Using the DynamoDB session handler with AWS SDK for PHP Version 3

The DynamoDB Session Handler is a custom session handler for PHP that enables developers to use Amazon DynamoDB as a session store. Using DynamoDB for session storage alleviates issues that occur with session handling in a distributed web application by moving sessions off of the local file system and into a shared location. DynamoDB is fast, scalable, easy to set up, and handles replication of your data automatically.

The DynamoDB Session Handler uses the session_set_save_handler() function to hook DynamoDB operations into PHP’s native session functions to allow for a true drop in replacement. This includes support for features such as session locking and garbage collection, which are a part of PHP’s default session handler.

For more information about the DynamoDB service, see the Amazon DynamoDB homepage.

Basic usage

Step 1: Register the handler

First, instantiate and register the session handler.

use Aws\DynamoDb\SessionHandler;

$dynamoDb = new Aws\DynamoDb\DynamoDbClient([
    'region'=>'us-east-1'  // Since version 3.277.10 of the SDK, 
]);                        // the 'version' parameter defaults to 'latest'.

$sessionHandler = SessionHandler::fromClient($dynamoDb, [
    'table_name' => 'sessions'
]);

$sessionHandler->register();

Step 2. Create a table to store your sessions

Before you can actually use the session handler, you need to create a table in which to store the sessions. You can do this ahead of time by using the AWS Console for Amazon DynamoDB, or by using the AWS SDK for PHP.

When creating this table use ‘id’ as the name of the primary key. Also it is recommended to setup a Time To Live attribute using the ‘expires’ attribute to benefit from automatic garbage collection of sessions.

Step 3. Use PHP sessions as you normally would

Once the session handler is registered and the table exists, you can write to and read from the session using the $_SESSION superglobal, just like you normally do with PHP’s default session handler. The DynamoDB Session Handler encapsulates and abstracts the interactions with DynamoDB and enables you to simply use PHP’s native session functions and interface.

// Start the session
session_start();

// Alter the session data
$_SESSION['user.name'] = 'jeremy';
$_SESSION['user.role'] = 'admin';

// Close the session (optional, but recommended)
session_write_close();

Configuration

You can configure the behavior of the session handler using the following options. All options are optional, but be sure to understand what the defaults are.

To configure the Session Handler, specify the configuration options when you instantiate the handler. The following code is an example with all of the configuration options specified.

$sessionHandler = SessionHandler::fromClient($dynamoDb, [
    'table_name'                    => 'sessions',
    'hash_key'                      => 'id',
    'data_attribute'                => 'data',
    'data_attribute_type'           => 'string',
    'session_lifetime'              => 3600,
    'session_lifetime_attribute'    => 'expires',
    'consistent_read'               => true,
    'locking'                       => false,
    'batch_config'                  => [],
    'max_lock_wait_time'            => 10,
    'min_lock_retry_microtime'      => 5000,
    'max_lock_retry_microtime'      => 50000,
]);

Pricing

Aside from data storage and data transfer fees, the costs associated with using DynamoDB are calculated based on the provisioned throughput capacity of your table (see the Amazon DynamoDB pricing details). Throughput is measured in units of write capacity and read capacity. The Amazon DynamoDB homepage says:

A unit of read capacity represents one strongly consistent read per second (or two eventually consistent reads per second) for items as large as 4 KB. A unit of write capacity represents one write per second for items as large as 1 KB.

Ultimately, the throughput and the costs required for your sessions table will correlate with your expected traffic and session size. The following table explains the amount of read and write operations that are performed on your DynamoDB table for each of the session functions.

Session locking

The DynamoDB Session Handler supports pessimistic session locking to mimic the behavior of PHP’s default session handler. By default, the DynamoDB Session Handler has this feature turned off because it can become a performance bottleneck and drive up costs, especially when an application accesses the session when using Ajax requests or iframes. Carefully consider whether your application requires session locking before enabling it.

To enable session locking, set the 'locking' option to true when you instantiate the SessionHandler.

$sessionHandler = SessionHandler::fromClient($dynamoDb, [
    'table_name' => 'sessions',
    'locking'    => true,
]);

Garbage collection

Setup a TTL attribute in your DynamoDB table, using the attribute ‘expires’. This will automatically garbage collect your sessions and avoid the need to garbage collect them yourself.

Alternatively, the DynamoDB Session Handler supports session garbage collection by using a series of Scan and BatchWriteItem operations. Due to the nature of how the Scan operation works, and to find all of the expired sessions and delete them, the garbage collection process can require a lot of provisioned throughput.

For this reason, we do not support automated garbage collection. A better practice is to schedule the garbage collection to occur during an off-peak time when a burst of consumed throughput will not disrupt the rest of the application. For example, you could have a nightly cron job trigger a script to run the garbage collection. This script would need to do something like the following.

$sessionHandler = SessionHandler::fromClient($dynamoDb, [
    'table_name'   => 'sessions',
    'batch_config' => [
        'batch_size' => 25,
        'before' => function ($command) {
            echo "About to delete a batch of expired sessions.\n";
        }
    ]
]);

$sessionHandler->garbageCollect();

You can also use the 'before' option within 'batch_config' to introduce delays on the BatchWriteItem operations that are performed by the garbage collection process. This will increase the amount of time it takes the garbage collection to complete, but it can help you spread out the requests made by the DynamoDB Session Handler to help you stay close to or within your provisioned throughput capacity during garbage collection.

$sessionHandler = SessionHandler::fromClient($dynamoDb, [
    'table_name'   => 'sessions',
    'batch_config' => [
        'before' => function ($command) {
            $command['@http']['delay'] = 5000;
        }
    ]
]);

$sessionHandler->garbageCollect();

Best practices

Required IAM permissions

To use the DynamoDB SessionHhandler, your configured credentials must have permission to use the DynamoDB table that you created in a previous step. The following IAM policy contains the minimum permissions that you need. To use this policy, replace the Resource value with the Amazon Resource Name (ARN) of the table that you created previously. For more information about creating and attaching IAM policies, see Managing IAM Policies in the IAM User Guide.

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Action": [
        "dynamodb:GetItem",
        "dynamodb:UpdateItem",
        "dynamodb:DeleteItem",
        "dynamodb:Scan",
        "dynamodb:BatchWriteItem"
      ],
      "Effect": "Allow",
      "Resource": "arn:aws:dynamodb:<region>:<account-id>:table/<table-name>"
    }
  ]
}