Handlers and Middleware

The primary mechanism for extending the SDK is through handlers and middleware. Each SDK client class owns an Aws\HandlerList instance that is accessible through the getHandlerList() method of a client. You can retrieve the HandlerList of a client and modify it to add or remove client behavior.

Handlers

A handler is a function that performs the actual transformation of a command and request into a result. A handler typically sends HTTP requests. Handlers can be composed with middlewares to augment their behavior. A handler is a function that accepts an Aws\CommandInterface and a Psr\Http\Message\RequestInterface and returns a promise that is fulfilled with an Aws\ResultInterface or rejected with an Aws\Exception\AwsException reason.

Here's a handler that returns the same mock result for each call:

use Aws\CommandInterface;
use Aws\Result;
use Psr\Http\Message\RequestInterface;
use GuzzleHttp\Promise;

$myHandler = function (CommandInterface $cmd, RequestInterface $request) {
    $result = new Result(['foo' => 'bar']);
    return Promise\promise_for($result);
};

You can then use this handler with an SDK client by providing a handler option in the constructor of a client:

// Set the handler of the client in the constructor.
$s3 = new Aws\S3\S3Client([
    'region'  => 'us-east-1',
    'version' => '2006-03-01',
    'handler' => $myHandler
]);

You can also change the handler of a client after it is constructed using the setHandler method of an Aws\ClientInterface:

// Set the handler of the client after it is constructed.
$s3->getHandlerList()->setHandler($myHandler);

Mock Handler

We recommend using the MockHandler when writing tests that use the SDK. You can use the Aws\MockHandler to return mocked results or throw mock exceptions. You enqueue results or exceptions, and the MockHandler will dequeue them in FIFO order.

use Aws\Result;
use Aws\MockHandler;
use Aws\DynamoDb\DynamoDbClient;
use Aws\CommandInterface;
use Psr\Http\Message\RequestInterface;

$mock = new MockHandler();

// Return a mocked result.
$mock->append(new Result(['foo' => 'bar']));

// You can provide a function to invoke. Here we throw a mock exception.
$mock->append(function (CommandInterface $cmd, RequestInterface $req) {
    return new AwsException('Mock exception', $command);
});

// Create a client with the mock handler.
$client = new DynamoDbClient([
    'region'  => 'us-west-2',
    'version' => 'latest',
    'handler' => $mock
]);

// Result object response will contain ['foo' => 'bar']
$result = $client->listTables();

// This will throw the exception that was enqueued
$client->listTables();

Middleware

Middleware are a special type of high level function that that augment the behavior of transferring a command and delegate to a "next" handler. Middleware functions accept an Aws\CommandInterface and a Psr\Http\Message\RequestInterface and return a promise that is fulfilled with an Aws\ResultInterface or rejected with an Aws\Exception\AwsException reason.

A middleware is a higher-order function that modifies a command, request, or result as it passes through the middleware. A middleware takes the following form:

use Aws\CommandInterface;
use Psr\Http\Message\RequestInterface;

$middleware = function () {
    return function (callable $handler) use ($fn) {
        return function (
            CommandInterface $command,
            RequestInterface $request = null
        ) use ($handler, $fn) {
            // Do something before calling the next handler
            // ...
            $promise = $fn($command, $request);
            // Do something in the promise after calling the next handler
            // ...
            return $promise;
        };
    };
};

A middleware receives a command to execute and an optional request object. The middleware can choose to augment the request and command or leave them as-is. A middleware then invokes the next handle in the chain or can choose to short-circuit the next handler and return a promise. The promise that is created by invoking the next handler can then be augmented using the then method of the promise to modify the eventual result or error before giving returning the promise back up the stack of middlewares.

HandlerList

The SDK uses an Aws\HandlerList to manage the middlewares and handlers used when executing a command. Each SDK client owns a HandlerList, and this HandlerList is cloned and added to each command that is created by a client. You can attach a middleware and default handler to use for each command created by a client by adding a middleware to the client's HandlerList. You can add and remove middlewares from specific commands by modifying the HandlerList owned by a specific command.

A HandlerList represents a stack of middlewares that are used to wrap a handler. To help manage the list of middleware and the order in which they wrap a handler, the HandlerList breaks the middleware stack into named steps that represents part of the lifecycle of transferring a command:

  1. init: add default parameters
  2. validate: validate required parameters
  3. build: serialize an HTTP request for sending
  4. sign: sign the serialized HTTP request
  5. <handler> (not a step, but performs the actual transfer)
init

This lifecycle step represents the initialization of a command, and a request has not yet been serialized. This step is typically used to add default parameters to a command.

You can add a middleware to the init step using the appendInit and prependInit methods where appendInit adds the middleware to the end of the prepend list while prependInit adds the middleware to the front of the prepend list.

use Aws\Middleware;

$middleware = Middleware::tap(function ($cmd, $req) {
    // observe the step
});

// Append to the end of the step with a custom name.
$client->getHandlerList()->appendInit($middleware, 'custom-name');
// Prepend to the beginning of the step
$client->getHandlerList()->prependInit($middleware, 'custom-name');
validate

This lifecycle step is used for validating the input parameters of a command.

You can add a middleware to the validate step using the appendValidate and prependValidate methods where appendValidate adds the middleware to the end of the validate list while prependValidate adds the middleware to the front of the validate list.

use Aws\Middleware;

$middleware = Middleware::tap(function ($cmd, $req) {
    // observe the step
});

// Append to the end of the step with a custom name.
$client->getHandlerList()->appendValidate($middleware, 'custom-name');
// Prepend to the beginning of the step
$client->getHandlerList()->prependValidate($middleware, 'custom-name');
build

This lifecycle step is used to serialize an HTTP request for the command being executed. Downstream lifecycle events will receive a command and PSR-7 HTTP request.

You can add a middleware to the build step using the appendBuild and prependBuild methods where appendBuild adds the middleware to the end of the build list while prependBuild adds the middleware to the front of the build list.

use Aws\Middleware;

$middleware = Middleware::tap(function ($cmd, $req) {
    // observe the step
});

// Append to the end of the step with a custom name.
$client->getHandlerList()->appendBuild($middleware, 'custom-name');
// Prepend to the beginning of the step
$client->getHandlerList()->prependBuild($middleware, 'custom-name');
sign

This lifecycle step is typically used to sign HTTP requests before they are sent over the wire. You should typically refrain from mutating an HTTP request after it is signed to avoid signature errors.

This it the last step in the HandlerList before the HTTP request is transferred by a handler.

You can add a middleware to the sign step using the appendSign and prependSign methods where appendSign adds the middleware to the end of the sign list while prependSign adds the middleware to the front of the sign list.

use Aws\Middleware;

$middleware = Middleware::tap(function ($cmd, $req) {
    // observe the step
});

// Append to the end of the step with a custom name.
$client->getHandlerList()->appendSign($middleware, 'custom-name');
// Prepend to the beginning of the step
$client->getHandlerList()->prependSign($middleware, 'custom-name');

Available Middleware

The SDK ships with several middlewares that can be used to augment the behavior of a client or to observe the execution of a command.

mapCommand

The Aws\Middleware::mapCommand middleware is useful when you need to modify a command before the command is serialized as an HTTP request. For example, mapCommand could be used to perform validation or add default parameters. The mapCommand function accepts a callable that accepts an Aws\CommandInterface object and returns an Aws\CommandInterface object.

use Aws\Middleware;
use Aws\CommandInterface;

// Here we've omitted the require Bucket parameter. We'll add it in the
// custom middleware.
$command = $s3Client->getCommand('HeadObject', ['Key' => 'test']);

// Apply a custom middleware named "add-param" to the "init" lifecycle step
$command->getHandlerList()->appendInit(
    Middleware::mapCommand(function (CommandInterface $command) {
        $command['Bucket'] = 'mybucket';
        // Be sure to return the command!
        return $command;
    }),
    'add-param'
);

mapRequest

The Aws\Middleware::mapRequest middleware is useful when you need to modify a request after it has been serialized but before it is sent. For example, this can be used to add custom HTTP headers to a request. The mapRequest function accepts a callable that accepts a Psr\Http\Message\RequestInterface argument and returns a Psr\Http\Message\RequestInterface object.

use Aws\Middleware;
use Psr\Http\Message\RequestInterface;

// Create a command so that we can access the handler list.
$command = $s3Client->getCommand('HeadObject', [
    'Key'    => 'test',
    'Bucket' => 'mybucket'
]);

// Apply a custom middleware named "add-header" to the "build" lifecycle step
$command->getHandlerList()->appendBuild(
    Middleware::mapRequest(function (RequestInterface $request) {
        // Return a new request with the added header
        return $request->withHeader('X-Foo-Baz', 'Bar');
    }),
    'add-header'
);

Now when the command is executed, it will be sent with the custom header.

Important

Notice that the middleware was appended to the handler list at the end of build step. This is to ensure that a request has been been built before this middleware is invoked.

mapResult

The Aws\Middleware::mapResult middleware is useful when you need to modify the result of a command execution. The mapResult function accepts a callable that accepts an Aws\ResultInterface argument and returns an Aws\ResultInterface object.

use Aws\Middleware;
use Aws\ResultInterface;

$command = $s3Client->getCommand('HeadObject', [
    'Key'    => 'test',
    'Bucket' => 'mybucket'
]);

$command->getHandlerList()->appendSign(
    Middleware::mapResult(function (ResultInterface $result) {
        // Add a custom value to the result
        $result['foo'] = 'bar';
        return $result;
    })
);

Now when the command is executed, it the returned result will contain a foo attribute.

history

The history middleware is useful for testing that the SDK executed the commands you expected, sent the HTTP requests you expected, and received the results you expected. It's essentially a middleware that acts similarly to the history of a web browser.

use Aws\History;
use Aws\Middleware;

$ddb = new Aws\DynamoDb\DynamoDbClient([
    'version' => 'latest',
    'region'  => 'us-west-2'
]);

// Create a history container to store the history data.
$history = new History();

// Add the history middleware that uses the history container.
$ddb->getHandlerList()->appendSign(Middleware::history($history));

An Aws\History history container will store 10 entries by default before purging entries. The number of entries can be customized by passing in the number of entries to persist to the constructor.

// Create a history container that stores 20 entries.
$history = new History(20);

You can inspect the history container after executing requests that pass a the history middleware.

// The object is countable, returning the number of entries in the container.
count($history);

// The object is iterable, yielding each entry in the container.
foreach ($history as $entry) {
    // You can access the command that was executed.
    var_dump($entry['command']);
    // The request that was serialized and sent.
    var_dump($entry['request']);
    // The result that was received (if successful).
    var_dump($entry['result']);
    // The exception that was received (if a failure occurred).
    var_dump($entry['exception']);
}

// You can get the last Aws\CommandInterface that was executed. This method
// will throw if no commands have been executed.
$command = $history->getLastCommand();

// You can get the last request that was serialized. This method will throw
// if no requests have been serialized.
$request = $history->getLastRequest();

// You can get the last return value (an Aws\ResultInterface or Exception).
// The method will throw if no value has been returned for the last
// executed operation (e.g., an async request has not completed).
$result = $history->getLastReturn();

// You can clear out the entries using clear
$history->clear();

tap

The tap middleware is used as an observer. This middleware can be used to invoke functions when sending commands through the chain of middlewares. The tap function accepts a callable that accepts the Aws\CommandInterface and an optional Psr\Http\Message\RequestInterface that is being executed.

use Aws\Middleware;

$s3 = new Aws\S3\S3Client([
    'region'  => 'us-east-1',
    'version' => '2006-03-01'
]);

$handlerList = $s3->getHandlerList();

// Create a tap middleware that observes the command at a specific step.
$handlerList->appendInit(
    Middleware::tap(function (CommandInterface $cmd, RequestInterface $req = null) {
        echo 'About to send: ' . $cmd->getName() . "\n";
        if ($req) {
            echo 'HTTP method: ' . $request->getMethod() . "\n";
        }
    }
);

Creating Custom handlers

A handler is simply a function that accepts an Aws\CommandInterface object and Psr\Http\Message\RequestInterface object and returns a GuzzleHttp\Promise\PromiseInterface that is fulfilled with an Aws\ResultInterface or rejected with an Aws\Exception\AwsException.

While the SDK has several @http options, a handler only needs to know how to use the following options:

Unless the option is specified as optional, a handler MUST be able to handle the option or it MUST return a rejected promise.

In addition to handling specific @http options, a handler MUST add a User-Agent header that takes the following form, where "3.X" can be replaced with Aws\Sdk::VERSION and "HandlerSpecificData/version ..." should be replaced with your handler specific User-Agent string.

User-Agent: aws-sdk-php/3.X HandlerSpecificData/version ...