DynamoDB Session Handler

Introduction

The DynamoDB Session Handler is a custom session handler for PHP that allows 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 setup, 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 like session locking and garbage collection which are a part of PHP's default session handler.

For more information on the Amazon DynamoDB service, please visit the Amazon DynamoDB homepage.

Basic Usage

1. Register the handler

The first step is to instantiate and register the session handler.

use Aws\DynamoDb\SessionHandler;

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

$sessionHandler->register();

2. Create a table for storing your sessions

Before you can actually use the session handler, you need to create a table in which to store the sessions. This can be done ahead of time through the AWS Console for Amazon DynamoDB, or using the SDK.

3. Use PHP sessions like normal

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 Amazon 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 may configure the behavior of the session handler using the following options. All options are optional, but you should make sure to understand what the defaults are.

table_name

The name of the DynamoDB table in which to store the sessions. This defaults to 'sessions'.

hash_key

The name of the hash key in the DynamoDB sessions table. This defaults to 'id'.

session_lifetime

The lifetime of an inactive session before it should be garbage collected. If it is not provided, then the actual lifetime value that will be used is ini_get('session.gc_maxlifetime').

consistent_read

Whether or not the session handler should use consistent reads for the GetItem operation. This defaults to true.

locking

Whether or not to use session locking. This defaults to false.

batch_config

Configuration used to batch deletes during garbage collection. These options are passed directly into DynamoDB WriteRequestBatch objects. You must manually trigger garbage collection via SessionHandler::garbageCollect().

max_lock_wait_time

Maximum time (in seconds) that the session handler should wait to acquire a lock before giving up. This defaults to 10 and is only used with session locking.

min_lock_retry_microtime

Minimum time (in microseconds) that the session handler should wait between attempts to acquire a lock. This defaults to 10000 and is only used with session locking.

max_lock_retry_microtime

Maximum time (in microseconds) that the session handler should wait between attempts to acquire a lock. This defaults to 50000 and is only used with session locking.

To configure the Session Handler, you must 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',
    'session_lifetime'         => 3600,
    '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 Amazon 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 is going to 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.

Read via session_start()	1 read operation (only 0.5 if consistent_read is false). (Conditional) 1 write operation to delete the session if it is expired.
Read via session_start() (Using session locking)	A minimum of 1 write operation. (Conditional) Additional write operations for each attempt at acquiring a lock on the session. Based on configured lock wait time and retry options. (Conditional) 1 write operation to delete the session if it is expired.
Write via session_write_close()	1 write operation.
Delete via session_destroy()	1 write operation.
Garbage Collection	0.5 read operations per 4 KB of data in the table to scan for expired sessions. 1 write operation per expired item to delete it.

Session Locking

The DynamoDB Session Handler supports pessimistic session locking in order to mimic the behavior of PHP's default session handler. By default the DynamoDB Session Handler has this feature turned off since it can become a performance bottleneck and drive up costs, especially when an application accesses the session when using ajax requests or iframes. You should carefully consider whether or not your application requires session locking or not 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

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 in order 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 where 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 session handler in order 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

1. Create your sessions table in a region that is geographically closest to or in the same region as your application servers. This will ensure the lowest latency between your application and DynamoDB database.
2. Choose the provisioned throughput capacity of your sessions table carefully, taking into account the expected traffic to your application and the expected size of your sessions.
3. Monitor your consumed throughput through the AWS Management Console or with Amazon CloudWatch and adjust your throughput settings as needed to meet the demands of your application.
4. Keep the size of your sessions small (ideally less than 1 KB). Small sessions will perform better and require less provisioned throughput capacity.
5. Do not use session locking unless your application requires it.
6. Instead of using PHP's built-in session garbage collection triggers, schedule your garbage collection via a cron job, or another scheduling mechanism, to run during off-peak hours. Use the 'batch_config' option to your advantage.

Required IAM Permissions

To use the DynamoDB session handler, 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 AWS Identity and Access Management 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>"
    }
  ]
}