Inter Process Communication (IPC) LECTURE 07

Interprocess Communication (IPC) Exchange of data between two or more separate, independent processes/threads. Mechanism for processes to communicate and to synchronize their actions. Message system – processes communicate with each other without resorting to shared variables. 27 November 2018

Interprocess Communication (IPC) IPC facility provides two operations: Send (message) – message size fixed or variable Receive (message) 27 November 2018

Interprocess Communication (IPC) If P and Q wish to communicate, they need to: establish a communication link between them exchange messages via send/receive 27 November 2018

Interprocess Communication (IPC) Implementation of communication link physical (e.g., shared memory, hardware bus) logical (e.g., logical properties) 27 November 2018

IPC – unicast and multicast In distributed computing, two or more processes engage in IPC using a protocol agreed upon by the processes. A process may be a sender at some points during a protocol, a receiver at other points. When communication is from one process to a single other process, the IPC is said to be a unicast, e.g., Socket communication. When communication is from one process to a group of processes, the IPC is said to be a multicast, e.g., Publish/Subscribe Message model, a topic that we will explore in a later chapter. 27 November 2018

Unicast vs. Multicast 27 November 2018

Interprocess Communications in Distributed Computing 27 November 2018

Interprocess Communication in basic HTTP Processing order: C1, S1, C2, S2, S3, C3, C4, S4 27 November 2018

Direct Communication Processes must name each other explicitly: send (P, message) – send a message to process P Receive (Q, message) – receive a message from process Q 27 November 2018

Direct Communication Properties of communication link Links are established automatically. A link is associated with exactly one pair of communicating processes. Between each pair there exists exactly one link. The link may be unidirectional, but is usually bi-directional. 27 November 2018

Indirect Communication Messages are directed and received from mailboxes (also referred to as ports). Each mailbox has a unique id. Processes can communicate only if they share a mailbox. 27 November 2018

Indirect Communication … Properties of communication link Link established only if processes share a common mailbox A link may be associated with many processes. Each pair of processes may share several communication links. Link may be unidirectional or bi-directional. 27 November 2018

Indirect Communication … Operations create a new mailbox send and receive messages through mailbox destroy a mailbox Primitives are defined as: send (A, message) receive (A, message) 27 November 2018

Indirect Communication … Mailbox sharing P1, P2, and P3 share mailbox A. P1, sends; P2 and P3 receive. Who gets the message? 27 November 2018

Indirect Communication … Solutions Allow a link to be associated with at most two processes. Allow only one process at a time to execute a receive operation. Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was. 27 November 2018

Synchronization Message passing may be either blocking or non-blocking. Blocking is considered synchronous Non-blocking is considered asynchronous send and receive primitives may be either blocking or non-blocking. 27 November 2018

Event Synchronization Interprocess communication may require that the two processes synchronize their operations: one side sends, then the other receives until all data has been sent and received. Ideally, the send operation starts before the receive operation commences. In practice, the synchronization requires system support. 27 November 2018

Synchronous vs. Asynchronous Communication The IPC operations may provide the synchronization necessary using blocking. A blocking operation issued by a process will block further processing of the process until the operation is fulfilled. Alternatively, IPC operations may be asynchronous or nonblocking. An asynchronous operation issued by a process will not block further processing of the process. Instead, the process is free to proceed with its processing, and may optionally be notified by the system when the operation is fulfilled. 27 November 2018

Synchronous send and receive Event Diagram Client Server Sender Receiver 27 November 2018

Asynchronous send and synchronous receive Event Diagram Client Server Sender Receiver 27 November 2018

Synchronous send and Async. Receive - 1 Data from P1 was received by P2 before issuing a non-blocking receive op in P2 27 November 2018

Synchronous send and Async. Receive - 2 Data from P1 arrived to P2 after P2 issued a non-blocking receive op 27 November 2018

Synchronous send and Async. Receive - 3 Data from P1 arrived to P2 before P2 issues a non-blocking receive op. P2 is notified of the arrival of data 27 November 2018

Asynchronous send and Asynchronous receive Does P1 need an acknowledgement from P2? 27 November 2018

Event diagram 27 November 2018

Buffering Queue of messages attached to the link; implemented in one of three ways. Zero capacity – No messages Sender must wait for receiver Bounded capacity – n messages Sender must wait if link full. Unbounded capacity – infinite length Sender never waits. 27 November 2018