The Horus Object Tools: Directory Server

This document is a part of the online Horus Documentation, under Horus Object Tools.

In the HOT Directory, data entries are (key, value) pairs. The key is a HorusString object; the value may be an instance of any subclass of HorusBase that supports stream-like message I/O operations.

There are two classes in HOT, HorusDirectoryServer and HorusDirectoryClient, that provide replicated-directory functionality.

The HorusDirectoryServer class implements a directory server. The only public methods of this class are constructors. For example:

	// create a directory-server object and join the group "lapa-dirsvr"
	HorusDirectoryServer svr("lapa-dirsvr");

NOTE: You may start up several copies of HorusDirectoryServer objects simultaneously. That will provide fault-tolerance (through active replication) and load-balancing of read requests.

The HorusDirectoryClient class implements the functionality of a directory client. It allows to lookup/install and lock/unlock data entries. The public methods are declared as follows:

  	// lock `key' (returns 1 if succeeds, 0 if fails)
	int lock(HorusString& key);	

	// unlock `key'
  	void unlock(HorusString& key);	

	// lookup the value of `key'
	// (returns 1 if succeeds, 0 if fails)
  	int lookup(HorusString& key, HorusBase& value); 
   
	// install `value' under `key'
  	void install(HorusString& key, HorusBase& value);

NOTE: lookup/install operations do not block even if the entry is locked. However, if an entry is locked, a following call to lock will block.

Example:

	// Note that in order to be used with HorusDirectory,
	// Foo must define the stream-like message I/O operators,
	// 	void operator>>(HorusMessage&) and
	// 	void operator<<(HorusMessage&).

	class Foo: public HorusBase {
	public:
		void operator >> (HorusMessage& msg) {
			msg << value;
		}

		void operator << (HorusMessage& msg) {
			msg >> value;
		}
	....
	private:
		int key, value;
	};

	HorusDirectoryClient dir;		// create a directory client object

	Foo f1, f2;
	....
	HorusString name("foo");

	dir.lock(name);				// lock the entry of "foo"
	dir.install(name, f1);			// install f1 under name "foo" 
	....
	dir.lookup(name, f2);			// lookup "foo" into f2
	dir.unlock(name);			// unlock the entry of "foo"

	HorusEntity ent;
	dir.lookup(name, ent);			// error! can't put Foo into HorusEntity

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