Editing Memcached (section)

==Example code==
''Note that all functions described on this page are [[pseudocode]] only. Memcached calls and programming languages may vary based on the API used.''

Converting database or object creation queries to use Memcached is simple. Typically, when using straight database queries, example code would be as follows:

<syntaxhighlight lang="c">
 function get_foo(int userid)
     data = db_select("SELECT * FROM users WHERE userid = ?", userid)
     return data

</syntaxhighlight>
After conversion to Memcached, the same call might look like the following
<syntaxhighlight lang="c">
 function get_foo(int userid)
     /* first try the cache */
     data = memcached_fetch("userrow:" + userid)
     if not data
         /* not found : request database */
         data = db_select("SELECT * FROM users WHERE userid = ?", userid)
         /* then store in cache until next get */
         memcached_add("userrow:" + userid, data)
     end

     return data

</syntaxhighlight>
The client would first check whether a Memcached value with the unique key "userrow:userid" exists, where userid is some number. If the result does not exist, it would select from the database as usual, and set the unique key using the Memcached API add function call.

However, if only this API call were modified, the server would end up fetching incorrect data following any database update actions: the Memcached "view" of the data would become out of date. Therefore, in addition to creating an "add" call, an update call would also be needed using the Memcached set function.
<syntaxhighlight lang="c">
 function update_foo(int userid, string dbUpdateString)
     /* first update database */
     result = db_execute(dbUpdateString)
     if result
         /* database update successful : fetch data to be stored in cache */
         data = db_select("SELECT * FROM users WHERE userid = ?", userid)
         /* the previous line could also look like data = createDataFromDBString(dbUpdateString) */
         /* then store in cache until next get */
         memcached_set("userrow:" + userid, data)
</syntaxhighlight>
This call would update the currently cached data to match the new data in the database, assuming the database query succeeds. An alternative approach would be to invalidate the cache with the Memcached delete function, so that subsequent fetches result in a cache miss. Similar action would need to be taken when database records were deleted, to maintain either a correct or incomplete cache.

An alternate cache-invalidation strategy is to store a random number in an agreed-upon cache entry and to incorporate this number into all keys that are used to store a particular kind of entry. To invalidate all such entries at once, change the random number. Existing entries (which were stored using the old number) will no longer be referenced and so will eventually expire or be recycled.

<syntaxhighlight lang="c">
  function store_xyz_entry(int key, string value)
      /* Retrieve the random number - use zero if none exists yet.
      *  The key-name used here is arbitrary. */ 
      seed = memcached_fetch(":xyz_seed:")
      if not seed
          seed = 0
      /* Build the key used to store the entry and store it.
      *  The key-name used here is also arbitrary. Notice that the "seed" and the user's "key"
      *  are stored as separate parts of the constructed hashKey string: ":xyz_data:(seed):(key)." 
      *  This is not mandatory, but is recommended. */
      string hashKey = sprintf(":xyz_data:%d:%d", seed, key)
      memcached_set(hashKey, value)

  /* "fetch_entry," not shown, follows identical logic to the above. */

  function invalidate_xyz_cache()
      existing_seed = memcached_fetch(":xyz_seed:")
      /* Coin a different random seed */
      do
          seed = rand()
      until seed != existing_seed
      /* Now store it in the agreed-upon place. All future requests will use this number. 
      *  Therefore, all existing entries become un-referenced and will eventually expire. */
      memcached_set(":xyz_seed:", seed)
</syntaxhighlight>