1 DISTRIBUTED SYSTEMS Principles and Paradigms Second Edition ANDREW S. TANENBAUM MAARTEN VAN STEEN Chapter 3 Processes Skip

2 3.3 Example: The XWindow System Figure 3-9. The basic organization of the XWindow System.

3 Design Issues –Server organization Iterative server or concurrent server –Where do clients contact a server? Daemon vs. superserverDaemon –Where and how a server can be interrupted? Ex: file transfer –Exit the client application or send out-of-band data –Whether the server is stateless or stateful soft state: the server maintain state on behalf of the client for a limited time Session state vs. permanent state –Does server keep a record on a client’s behavior 3.4 Servers

4 General Design Issues (1) Figure (a) Client-to-server binding using a daemon. (port)

5 General Design Issues (2) Figure (b) Client-to-server binding using a superserver.

Server Clusters (1) Figure The general organization of a three-tiered server cluster.

7 Server Clusters (2) Figure The principle of TCP handoff. Advanced server selection: payload or content-aware

8 Distributed Servers Figure Route optimization (MIPv6) in a distributed server.

Managing Server Clusters Common approaches –Log into a node and execute local managing commands –Provide an interface at an administration machine Not for large server clusters –Self management

10 Example: PlanetLab Figure The basic organization of a PlanetLab node. A slice is a set of vservers Node manager Resource specification

11 PlanetLab (1) PlanetLab management issues: Nodes belong to different organizations. –Each organization should be allowed to specify who is allowed to run applications on their nodes,and restrict resource usage appropriately. Monitoring tools available assume a very specific combination of hardware and software. –All tailored to be used within a single organization. Programs from different slices but running on the same node should not interfere with each other.

12 PlanetLab (2) Figure The management relationships between various PlanetLab entities.

13 PlanetLab (3) Relationships between PlanetLab entities: 1.A node owner puts its node under the regime of a management authority, possibly restricting usage where appropriate. 2.A management authority provides the necessary software to add a node to PlanetLab. 3.A service provider registers itself with a management authority, trusting it to provide well- behaving nodes.

14 PlanetLab (4) Relationships between PlanetLab entities: 4.A service provider contacts a slice authority to create a slice on a collection of nodes. 5.The slice authority needs to authenticate the service provider. 6.A node owner provides a slice creation service for a slice authority to create slices. It essentially delegates resource management to the slice authority. 7.A management authority delegates the creation of slices to a slice authority.

Code Migration Approaches to code migration –Process migration –Mobile agent Migration and local resources Migration in heterogeneous systems

16 Reasons for Migrating Code Figure The principle of dynamically configuring a client to communicate to a server. The client first fetches the necessary software, and then invokes the server. Allocation and redistribution of tasks Performance improvement Flexibility

17 Models for Code Migration Figure Alternatives for code migration. Code segment Execution segment Resource segment

18 Migration and Local Resources Figure Actions to be taken with respect to the references to local resources when migrating code to another machine.

19 Migration in Heterogeneous Systems Three ways to handle memory migration (which can be combined) Pushing memory pages to the new machine and resending the ones that are later modified during the migration process. Stopping the current virtual machine; migrate memory, and start the new virtual machine. Letting the new virtual machine pull in new pages as needed, that is, let processes start on the new virtual machine immediately and copy memory pages on demand. Script Portable language Environment: memory image and local resource binding