Home > Articles > Web Services > XML

  • Print
  • + Share This
This chapter is from the book

3.3 Synchrony

Synchronous and asynchronous communications are two different forms of interaction that both require the support of a generic technology for distributed systems.

Synchronous communication is characterized by the immediate responses of the communication partners. The communication follows a request/reply pattern that enables the free flow of conversation, often based on the use of busy waits. Applications with user interaction specifically require this conversational mode of interaction. Synchronous communication requires that the client and server are always available and functioning.

Asynchronous communication is less stringent. Both communication partners are largely decoupled from each other, with no strict request/reply pattern. Typically, one party creates a message that is delivered to the recipient by some mediator, and no immediate response is needed. The sender can store context information and retrieve it when the recipient returns the call, but there is not necessarily a response. In contrast to a synchronous request-reply mechanism, asynchronous communication does not require the server to be always available, so this type can be used to facilitate high-performance message-based systems.

Typically, synchronous communication is implemented by RPC-style communication infrastructures, while asynchronous mechanisms are implemented by MOM. However, it is entirely possible to implement synchronous communication based on MOM, and it is also possible to build MOM-style interaction over RPC. Nevertheless, RPC is more suitable if immediate responses are required, and MOM is the technology of choice for decoupled, asynchronous communication.

Due to the manifold requirements of most real-world scenarios, typical enterprise systems embody both synchronous and asynchronous communication. For this purpose, a variety of different communication infrastructures is used, ranging from simple FTP (File Transfer Protocol) to more advanced middleware platforms, such as RPC and MOM. In addition, there are also communication infrastructures that support both communication modes—for example, pipelining RPC, which supports asynchronous communication in addition to the standard synchronous RPC communication.

To conclude this discussion on synchrony, we will provide an overview of the most common ways of implementing synchronous and asynchronous communication with both RPC/ORB and MOM technology. We will look at the following examples:

  • Simulated synchronous services with queues

  • Asynchronous one-way: fire-and-forget RPC

  • Callbacks and polling services

The first example, simulated synchronous communication, can often be found in mainframe environments, where a message queuing system has been chosen as the standard means for remote interaction with the host, such as OS/390 with MQSeries access. This is a common scenario in many large enterprises. Often, these companies have gone one step further, developing frameworks on top of this combination of OS/390 and MQSeries that enable service-like interfaces to the most widely used transactions on the mainframe. This is done by implementing client-service wrappers that shield the underlying MQ infrastructure from the client developer. These service wrappers often simulate synchronous interactions with the mainframe by combining two underlying queues, one with request semantics and the other with reply semantics, using correlation IDs to pair messages into request/reply tuples. Effectively, this relegates the message queuing system to playing a low-level transport function only, not generally leveraging any of the advanced features of the messaging system. Figure 3-9 provides an overview of this approach.

Figure 3.9Figure 3-9 Simulated synchronous services with queues. A correlation ID maps a reply message to the corresponding request. On the client side, this is hidden by a service wrapper, which gives the caller the impression of synchrony.

The second example, fire-and-forget RPC, assumes an RPC or ORB implementation with asynchronous one-way semantics: The client fires off a request to the server without expecting an answer. This can be achieved either by defining an operation signature that does not include any return values or by using specific features of the middleware, such as a CORBA IDL operation using the keyword oneway. Figure 3-10 provides an overview of this approach.

Figure 3.10Figure 3-10 A synchronous one-way call implies fire-and-forget semantics. The request is fired off by the client without a reply from the server.

There are two key issues with this approach: The first is that the client has no guarantee that the server will receive and process the request appropriately. This problem reduces the applicability of this method significantly. The second problem is that most RPCs/ORBs typically use a reliable communication protocol such as TCP. Sending a one-way request through TCP generally means that the client is blocked until delivery to the server on the TCP level has been completed. If a server is getting swamped with requests, it might become unable to process all incoming one-way requests on the TCP layer. Effectively, this means that the client is blocked until the server is at least able to read the request from the network. Therefore, it is not advisable to use this approach to implement large-scale event notification. Instead, an appropriate MOM should be chosen.

The third example, callbacks and polling services, is the most common way of decoupling clients and server in RPC-style applications, without having to move to a fully fledged MOM solution. The basic idea is similar to the conventional callback, as it is realized in functional programming languages with function pointers, or in OO languages using object references: A client sends a request to the server, and the server stores the request and returns control back to the client (possibly sending an acknowledgment that it received the request). After having processed the request, the server (now acting as a client) sends the result back to the client (now acting as a server), using the standard RPC/ORB invocation mechanism (see Figure 3-11). Sometimes, it is not possible for the client to act as a server (e.g., due to firewall restrictions). In these cases, the client can periodically poll the server for the availability of the result.

Figure 3.11Figure 3-11 Callbacks and polling services: A client sends a request to a server ("trigger"). The server stores the requested activity in a database before replying with an acknowledgment to the client. The server has a thread that takes pending requests from the database, processes them, and sends back the result to the originating client using callback.

This approach can sometimes provide a good compromise for satisfying the need to decouple clients and servers without having to add technology such as a MOM. However, often the implementation of such a remote callback can be more difficult than it originally appears. This is especially true if the application requires a high degree of reliability. In these cases, it is necessary to introduce some kind of mechanism for ensuring reliability, such as through combining a server-side database with some kind of custom-built acknowledgment protocol. Also, the server-side logic can become quite complex: To ensure that all requests are eventually processed, the database must be constantly polled for pending requests, potentially adding a huge burden on database performance. For this reason, one should carefully weigh the use of database triggers. Here, it is important to ensure that the execution of the trigger is not part of the same transaction that puts the new request in the database. In this case, you could encounter a situation where the client is blocked because it has to wait not only until the server has stored the request in the database before returning an acknowledgment to the client, but also until the database trigger has been executed. This will effectively eliminate the decoupling effect of the callback implementation.

Figure 3.12Figure 3-12 Callbacks and queues. Similar to the previous example, except that queues are introduced on the server side to ensure better decoupling on the server side.

As shown in Figure 3-12, the server-side implementation can alternatively use internal message queues to ensure an efficient means of storing incoming requests in a reliable and efficient manner, thus avoiding many of the issues with the pure-database approach described previously.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020