Conflict Resolution:
PHP

For reasons explained in the Introduction to conflict resolution, we strongly recommend adopting a conflict resolution strategy that requires applications to resolve siblings according to use-case-specific criteria. Here, we’ll provide a brief guide to conflict resolution using the official Riak PHP client.

How the PHP Client Handles Conflict Resolution

Every \Basho\Riak\Object command returns a \Basho\Riak\Command\Object\Response object, which provides what is needed to handle object conflicts. If siblings exist and have been returned from the server within the response body, they will be available within the response object. See below:

$response = (new \Basho\Riak\Command\Builder\FetchObject($riak))
    ->buildLocation('conflicted_key', 'bucket_name', 'bucket_type')
    ->build()
    ->execute();

echo $response->getStatusCode(); // 300
echo $response->hasSiblings(); // 1
echo $response->getSiblings(); // \Basho\Riak\Object[]

Basic Conflict Resolution Example

Let’s say that we’re building a social network application and storing lists of usernames representing each user’s “friends” in the network. Each user will bear the class User, which we’ll create below. All of the data for our application will be stored in buckets that bear the bucket type siblings, and for this bucket type allow_mult is set to true, which means that Riak will generate siblings in certain cases—siblings that our application will need to be equipped to resolve when they arise.

The question that we need to ask ourselves now is this: if a given user has sibling values, i.e. if there are multiple friends lists and Riak can’t decide which one is most causally recent, which list should be deemed “correct” from the standpoint of the application? What criteria should be applied in making that decision? Should the lists be merged? Should we pick a User object at random?

This decision will always be yours to make. Here, though, we’ll keep it simple and say that the following criterion will hold: if conflicting lists exist, the longer list will be the one that our application deems correct. So if the user user1234 has a sibling conflict where one possible value has friends lists with 100, 75, and 10 friends, respectively, the list of 100 friends will win out. While this might not make sense in real-world applications, it’s a good jumping-off point. We’ll explore the drawbacks of this approach, as well as a better alternative, in this document as well.

Creating Our Data Class

We’ll start by creating a User class for each user’s data. Each User object will consist of a username as well as a friends property that lists the usernames, as strings, of the user’s friends. We’ll use a Set for the friends property to avoid duplicates.

class User {
    public $username;
    public $friends;

    public function __construct($username, array $friends = [])
    {
        $this->username = $username;
        $this->friends = $friends;
    }

    public function __toString()
    {
        return json_encode([
            'username' => $this->username,
            'friends' => $this->friends,
            'friends_count' => count($this->friends)
        ]);
    }
}

Here’s an example of instantiating a new User object:

$bashobunny = new User('bashobunny', ['fred', 'barney']);

Implementing a Conflict Resolution Function

Let’s say that we’ve stored a bunch of User objects in Riak and that a few concurrent writes have led to siblings. How is our application going to deal with that? First, let’s say that there’s a User object stored in the bucket users (which is of the bucket type siblings, as explained above) under the key bashobunny. We can fetch the object that is stored there and see if it has siblings:

$response = (new \Basho\Riak\Command\Builder\FetchObject($riak))
    ->buildLocation('bashobunny', 'users', 'siblings')
    ->build()
    ->execute();

echo $response->hasSiblings(); // 1

If we get true, then there are siblings. So what do we do in that case? At this point, we need to write a function that resolves the list of siblings, i.e. reduces the $response->getSiblings() array down to one member. In our case, we need a function that takes a Riak response object as its argument, applies some logic to the list of values contained in the siblings property of the object, and returns a single value. For our example use case here, we’ll return the sibling with the longest friends list:

use \Basho\Riak;
use \Basho\Riak\Command;

function longest_friends_list_resolver(Command\Object\Response $response)
{
    if ($response->hasSiblings()) {
        $siblings = $response->getSiblings();
        $max_key = 0;
        foreach ($siblings as $key => $sibling) {
            if ($sibling->getData()['friends_count'] > $siblings[$max_key]->getData()['friends_count']) {
                $max_key = $key;
            }
        }
    }

    return $siblings[$max_key];
}

We can then embed this function into a more general function for fetching objects from the users bucket:

function fetch_user_by_username($username, Riak $riak)
{
    $response = (new Command\Builder\FetchObject($riak))
      ->buildLocation($username, 'users', 'siblings')
      ->build()
      ->execute();

    return longest_friends_list_resolver($response);
}

bashobunny = fetch_user_by_username('bashobunny', $riak);

Now, when a User object is fetched (assuming that the username acts as a key for the object), a single value is returned for the friends list. This means that our application can now use a “correct” value instead of having to deal with multiple values.

Conflict Resolution and Writes

In the above example, we created a conflict resolver that resolves a list of discrepant User objects and returns a single User. It’s important to note, however, that this resolver will only provide the application with a single “correct” value; it will not write that value back to Riak. That requires a separate step. When this step should be undertaken depends on your application. In general, though, we recommend writing objects to Riak only when the application is ready to commit them, i.e. when all of the changes that need to be made to the object have been made and the application is ready to persist the state of the object in Riak.

Correspondingly, we recommend that updates to objects in Riak follow these steps:

  1. Read the object from Riak
  2. Resolving sibling conflicts if they exist, allowing the application to reason about one “correct” value for the object (this step is the subject of this tutorial)
  3. Modify the object
  4. Write the object to Riak once the necessary changes have been made

You can find more on writing objects to Riak, including examples from the official PHP client library, in the Developing with Riak KV: Usage section.

More Advanced Example

Resolving sibling User values on the basis of which user has the longest friends list has the benefit of being simple but it’s probably not a good resolution strategy for our social networking application because it means that unwanted data loss is inevitable. If one friend list contains A, B, and C and the other contains D and E, the list containing A, B, and C will be chosen. So what about friends D and E? Those usernames are essentially lost. In the sections below, we’ll implement an alternative strategy as an example.

Merging the Lists

To avoid losing data like this, a better strategy would be to merge the lists. We can modify our original resolver function to accomplish precisely that and will also store the resulting User object.

The drawback to this approach is that it’s more or less inevitable that a user will remove a friend from their friends list, and then that friend will end up back on the list during a conflict resolution operation. While that’s certainly not desirable, that is likely better than the alternative proposed in the first example, which entails usernames being simply dropped from friends lists. Sibling resolution strategies almost always carry potential drawbacks of this sort.

Riak Data Types

An important thing to always bear in mind when working with conflict resolution is that Riak offers a variety of Data Types that have specific conflict resolution mechanics built in. If you have data that can be modeled as a counter, set, or map, then you should seriously consider using those Data Types instead of creating your own application-side resolution logic.

In the example above, we were dealing with conflict resolution within a set, in particular the friends list associated with each User object. The merge operation that we built to handle conflict resolution is analogous to the resolution logic that is built into Riak sets. For more information on how you could potentially replace the client-side resolution that we implemented above, see our tutorial on Riak sets.