Creating A Generic Proxy For Retry Semantics In ColdFusion

RetryProxy.cfc

component
	output = false
	hint = "I provide automatic retry functionality around the target component."
	{

	/**
	* I initialize the retry proxy with the given target component. Retries will
	* only be applied to "transient" errors. And, since the proxy doesn't know which
	* errors are transient / retriable, it must check with the isTransientError()
	* function.
	*
	* @target I am the component being proxied.
	* @isTransientError I determine if the thrown error is safe to retry (returns a Boolean).
	* @retryCount I am the number of retries that will be attempted before throwing an error.
	* @includeMethods I am the collection of method names for which to explicitly apply retry semantics.
	* @excludeMethods I am the collection of method names for which to explicitly omit retry semantics.
	* @output false
	*/
	public any function init(
		required any target,
		required function isTransientError,
		numeric retryCount = 2,
		array includeMethods = [],
		array excludeMethods = []
		) {

		variables.target = arguments.target;
		variables.isTransientError = arguments.isTransientError;
		variables.retryCount = arguments.retryCount;

		generateProxyMethods( includeMethods, excludeMethods );

		return( this );

	}


	// ---
	// PUBLIC METHODS.
	// ---


	// ... proxy methods will be duplicated and injected here ...


	// ---
	// PRIVATE METHODS.
	// ---


	/**
	* I inspect the target component and create local, public proxy methods that match
	* the invocable methods on the target component. All target methods will be proxied;
	* however, the proxy will be a RETRY proxy or a BLIND proxy based on the include /
	* exclude method name collections.
	*
	* @includeMethods I am the collection of method names for which to explicitly apply retry semantics.
	* @excludeMethods I am the collection of method names for which to explicitly omit retry semantics.
	* @output false
	*/
	private void function generateProxyMethods(
		required array includeMethods,
		required array excludeMethods
		) {

		// Look for public methods / closures on the target component and create a
		// local proxy method for each invocable property. By explicitly stamping out
		// clones of the proxy method, we don't have to rely on the onMissingMethod()
		// functionality, which I personally feel makes this a cleaner approach.
		for ( var publicKey in structKeyArray( target ) ) {

			var publicProperty = target[ publicKey ];

			if ( isInvocable( publicProperty ) ) {

				// Determine if the given method is being implicitly or explicitly
				// excluded from the proxy's retry semantics.
				var isIncluded = ( ! arrayLen( includeMethods ) || arrayFindNoCase( includeMethods, publicKey ) );
				var isExcluded = arrayFindNoCase( excludeMethods, publicKey );

				this[ publicKey ] = ( isIncluded && ! isExcluded )
					? proxyRetryTemplate
					: proxyBlindTemplate
				;

			}

		}

	}


	/**
	* I return the back-off duration, in milliseconds, that should be waited after
	* the given attempt has failed to execute successfully.
	*
	* @attempt I am the attempt number (starting at zero) that just failed.
	* @output false
	*/
	private numeric function getBackoffDuration( required numeric attempt ) {

		return( 1000 * ( attempt + rand() ) );

	}


	/**
	* I determine if the given value is invocable.
	*
	* @value I am the public property that was plucked from the target component.
	* @output false
	*/
	private boolean function isInvocable( required any value ) {

		return( isCustomFunction( value ) || isClosure( value ) );

	}


	/**
	* I provide the template for "blind pass-through" proxy methods. These implement
	* no retry logic.
	*
	* @output false
	*/
	private any function proxyBlindTemplate( /* ...arguments */ ) {

		// Gather the proxy invocation parameters. Since the proxyBlindTemplate() has
		// been cloned for each public method on the target, we can get the name of the
		// target method by introspecting the name of "this" method.
		var methodName = getFunctionCalledName();
		var methodArguments = arguments;

		return( invoke( target, methodName, methodArguments ) );

	}


	/**
	* I provide the template for "retry" proxy methods.
	*
	* @output false
	*/
	private any function proxyRetryTemplate( /* ...arguments */ ) {

		// For the purposes of the error message, we'll record the duration of the
		// attempted proxy execution.
		var startedAt = getTickCount();

		// Gather the proxy invocation parameters. Since the proxyRetryTemplate() has
		// been cloned for each public method on the target, we can get the name of the
		// target method by introspecting the name of "this" method.
		var methodName = getFunctionCalledName();
		var methodArguments = arguments;

		for ( var attempt = 0 ; attempt <= retryCount ; attempt++ ) {

			try {

				return( invoke( target, methodName, methodArguments ) );

			} catch ( any error ) {

				// If this is not a retriable error, then rethrow it and let it bubble
				// up to the calling context.
				if ( ! isTransientError( error ) ) {

					rethrow;

				}

				// If this was our last retry attempt on the target method, throw an
				// error and let it bubble up to the calling context.
				if ( attempt >= retryCount ) {

					throw(
						type = "RetryError",
						message = "Proxy method failed even after retry.",
						detail = "The proxy method [#methodName#] could not be successfully executed after [#( retryCount + 1 )#] attempts taking [#numberFormat( getTickCount() - startedAt )#] ms.",
						extendedInfo = serializeJson( duplicate( error ) )
					);

				}

				// Since we're encountering a transient error, let's sleep the thread
				// briefly and give the underlying system time to recover.
				sleep( getBackoffDuration( attempt ) );

			}

		}

		// CAUTION: Control flow will never get this far since the for-loop will either
		// return early or throw an error on the last iteration.

	}

}

test.cfm

<cfscript>

	// Setup some general error-checking functions (closures work as well). Each of
	// these function accepts the Error instance in question and must return a boolean
	// indicating that the error is transient (true) or non-retriable (false).

	function isMySqlLockTimeoutError( required any error ) {

		// Read more: https://dev.mysql.com/doc/refman/5.7/en/error-messages-server.html#error_er_lock_deadlock
		return(
			( error.type == "Database" ) &&
			( error.errorCode == "40001" )
		);

	}

	function isSqlServerLockTimeoutError( required any error ) {

		// Read more: https://technet.microsoft.com/en-us/library/cc645860(v=sql.105).aspx
		return(
			( error.type == "Database" ) &&
			( error.errorCode == "1222" )
		);

	}

	function isAlwaysTransientError( required any error ) {

		return( true );

	}

	function isNeverTransientError( required any error ) {

		return( false );

	}

	// ------------------------------------------------------------------------------- //
	// ------------------------------------------------------------------------------- //

	// Create our retry proxy using the given transient error test.
	proxy = new RetryProxy( new TestTarget(), isAlwaysTransientError );

	try {

		writeDump( proxy.works() );
		writeDump( proxy.breaks() );

	} catch ( any error ) {

		// This should be the "breaks()" method error.
		writeDump( error );

	}

</cfscript>