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Questions and Answers
Blocking receive will halt execution until an appropriate message is available.
Blocking receive will halt execution until an appropriate message is available.
True
Non-blocking receive allows a process to continue execution even if no message is currently available.
Non-blocking receive allows a process to continue execution even if no message is currently available.
True
Message queues ensure that messages are delivered more than once to guarantee reliability.
Message queues ensure that messages are delivered more than once to guarantee reliability.
False
Distributed Shared Memory (DSM) allows processes to access memory as if it is shared, despite being physically distributed.
Distributed Shared Memory (DSM) allows processes to access memory as if it is shared, despite being physically distributed.
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The scalability of message queues is considered limited compared to publish-subscribe systems.
The scalability of message queues is considered limited compared to publish-subscribe systems.
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In direct communication, the sender and receiver know of each other and exist at the same time.
In direct communication, the sender and receiver know of each other and exist at the same time.
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In indirect communication, the sender and receiver are coupled and must be available simultaneously.
In indirect communication, the sender and receiver are coupled and must be available simultaneously.
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Group communication guarantees ordered delivery of messages.
Group communication guarantees ordered delivery of messages.
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A group is considered open if only its members may multicast to it.
A group is considered open if only its members may multicast to it.
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Basic multicast does not require agreement in message delivery across all processes in a group.
Basic multicast does not require agreement in message delivery across all processes in a group.
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Message queues are a technique used in indirect communication methods.
Message queues are a technique used in indirect communication methods.
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Unicast communication means sending a message to one specific recipient.
Unicast communication means sending a message to one specific recipient.
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Fault tolerance is not a feature of group communication.
Fault tolerance is not a feature of group communication.
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In a publish-subscribe system, notifications are sent synchronously by event generating publishers.
In a publish-subscribe system, notifications are sent synchronously by event generating publishers.
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Publishers in a publish-subscribe system must always declare the styles of events they will generate.
Publishers in a publish-subscribe system must always declare the styles of events they will generate.
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A subscriber can revoke their interest in receiving notifications in a publish-subscribe system by using an unsubscribe operation.
A subscriber can revoke their interest in receiving notifications in a publish-subscribe system by using an unsubscribe operation.
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Message queues ensure that a single process retains control over the message until it is delivered.
Message queues ensure that a single process retains control over the message until it is delivered.
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In a topic-based approach of subscription, subscribers express their interests in specific topics.
In a topic-based approach of subscription, subscribers express their interests in specific topics.
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In a message queue, both sender and receiver must be aware of the identity of the queue.
In a message queue, both sender and receiver must be aware of the identity of the queue.
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There is only one programming model for message queues regardless of the application.
There is only one programming model for message queues regardless of the application.
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In a publish-subscribe system, the delivery guarantees of notifications are not influenced by the application requirements.
In a publish-subscribe system, the delivery guarantees of notifications are not influenced by the application requirements.
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FIFO ordering ensures that messages are delivered in the order they are sent to all processes in the group.
FIFO ordering ensures that messages are delivered in the order they are sent to all processes in the group.
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Causal ordering delivers messages without considering the sequence in which they were sent.
Causal ordering delivers messages without considering the sequence in which they were sent.
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Total ordering guarantees that all processes will deliver messages in the same order.
Total ordering guarantees that all processes will deliver messages in the same order.
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A group membership service is responsible for managing group address expansion.
A group membership service is responsible for managing group address expansion.
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Multicast delivery is fully coordinated with changes in group membership.
Multicast delivery is fully coordinated with changes in group membership.
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In a publish-subscribe system, publishers are responsible for expressing interest through subscriptions.
In a publish-subscribe system, publishers are responsible for expressing interest through subscriptions.
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Group membership services do not involve failure detection.
Group membership services do not involve failure detection.
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The primary purpose of a causal ordering system is to prevent message collisions during delivery.
The primary purpose of a causal ordering system is to prevent message collisions during delivery.
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Study Notes
Indirect Communication
- Indirect communication decouples senders and receivers in time and space.
- Sender can send a message even if the receiver is unavailable or inaccessible.
- Sender does not necessarily know to whom or how many receivers there are.
Indirect Communication Techniques
- Group communication
- Publish-subscribe systems
- Message queues
- Distributed shared memory (DSM)
Group Communication
- A service that sends a message to a defined group, and then that message is delivered to all group members.
- Features include well-defined, managed groups, ordered message delivery, fault tolerance and delivery guarantees, and handling of multiple senders.
- Different communication methods include point-to-point (unicast), one-to-all (broadcast), and one-to-multiple (multicast). Multicast can have closed or open groups.
- Closed groups only permit members to send messages. Open groups allow both internal and external parties to send.
- Ordering of messages is necessary, including Integrity (message received matches sent, no duplicate deliveries) and Validity (all outgoing messages eventually delivered). If one process receives a message, then all other processes in the group receive it.
Ordering of Messages
- FIFO (First-In, First-Out): Messages are delivered in the order they were sent.
- Causal Ordering: If one message happens before another in the distributed system, the order is preserved in the delivery. This ensures the correct sequencing of related message exchanges.
- Total Ordering: Messages are delivered to all processes in the same order.
Group Membership Service
- Four main tasks:
- An interface for managing group membership changes.
- A failure detector for identifying failures in the system.
- Membership change notification.
- Group address expansion.
- Multicast delivery is not coordinated with membership changes, making it a weak system for managing group membership, as membership changes are not reflected for receiving parties.
Publish-Subscribe Systems
- A system where publishers publish events and subscribers express interest through subscriptions.
- The system matches subscriptions against published events to ensure delivery of event notifications.
- Examples include a dealing room system.
- Characteristics: heterogeneity (objects publish and subscribe to patterns), and asynchronicity (notifications sent asynchronously from publisher to subscriber).
Delivery Guarantees
- Different delivery guarantees for notifications depend on application requirements and the failure model for the communication type.
Programming Model
- Publish-subscribe systems use a set of operations to manage communication.
- Subscribe: Subscribers declare their interest in specific types or patterns of events.
- Advertise/Publish: Publishers declare the type and style of events they produce.
- Notify: Events are delivered to subscribed parties.
- Unsubscribe: Subscribers can remove their interest in specific events.
Subscription (Filter) Model
- Ways to define event subscriptions include: Channel-based, Topic-based, Content-based, and Type-based approaches.
Message Queues (MQ)
- The sender places a message in a queue, where it is removed by a single process. Senders and receivers must know the queue identity.
- Queue Middleware should be active.
- Programming Model: Producer processes send to the queue and consumer processes retrieve. Receive can be Blocking (waits until message available), Non-blocking (checks and returns message if available), or Notify (when a message becomes available).
- Reliable delivery features:*
- Messages are persistent and stored until consumption.
- Integrity: The message received matches the message sent and no messages are delivered twice.
- Validity: All sent messages will eventually be received.
Distributed Shared Memory (DSM)
- DSM provides an abstraction for sharing data across distributed computers that do not share a physical memory space.
- Processes interact with DSM as if it were a single shared memory, but the memory is distributed.
- Processes access DSM by reading and updating memory.
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Description
Explore the concept of indirect communication, where messages can be sent without the immediate availability of the receiver. This quiz covers various techniques such as group communication, publish-subscribe systems, and message queues, alongside the principles of ordered message delivery and fault tolerance.
