Alejandro Imass > POE-Component-Server-AsyncEndpoint > POE::Component::Server::AsyncEndpoint

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NAME ^

POE::Component::Server::AsyncEndpoint - SOA Asynchronous Endpoint Server

  B<IMPORTANT:> The API has changed for this (0.10) release. SOAP
  client is now asynchronous. You will have to port your existing
  Endpoint code to the new pattern. The easiest way to do this is to
  backup you Endpoint code and generate new ones with the scaffolding
  helper scripts. Then port the old functionallity to the new
  pattern. Although it may seem a lot of work, is actually easier
  because the new pattern has a lot less code than the 0.02 version.

SYNOPSIS ^

  1) Create the server and your endpoints:

  shell$ mkdir MyAES; cd MyAES; aescreate
  shell$ cd endpoints/
  shell$ endpointcreate

  Follow the prompts! (read on for full explanation)

  2) Configure the server and develop the endpoint code
  see POE::Component::Server::AsyncEndpoint::ChannelAdapter

  3) Run or daemonize the server
  shell$ ./aes

  4) Monitor the server using the Web Interface or SNMP
    (Web and SNMP are still in development)

DESCRIPTION ^

WebServices are very popular

Note on the term AES

Please note that at times we may abbreviate this project as AES (Asynchronous Endpoint Server), and other times we may use "AES" to refer to the master AES process. It should be clear by context when we use it for one or the other.

Library or Platform?

More than a library, is a ready-to-use and complete SOA Application Integration Platform. Once installed, you will use the helper scripts to generate your server and message endpoints. These scripts will scaffold most of the code and configuration files for you and all you have to do is implement the code and your interface will be running in a few minutes. It is also a CPAN library of course, which can be easily extended and specialized to develop new software.

The initial motivation to develop this library was to facilitate the deployment of asynchronous (non-blocking) outbound soap services, but we decided to build a complete end-to-end integration platform that would be scalable and very easy to implement. It is not targeted at the expert hacker, but rather to the integration consultant who wants to get the job done, fast and easy.

The "Asynchronous Endpoint Server" implements a design pattern to aid in the development and deployment of a distributed (as opposed to the centralized spirit of the so-called "Enterprise Service Bus" or ESB) Service Oriented Architecture (SOA). The concepts and terms come from several sources, but the actual names of the components are inspired by the book "Enterprise Integration Patterns" by Gregor Hohpe and Bobby Wolf (http://www.enterpriseintegrationpatterns.com). The AsyncEnpoint Server implements a component called a "Message Endpoint" based on the specialization of the "Channel Adapter" concept coined in the book, plus a series of helpers and facilities to deploy your interface in just a few hours.

Current Protocol Support

Right now, the Channel Adapters support for SOAP on the WS side and STOMP on the MQ side. More protocols can be easily added by specializing the ChannelAdpater class. We hope to receive feedback and incorporate new protocols as needed by the user community.

Is this an Enterprise Service Bus?

To date, there is certain controversy on whether SOA should be implemented in a hub-and-spoke versus a bus fashion, and it seems that the "Enterprise Service Bus" is gaining some traction in the "Enterprise" and also in the Free and Open Source Communities.

This project is the contrary of the bus pattern and promotes a more distributed architecture where "the ESB (if it exists) is pushed to the endpoints" (quote from Dr. Jim Webbers's "Guerrilla SOA" presentation Jim.Webber@ThoughtWorks.com http://jim.webber.name).

ESB pretends to hide the complexities of EAI inside the bus, when in reality, the real complexity lies in the writing of the web service at the endpoint. That is, exposing your current working code through a web-service API, is the real hard work. Once you get your current system API exposed via a "Web Service", one small problem remains: the HTTP is (a) synchronous and (b) it must be initiated from the outside. This means that even though you may write a Web Service that sends "OUTBOUND" data to another system, it must be "pulled" or invoked from the "outside" because your web service cannot initiate the HTTP connection in a "push" fashion.

What the AES does, is that it allows for you to "pull" your OUTBOUND data from an OUTBOUND Web Service and put it in a Message Queue where it can be "pushed" to subscribed clients on the other end. It also, allow for you to write the INBOUND clients as well, that receive the data via the Message Queue and push it to the destination system with an INBOUND Web Service.

In a Nutshell

Basically, you start by generating a server with the aescreate helper script (inside a pre existing empty directory). This will create a directory structure, configuration files, and the aes executable. To create a new Endpoint, you invoke the endpointcreate helper script (inside the endpoints directory) and it will generate a directory for the endpoint and scaffolding code. The endpointcreate will ask you a few questions and all you have to do is follow the prompts.

Once you have your Endpoint code ready, you run the master aes executable and it, will in turn scan your directory structure and load your endpoints automatically. The watchdog in the aes will monitor your Endpoints and re-start them if they should die or hang for some reason.

Each Endpoint is a distinct executable and will run as a separate PID on your machine. This means that to a certain degree, you can develop and debug your Endpoint code individually, although you won't have access to the Message Queue server provided by the main aes process. We hope to get some feedback on how we can further facilitate the individual development of the Endpoints, perhaps by simulating the MQ and other parts of the system. In the mean time, you may just comment the things that rely on the aes process. There is a conceived facility to allow each Endpoint to be started and stopped via a web interface, but this feature is currently under development.

Programming Style

The programming technique is event-based (EDA) using POE (Perl Object Environment) so you should have some familiarity with POE before coding your Endpoints. Nevertheless, the scaffolding already generates typical endpoint skeletons for you, so you don't have to be an expert in POE.

Inbound (IB) and Outbound (OB) Endpoints

When creating and Endpoint you must chose between an IB or OB Endpoint base. The OB Endpoint is designed to poll an OB Web Service (SOAP at the moment) and publish the OB data to the MQ. The IB Endpoints subscribe to an MQ and push to the target system via an IB Web Service (SOAP at the moment).

The OB endpoints can be single or multi phased. Please read the "Poling or Event Driven Endpoints" subsection below for some important information on the OB Endpoint design and considerations.

Poling or Event Driven Endpoints

To implement a non-blocking, asynchronous outbound interface for any application, you must somehow signal the outbound event to the destination process. To guarantee that the signal delivery is non-blocking, this signal must not depend on instantaneous acknowledgement of any other process, and at the same time, the signal must not be lost. The most obvious way to do this is to store the signal into permanent storage for later retrieval by the destination process, and usually, you would want to store these outbound signals in a database table or to a file on disk.

Retrieving the signals from permanent storage requires either a polling technique, or an event-based interface to the database table or file (via a db trigger, or something in the likes of POE::Wheel::FollowTail). Access to the table or file requires a database connection or physical access to the file (locally or via the network) which can be a security risk for many set-ups. Most systems administrators do not want to deal with complicated set-ups, opening arbitrary IP ports, etc. And also, she would want the flexibility to run the interface on any server, not on one particular server.

Because of these issues, we assumed in our design that ALL interaction with the business systems on ALL ends are STRICTLY through the use of a Web Service, and for our first deployment, we have chosen SOAP over HTTP transport. If you need something different, you can either extend our classes, write us, or send us a patched version of your extension. In any case, please contact the mailing list to discuss the needs of your particular implementation.

In our model, the OB Endpoint must periodically invoke a Web Service that retrieves the outbound signals from the source system's permanent storage (in other words, polling the "signals" file with a Web Service). We call this "signals" file a FIFO and the Web Service that reads it, is called the FIFO Popper. The FIFO must never be confused or treated like a Message Queue. The FIFO is just a temporary stack to make the outbound signals non-blocking and allow for the complete application interface to be developed through Web Services.

The Endpoint design pattern, can only process one single FIFO record at any given time and will only mark the FIFO as done when the data is sent to the MQ and we get a receipt. This assures that our Endpoint code can hang or die at any moment and we are sure that we have not lost any data. Also, by changing the status of the FIFO only when we get a receipt from the MQ, we can rest assured that the EP will process the failed FIFO when it re-starts. It is very important that you follow this pattern in your code so you never treat the FIFO or your Endpoint as a Queue. The actual Queue, is an industry-grade Message Queue that is available for POE (see POE::Component::MessageQueue).

Single or Multi Phased OB Endpoint

The OB Endpoint can be single-phased or multi-phased depending on the complexity ("orchestration", "choreography") and business needs of each particular interface.

In the single-phased OB Endpoints, the FIFO record by itself contains all the data needed to be pushed to the other system. This is practical for simple interfaces such as passing document states between two systems, or synchronizing master-slave value list data between two databases. So if the data is not too complex, or you don't need any complex "orchestration" or "choreography" it is usually enough to implement a single-phased OB Endpoint.

Double-phased OB Endpoints are needed when it's not practical or feasible to save the OB data into the FIFO table or file directly. For example, an interface that needs to send a complex document from one system to the other. A complex document will have multiple parts and dependant data (like foreign key records), that must be processed in a particular order. In these cases, the FIFO record will probably just have the id of the document, and another larger and complex WebService will actually construct the OB data package in a second phase.

In the double or multi-phased Endpoints, one phase extracts the basic information with the FIFO, and then invokes the actual Web Service that does the complex packaging of the Outbound data. More stages can be added easily but two phases should accomodate most of your integration needs.

The standard OB Endpoints generated by the helper scripts will let you choose between single-phased or double-phased. Multi-phased are implementation dependant and should be based on the double-phased type.

Technical Details ^

The idea of this section is that you can quickly understand the library code and help us make this project better by hacking it and sending us reviews and patches. We would very much welcome comments and ideas on the code and most importantly, if you can send us references on how you are using it would be excellent also.

As you can tell by the name-space, POE::Component::Server::AsyncEndpoint resides in the Server components of POE. This means that is mostly ready to use, and large part of the code is actually based on other Components of POE and of CPAN in general (please note that we use POE::Component and the abbreviated form "PoCo" from this point on).

For the experienced Perl hacker, you may find that our classes are a bit restrictive, and that many of them will refuse to start and croak if not implemented correctly. This is done on purpose because it's targeted for the final user, and by reducing the flexibility we hope to reduce the pain. Any comments are, of course, very welcome on the mailing list.

Some of the main packages that we build upon for the main server are:

        POE
        POE::Wheel::Run
        POE::Component::Logger
        POE::Component::MessageQueue
        POE::Component::Server::HTTP (planned for release 0.2x)
        POE::Component::Server::SNMP (planned for release 0.3x)

Basically, the AsyncEndpoint Server spawns a PoCo::MessageQueue session and then scans the directory structure to find and run the individual Endpoints via the package PoCo::Server::AsyncEndpoint::Endpoints. The package PoCo::Server::AsyncEndpoint::Config provides the configuration facilities and PoCo::Server::AsyncEndpoint::WebServer provides the Web Interface, which in turn is an implementation of POE::Component::Server::HTTP.

The Endpoints are based on the class PoCo::Server::AsyncEndpoint::ChannelAdapter which in turn implements other POE components such as:

        SOAP::Lite (for SOAP support)
        POE::Component::Client::Stomp (for STOMP support)

Through these wrappers respectively:

        POE::Component::Server::AsyncEndpoint::ChannelAdapter::SOAP
        (the above uses POE::Component::Client::SOAP)
        POE::Component::Server::AsyncEndpoint::ChannelAdapter::Stomp

It also provides a configuration file interface through

        POE::Component::Server::AsyncEndpoint::ChannelAdapter::Config

The AES starts the Endpoints with POE::Wheel::Run and communicates with them via POE IKC on a predefined port. So the endpoints wind up "speaking" three different languages: SOAP to communicate with the Web Services, STOMP (http://stomp.codehaus.org/) to communicate with the MQ and SEP (Simple Endpoint Protocol) to communicate with the main server process (via IKC). Endpoint programmers need not know about the IKC and associated SEP protocol, as this is encapsulated in the ChannelAdapter superclass.

Functionally, the Outbound (OB) Endpoints invoke a Web Service and push the data using STOMP to the MQ Server. The IB Endpoints, on the other hand, subscribe to the channels and push the incoming data through a Web Service on their side. So in a nutshell, the AES as a whole is a platform to link systems that offer plain SOAP services, with an emphasis on easily implementing asynchronous outbound SOAP services.

Event and Process Model

The publishing of SEDA (Matt Welsh 2001) raised many polemic discussions on whether EDA is better than thread/process model and others. We think that each one has it's benefits so we opted for a "Salomonic" solution: Each Endpoint is an Operating System Process, but the programming technique of the Endpoint code is EDA (thanks to POE).

TIPS AND PITFALLS ^

The OB Endpoint must be coded in such a way, that it can only handle ONE FIFO RECORD at any given time, and should not mark the FIFO record as "POPPED", until the endpoint gets a STOMP/RECEIPT. This way, you don't have to handle stacks or queues in your code and place all responsibility on the MQ, where it should be. In the IB Endpoint, you should not STOMP/ACK the message until you have made sure that the IB Web Service has succeeded. The stub code generated by the helper scripts try to enforce these practices, but ultimately it's up to the final programmer to follow them.

Note that an obvious weak spot is if the OB Endpoint process dies right after getting the STOMP RECEIPT and before the invocation of the POPPED Web Service (the one that marks the FIFO record as effectively popped). Also, if the POPPED Web Service is failing in any way, as long as you don't shutdown the Endpoint, make sure YOU DON'T process any more FIFO records until you can successfully mark the current one as POPPED (i.e. you get some-kind of OK from your Web Service). We are currently evaluating all borderline conditions an will not only develop automated tests for each one, but will also work on a safe shutdown sequence that warns about these conditions.

Even with all these precautions, if the OB Endpoint dies, or the POPPED Service never recovers before a re-start of the endpoint, this WILL RESULT IN A DUPLICATE MESSAGE DOWN THE MQ, so please take this into account. Unless you are processing TRANSACTIONS (GL entries, for example) all this should not worry you too much. If you do process transactions, the use of a correlative and/or distinct id, might aid to reduce or effectively eliminate the possibility of duplicate records on the target system. Some interfaces rely on the concept of master and slave tables, where the unique identifiers of the slave tables are always re-written by the owner of the record (the Master).

The good news is that the way this library is written (and if you follow all the recommendations) you should never lose events, on the contrary, the problem is actually the possibility of duplication as stated above, which can easily be solved with unique identifiers and good coding of the Web Services themselves. Please discuss your specific needs on the list so we can work on these issues.

In any case, the use of serial id's should always be preferred over the use of alpha-numeric identifiers for records, so FIFO tables and files, should always have a serial id column (this is regardless of the "logic" correlative that you may have for transaction based FIFOs). The examples and the generated stub code already set-up many of these practices for you.

Using XML for the actual Message Data

Even though traditionally, most Web Services use XML for the serialization of data, we have found that using XML is, in many cases, a useless overhead and an unnecessary complication. The use of simple and universal serialization languages, such as JSON, is a great alternative for many interfaces. Of course, you use still have to use XML to define the WS (and for the SOAP envelope), but the actual data (the "message parts" in WSDL) are just XSD::string (JSON encoded strings) that carry associative arrays and perhaps even simple objects. All our examples and scaffolding code use this technique, but of course, it's up to you if you want to use XML or another serialization technique.

Encoding

We assume that all interface components as well as the Web Services that provide the data use UTF-8 encoded information. Support for other encoding systems is up to the developer, but please discuss it on the list if you require a different one and it's not working.

OPERATING SYSTEM SUPPORT

The system has been developed and tested on Linux and FreeBSD. We should say that it runs everywhere that Perl runs but we don't have the resources or the need for it to run outside of Linux and FreeBSD (or most Unix or Unix-like OSs). Testing and portability issues to other platforms may be an added bonus in the future, and if anyone needs this to run in a non-UNIX platform please let us know. In general, we think that should run anywhere POE runs, but we can't be sure. Send us your comments and we will do our best to help.

COMMUNITY SUPPORT

This software is part of the P2EE project (http://www.p2ee.org) and can be directly discussed on the p2ee development mailing list here: https://lists.sourceforge.net/lists/listinfo/p2ee-devel.

COMMERCIAL SUPPORT

Corcaribe Tecnología C.A. (http://www.corcaribe.com) has funded this particular development and provides commercial support and enhancements to this software. They plan to release any enhancements to the community and keep this software free.

EXPORT

None by default.

SEE ALSO ^

POE::Component::Server::AsyncEndpoint::ChannelAdapter POE::Component::Server::AsyncEndpoint::ChannelAdapter::SOAP POE::Component::Server::AsyncEndpoint::ChannelAdapter::Stomp POE::Component::Server::AsyncEndpoint::ChannelAdapter::Config

POE POE::Wheel::Run POE::Component::Logger POE::Component::MessageQueue POE::Component::Server::HTTP POE::Component::Server::SNMP POE::Component::Client::SOAP POE::Component::Client::Stomp SOAP::Lite JSON

AUTHOR ^

Alejandro Imass <ait@p2ee.org> Alejandro Imass <aimass@corcaribe.com>

COPYRIGHT AND LICENSE ^

Copyright (C) 2008 by Alejandro Imass / Corcaribe Tecnología C.A. for the P2EE Project

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.8 or, at your option, any later version of Perl 5 you may have available.

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