CSE 486 Final Question and answers
verified to pass 2025/2026
In gossip protocols, as in epidemiology, what are the options for a process to be categorized as and what
does each mean? - correct answer ✔-Susceptible to infections by a new message
-Infected by a new message and capable of retransmitting it
-Removed from the set of infected processes (and now "immune" to the message)
Why is Simple Multicast problematic? - correct answer ✔If a message is lost or if the process fails, only
some processes will receive the message while some might not receive the message
Describe Reliable Multicast - correct answer ✔-One process casts a message to every process
-Each process then sends that message to every process
-Each process will received that message once for each process in the system
-If any correct process receives the message, all correct processes receive the message
DOWNSIDE:
-Requires more message: |G|^2 messages [G = number of processes]
Describe Simple Gossip - correct answer ✔-A Process wants to multicast a message to all processes
-That processes will send that message to a few (randomly selected) processes
-The processes that received the message will then send that message to a few (randomly selected)
processes
-If a process receives a message that It already heard it will not send it again
DOWNSIDE:
- There is a chance that a process will not get the message; if processes happen to only send to
processes that already heard it
,What are the Disadvantages of Gossip? - correct answer ✔-Some processes may not received a
message even without failure [processes happen to only send messages to processes that already heard
it]
-Small groups require k to be roughly equal to |G| anyway (K = the number of randomly selected
processes being sent the message)
- Delay between first transmission and final infection can be large
What are the benefits of Gossip? - correct answer ✔- Far fewer messages
- only one process must hear the message to start an epidemic
- every process receives every message with high probability
What does LPB (Lightweight Probabilistic Broadcast) use gossip for? What does it allow? - correct
answer ✔-Message distribution
-Group membership
Allows:
- Large groups
- Dynamic membership
- Configuration reliability
-low message traffic
What actions can a process take in LPB (Lightweight Probabilistic Broadcast)? - correct answer ✔-
Subscribe to a topic (join a group)
-Unsubscribe from a topic (leave a group)
-Send Notification (message) to a topic (group)
In LPB (Lightweight Probabilistic Broadcast), processes keep track of three lists, what are they? - correct
answer ✔- Recently subscribed processes in subs
- Recently unsubscribed processes in unSubs
, - Exactly I processes believed to be subscribed in view
What does every message LPB (lightweight probabilistic Broadcast) contain? - correct answer ✔- A list
of all new notifications since the last message
- A list of event IDs for some recent notifications
- A list of some recent subscriptions
- A list of some recent unsubsriptions
How does subscribing work in LPB (Lightweight Probabilistic Broadcast)? - correct answer ✔- To
subscribe to a the topic, a process must send a request to any subscribed process [includes itself in new
subscribed]
- if it does not start receiving notifications, it tries again
- A subscribed process periodically gossips its subscription
- failed processes are eventually forgotten
What is a way to prevent LPB (lightweight Probabilistic Broadcast) from partitioning? - correct answer
✔- Some privileged processes can be kept by all processes to prevent partition
What are the benefits of ipbcast (Lightweight Probabilistic Broadcast)? - correct answer ✔- adds
memebrship management to simple gossip
- adds reliability through events and eventIDs
- uses a relatively constant bandwidth
When is gossip appropriate to use? - correct answer ✔- The occasional lost message can be tolerated
- simple multicast is not reliable enough
- reliable multicast is too expensive
- group membership is unstable
verified to pass 2025/2026
In gossip protocols, as in epidemiology, what are the options for a process to be categorized as and what
does each mean? - correct answer ✔-Susceptible to infections by a new message
-Infected by a new message and capable of retransmitting it
-Removed from the set of infected processes (and now "immune" to the message)
Why is Simple Multicast problematic? - correct answer ✔If a message is lost or if the process fails, only
some processes will receive the message while some might not receive the message
Describe Reliable Multicast - correct answer ✔-One process casts a message to every process
-Each process then sends that message to every process
-Each process will received that message once for each process in the system
-If any correct process receives the message, all correct processes receive the message
DOWNSIDE:
-Requires more message: |G|^2 messages [G = number of processes]
Describe Simple Gossip - correct answer ✔-A Process wants to multicast a message to all processes
-That processes will send that message to a few (randomly selected) processes
-The processes that received the message will then send that message to a few (randomly selected)
processes
-If a process receives a message that It already heard it will not send it again
DOWNSIDE:
- There is a chance that a process will not get the message; if processes happen to only send to
processes that already heard it
,What are the Disadvantages of Gossip? - correct answer ✔-Some processes may not received a
message even without failure [processes happen to only send messages to processes that already heard
it]
-Small groups require k to be roughly equal to |G| anyway (K = the number of randomly selected
processes being sent the message)
- Delay between first transmission and final infection can be large
What are the benefits of Gossip? - correct answer ✔- Far fewer messages
- only one process must hear the message to start an epidemic
- every process receives every message with high probability
What does LPB (Lightweight Probabilistic Broadcast) use gossip for? What does it allow? - correct
answer ✔-Message distribution
-Group membership
Allows:
- Large groups
- Dynamic membership
- Configuration reliability
-low message traffic
What actions can a process take in LPB (Lightweight Probabilistic Broadcast)? - correct answer ✔-
Subscribe to a topic (join a group)
-Unsubscribe from a topic (leave a group)
-Send Notification (message) to a topic (group)
In LPB (Lightweight Probabilistic Broadcast), processes keep track of three lists, what are they? - correct
answer ✔- Recently subscribed processes in subs
- Recently unsubscribed processes in unSubs
, - Exactly I processes believed to be subscribed in view
What does every message LPB (lightweight probabilistic Broadcast) contain? - correct answer ✔- A list
of all new notifications since the last message
- A list of event IDs for some recent notifications
- A list of some recent subscriptions
- A list of some recent unsubsriptions
How does subscribing work in LPB (Lightweight Probabilistic Broadcast)? - correct answer ✔- To
subscribe to a the topic, a process must send a request to any subscribed process [includes itself in new
subscribed]
- if it does not start receiving notifications, it tries again
- A subscribed process periodically gossips its subscription
- failed processes are eventually forgotten
What is a way to prevent LPB (lightweight Probabilistic Broadcast) from partitioning? - correct answer
✔- Some privileged processes can be kept by all processes to prevent partition
What are the benefits of ipbcast (Lightweight Probabilistic Broadcast)? - correct answer ✔- adds
memebrship management to simple gossip
- adds reliability through events and eventIDs
- uses a relatively constant bandwidth
When is gossip appropriate to use? - correct answer ✔- The occasional lost message can be tolerated
- simple multicast is not reliable enough
- reliable multicast is too expensive
- group membership is unstable
