CS452 - Real-Time Programming - Fall 2009

Lecture 11 - The Name Server

Reminders

Why Do We Need a Name Server

Names	constant across applications & executions	interface	Associated with a set of services (an API)
Task Ids	vary across applications & executions	implementation	Associated with a particular set of instructions and data (an execution)

How do You Get the Task Id of the Name Server?

Name Server API

int RegisterAs( char *name );

One task can be registered under two names.
Each name is associated with a single task.
Name is \0 terminated.

int WhoIs( char *name );

Name is \0 terminated.

Name Server Semantics

RegisterAs

Errors
1. Not a legal name.
2. tid is not a task
3. tid is not the Name Server
4. Already somebody registered with that name
  - What does the caller do?

WhoIs

Errors
1. Not a legal name.
2. tid is not a task
3. tid is not the Name Server
4. No task registered under that name
  - What does the caller do?

Comments

Are we likely to want an Unregister primitive?

Name Server Implementation

User Code

typedef struct {
    int type;
    char *name;
} NSstruct;
int RegisterAs( char *name ) {
    int result;
    NSstruct req;
    req.type = REGISTERAS;
    req.name = name;
    bytes = Send( NSTid, &req, sizeof(NSstruct), (char *) result, sizeof(int) );
    if ( bytes != sizeof(int) ) {
        // Do something error-like
    } else return result;
}

There are lots of possible variations, and this one is by no means the best.

Server Code

typedef struct {
    int type;
    char *name;
} NSstruct;
int bytes, tid;
NSstruct req;
// initialize tables
FOREVER {
    bytes = Receive( &tid, &req, sizeof(NSstruct) );
    if ( bytes != sizeof(NSstruct) ) {
        // Reply with error
    } else {
        switch( req.type ) {
        case REGISTERAS:
            insert( name, tid );
            Reply( tid, SUCCESS, sizeof(int) );
            break;
        case WHOIS:
            result = lookup( name );
            Reply( tid, result, sizeof(int) );
            break;
        default
            Reply( tid, ERROR, sizeof(int) );
            break;
        }
    }
}

Comments

This code wouldn't work. Why?
How much will this code run?
- When will it run?
How would you implement insert & lookup?
You could send a hash of name.
What should be allowable as a name?

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