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IT-SC book: Advanced CORBA Programming with C++
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Part VI: Power CORBA 21 Multithreaded Applications
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211 Overview
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In this chapter we explore issues related to multithreaded CORBA applications Section 213 explains the benefits that multithreading brings to CORBA applications Sections 214 and 215 discuss fundamental multithreading techniques and explain how the ORB and POA help support them In Section 216 we convert the servant locator example first presented in 12 to work properly in the presence of multiple threads Finally, in Section 217 we briefly discuss multithreading problems related to servant activators
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212 Introduction
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Practical CORBA applications must be able to scale well in several dimensions These dimensions include the number of objects that an application can support, the number of requests it can handle simultaneously, the number of connections it allows, and the amount of CPU and memory resources it uses One important method of making applications scale well is to employ multithreaded programming techniques Although multiple threads allow true concurrent programming only on multi-CPU machines, using them can simplify program logic as well as enhance program scalability and performance This chapter provides a high-level overview of how multithreaded programming techniques can be used to develop CORBA applications We do not intend to provide an in-depth tutorial on threads because doing this properly would itself require a book Fortunately, a number of good books and articles on multithreaded programming and concurrency for distributed applications exist [2] [10] [12>] [35] [36] [37], and we recommend that you read and study them if you need to brush up on your multithreaded programming skills
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213 Motivation for Multithreaded Programs
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Ordinarily, processes on commonly used operating systems such as Windows NT and various flavors of UNIX are single-threaded All actions taken by a single-threaded process, from accessing a variable on the run-time stack to sending and receiving network packets through a socket, are performed by the single thread of control that runs within the process
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IT-SC book: Advanced CORBA Programming with C++
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Unfortunately, it is often difficult to develop and use server applications that use only a single thread of control The following sections explain why 2131 Request Serialization When only a single thread is available, the server must serialize the processing of client requests New requests arriving at the server are queued in the POA that hosts the target object As described in 11, a POA with the SINGLE_THREAD_MODEL value for its ThreadPolicy must be capable of queuing incoming requests while an object is already busy processing a request If processing any request takes a long time, it prevents the processing of all other requests for objects in the same POA It is thus possible for the request queue in the POA to grow too large If this occurs, the POA raises a TRANSIENT exception back to the client to tell it to retry the request, with the hope that the number of queued requests will have been reduced by the time the retry is received 2132 Event Handling Because server applications wait for requests to arrive on their advertised network ports, they are often described as reactive [34] To detect when requests arrive, most ORB implementations employ an event loop to monitor their network connections For example, ORB implementations often use the UNIX select system call to watch for events occurring on the set of file descriptors corresponding to the server's network connections When a request arrives, the ORB reacts by dispatching the request to the application so that it can be carried out For event handling to occur properly, the ORB must be able to gain the thread of control from time to time Unfortunately, if a long-running request ties up the single thread doing other things, such as performing a complicated calculation, it denies the ORB the ability to wait for requests; the server cannot read incoming messages from its network connections When this occurs, it can cause the network transport to apply flow control to make the clients' network transports stop sending messages If each client ORB continues to attempt to send requests under these conditions, it will start getting errors from its own network transport layer and thus each client also may have to buffer requests Thus, a single long-running request can deny service to numerous clients One way to prevent these conditions is to ensure that no requests take a long time to complete If the nature of a request is such that it will take a while to process, the request can be broken up in two ways Break the IDL operation into two parts: one to start the request and the other to obtain the results For example, assume that an interface has one operation that we know will take a long time to execute
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interface Rocket {
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