1. Inter Process Communication (IPC)
LECTURE 07

2. 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.
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3. Interprocess Communication (IPC)
IPC facility provides two operations:
Send (message) – message size fixed or variable
Receive (message)
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4. Interprocess Communication (IPC)
If P and Q wish to communicate, they need to:
establish a communication link between them
exchange messages via send/receive
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5. Interprocess Communication (IPC)
Implementation of communication link
physical (e.g., shared memory, hardware bus)
logical (e.g., logical properties)
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6. 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.
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7. Unicast vs. Multicast
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8. Interprocess Communications in Distributed Computing
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9. Interprocess Communication in basic HTTP
Processing order: C1, S1, C2, S2, S3, C3, C4, S4
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10. 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
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11. 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.
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12. 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.
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13. 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.
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14. 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)
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15. Indirect Communication …
Mailbox sharing
P1, P2, and P3 share mailbox A.
P1, sends; P2 and P3 receive.
Who gets the message?
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16. 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.
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17. 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.
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18. 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.
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19. 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.
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20. Synchronous send and receive
Event Diagram
Client Server
Sender Receiver
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21. Asynchronous send and synchronous receive
Event Diagram
Client Server
Sender Receiver
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22. Synchronous send and Async. Receive - 1
Data from P1 was received by P2
before issuing a non-blocking receive op in P2
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23. Synchronous send and Async. Receive - 2
Data from P1 arrived to P2 after P2 issued a
non-blocking receive op
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24. 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
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25. Asynchronous send and Asynchronous receive
Does P1 need an acknowledgement from P2?
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26. Event diagram
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27. 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.
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