Redis

A case study: Design and implementation of a simple Twitter clone using only PHP and the Redis key-value store

In this article I'll describe the design and implementation of a simple clone of Twitter written using PHP with Redis as the only database. The programming community has traditionally considered key-value stores as a special purpose database that couldn't be used as a drop in replacement for a relational database for the development of web applications. This article will try to correct this impression.

Our Twitter clone, called Retwis, is structurally simple, has very good performance, and can be distributed among any number of web and Redis servers with very little effort. You can find the source code here.

I use PHP for the example since it can be read by everybody. The same (or... much better) results can be obtained using Ruby, Python, Erlang, and so on.

Note: Retwis-RB is a port of Retwis to Ruby and Sinatra written by Daniel Lucraft! Full source code is included of course, and a link to its Git repository appears in the footer of this article. The rest of this article targets PHP, but Ruby programmers can also check the Retwis-RB source code since it's conceptually very similar.

Note: Retwis-J is a port of Retwis to Java, using the Spring Data Framework, written by Costin Leau. Its source code can be found on GitHub, and there is comprehensive documentation available at springsource.org.

Key-value store basics

The essence of a key-value store is the ability to store some data, called a value, inside a key. The value can be retrieved later only if we know the specific key it was stored in. There is no way to search for a key by value. In a sense, it is like a very large hash/dictionary, but it is persistent, i.e. when your application ends, the data doesn't go away. So, for example, I can use the command SET to store the value bar in the key foo:

SET foo bar

Redis stores data permanently, so if I later ask "What is the value stored in key foo?" Redis will reply with bar:

GET foo => bar

Other common operations provided by key-value stores are DEL, to delete a given key and its associated value, SET-if-not-exists (called SETNX on Redis), to assign a value to a key only if the key does not already exist, and INCR, to atomically increment a number stored in a given key:

SET foo 10
INCR foo => 11
INCR foo => 12
INCR foo => 13

Atomic operations

There is something special about INCR. Think about why Redis provides such an operation if we can do it ourselves with a bit of code? After all, it is as simple as:

x = GET foo
x = x + 1
SET foo x

The problem is that incrementing this way will work as long as there is only one client working with the key foo at one time. See what happens if two clients are accessing this key at the same time:

x = GET foo (yields 10)
y = GET foo (yields 10)
x = x + 1 (x is now 11)
y = y + 1 (y is now 11)
SET foo x (foo is now 11)
SET foo y (foo is now 11)

Something is wrong! We incremented the value two times, but instead of going from 10 to 12, our key holds 11. This is because the increment done with GET / increment / SET is not an atomic operation. Instead the INCR provided by Redis, Memcached, ..., are atomic implementations, and the server will take care of protecting the key during the time needed to complete the increment in order to prevent simultaneous accesses.

What makes Redis different from other key-value stores is that it provides other operations similar to INCR that can be used to model complex problems. This is why you can use Redis to write whole web applications without using an SQL database and without going crazy.

Beyond key-value stores

In this section we will see which Redis features we need to build our Twitter clone. The first thing to know is that Redis values can be more than strings. Redis supports Lists and Sets as values, and there are atomic operations to operate on them so we are safe even with multiple accesses of the same key. Let's start with Lists:

LPUSH mylist a (now mylist holds 'a')
LPUSH mylist b (now mylist holds 'b','a')
LPUSH mylist c (now mylist holds 'c','b','a')

LPUSH means Left Push, that is, add an element to the left (or to the head) of the list stored in mylist. If the key mylist does not exist it is automatically created as an empty list before the PUSH operation. As you can imagine, there is also an RPUSH operation that adds the element to the right of the list (on the tail). This is very useful for our Twitter clone. User updates can be added to a list stored in username:updates, for instance.

There are operations to get data from Lists, of course. For instance, LRANGE returns a range from the list, or the whole list.

LRANGE mylist 0 1 => c,b

LRANGE uses zero-based indexes - that is the first element is 0, the second 1, and so on. The command arguments are LRANGE key first-index last-index. The last-index argument can be negative, with a special meaning: -1 is the last element of the list, -2 the penultimate, and so on. So, to get the whole list use:

LRANGE mylist 0 -1 => c,b,a

Other important operations are LLEN that returns the number of elements in the list, and LTRIM that is like LRANGE but instead of returning the specified range trims the list, so it is like Get range from mylist, Set this range as new value but does so atomically. We will use only these List operations, but make sure to check the Redis documentation to discover all the List operations supported by Redis.

The set data type

There are more data types than just Lists. Redis also supports Sets, which are unsorted collections of elements. It is possible to add, remove, and test for existence of members, and perform the intersection between different Sets. Of course it is possible to get the elements of a set. Some examples will make it more clear. Keep in mind that SADD is the add to set operation, SREM is the remove from set operation, sismember is the test if member operation, and SINTER is the perform intersection operation. Other operations are SCARD to get the cardinality (the number of elements) of a Set, and SMEMBERS to return all the members of a Set.

SADD myset a
SADD myset b
SADD myset foo
SADD myset bar
SCARD myset => 4
SMEMBERS myset => bar,a,foo,b

Note that SMEMBERS does not return the elements in the same order we added them since Sets are unsorted collections of elements. When you want to store in order it is better to use Lists instead. Some more operations against Sets:

SADD mynewset b
SADD mynewset foo
SADD mynewset hello
SINTER myset mynewset => foo,b

SINTER can return the intersection between Sets but it is not limited to two sets. You may ask for the intersection of 4,5, or 10000 Sets. Finally let's check how SISMEMBER works:

SISMEMBER myset foo => 1
SISMEMBER myset notamember => 0

Okay, we are ready to start coding!

Prerequisites

If you haven't downloaded the Retwis source code already please grab it now. It's a simple tar.gz file containing a few PHP files. The implementation is very simple. You will find the PHP library client (redis.php) inside that is used to talk with the Redis server from PHP. This library was written by Ludovico Magnocavallo and you are free to reuse this in your own projects. For an updated version of the library please download the Redis distribution. (Note: there are now better PHP libraries available. Check our clients page.

Another thing you probably want is a working Redis server. Just get the source, build with make, run with ./redis-server, and you're ready to go. No configuration is required at all in order to play with or run Retwis on your computer.

Data layout

When working with a relational database, database schema must be designed so that we'd know the tables, indexes, and so on that the database will contain. We don't have tables in Redis, so what do we need to design? We need to identify what keys are needed to represent our objects and what kind of values this keys need to hold.

Let's start with Users. We need to represent users, of course, with their username, userid, password, the users who follow the user, the users who the user follows, and so on. The first question is, how should we identify a user? The username might be a good idea since it is unique, but it is also too big since we want to stay low on memory usage. So like in a relational DB we can associate a unique ID with every user. Every other reference to this user will be done by id. Creating unique IDs is very simple to do by using our atomic INCR operation! When we create a new user we can do something like this, assuming the user is called "antirez":

INCR global:nextUserId => 1000
SET uid:1000:username antirez
SET uid:1000:password p1pp0

We use the global:nextUserId key in order to always get an unique ID for every new user. Then we use this unique ID to name all the other keys holding the user's data. This is a Design Pattern with key-values stores! Keep it in mind. Besides the fields already defined, we need some more stuff in order to fully define a User. For example, sometimes it can be useful to be able to get the user ID from the username, so we set this key too:

SET username:antirez:uid 1000

This may appear strange at first, but remember that we are only able to access data by key! It's not possible to tell Redis to return the key that holds a specific value. This is also our strength. This new paradigm is forcing us to organize data so that everything is accessible by primary key, speaking in relational DB terms.

Followers, following, and updates

There is another central need in our system. A user might have users who follow them, which we'll call their followers. A user might follow other users, which we'll call a following. We have a perfect data structure for this! That is... Sets. So let's add these two new fields to our schema:

uid:1000:followers => Set of uids of all the followers users
uid:1000:following => Set of uids of all the following users

Another important thing we need is a place were we can add the updates to display in the user's home page. We'll need to access this data in chronological order later, from the most recent update to the oldest, so the perfect kind of data structure for this is a List. Basically every new update will be LPUSHed in the user updates key, and thanks to LRANGE, we can implement pagination and so on. Note that we use the words updates and posts interchangeably, since updates are actually "little posts" in some way.

uid:1000:posts => a List of post ids - every new post is LPUSHed here.

Authentication

OK, we have more or less everything about the user except for authentication. We'll handle authentication in a simple but robust way: we don't want to use PHP sessions or other things like this, our system must be ready to be distributed among different servers, so we'll keep the whole state in our Redis database. So all we need is a random string to set as the cookie of an authenticated user, and a key that will contain the user ID of the client holding the string. We need two keys in order to make this thing work in a robust way:

SET uid:1000:auth fea5e81ac8ca77622bed1c2132a021f9
SET auth:fea5e81ac8ca77622bed1c2132a021f9 1000

In order to authenticate a user we'll do these simple steps (login.php): * Get the username and password via the login form * Check if the username:<username>:uid key actually exists * If it exists we have the user id, (i.e. 1000) * Check if uid:1000:password matches, if not, create an error message * Ok authenticated! Set "fea5e81ac8ca77622bed1c2132a021f9" (the value of uid:1000:auth) as "auth" cookie

This is the actual code:

include("retwis.php");

# Form sanity checks
if (!gt("username") || !gt("password"))
    goback("You need to enter both username and password to login.");

# The form is OK, check if the username is available
$username = gt("username");
$password = gt("password");
$r = redisLink();
$userid = $r->get("username:$username:id");
if (!$userid)
    goback("Wrong username or password");
$realpassword = $r->get("uid:$userid:password");
if ($realpassword != $password)
    goback("Wrong useranme or password");

# Username / password OK, set the cookie and redirect to index.php
$authsecret = $r->get("uid:$userid:auth");
setcookie("auth",$authsecret,time()+3600*24*365);
header("Location: index.php");

This happens every time a user logs in, but we also need a function isLoggedIn in order to check if a given user is already authenticated or not. These are the logical steps preformed by the isLoggedIn function: * Get the "auth" cookie from the user. If there is no cookie, the user is not logged in, of course. Let's get the value of the cookie <authcookie> * Check if auth:<authcookie> exists, and what the value (the user id) is (1000 in the example). * In order to be sure, check that uid:1000:auth matches. * OK the user is authenticated, and we loaded a bit of information in the $User global variable.

The code is simpler than the description, possibly:

function isLoggedIn() {
    global $User, $_COOKIE;

    if (isset($User)) return true;

    if (isset($_COOKIE['auth'])) {
        $r = redisLink();
        $authcookie = $_COOKIE['auth'];
        if ($userid = $r->get("auth:$authcookie")) {
            if ($r->get("uid:$userid:auth") != $authcookie) return false;
            loadUserInfo($userid);
            return true;
        }
    }
    return false;
}

function loadUserInfo($userid) {
    global $User;

    $r = redisLink();
    $User['id'] = $userid;
    $User['username'] = $r->get("uid:$userid:username");
    return true;
}

Having loadUserInfo as a separate function is overkill for our application, but it's a good approach in a complex application. The only thing that's missing from all the authentication is the logout. What do we do on logout? That's simple, we'll just change the random string in uid:1000:auth, remove the old auth:<oldauthstring>, and add a new auth:<newauthstring>.

Important: the logout procedure explains why we don't just authenticate the user after looking up auth:<randomstring>, but double check it against uid:1000:auth. The true authentication string is the latter, auth:<randomstring> is just an authentication key that may even be volatile, or, if there are bugs in the program or a script gets interrupted, we may even end with multiple auth:<something> keys pointing to the same user id. The logout code is the following (logout.php):

include("retwis.php");

if (!isLoggedIn()) {
    header("Location: index.php");
    exit;
}

$r = redisLink();
$newauthsecret = getrand();
$userid = $User['id'];
$oldauthsecret = $r->get("uid:$userid:auth");

$r->set("uid:$userid:auth",$newauthsecret);
$r->set("auth:$newauthsecret",$userid);
$r->delete("auth:$oldauthsecret");

header("Location: index.php");

That is just what we described and should be simple to understand.

Updates

Updates, also known as posts, are even simpler. In order to create a new post in the database we do something like this:

INCR global:nextPostId => 10343
SET post:10343 "$owner_id|$time|I'm having fun with Retwis"

As you can see, the user id and time of the post are stored directly inside the post string, so we don't need to lookup by time or user id in the example application. It is better to compact everything inside the post string.

After we create a post we obtain the post id. We need to LPUSH this post id in every user that's following the author of the post, and of course in the list of posts of the author. This is the file update.php that shows how this is performed:

include("retwis.php");

if (!isLoggedIn() || !gt("status")) {
    header("Location:index.php");
    exit;
}

$r = redisLink();
$postid = $r->incr("global:nextPostId");
$status = str_replace("\n"," ",gt("status"));
$post = $User['id']."|".time()."|".$status;
$r->set("post:$postid",$post);
$followers = $r->smembers("uid:".$User['id'].":followers");
if ($followers === false) $followers = Array();
$followers[] = $User['id']; /* Add the post to our own posts too */

foreach($followers as $fid) {
    $r->push("uid:$fid:posts",$postid,false);
}
# Push the post on the timeline, and trim the timeline to the
# newest 1000 elements.
$r->push("global:timeline",$postid,false);
$r->ltrim("global:timeline",0,1000);

header("Location: index.php");

The core of the function is the foreach loop. We use SMEMBERS to get all the followers of the current user, then the loop will LPUSH the post against the uid:<userid>:posts of every follower.

Note that we also maintain a timeline for all the posts. This requires just LPUSHing the post against global:timeline. Let's face it, did you start thinking it was a bit strange to have to sort things added in chronological order using ORDER BY with SQL? I think so.

Paginating updates

Now it should be pretty clear how we can use LRANGE in order to get ranges of posts, and render these posts on the screen. The code is simple:

function showPost($id) {
    $r = redisLink();
    $postdata = $r->get("post:$id");
    if (!$postdata) return false;

    $aux = explode("|",$postdata);
    $id = $aux[0];
    $time = $aux[1];
    $username = $r->get("uid:$id:username");
    $post = join(array_splice($aux,2,count($aux)-2),"|");
    $elapsed = strElapsed($time);
    $userlink = "<a class=\"username\" href=\"profile.php?u=".urlencode($username)."\">".utf8entities($username)."</a>";

    echo('<div class="post">'.$userlink.' '.utf8entities($post)."<br>");
    echo('<i>posted '.$elapsed.' ago via web</i></div>');
    return true;
}

function showUserPosts($userid,$start,$count) {
    $r = redisLink();
    $key = ($userid == -1) ? "global:timeline" : "uid:$userid:posts";
    $posts = $r->lrange($key,$start,$start+$count);
    $c = 0;
    foreach($posts as $p) {
        if (showPost($p)) $c++;
        if ($c == $count) break;
    }
    return count($posts) == $count+1;
}

showPost will simply convert and print a Post in HTML while showUserPosts gets a range of posts and then passes them to showPosts.

Following users

If user id 1000 (antirez) wants to follow user id 1001 (pippo), we can do this with just two SADD:

SADD uid:1000:following 1001 SADD uid:1001:followers 1000

Note the same pattern again and again. In theory with a relational database the list of following and followers would be contained in a single table with fields like following_id and follower_id. You can extract the followers or following of every user. With a key-value DB things are a bit different since we need to set both the 1000 is following 1001 and 1001 is followed by 1000 relations. This is the price to pay, but on the other hand accessing the data is simpler and ultra-fast. And having these things as separate sets allows us to do interesting stuff. For example, using SINTER we can have the intersection of 'following' of two different users, so we may add a feature to our Twitter clone so that it is able to tell you very quickly when you visit somebody else's profile, "you and foobar have 34 followers in common", and things like that.

You can find the code that sets or removes a following/follower relation at follow.php. It is trivial, as you can see.

Making it horizontally scalable

Gentle reader, if you reached this point you are already a hero. Thank you. Before talking about scaling horizontally it is worth checking performance on a single server. Retwis is amazingly fast, without any kind of cache. On a very slow and loaded server, an apache benchmark with 100 parallel clients issuing 100000 requests measured the average pageview to take 5 milliseconds. This means you can serve millions of users every day with just a single Linux box, and this one was monkey ass slow! Imagine the results with more recent hardware.

So, first of all, probably you will not need more than one server for a lot of applications, even when you have a lot of users. But let's assume we are Twitter and need to handle a huge amount of traffic. What to do?

Hashing the key

The first thing to do is to hash the key and issue the request to different servers based on the key hash. There are a lot of well known algorithms to do so. For example, check the Redis Ruby library client that implements consistent hashing, but the general idea is that you can turn your key into a number, and than take the reminder of the division of this number by the number of servers you have:

server_id = crc32(key) % number_of_servers

This has a lot of problems since if you add one server you need to move too many keys and so on, but this is the general idea.

Ok, are key accesses distributed among the key space? Well, all the user data will be partitioned among different servers. There are no inter-key operations used (like SINTER, otherwise you need to care that things you want to intersect will end up on the same server. This is why Redis, unlike memcached, does not force a specific hashing scheme. It's application specific). Btw there are keys that are accessed more frequently.

Special keys

For example, every time we post a new message, we need to increment the global:nextPostId key. How to fix this problem? A single server will get a lot of increments. The simplest way to handle this is to have a dedicated server just for increments. This is probably overkill unless you have really a lot of traffic. There is another trick. The ID does not really need to be an incremental number, but just it needs to be unique. So you can get a random string long enough to be unlikely (almost impossible, if it's md5-size) to collide, and you are done. We successfully eliminated our main problem to make it really horizontally scalable!

There is another one: global:timeline. There is no fix for this, if you need to retrieve something in order you can split among different servers and then merge when you need to get the data back, or take it ordered and use a single key. Again, if you really have that many posts per second, you can use a single server just for this. Remember that with commodity hardware Redis is able to handle 100000 writes per second. That's enough even for Twitter, I guess.

Please feel free to use the comments below for questions and feedbacks.