFree, (CompareFunc_t)FnCompare, 150000, PR_SecondsToInterval(7200)))
{
   ereport(LOG_FAILURE, "dr_cache_init() failed");
   return(REQ_ABORTED);
}

dr_cache_refresh

The dr_cache_refresh function provides a way of refreshing a cache entry when the plugin requires it. This can be achieved by passing NULL for the ref parameter in dr_cache_init and by passing DR_CHECK in a dr_net_write call. If DR_CHECK is passed to dr_net_write and it returns with DR_EXPIR, the plugin should generate a new content in the entry and call dr_cache_refresh with that entry before calling dr_net_write again to send the response.

The plugin may simply decide to replace the cached entry even if it has not expired (based on some other business logic). The dr_cache_refresh function is useful in this case. This way the plugin does the cache refresh management actively by itself.

Syntax
PRInt32 dr_cache_refresh(DrHdl hdl, const char *key, PRUint32 klen, PRIntervalTime timeout, Entry *entry, Request *rq, Session *sn);

Returns
1 if successful.

0 if an error occurs.

Parameters
DrHdl hdl is a persistent handle created by the dr_cache_init function.

const char *key is the key to cache, search, or refresh.

PRUint32 klen is the length of the key in bytes.

PRIntervalTime timeout is the expiration time of this entry. If a value of 0 is passed, the maxAge value passed to dr_cache_init is used.

Entry *entry is the not NULL entry to be cached.

Request *rq is a pointer to the request.

Session *sn is a pointer to the session.

Example
Entry entry;
char *key = "MOVIES"
GenNewMovieList(&entry.data, &entry.dataLen);                                           // Implemented by
                                          // plugin developer
if(!dr_cache_refresh(hdl, key, strlen(key), 0, &entry, rq, sn))
{
   ereport(LOG_FAILURE, "dr_cache_refresh() failed");
   return REQ_ABORTED;
}

dr_net_write

The dr_net_write function sends a response back to the requestor after constructing the full page with hdr, the content of the cached entry as the body (located using the key), and ftr. The hdr, ftr, or hdl can be NULL, but not all of them can be NULL. If hdl is NULL, no cache lookup is done; the caller must pass DR_NONE as the flag.

By default, this function refreshes the cache entry if it has expired by making a call to the ref function passed to dr_cache_init. If no cache entry is found with the specified key, this function adds a new cache entry by calling the ref function before sending out the response. However if the DR_CHECK flag is passed in the flags parameter and if either the cache entry has expired or the cache entry corresponding to the key does not exist, dr_net_write does not send any data out. Instead it returns with DR_EXPIR.

If ref (passed to dr_cache_init) is NULL, the DR_CHECK flag is not passed in the flags parameter, and the cache entry corresponding to the key has expired or does not exist, dr_net_write fails with DR_ERROR. However, dr_net_write refreshes the cache if ref is not NULL and DR_CHECK is not passed.

If ref (passed to dr_cache_init) is NULL and the DR_CHECK flag is not passed but DR_IGNORE is passed and the entry is present in the cache, dr_net_write sends out the response even if the entry has expired. However, if the entry is not found, dr_net_write returns DR_ERROR.

If ref (passed to dr_cache_init) is not NULL and the DR_CHECK flag is not passed but DR_IGNORE is passed and the entry is present in the cache, dr_net_write sends out the response even if the entry has expired. However, if the entry is not found, dr_net_write calls the ref function and stores the new entry returned from ref before sending out the response.

Syntax
PRInt32 dr_net_write(DrHdl hdl, const char *key, PRUint32 klen, const char *hdr, const char *ftr, PRUint32 hlen, PRUint32 flen, PRIntervalTime timeout, PRUint32 flags, Request *rq, Session *sn);

Returns
IO_OKAY if successful.

IO_ERROR if an error occurs.

DR_ERROR if an error in cache handling occurs.

DR_EXPIR if the cache has expired.

Parameters
DrHdl hdl is a persistent handle created by the dr_cache_init function.

const char *key is the key to cache, search, or refresh.

PRUint32 klen is the length of the key in bytes.

const char *hdr is any header data (which can be NULL).

const char *ftr is any footer data (which can be NULL).

PRUint32 hlen is the length of the header data in bytes (which can be 0).

PRUint32 flen is the length of the footer data in bytes (which can be 0).

PRIntervalTime timeout is the timeout before this function aborts.

PRUint32 flags is ORed directives for this function (see Flags).

Request *rq is a pointer to the request.

Session *sn is a pointer to the session.

Flags
DR_NONE specifies that no cache is used, so the function works as net_write does; DrHdl can be NULL.

DR_FORCE forces the cache to refresh even if it has not expired.

DR_CHECK returns DR_EXPIR if the cache has expired. If the calling function has not provided a refresh function and this flag is not used, DR_ERROR is returned.

DR_IGNORE ignores cache expiration and sends out the cache entry even if it has expired.

DR_CNTLEN supplies the Content-length header and does a PROTOCOL_START_RESPONSE.

DR_PROTO does a PROTOCOL_START_RESPONSE.

Example
if(dr_net_write(Dr, szFileName, iLenK, NULL, NULL, 0, 0, 0, DR_CNTLEN | DR_PROTO, rq, sn) == IO_ERROR)
{
   return(REQ_EXIT);
}

fc_net_write

The fc_net_write function is used to send a header and/or footer and a file that exists somewhere in the system. The fileName should be the full path name of a file.

Syntax
PRInt32 fc_net_write(const char *fileName, const char *hdr, const char *ftr, PRUint32 hlen, PRUint32 flen, PRUint32 flags, PRIntervalTime timeout, Session *sn, Request *rq);

Returns
IO_OKAY if successful.

IO_ERROR if an error occurs.

FC_ERROR if an error in file handling occurs.

Parameters
const char *fileName is the file to be inserted.

const char *hdr is any header data (which can be NULL).

const char *ftr is any footer data (which can be NULL).

PRUint32 hlen is the length of the header data in bytes (which can be 0).

PRUint32 flen is the length of the footer data in bytes (which can be 0).

PRUint32 flags is ORed directives for this function (see Flags).

PRIntervalTime timeout is the timeout before this function aborts.

Request *rq is a pointer to the request.

Session *sn is a pointer to the session.

Flags
FC_CNTLEN supplies the Content-length header and does a PROTOCOL_START_RESPONSE.

FC_PROTO does a PROTOCOL_START_RESPONSE.

Example
const char *fileName = "/docs/myads/file1.ad";
char *hdr = GenHdr(); // Implemented by plugin
char *ftr = GenFtr(); // Implemented by plugin

if(fc_net_write(fileName, hdr, ftr, strlen(hdr), strlen(ftr),
   FC_CNTLEN, PR_INTERVAL_NO_TIMEOUT, sn, rq) != IO_OKEY)
{
   ereport(LOG_FAILURE, "fc_net_write() failed");
   return REQ_ABORTED;
}