Lecture 8 Epidemic communication, Server implementation

EECE 411: Design of Distributed Software Applications Logistics / reminders Subscribe to the mailing list! Assignment1 marks You should receive them today Assignment2: due this Friday 11:59pm Quizzes: Q1: Thursday next week (10/14) Q2: 11/16

EECE 411: Design of Distributed Software Applications Roadmap A distributed system is: a collection of independent computers that appears to its users as a single coherent system Components need to: Communicate point-to-point communictaion Sockets, RPC, RMI point-to-multipoint [last time] Multicast Epidemic communication Cooperate [next]

EECE 411: Design of Distributed Software Applications Quiz question A distributed service operates on a large cluster. The service has one component running on each cluster node. Cluster nodes might be shut down for maintenance, they might simply fail, or they might come back online. To function correctly each service component needs an accurate list of all other nodes/service components that are active. TO DO: Design a mechanism that provides this list Describe the mechanism in natural language Provide the pseudocode.

EECE 411: Design of Distributed Software Applications Detour: processes and threads

EECE 411: Design of Distributed Software Applications Threads & distributed systems: Client side issues Essence: Client side often tailored to provide transparency access transparency: client-side stubs for RPCs location/migration transparency: let client-side software keep track of actual location replication transparency: multiple invocations handled by client stub: failure transparency: can often be placed only at client (we’re trying to mask server and communication failures).

EECE 411: Design of Distributed Software Applications Threads & distributed systems: Client side issues Goal: hiding network latency. Example I: Multi-threaded Web client: Web browser scans an incoming HTML page, and finds that more files need to be fetched. Each file is fetched by a separate thread, each doing a (blocking) HTTP request. As files come in, the browser displays them. Example II: Multiple request-response calls to other machines (RPC): A client does several calls at the same time, each one by a different thread. It then waits until all results have been returned. Q: Why exactly does multi-threaded processing help?

EECE 411: Design of Distributed Software Applications Server side issues: How to handle incoming requests? iteratively vs. concurrently Why multiple threads can be a good idea? Multithreaded File Server Example

EECE 411: Design of Distributed Software Applications How to handle incoming requests? Iterative vs. concurrent If concurrent one more choice: Blocking vs. non- blocking I/O Concurrent with blocking I/O: Choice: Processes vs. threads. Concurrent with non-blocking I/O Finite state machine based design Event driven programming

EECE 411: Design of Distributed Software Applications Threads & distributed systems: Server side issues Main issues are performance and structure. Improve performance: Having an iterative server prohibits scaling in a multiprocessor system. As with clients: concurrent servers reduce latency (by reacting to next request while previous one is being processed) and increase throughput Starting a thread to handle an incoming request is much cheaper than starting a new process. Better structure: Most servers have high I/O load. Using simple, well-understood blocking calls may simplify the overall structure. Multithreaded programs tend to be smaller and easier to understand due to simplified flow of control. Better isolation may sometimes be an argument for process based servers

EECE 411: Design of Distributed Software Applications Other issues in Server Design How do clients find server’s location? Stateless vs. stateful server design Server clusters

EECE 411: Design of Distributed Software Applications Servers: General organization Basic server model: A server is a process that waits for incoming requests at a specific transport address. In practice, there is a one-to-one mapping between a port and a service: superserver (inetd) vs. ‘normal’ server.

EECE 411: Design of Distributed Software Applications Servers and State: File system server example

EECE 411: Design of Distributed Software Applications Servers and State (I): Stateless servers Stateless servers: Never keep accurate information about the status of a client after having handled a request: Don’t record whether a file has been opened (simply close it again after access) Don’t promise to invalidate a client’s cache Don’t keep track of your clients Consequences: Clients and servers are completely independent State inconsistencies resulted from client or server crashes are reduced. Possible loss of performance because, e.g., a server cannot anticipate client behavior (think of prefetching file blocks)

EECE 411: Design of Distributed Software Applications Servers and State (II): Stateful servers Stateful servers: Keep track of client status: E.g., record that a file has been opened, so that prefetching can be done E.g., Knows which data a client has cached, and allows clients to keep local copies of shared data and my promise to invalidate them Observation: The performance of stateful servers can be extremely high, provided clients are allowed to keep local copies. [Depending on application] reliability may not a major problem.

EECE 411: Design of Distributed Software Applications Server clusters Observation: Many server clusters are organized along multiple tiers Important: The first tier is generally responsible for passing requests to an appropriate server.

EECE 411: Design of Distributed Software Applications Request Handoff Observation: Having the first tier handle all communication from/to the cluster may lead to a bottleneck. Solution: Various, but a popular one is TCP-handoff

EECE 411: Design of Distributed Software Applications Summary so far Client and server design: processes focus Sequential vs. concurrent, Processes vs. threads blocking vs. non-blocking Concurrent & non-blocking: Finite state machine design Design issues for clients and servers Cluster servers

EECE 411: Design of Distributed Software Applications Next A distributed system is: a collection of independent computers that appears to its users as a single coherent system Components need to: Communicate Cooperate => support needed Naming Synchronization

EECE 411: Design of Distributed Software Applications Naming systems Functionality Map: names  access points (addresses) Names are used to denote entities in a distributed system. To operate on an entity, we need to access it at an access point (address). Note: A location-independent name for an entity E, is independent from the addresses of the access points offered by E.

EECE 411: Design of Distributed Software Applications Names are valuable! NYT, August’00

EECE 411: Design of Distributed Software Applications Naming systems Functionality Map: names  access points (addresses) One challenge: scaling #of names, #clients geographical distribution, Management!