Memcached and “N” things you can do with it – Part 1

In my last post MySQL Query Cache, WP-Cache, APC, Memcache – What to choose, I discussed in brief about 4 caching technologies which you might have used knowingly or unknowingly.

Towards the end we came to a conclusion that memcached is the best caching solution when you are looking for speed and number of hits per second. By my experience, memcached is capable of handling more than a 100 Million PV’s per month without any problem. However, towards the end I did also discussed why memcached is unreliable and unsecure.

In this post I will dig a level deeper into memcached. For ease here is the so called table of content:

  1. Basics: Memcached – Revisited
  2. Code Sample: A memcached class and how to use it
  3. N things: What else can I do with memcached
  4. Case Study: How Facebook uses memcached
  5. DONT’s: A few things to be taken care

Basics: Memcached – Revisited
Memcached was developed by Brad when live journal was hitting more than 20 Million PV’s per day. Handling 20 Million PV’s was no joke and he needed a better solution to handle such a high traffic. Since most of the blogs doesn’t change once published, he thought of having a model where he can skip the database read for the same posts again and again or atleast reduce the number of database reads. And hence came Memcached. Memcached is a deamon which runs in the background. By deamon you may think of a process running in the background doing its job.

If you are using ubuntu or debian like me, here are the steps for installing memcached:

  1. sudo apt-get install php5-memcache
  2. Go to /etc/php5/conf.d/memcache.ini and uncomment the line ;extension=memcache.so to enable this module
  3. sudo pecl install memcache
  4. Go to php.ini file and add this line: extension=memcache.so
  5. sudo apt-get install memcached
  6. sudo /etc/init.d/memcached start
  7. Restart Apache

If you are on windows, here are the steps which will help you running memcached on windows machine:

  1. Download the memcached win32 binary from my code.google vault.
  2. Unzip the downloaded file under C:memcached
  3. As we need to install memcached as a service, run this from a command line: C:memcachedmemcached.exe -d install from the command line
  4. Memcache by default loads with 64Mb of memory which is just not sufficient enough. Navigate to HKEY_LOCAL_MACHINESYSTEMCurrentControlSetServicesmemcached Server in your registry and find the ImagePath entry. Change that to “C:memcachedmemcached.exe” -d runservice -m 512
  5. Start the server by running this command: C:memcachedmemcached.exe -d start
  6. Open folder C:PHPext and check for php_memcache.dll. If you are unlucky not to have that download from here for PHP-4.x and from here for PHP-5.x
  7. Add extension=php_memcache.dll in your php.ini file
  8. Restart Apache
  9. Download the full package consisting of exe’s, classes and dll’s from here.

A few other options which you can use for starting memcached are:
memcached -d -m 2048 -l 10.0.0.40 -p 11211 , This will start memcached as a daemon, using 2GB of memory, and listening on IP 10.0.0.40, port 11211. Port 11211 is the default port for memcached.

By now I assume you have a 🙂 on your face, because you have memcached deamon running on your machine. Windows users can check that by opening up the task manager and looking for a memcached.exe process. I don’t see any reason for you not smiling, but if in case you are that unlucky windows user, please leave that system and move to a unix machine. Atleast try running Ubuntu on windows by reading this post of mine How to configure Ubuntu and LAMP on Windows.

Code Sample: Memcached class
So we have memcached setup on our system. Now we will try to hook up a simple code with it, which will do all the necessary talking with the deamon. Below I will demonstrate a very basic and simple application which will help you getting started. This application contains a total of 5 files namely: database.class.php, memcache.class.php, log.class.php, index.php, memcacheExtendedStats.php.

log.class.php
This is a very basic logger class which will log every thing for you to inspect later. Here is the code:

  class logAPI {
    var $LogFile = "log.txt";

    function Logger($Log) {
      $fh = fopen($this->LogFile,"a");
      fwrite($fh,$Log);
      fclose($fh);
    }
  }

When ever the code connects with memcached deamon, or database or fail to connect to either of them , this class will log the appropriate message in log.txt

database.class.php
This is another basic database class which have methods like getData() and setData(). getData() is used while trying to retrieve rows from the database while setData() is used while updating or inserting new rows in the database. For this demo application we will only be using the getData() method. Here is the code:

  include_once("memcache.class.php");
  include_once("log.class.php");

  class databaseAPI {

    /************************/
    /** Database information **/
    /************************/
    var $dbhost = "localhost";
    var $dbuser = "root";
    var $dbpass = "";
    var $dbname = "gtalkbots";
    var $db = NULL;

    /******************************/
    // CONSTRUCTOR DEFINITION //
    /******************************/
    function __construct() {
      $this->connect();
    }

    /*************************************************/
    // Function establishes a connection to database //
    /*************************************************/
    function connect() {
      // Connect to the dbhost
      $connection = mysql_connect($this->dbhost,$this->dbuser,$this->dbpass) or die(mysql_error());

	  // If connection fails send a mail to $dbmail about the same
      if(!$connection) {
        echo "Failed to establish a connection to host";
        exit;
      }
      else {
        // Connect to dbname
        $database = @mysql_select_db($this->dbname);

	// If fails to connect to the database send a mail to $dbmail
        if(!$database) {
          echo "Failed to establish a connection to database";
          exit;
        }
        else {
          $this->db = $connection;
        }
      }
    }

    /*******************************************/
    // Function closes the database connection //
    /*******************************************/
    function close() {
      mysql_close($this->db);
    }

    /***********************************************************/
    // Function executes the query against database and returns the result set   //
    // Result returned is in associative array format, and then frees the result //
    /***********************************************************/
    function getData($query,$options=array("type"=>"array","cache"=>"on"),$resultset="") {
        // Lookup on memcache servers if cache is on
	if($options['cache'] == "on") {
	    $obj = new memcacheAPI();
	    if($obj->connect()) {
	        // Try to fetch from memcache server if present
		$resultset = $obj->getCache($query);
	    }
    	    else {
	        // Fetch query from the database directly
	    }
	}
	// If $resultset == "" i.e. either caching is off or memcache server is down
        // OR $resultset == null i.e. when $query is not cached
	if($resultset == "" || $resultset == null) {
	    $result = mysql_query($query,$this->db);
	    if($result) {
		if($options['type'] == "original") {
		    // Return the original result set, if passed options request for the same
		    $resultset = $result;
		}
                else if($options['type'] == "array") {
	            // Return the associative array and number of rows
	    	    $mysql_num_rows = mysql_num_rows($result);
		    $result_arr = array();
		    while($info = mysql_fetch_assoc($result)) {
		        array_push($result_arr,$info);
		    }
		    $resultset = array("mysql_num_rows" => $mysql_num_rows,"result" => $result_arr,"false_query" => "no");
		}
  		// Cache the $query and $resultset
		$obj->setCache($query,$resultset);
		$obj->close();
		return $resultset;

                // Free the memory
		mysql_free_result($result);
            }
            else {
	        $resultset = array("false_query" => "yes");
	        return $resultset;
            }
	}
	else {
	    // If $query was found in the cache, simple return it
	    $obj->close();
	    return $resultset;
	}
    }

    /**********************************************************/
    // Function executes the query against database (INSERT, UPDATE) types   //
    /************************************************************/
    function setData($query) {
      // Run the query
      $result = mysql_query($query,$this->db);
      // Return the result
      return array('result'=>$result,'mysql_affected_rows'=>mysql_affected_rows());
    }

  }

memcache.class.php
Memcache class consists of two methods: getCache() and setCache(). getCache() will look up for the (key,value) pair in memory. If it exists, the method unserialize it and returns back. setCache() is used to set (key,value) pair in memory. It accepts the key and value, serialize the value before storing in cache.

  class memcacheAPI {

	/* Define the class constructor */
	function __construct() {
	  $this->connection = new Memcache;
	  $this->log = new logAPI();
	  $this->date = date('Y-m-d H:i:s');
	  $this->log->Logger("[[".$this->date."]] "."New Instance Created
n"); } /* connect() connects to the Memcache Server */ /* returns TRUE if connection established */ /* returns FALSE if connection failed */ function connect() { $memHost = "localhost"; $memPort = 11211; if($this->connection->connect($memHost,$memPort)) { $this->log->Logger("[[".$this->date."]] "."Connection established with memcache server
n"); return TRUE; } else { $this->log->Logger("[[".$this->date."]] "."Connection failed to establish with memcache server
n"); return FALSE; } } /* close() will disconnet from Memcache Server */ function close() { if($this->connection->close()) { $this->log->Logger("[[".$this->date."]] "."Connection closed with memcache server
n"); $this->log->Logger("=================================================================================================
nn"); return TRUE; } else { $this->log->Logger("[[".$this->date."]] "."Connection didn't close with memcache server
n"); $this->log->Logger("=======================================================================================================
nn"); return FALSE; } } /* getCache() function will fetch the passed $query resultset from cache */ /* returned resultset is null if $query not found in cache */ function getCache($query) { /* Generate the key corresponding to query */ $key = base64_encode($query); /* Get the resultset from cache */ $resultset = $this->connection->get($key); /* Unserialize the result if found in cache */ if($resultset != null) { $this->log->Logger("[[".$this->date."]] "."Query ".$query." was found already cached
n"); $resultset = unserialize($resultset); } else { $this->log->Logger("[[".$this->date."]] "."Query ".$query." was not found cached in memcache server
n"); } return $resultset; } /* setCache() function will set the serialized resultset on Memcache Server */ function setCache($query,$resultset,$useCompression=0,$ttl=600) { /* Generate the key corresponding to query */ $key = base64_encode($query); /* Set the value on Memcache Server */ $resultset = serialize($resultset); if($this->connection->set($key,$resultset,$useCompression,$ttl)) { $this->log->Logger("[[".$this->date."]] "."Query ".$query." was cached
n"); return TRUE; } else { $this->log->Logger("[[".$this->date."]] "."Query ".$query." was not able to cache
n"); return FALSE; } } }

index.php
With everything in place, its time to test memcached. We will check if memcached is working fine by running this code file twice one by one. Open command line and point to this code. Run from command line: php index.php . Then again run from command line php index.php.

  include_once("database.class.php");
  $mdb = new databaseAPI();

  $query = "SELECT * from status LIMIT 0,1000";
  $resultset = $mdb->getData($query);
  $mdb->close();

  echo "
";
  print_r($resultset);
  echo "

";

If everything is working fine, you will see a log.txt file being generated which will look as follows.

log.txt

[[2009-01-18 09:52:57]] New Instance Created
[[2009-01-18 09:52:57]] Connection established with memcache server
[[2009-01-18 09:52:57]] Query SELECT * from status LIMIT 0,1000 was not found cached in memcache server
[[2009-01-18 09:52:57]] Query SELECT * from status LIMIT 0,1000 was cached
[[2009-01-18 09:52:57]] Connection closed with memcache server
=================================================================================================
[[2009-01-18 09:53:08]] New Instance Created
[[2009-01-18 09:53:08]] Connection established with memcache server
[[2009-01-18 09:53:08]] Query SELECT * from status LIMIT 0,1000 was found already cached
[[2009-01-18 09:53:08]] Connection closed with memcache server
=================================================================================================

From the log file we can see that the 1st time results were fetched from the database and for the second time from memcached 🙂

Before we proceed further lets explain the flow of the above scripts. In index.php we create a new instance of database.class.php $mdb. Then we try to $query for 100 rows from the database. $mdb->getData($query) initiates this database fetch. As the program control goes to getData() method of database.class.php, it passed the control to getCache() method of memcache.class.php. There the code create a $key = base64_encode($query) and checks if we have the result set cached in memcached. If it doesn’t exists, it passed control back to getData() of database.class.php which fetches it from the database. After the fetch, it passes the $resultset back to setCache() method of memcache.class.php. There the setCache() method serialize the $resultset and cache it as ($key,$value) = (base64_encode($query), serialize($resultset)) in memcache.

Next time when the same query is fired and control goes to getCache() method of memcache.class.php, it fetches the result from cache, unserialize it and returns back the result to getData() of database.class.php. And thats why you see a log similar to above.

memcache.extendedstats.php
Finally it’s time to see some statistics. Here is a simple file which will show memcache status:

  $memcache_obj = new Memcache;
  $memcache_obj->addServer('localhost', 11211);

  $stats = $memcache_obj->getExtendedStats();

  echo "
";
  print_r($stats);
  echo "

";

Running it from command line using: php memcache.extendedstats.php will give you a statistic array like this.

Array
(
    [localhost:11211] => Array
        (
            [pid] => 5472
            [uptime] => 17
            [time] => 1232303504
            [version] => 1.2.5
            [pointer_size] => 32
            [curr_items] => 1
            [total_items] => 1
            [bytes] => 271631
            [curr_connections] => 2
            [total_connections] => 5
            [connection_structures] => 3
            [cmd_get] => 2
            [cmd_set] => 1
            [get_hits] => 1
            [get_misses] => 1
            [evictions] => 0
            [bytes_read] => 271705
            [bytes_written] => 271614
            [limit_maxbytes] => 536870912
            [threads] => 1
        )

)

This array tells you of a number of things about how your memcached deamon and caching architecture is performing. In short here is what each of the variable would mean:

  1. pid: Process id of this server process
  2. uptime: Number of seconds this server has been running
  3. time: Current UNIX time according to the server
  4. version: Version string of this server
  5. rusage_user: Accumulated user time for this process
  6. rusage_system: Accumulated system time for this process
  7. curr_items: Current number of items stored by the server
  8. total_items: Total number of items stored by this server ever since it started
  9. bytes: Current number of bytes used by this server to store items
  10. curr_connections: Number of open connections
  11. total_connections: Total number of connections opened since the server started running
  12. connection_structures: Number of connection structures allocated by the server
  13. cmd_get: Cumulative number of retrieval requests
  14. cmd_set: Cumulative number of storage requests
  15. get_hits: Number of keys that have been requested and found present
  16. get_misses: Number of items that have been requested and not found
  17. bytes_read: Total number of bytes read by this server from network
  18. bytes_written: Total number of bytes sent by this server to network
  19. limit_maxbytes: Number of bytes this server is allowed to use for storage.

However at this stage the figures which you might be interested in knowing are get_hits and get_misses. get_misses means number of times you requested for a key in memcache and it was not found while get_hits means number of times your requested key was successfully retrieved from memcached. Hence as expected we currently have get_misses = 1 and get_hits = 1. Try running php index.php once more and get_hits will get incremented by one.

N things: What else can I do
Till now you have memcached deamon running on your system and you know how to communicate with the deamon. You also know a very basic usage of memcached now, i.e. cache the queries to reduce load from your database. However there is a lot more you can do with memcached.

Here I would like to present before you a few applications of memcached which I learnt from my experiences. I hope they will be enough to make you think otherwise.

  1. Restricting spammers on your site : Often you will find on social networking sites like Digg, Facebook and Orkut that if you try to add several users as your friend within a short span, they (facebook) will show you a warning or they (digg) will restrict you from doing it. Similarly there are cases when you want to send a shout to more than 200 users on digg and you are restricted from doing so. How will you implement this on your site?

    Ordinary user’s solution: One solution is if a user ‘X’ add another user ‘Y’ as a friend, you will check how many friends has ‘X’ added in the past 10 minutes. If that exceeds 20, you won’t allow him to add more friends or show him a warning message. Simple enough and will work coolly. But what if your site have more than a million users, with many hackers around the world trying to keep your servers busy. As a memcached user you must have following solution in your mind:

    Your solution: As ‘X’ adds ‘Y’ in his friend list, you will set a (key,value) pair in memcached, where $key = “user_x” with $TTL = 10 minutes. For the first friend added, $value = 1. Now as ‘X’ tries to add another friend, your simply increment $value for $key = “user_x”. As this $value equals 20, and ‘X’ tried to add another friend, your code will check the value of $key = “user_x” and see if it’s present in memcached. If it’s present check for it’s value. If it’s value is equal to 20, you show a warning message to the user. Hence restricting him from adding more than 20 friends within a time span of 10 minutes. After 10 minutes, $key = “user_x” will automatically expires and your code will allow ‘X’ to add more friends. Similar solution exists if you want to stop spammers from sending message or commenting over a limit on your site. Now I see confidence building in you as a memcached programmer 😀

  2. Detecting online active/inactive users : Often you want a method which can tell your registered users, about their online friends. Or in a forum you want to tell the current reading user, about who all are reading this post. I won’t tell how an ordinary user will implement this, but as a memcached user your solution should be:

    Ordinary user’s solution: You don’t want to think of this.

    Your solution: As user ‘X’ visit post#12345 in the forum, not only you will fetch post data from the database, you also set two (key,value) pairs.

    $key1 = “post_12345”
    $value1 = [[comma separated list of user names]]
    $TTL1 = [[some arbitrary large value is fine]]

    $key2 = “user_x”
    $value2 = “post_12345”
    $TTL2 = 10 minutes, We assume a user to be inactive if he is on a particular post for more than 10 minutes (tunable), and we will mark him as inactive.

    (key1,value1) is a post specific data while (key2,value2) is a user specific data. Now every time a user visits post#12345, you will do two things. Read all comma separated user names from $value1, and then check for their corresponding $key2 value. If corresponding $key2 exists and equals to $value2 = “post_12345”, i.e. on the same post and not idle, we will keep that user name in value of $key1. However if $key2 is not present (i.e. user gone away) or value of $key2 equals to some other post, we will remove his user name from $value1. Confused 😛 , read twice and the picture will be clear.

    Can you think of a better solution? Please let me and others know about it. (Remember we are trying to detect only active/inactive users, which is not same as online/offline users)

  3. Building scalable web services : Another application of memcached lies in building scalable web services and web widgets. Gtalkbots offer a cool widget which you can put on your blog and sites to show off your aggregated status messages. See this widget live in the right hand sidebar. While building this widget, one thing which I kept in mind was that, what if someone with million hits per day put my widget on his site. Even though, Gtalkbots gets a few thousand of hits per day, it will crash, mainly because of this particular widget being placed on a high traffic site. So as a memcached user I have done the following thing.

    Ordinary user’s solution: Deprecated

    Your solution: I simply cache the widget data in memcache with $TTL = 1 hour. So every time this million hits per day site is accessed, which loads my widget million times a day, the query will be returned from cache. And hence saving my server from crashing. Fetch Gtalkbots widget from here and try putting on your site.

Alright by now you can impress your bosses with your cool memcache implementations. But wait there is a lot more you need to know. I can go on putting hundred’s of memcached applications here, but main point is, setting your mind as a memcached user. I personally have this habit of linking everything to memcached while designing a system, and if it suits my need, Bingo!.

  1. Versioning your cache keys : One disadvantage of using cache at times is that, if unfortunately something goes wrong with your data and that buggy data is cached, your users will keep seeing that buggy data until your cache expires or you manually clear off your cache. Suppose you clear your cache and then one of your fucking engineer comes running saying the data is fine.

    Ordinary user’s solution: Stop it, No more plz

    Your solution: As a memcached user, i would love to keep a Versioning system for my caches. Nothing complex, simply append “v1” (version number) to all your keys. i.e. $key = “user_x” will now become $key = “v1:user_x”. and at a place in your code you have $current_cache_version = “v1”. Now suppose you are told your data is buggy, so by the time your engineers are investigation change $current_cache_version = “v2”. This will keep your old caches, which you may want to recover after your investigation and at the same time show new data to your users.

  2. Not so frequent update of Db for trivial data : This is a site dependent application. Suppose you run a site where you are not so serious about database columns like “last_update_date”, “last_logged_in” and so on. However you still keep a track of such for analysis purpose and don’t mind if it’s not so accurate.

    Your solution: One simple solution to this problem is keep such trivial data in memcached and set up a cron job which will run every 10 minutes and populate such data in the database. 🙂

I will leave you with a presentation on memcache which I gave sometime back at office. I hope it will help you gain more understanding of memcache.

Memcache

View SlideShare presentation or Upload your own. (tags: caching memcache)

I hope after reading this you are well equipped on how to design scalable systems. All the best! , do leave a comment or suggestions if any. If you liked this post, do subscribe for the next post in memcache series.

MySQL Query Cache, WP-Cache, APC, Memcache – What to choose

Hello Cache Freaks,

Ever since I changed my job (from Business Intelligence to Web development) and started working with my present employer, I have had a chance to work on a lot of scalable projects. From making my project to scale from 20 Million PV’s to 100 Million PV’s to development of an internal tool, the answer to all scalable applications have been caching.

There are a lot of caching techniques which are being employed by sites worldwide.

  1. WP-Cache used in wordpress – a file system based caching mechanism
  2. APC Cache – an opcode based caching system
  3. Memcache – an in memory caching system
  4. Query Cache – caching mechanism employed in MySQL

Here in this post I would like to pen down my experiences while working with all the caching mechanism. Their pros and cons. What things you need to take care while working with them and every little tit bit which comes to my mind while writing this post.

Query Cache – inbuilt cache mechanism in MySQL
Query cache is an inbuilt cache mechanism for MySQL. Basically when you fire a query against a MySQL database, it goes through a lot of core modules. e.g. Connection Manager, Thread Manager, Connection Thread, User Authentication Module, Parse Module, Command Dispatcher, Optimizer Module, Table Manager, Query Cache Module and blah blah. Discussing these modules is out of scope of this blog post. The only module we are interested here is Query Cache Module.

Suppose I fire a query:
$query = “Select * from users”;

MySQL fetches it for the first time from the database and caches the result for further similar query. Next time when you fire a similar query, it picks the result from the cache and deliver it back to you.

However, there are a few drawbacks with Query Cache:

  • If there is a new entry in the users table, the cache is cleared.
  • Secondly, even if the result of your query is cached, MySQL has to go through a number of core modules before it give back the cached result to you.
  • Thirdly, even if your results are caches, you need to connect to your MySQL database, which generally have a bottleneck with number of connections allowed.

One thing which you should take care while replying on Query Cache is that, your query must not have any parameter which is random or changes quite often. For e.g. If you wish to fetch url’s of 100 photo from the database, and then you want to present them in a random fashion every time, you might be tempted to use rand() somewhere in your MySQL queries. However by using rand() you ensure that the query is never returned from cache, since rand() parameter always makes the query look different.

Similarly, If you have a case where you need to show data not older than 15 days, and by mistake you also include the seconds parameter in your SQL query, then the query will never return from cache.

Hence for a scalable site, with 100 Million PV’s you can’t really survive with a simple query cache provided by MySQL database.

WP-Cache – Caching mechanism for WordPress blogs
WP-Cache is a file system based caching mechanism i.e. it caches your blog posts in form of simple text files which are saved on your file system. You can have a look at these cached files by visiting wp-content/cache folder inside your blog directory. Generally you will find two set of files for a single blog post. One .html and another .meta file.

.html file generally contains the static html content for your blog post. Once published, the blog post content is static, hence instead of fetching it’s data from the database, WP-Cache serves it from the cache directory.

.meta file contains serialized information such as Last Modified, Content-Type, URI which WP-Cache uses to maintain cache expiry on your blog.

WP-Cache works really well, however if the traffic starts increasing on your blog, then the bottleneck will be maximum number of child processes which apache can create (For starters you can think, each user connecting to your blog as one apache process, hence there is a restriction on number of users who can connect to your blog at a particular time). In such case the solution can be either to have multiple front end servers with a load balancer to distribute the traffic among front end servers, or to have a better cache solution such as a memory based caching mechanism (e.g. Memcache). Also since memory read is always faster than file read, you must go for a memory based cache system.

APC Cache – An opcode based cache for PHP
APC stands for Alternative PHP Cache. In one of my previous post How does PHP echo’s a “Hello World”? – Behind the scene, I talk about how PHP churns out “Hello World” for you.

PHP takes your code through various steps:

  1. Scanning – The human readable source code is turned into tokens.
  2. Parsing – Groups of tokens are collected into simple, meaningful expressions.
  3. Compilation – Expressions are translated into instruction (opcodes)
  4. Execution – Opcode stacks are processed (one opcode at a time) to perform the scripted tasks.

Opcode caches let the ZEND engine perform the first three of these steps, then store that compiled form (opcodes) so that the next time a given script is used, it can use the stored version without having to redo those steps only to come to the same result.

However the problem with APC cache is that it is not a distributed cache system. By distributed cache I mean, if you have 3 frontend server then you need to have a copy of this opcode on all the three fronend server. Also like WP-Cache APC is again a file system driven cache system, which is not the optimal solution. Also with APC cache, PHP still has to go through the last step as described above.

Memcache – In memory based cache mechanism
Memcache is the solution when you talk about million PV’s on your site. It is a high-performance, distributed memory object caching system, generic in nature, but intended for use in speeding up dynamic web applications by alleviating database load.

For starters, Memcache is not PHP, nor Python or any other thing as you may think. It’s a deamon which runs in the background on your server. Your code connects to it and cache query results, JS, CSS and other cachable data in the server’s memory (RAM). Since its an in-memory caching system, it is faster than any of the discussed caching system above. However it is unreliable and unsecure but then there are ways to tackle this unreliable and unsecure nature of memcache.

Memcache is unreliable because it resides in your system memory. So an event like system reboot or power failure will result in loss of all your cache. Also memcache provides no mechanism to take backup of your caches. Hence once lost, you need to programmatically warmup your caches.

Memcache is unsecure because it doesn’t require any authentication mechanism. There is no username or password with which your code connects to it. (Hence it is super fast, unlike Query cache which has to go through auth module even if the query result is cached). It usually runs at port 11211 on your server and if not taken care, anyone can telnet to port 11211 on your server and steal your caches.

Below are the steps which are being followed on a memcache enabled website:

  1. User enter your site’s url in his browser, say http://localhost
  2. There are about 6 queries which drives your opening page
  3. Lets assume one of the query for this page is $query = “SELECT photo_url from photos LIMIT 0,10”
  4. When the user visit http://localhost, your code will first connect to memcache deamon on port 11211
  5. If memcache deamon is running, it checks if the result of this query are already cached. Generally data is cached in memcache as (key,value) pair
  6. Since this is the first visit on your site, ofcourse there is no query being cached till now. Hence your code now connect to your MySQL database and fetched the resultset. Something like this probably. $resultset = mysql_query($query);
  7. After successfully fetching the resultset, your code will cache this resultset in memcache. The code connects to memcache deamon and saves this (key,value) pair in memory, where $key = md5($query) and $value = serialize($resultset)
  8. After caching the (key,value) pair, your code returns back the fetched resultset to the frontpage where it is being displayed to the user
  9. Similarly all the 6 queries which drives your front page are being cached one by one
  10. Now the next time when another user visit this page i.e. http://localhost, your code will first see if $key = md5($query) is already present in cache. It will find the $key being cached, fetches the serialized resultset from memory, unserialize it and throws back to the front page where it is displayed as intended
  11. While caching (key,value) pair you also have a provision to specify a TTL (time to live) after which memcache will automatically expire your cached result
  12. Suppose you specifies a TTL = 15 Minutes for all the above queries and now a visitor visit http://localhost after 30 minutes
  13. Your code will again first connect to memcache deamon and check if $key = md5($query) is present in cache. Memcache deamon will see that this $key is present but it has expired. It will return a result saying $key is not cached and internally flushes out $key. Your code then again connect to MySQL database, fetches the resultset and cache back the results in memcache for further use

I will leave you with a presentation on memcache which I gave sometime back at office. I hope it will help you gain more understanding of memcache.

Memcache

View SlideShare presentation or Upload your own. (tags: caching memcache)

In my next posts, I will be covering a few code samples and use cases of memcache which you wouldn’t even heard of. If you liked the post don’t forget to promote it on social networking sites and do subscribe to my blog. 🙂

New Year, New Server and a New Theme

Hello Everyone,

I wish I could have finished this task by 1st January, but nevertheless it’s finally done. Finally I have a new self designed wordpress theme named “Jhantu”. Also I have moved my blog from a shared hosting to my VPS due to increasing traffic.

Transition from one server to another is always painful. Here are the steps which I followed to make my server move smooth:

  • I took a dump of my blog last week and started developing new theme locally on my windows system
  • Yesterday before moving to VPS, I took the latest dump of wordpress blog (click export and download the xml file which guarantees for a smooth move)
  • However the database restoration wasn’t smooth. I found one of my previously published blog has been imported as a draft with no content inside and all comments lost. WTF
  • I had to republish it by extracting the blog content from the downloaded xml (If you are planning to move your blog, check out)
  • Then I changed the DNS settings in Godaddy, so that my domain now points to my VPS (something I discussed here before)
  • However it takes sometime before this new lookup (domain name => IP Address) is propagated worldwide. Infact even though you change the settings at Godaddy, you might continue seeing your old blog
  • I did 3 things, so that I can atleast setup the blog. Cleared browser cache, flushed my DNS lookups (run ipconfig /flushdns on command line), went to C:WINDOWSsystem32driversetchosts and made a new entry which reads: 74.86.123.245 abhinavsingh.com
  • This last entry will force your system to lookup at 74.86.123.245 for abhinavsingh.com

I guess these steps might help you if ever you plan to take the pain of server move.

Finally a bit about plugin’s powering “Jhantu”:

  • Akismet as always is guarding and saving us from spam
  • WP-Toolbar , a custom made plugin for showing a toolbar for outgoing links from my blogpost
  • My Adsense Manager a self written plug-in to embed adsense codes in between the blog posts. Will open source after some testing
  • Show your stats , yet another custom plugin to display my weekly stats on right tabs. Data derived from wordpress stats database using a cron job.
  • All in one SEO Pack, which automatically generates meta description and keyword for each blog post
  • With increasing number of visitors from european and south american citizens, I decided to have a language translator. Thanks to Global Translator for providing such an excellent plugin for translation
  • Google XML Sitemaps plugin for generating instant sitemaps as soon as I publish a new post
  • Thanks to MCEComments which will now allow you to format your comments 🙂
  • Sociable, which you will be using the most to share, print and email these posts if required. However I have customized it to also display tinyurl for the blog post and the RSS/Mail subscription links.
  • Subscribe To Comments will help you to keep upto date with an ongoing discussion
  • Get Recent Comments for keeping you in the know of latest discussions
  • WordPress Related Posts for feeding you with something similar to your taste 😉
  • WP-PageNavi for showing pagination instead of plain previous and next blog links
  • WP-Cache to handle increasing traffic
  • Syntax Highlighter making it easy for you to read through my code snippets
  • Finally WordPress.com Stats to keep a track of incoming traffic 🙂

In future “Jhantu” will be made open source, so that you can run your blog using it. Do let me know your feedbacks about the new theme inspired from Google Code.

How to get dzone feeds as IM using JAXL? Add dzone@gtalkbots.com

The most funny part of knowing XMPP protocol is that you can execute all your daily web needs using it. Be it playing anagram online or achieving your status messages. And your life gets easy if you are using an XMPP Client Library like JAXL.

Here I have tried to create a simple bot which allows you to read newest links posted on dzone category wise. Simply add dzone@gtalkbots.com in your gtalk messenger and start by typing options.

Available options are:

  • frontpage: Will send you latest links on frontpage.
  • queue: Will send you latest links in queue.
  • tag: Send in a tag e.g. php,java,xml and you will get latest links for the particular tag

Here is modified eventMessage() method of JAXL library powering dzone@gtalkbots.com:

    function eventMessage($fromJid, $content, $offline = FALSE) {

      // If the message is not an offliner
      if(!$offline) {
        if($this->restrictJid && !in_array($this->getBareJid($fromJid),$this->authorizedJid)) { // Block un-authorized users
          $this->logger->logger($fromJid." tried using this service by sending:n".$content);
          $message = 'You are not authorized to use this service';
          $this->sendMessage($fromJid,$message);
          return TRUE;
        }
        else {
          $content = strtolower(trim($content));

          // Build RSS Feed URL depending upon passed IM
          if($content == "options") {
            $message = "Available options are:n";
            $message .= "*1. frontpage:* Will send you latest links on frontpage.n";
            $message .= "*2. queue:* Will send you latest links in queue.n";
            $message .= "*3. tag:* Send in a tag e.g. php,java,xml and you will get latest links for the particular tagn";
            $this->sendMessage($fromJid,$message);
            return TRUE;
          }
          else if($content == "frontpage") $url = "http://www.dzone.com/links/feed/frontpage/rss.xml";
          else if($content == "queue") $url = "http://www.dzone.com/links/feed/queue/rss.xml";
          else $url = "http://www.dzone.com/links/feed/frontpage/".$content."/rss.xml";

          // Generate cache file location
          $cache_file = "cache/dzone/".md5($url)."-".$content.".rss";

          // If cache file is not older than 900 seconds (15 min)
          if(file_exists($cache_file) && (filemtime($cache_file) > time() - 900)) {
            $this->logger->logger("RSS already found cached...");
          }
          else { // else fetch a fresh copy of RSS
            $this->logger->logger("Fetching RSS for url:n".$url);
            $head = $this->fetchWebURL($url);
            if($head['errno'] == 0 && $head['http_code'] == 200) {
              $this->logger->logger("Fetched successfully...");
              $data = $head['content'];
              $fh = fopen($cache_file,"w");
              fwrite($fh,$data);
              fclose($fh);
            }
            else {
              // Send a message saying no such feed exists
              $this->logger->logger("Failed to fetch the RSS feed...");
              $message = 'No feed exists for the passed keyword: '.$content;
              $this->sendMessage($fromJid,$message);
              return TRUE;
            }
          }

          // Parse RSS Feed
          $r = &new XML_RSS($cache_file);
          $r->parse();
          $NumberOfFeeds = count($r->getItems());
          if($NumberOfFeeds > 0) {
            $message = '';
            foreach($r->getItems() as $key => $item) {
              if($this->count < $this->maxResults) {
                $message .= "*".$item["title"]."*n";
                $message .= $item["link"]."n";
                $message .= $item["description"]."nn";
                $this->count++;
              }
              else {
                break;
              }
            }
            $message .= "powered by *Jaxl* http://code.google.com/p/jaxl"."nn";
            $this->sendMessage($fromJid,$message);
            $this->count = 0;
            return TRUE;
          }
          else {
            // Send a message saying no feed found
            $message = 'No feed was found for passed keyword: '.$content;
            $this->sendMessage($fromJid,$message);
            return TRUE;
          }
        }
      }
      else {
        // Send an appropriate message
        $this->logger->logger($fromJid." sent an offliner saying:n".$content);
        return TRUE;
      }
    }

To run this sample application you need:

  • JAXL Library (http://code.google.com/p/jaxl)
  • XML_RSS Package (pear install XML_RSS)
  • Create a directory cache/dzone where this application will cache all rss files
  • Include jaxl4dzone.class.php in index.php, instead of jaxl.class.php and run ‘php index.php’ from command line to start this application

Checkout the complete code and JAXL library from http://code.google.com/p/jaxl.

Now browse new links even when dzone is down for maintenance 😛
Do let me know about your feedbacks 🙂

Introducing JAXL – Open Source Jabber XMPP Library

Introduction

JAXL stands for “Jabber XMPP Library“. For fun, it stands for “Just Another XMPP Library

This library currently supports following features:

  • Connect to a Jabber Server (e.g. Gtalk)
  • TLS Encryption
  • DIGEST-MD5 and PLAIN authentication mechanisms
  • Roster Support

Library comes with the following class files:

  1. config.ini.php : Holds your jabber account and mysql connection information
  2. mysql.class.php : Basic MySQL connection class used to insert received messages and presence into MySQL database
  3. logger.class.php : A very basic logger class which allows you to log all XML stanza’s send and received from jabber server
  4. xmpp.class.php : Base XMPP class library which implements the XMPP protocol
  5. jaxl.class.php : JAXL class which extends XMPP class library. Should be the starting point for your application.
  6. index.php : After building your application in jaxl.class.php, you finally initializa and call your methods here

Source Code
JAXL is hosted at Google Code. Checkout full source code from here:
http://code.google.com/p/jaxl

Google Groups
jaxl
Visit this group

How to use JAXL:
JAXL Client Library is highly structured. There is a base XMPP class library (xmpp.class.php) and a JAXL class library (jaxl.class.php) which is derived from the base XMPP class library.

Base XMPP class library implements the XMPP protocol and it also provides you with two extendable methods named eventMessage() and eventPresence(). These methods are internally called when a message or presence XML Stanza is received from the Jabber server.

Jaxl.class.php must be the starting point for all your applications. Simply customize the eventMessage() and eventPresence() Method for your application. Other methods which might interest you while customizing them are:

  1. sendMessage($jid,$message) : For sending message to a particular Jid
  2. sendStatus() : To set your status message
  3. roster(‘get’) : To get list of roster
  4. roster(‘add’,$jid) : Add a new contact in roster list
  5. roster(‘remove’,$jid) : Remove a contact from roster list
  6. roster(‘update’,$jid,$name,$groups) : Update a particular contact in roster
  7. subscribe($jid) : Subscribe for presence of a particular $jid

This library includes the following files:

Config File (config.ini.php)
You specify all your jabber account username and password in this file. For development environment keep $env = “devel” and for production simply change it to $env = “prod”

  // Set an enviornment
  $env = "prod";

  // Log Level for logger class
  $logEnable = TRUE;

  // Log in MySQL database
  $logDB = FALSE;

  $key = array("prod"=>array("user"=>"myproductionuser",
                             "pass"=>"password",
                             "host"=>"talk.google.com",
                             "port"=>5222,
                             "domain"=>"gmail.com"
                            ),
              "devel"=>array("user"=>"mydevelopmentuser",
                             "pass"=>"password",
                             "host"=>"localhost",
                             "port"=>5222,
                             "domain"=>"localhost"
                            )
              );

  $db = array("prod"=>array("dbuser"=>"root",
                            "dbpass"=>"password",
                            "dbhost"=>"localhost",
                            "dbname"=>"jaxl"
                           ),
             "devel"=>array("dbuser"=>"root",
                            "dbpass"=>"password",
                            "dbhost"=>"localhost",
                            "dbname"=>"jaxl"
                           )
             );


MySQL Class (mysql.class.php)
This is a basic MySQL connection class used to insert received messages and presence into MySQL database

Base XMPP Class (xmpp.class.php)
You should not worry about this class. Until and unless you are aware of what are you trying to achieve you should not touch this class file.

Logger Class (logger.class.php)
This is a basic logger class. It help you log XML send to and received from jabber server. Might prove helpful in debugging and if you are interested in learning the core of XMPP Protocol.

Extended JAXL Class (jaxl.class.php)
You will be customizing eventMessage() and eventPresence() methods here.

    function eventMessage($fromJid, $content, $offline = FALSE) {
      if($offline) {
        $this->sendMessage($fromJid,"Hi, Thanks for your offline message");
      }
      else {
        $this->sendMessage($fromJid,"Hi, Thanks for your message");
      }

      if($this->logDB) {
        // Save the message in the database
        $timestamp = date('Y-m-d H:i:s');
        $query = "INSERT INTO message (FromJid,Message,Timestamp) value ('$fromJid','$content','$timestamp')";
        $this->mysql->setData($query);
      }
    }

    function eventPresence($fromJid, $status, $photo) {
      // Change your status message to your friend's status
      $this->sendStatus($status);

      if($this->logDB) {
        // Save the presence in the database
        $timestamp = date('Y-m-d H:i:s');
        $query = "INSERT INTO presence (FromJid,Status,Timestamp) value ('$fromJid','$status','$timestamp')";
        $this->mysql->setData($query);
      }
    }

In above example I have done 4 things:

  • Sends back a message saying “Hi, Thanks for your offline message”, when I receive a offliner.
  • Sends back a message saying “Hi, Thanks for your message”, when I receive an IM from my friend.
  • Change my status message, as and when I receive a status update from my friends.
  • Save messages and presence into database if $logDB = TRUE in config.ini.php

Final Call (index.php)

  /* Include Key file */
  include_once("config.ini.php");

  /* Include JAXL Class */
  include_once("jaxl.class.php");

  /* Create an instance of XMPP Class */
  $jaxl = new JAXL($key[$env]['host'],    // Jabber Server Hostname
                   $key[$env]['port'],    // Jabber Server Port
                   $key[$env]['user'],    // Jabber User
                   $key[$env]['pass'],    // Jabber Password
                   $key[$env]['domain'],  // Jabber Domain
                   $db[$env]['dbhost'],   // MySQL DB Host
                   $db[$env]['dbname'],   // MySQL DB Name
                   $db[$env]['dbuser'],   // MySQL DB User
                   $db[$env]['dbpass'],   // MySQL DB Pass
                   $logEnable,            // Enable Logging
                   $logDB                 // Enable MySQL Inserts
                  );

  try {
    /* Initiate the connection */
    $jaxl->connect();

    /* Communicate with Jabber Server */
    while($jaxl->isConnected) {
      $jaxl->getXML();
    }
  }
  catch(Exception $e) {
    die($e->getMessage());
  }

Instant Messenger Powered by JAXL
I am pleased to announce 3 months after release of JAXL, 1st Instant Messenger powered by JAXL. Currently the instant messenger is in testing phase and will be released after some 3-4 weeks of thorough testing. Meanwhile here is the first clip shot:

Disclaimer
Currently this library is being developed and used at Gtalkbots.com . Though it is thoroughly tested and being used, I still recommend not to use it on production servers. Mainly, since it is heavily customized for Gtalkbot’s usage. I am releasing it, as I see it of use for the community worldwide. If you want to use it, use at your own risk. Let me know of any changes you make to this library. In future I look to make this library more generic.

Enjoy and let me know of any bugs, feedbacks, enhancements or any abuse you may want to pass through if it doesn’t work for you 😛

You should be running index.php using command line and not browser (recommended). This class is currently highly customized for Gtalkbots usage and works perfectly with Gtalk Servers.

Behind the scenes – How and What XML’s are exchanged by JAXL

Pre-requisite
Though it’s not mandatory but will be helpful if you have given a casual reading to the following RFC’s:

Case Study: Google Talk Server

  • JAXL Sends >>
    1. <!–?xml version=”1.0″?–>
    2. <stream:stream xmlns:stream=“http://etherx.jabber.org/streams” version=“1.0” xmlns=“jabber:client” to=“gmail.com” xml:lang=“en” xmlns:xml=“http://www.w3.org/XML/1998/namespace”></stream:stream>
  • Gtalk Acknowledges With >>
    1. <!–?xml version=”1.0″ encoding=”UTF-8″?–>
    2. <stream:stream from=“gmail.com” id=“981E0522D7363BDF” version=“1.0” xmlns:stream=“http://etherx.jabber.org/streams” xmlns=“jabber:client”>
    3. <stream:features>
    4.   <starttls xmlns=“urn:ietf:params:xml:ns:xmpp-tls”>
    5.     <required>
    6.   </required></starttls>
    7.   <mechanisms xmlns=“urn:ietf:params:xml:ns:xmpp-sasl”>
    8.     <mechanism>X-GOOGLE-TOKEN</mechanism>
    9.   </mechanisms>
    10. </stream:features></stream:stream>
  • JAXL Sends >>
    1. <starttls xmlns=“urn:ietf:params:xml:ns:xmpp-tls”></starttls>
  • Gtalk Acknowledges With >>
    1. <proceed xmlns=“urn:ietf:params:xml:ns:xmpp-tls”></proceed>
  • JAXL Sends >>
    1. <!–?xml version=”1.0″?–>
    2. <stream:stream xmlns:stream=“http://etherx.jabber.org/streams” version=“1.0” xmlns=“jabber:client” to=“gmail.com” xml:lang=“en” xmlns:xml=“http://www.w3.org/XML/1998/namespace”></stream:stream>
  • Gtalk Acknowledges With >>
    1. <!–?xml version=”1.0″ encoding=”UTF-8″?–>
    2. <stream:stream from=“gmail.com” id=“C01610C43D6A37A2” version=“1.0” xmlns:stream=“http://etherx.jabber.org/streams” xmlns=“jabber:client”>
    3. <stream:features>
    4.   <mechanisms xmlns=“urn:ietf:params:xml:ns:xmpp-sasl”>
    5.     <mechanism>PLAIN</mechanism>
    6.     <mechanism>X-GOOGLE-TOKEN</mechanism>
    7.   </mechanisms>
    8. </stream:features></stream:stream>
  • JAXL Sends >>
    1. <auth xmlns=“urn:ietf:params:xml:ns:xmpp-sasl” mechanism=“PLAIN”>AGFjY192nbRbm3J1cGvxYWRzAG15QVRNcGlukT9zcpMxAjI=</auth>
  • Gtalk Acknowledge With >>
    1. <success xmlns=“urn:ietf:params:xml:ns:xmpp-sasl”></success>
  • JAXL Sends >>
    1. <!–?xml version=”1.0″?–>
    2. <stream:stream xmlns:stream=“http://etherx.jabber.org/streams” version=“1.0” xmlns=“jabber:client” to=“gmail.com” xml:lang=“en” xmlns:xml=“http://www.w3.org/XML/1998/namespace”></stream:stream>
  • Gtalk Acknowledge With >>
    1. <!–?xml version=”1.0″ encoding=”UTF-8″?–>
    2. <stream:stream from=“gmail.com” id=“1DA3DFD778DA0116” version=“1.0” xmlns:stream=“http://etherx.jabber.org/streams” xmlns=“jabber:client”>
    3. <stream:features>
    4.   <bind xmlns=“urn:ietf:params:xml:ns:xmpp-bind”>
    5.   <session xmlns=“urn:ietf:params:xml:ns:xmpp-session”>
    6. </session></bind></stream:features></stream:stream>
  • JAXL Sends >>
    1. <iq type=“set” id=“1”>
    2.   <bind xmlns=“urn:ietf:params:xml:ns:xmpp-bind”>
    3.     <resource>jaxl</resource>
    4.   </bind>
    5. </iq>
  • Gtalk Acknowledge With >>
    1. <iq id=“1” type=“result”>
    2.   <bind xmlns=“urn:ietf:params:xml:ns:xmpp-bind”>
    3.     <jid>myproductionuser@gmail.com/jaxl25C3CD9A</jid>
    4.   </bind>
    5. </iq>
  • JAXL Sends >>
    1. <iq type=“get” to=“gmail.com”>
    2.   <query xmlns=“http://jabber.org/protocol/disco#info”>
    3. </query></iq>
  • Gtalk Acknowledge With >>
    1. <iq to=“myproductionuser@gmail.com/jaxl25C3CD9A” from=“gmail.com” type=“result”>
    2.   <query xmlns=“http://jabber.org/protocol/disco#info”>
    3.     <identity category=“server” type=“im” name=“Google Talk”>
    4.     <feature var=“http://jabber.org/protocol/disco#info”>
    5.     <feature var=“google:jingleinfo”>
    6.     <feature var=“google:roster”>
    7.     <feature var=“google:nosave”>
    8.     <feature var=“google:setting”>
    9.     <feature var=“google:shared-status”>
    10.     <feature var=“http://jabber.org/protocol/archive#otr”>
    11.     <feature var=“google:mail:notify”>
    12.     <feature var=“http://jabber.org/protocol/archive#save”>
    13.   </feature></feature></feature></feature></feature></feature></feature></feature></feature></identity></query>
    14. </iq>
  • JAXL Sends >>
    1. <iq type=“get” id=“2”>
    2.   <query xmlns=“jabber:iq:roster”>
    3. </query></iq>
  • Gtalk Acknowledge With >>
    1. <iq to=“myproductionuser@gmail.com/jaxl25C3CD9A” id=“2” type=“result”>
    2.   <query xmlns=“jabber:iq:roster”>
    3.     <item jid=“friend_1@gtalkbots.com” subscription=“both”><group>Buddies</group></item>
    4.     <item jid=“friend_2@gmail.com” subscription=“both”>
    5.     <item jid=“friend_3@gmail.com” subscription=“both”><group>Buddies</group></item>
    6.     <item jid=“friend_4@gmail.com” subscription=“both” name=“Abhinav Singh”>
    7.     <item jid=“friend_5@gmail.com” subscription=“both”><group>Buddies</group></item>
    8.     <item jid=“friend_6@yahoo.co.in” subscription=“none” ask=“subscribe”>
    9.   </item></item></item></query>
    10. </iq>
  • JAXL Sends >>
    1. <presence from=“myproductionuser@gmail.com/jaxl25C3CD9A” to=“friend_6@yahoo.co.in” type=“subscribed”>
    2. <presence>
    3.   <show>chat</show>
    4.   <status>Online using JAXL – Jabber XMPP Library</status>
    5. </presence></presence>
  • Gtalk Acknowledge With >>
    1. <presence from=“friend_1@gtalkbots.com/gtalkbots.BCFBAC47” to=“myproductionuser@gmail.com/jaxl25C3CD9A”>
    2.   <status>I am online using Gtalkbot’s Client Library (JAXL)</status>
    3.   <x xmlns=“vcard-temp:x:update”>
    4. <photo>7d29d807158fd64820b109b4b42b2a23ca5a9d5a</photo>
    5.   </x>
    6. </presence>

And in this fashion JAXL Client and Gtalk Server keeps exchanging XML Streams when ever there is a new message or an update in friend’s status message.

For more detail and in-depth knowledge kindly refer to the RFC documents linked at the top.

PS: After publishing this post on January 2, 2009, it mysteriously vanished sometime between between March 8, 2010 and April 18, 2010. Today on Feb 1, 2015 while updating my blog theme I found about it via google webmaster tool and was able to recover using web.archieve.org