Finishing up MOM

EVEN more on MOM... ugh

why is temporal decoupling (queuing) useful

there doesn’t have to be a consumer running when the event is produced as long as one turns up eventually

issues with queues

how large is a queue? how can this change? what happens in the extremes?

extreme 1: one event queue

channel can hold one event at most. first producer deposits event and blocks until its been consumed. second producer blocks until the first producer unblocks

extreme 2: grow the queue

the queue expands as events arrive, producers never block and consumers only block when there are no events

pro of growing the queue

simple straightforward model

cons of growing the queue

can generate memory overflows - we need to guarantee a maximum size the queue can grow to

bounded queue

fix the maximum size of the queue somewhere greater than one

allow events to drop

drop events from the queue when it is nearly full or full

note on dropping events

whether or not this is a good idea depends critically on what events mean to the system

event delivery QoS 1

guaranteed delivery - no matter what, the event will be delivered

event delivery QoS 2

at least once delivery - the event will be delivered, potentially several times

event delivery QoS 3

at most once delivery - the event will be dropped or delivered exactly once

event delivery QoS 4

best effort delivery - we try to deliver the event but you must be ready for the possibility of it not arriving

routing of event options

duplicate all events on all channels, send each event to a single channel or route based on the event’s content

duplication

events get duplicated for robustness but this means they may be delivered several times

duplication solutions

single path, unique stamps on events, idempotent events

message system

guarantees exactly once delivery

challenges of message systems

making sure messages are transferred reliably, surviving network partitions and machine failures

solution to message system challenges

using stable storage in the message queue

heuristic solutions to out of order results

timestamps, vector clocks

benefits of MOM having a larger framework 1

defines an architecture for a system

benefits of MOM having a larger framework 2

allows programmers to focus on business logic

benefits of MOM having a larger framework 3

less diversity and complexity for the component programmers