Cache API

The cache API enables plugins to read, write, and remove objects in the Traffic Server cache. All cache APIs are keyed by an object called an TSCacheKey; cache keys are created via TSCacheKeyCreate; keys are destroyed via TSCacheKeyDestroy. Use TSCacheKeyDigestSet to set the hash of the cache key.

Note that the cache APIs differentiate between HTTP data and plugin data. The cache APIs do not allow you to write HTTP docs in the cache; you can only write plugin-specific data (a specific type of data that differs from the HTTP type).

Example:

  1. const unsigned char *key_name = "example key name";
  3. TSCacheKey key;
  4. TSCacheKeyCreate (&key);
  5. TSCacheKeyDigestSet (key, (unsigned char *) key_name , strlen(key_name));
  6. TSCacheKeyDestroy (key);

Cache Reads

TSCacheRead does not really read - it is used for lookups (see the sample Protocol plugin). Possible callback events include:

  • TS_EVENT_CACHE_OPEN_READ - indicates the lookup was successful. The data passed back along with this event is a cache vconnection that can be used to initiate a read on this keyed data.
  • TS_EVENT_CACHE_OPEN_READ_FAILED - indicates the lookup was unsuccessful. Reasons for this event could be that another continuation is writing to that cache location, or the cache key doesn’t refer to a cached resource. Data payload for this event indicates the possible reason the read failed (TSCacheError).

Cache Writes

Use TSCacheWrite to write to a cache (see the sample Protocol plugin). Possible callback events include:

  • TS_EVENT_CACHE_WRITE_READ - indicates the lookup was successful. The data passed back along with this event is a cache vconnection that can be used to initiate a cache write.
  • TS_EVENT_CACHE_OPEN_WRITE_FAILED - event returned when another continuation is currently writing to this location in the cache. Data payload for this event indicates the possible reason for the write failing (TSCacheError).

Cache Removes

Use TSCacheRemove to remove items from the cache. Possible callback events include:

  • TS_EVENT_CACHE_REMOVE - the item was removed. There is no data payload for this event.
  • TS_EVENT_CACHE_REMOVE_FAILED - indicates the cache was unable to remove the item identified by the cache key. TSCacheError data indicates why the remove failed.

Errors

Errors pertaining to the failure of various cache operations are indicated by TSCacheError (enumeration). They are as follows:

  • TS_CACHE_ERROR_NO_DOC - the key does not match a cached resource
  • TS_CACHE_ERROR_DOC_BUSY - e.g, another continuation could be writing to the cache location
  • TS_CACHE_ERROR_NOT_READY - the cache is not ready

Example

In the example below, suppose there is a cache hit and the cache returns a vconnection that enables you to read the document from cache. To do this, you need to prepare a buffer (cache_bufp) to hold the document; meanwhile, use TSVConnCachedObjectSizeGet to find out the actual size of the document (content_length). Then, issue TSVConnRead to read the document with the total data length required as content_length. Assume the following data:

  1. TSIOBuffer       cache_bufp = TSIOBufferCreate ();
  2. TSIOBufferReader cache_readerp = TSIOBufferReaderAlloc (out_bufp);
  3. TSVConn          cache_vconnp = NULL;
  4. TSVIO            cache_vio = NULL;
  5. int               content_length = 0;

In the TS_CACHE_OPEN_READ handler:

  1. cache_vconnp = (TSVConn) data;
  2.     TSVConnCachedObjectSizeGet (cache_vconnp, &content_length);
  3.     cache_vio = TSVConnRead (cache_vconn, contp, cache_bufp, content_length);

In the TS_EVENT_VCONN_READ_READY handler:

  1. (usual VCONN_READ_READY handler logic)
  2. int nbytes = TSVIONBytesGet (cache_vio);
  3. int ntodo  = TSVIONTodoGet (cache_vio);
  4. int ndone  = TSVIONDoneGet (cache_vio);
  5. (consume data in cache_bufp)
  6. TSVIOReenable (cache_vio);

Do not try to get continuations or VIOs from TSVConn objects for cache vconnections. Also note that the following APIs can only be used on transformation vconnections and must not be used on cache vconnections or net vconnections:

  • TSVConnWriteVIOGet
  • TSVConnReadVIOGet
  • TSVConnClosedGet

APIs such as TSVConnRead, TSVConnWrite, TSVConnClose, TSVConnAbort, and TSVConnShutdown can be used on any kind of vconnections.

When you are finished:

TSCacheKeyDestroy (key);