Client and Server on the Same System

When the components are on the same physical machine, there are several different mechanisms that may be used for this connecting link. They include:

• Shared memory connections: These are usually the fastest and most reliable connections between co-located clients and servers. Both client and server attach to the same shared memory segment and communicate with each other by leaving and retrieving messages in shared memory. If you are setting up or tuning a large UNIX system in which all or most of the client activity occurs on the single UNIX computer, there are significant performance gains in using shared memory connections, assuming you have enough shared memory to go around. The major drawback of shared memory connections is that since shared memory must be readable and writable by all users, they are less secure than TCP/IP or stream pipe connections. It is theoretically possible for a hacker to write code that will read the contents of shared memory and eavesdrop on client/server communications. It is also possible for a rogue program to write to the shared memory area, crashing the database engine.

• Network connections: These connections use the either the TLI (transport-level interface), IPX/SPX protocols, or the sockets interface which forms the basis of Internet communications. Network connections are the only type of communications for clients and servers on separate host machines and is set up by default in most cases where the client and server co-reside on the same host. In this case, Informix uses a local loopback communication and doesn't actually send packets out onto the network. This is both the most common communications method and the most difficult to set up because of the necessary interrelation between Informix and the networking system. Because of this, we will concentrate on TCP/IP and sockets in this chapter.

• Streams connections: These connections became available with IDS 7.1 and are available on some UNIX systems. Check your release notes to see if they are supported in your environment. Streams connections use UNIX pipes to provide interprocess communication between client and server. They are good performers and have the advantage of being somewhat more secure than shared memory connections.

Different ports of the OnLine and IDS engines support different combinations of these transport mechanisms. To find out which mechanisms are supported in any particular port, look at the files in the release notes. These release notes will be found in the $INFORMIXDlR/release directory structure. In OnLine systems, the notes will be found in the release directory. In later IDS engines, the release notes will be buried deeper in this directory structure, in subdirectories based on the language chosen (GLS) for the application. Just drill deeper into the directory until you find a subdirectory with several text files in it. If your system actually supports multiple languages, search until you can find notes in a language you can read. These different communication mechanism are not mutually exclusive. If your system supports the connection method, you can use one or all of the mechanisms for connecting local clients to local servers. This will be illustrated later in this chapter.