The main reason servers are typically more complex is that they serve multiple clients. Servers usually are initiated upon the first linkage request from a client. There are usually critical sections and other locking issues to consider. Servers must provide graceful termination of clients if a service is terminated. Some simple services are still implemented using a server library.

Server Libraries

This is the simplest form of a server. The program requires an EXPORT list, which defines the collection of available library objects, and a FREEZE statement, which defines the point in the execution logic that the service is available. This implementation strategy is supported by most languages implementing libraries, although in many cases, the FREEZE statement is performed implicitly.

Examples

The following is an example of an ALGOL EXPORT list:

PROCEDURE REQUEST (BUF);
	ARRAY BUF [*];
	BEGIN
	. . .
	END REQUEST;
EXPORT REQUEST;

FREEZE (TEMPORARY);

The following is an example of a COBOL EXPORT list:

IDENTIFICATION DIVISION.
PROGRAM-ID.
		REQUEST.
DATA DIVISION.
WORKING STORAGE SECTION.
	77 BUF PIC X(80).
PROCEDURE DIVISION USING BUF.
. . .

Connection Libraries

This is an advanced server, only available in ALGOL and NEWP. The program requires a connection block TYPE declaration, which holds declarations for the LIBRARY OBJECTS that make up the interface, a LIBRARY interface declaration of the connection library type, and a READYCL statement for that library interface.

Example

The following is an example of an ALGOL connection block TYPE declaration:

TYPE CONNECTION BLOCK SERVICETYPE;
	BEGIN
	PROCEDURE REQUEST (BUF);
		ARRAY BUF [*];
		BEGIN
		. . .
		END REQUEST;
	EXPORT REQUEST;
	END SERVICETYPE;

SERVICETYPE LIBRARY SERVICE;

READYCL (SERVICE);

The reasons for choosing a connection library over a server library are