vault backup: 2024-10-11 17:44:32

BUCA/Queues.md

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+Challenges
+- Atomicity
+	- you write to the queue first
+	- you write to the db first
+	- we need a transaction!
+- Idempotence
+	- we need locking mechanism on the queue, to avoid multiple process to process the same element
+	- solution: replay protection (lastUpdate in the db), but still I have the problem of multiple process reading the same element
+
+## Data replication
+- Waiting for all replicas
+	- definitely slower
+	- safer for the data
+- Waiting only for the primary replica
+	- faster but...
+	- replica may fail
+	- not getting always the same value
+	- ok for maybe Instagram feed, but not for e-mails
+
+## Paxos
+Paxos overview:
+- partecipnats are: Writers & Replicas
+- 2 phase protocol:
+	- **prepare** reserves right to supply value, and robs other
+	- **accepts** supplies the value **v**
+	- replicas are stateful
+	- communication is ordered by "proposal number" **n**
+	- uses "viral propagation"
+
+#### Algorithm
+- Prepare phase
+	- Success if replica hasn't prepared a higher proposal
+	- Fail otherwise
+	- Success with value if we have accepted another value (viral property)
+
+Majority succeded -> Go to Accept Phase
+
+- Accept phase
+	- Success only if replica hasn't Prepared a higher proposal (same is ok)
+
+### Mesh
+- full mesh
+- subsetting: not every node connected to every other node: in case of a query of death, only connected nodes will fail
+- proxying: adding a layer of proxies (load balancers), that use a lookup service to route the request
+
+### Strong stickiness
+Some applications like chat, online game servers, video calls ecc. need a very strong stickiness.
+In these cases, we may not want to use consistent hashing. Maybe we can use it to pick a server and then stick with it.
+
+Also, it can happen that some users are not able to reach a specific server, but other does.
+Using leases might be a solution.
+
+- Request from Alice
+- lookup server response is cached
+- we have a failure
+- we call Set to change the server, but the call stalls until the lease expires. Also, the lookup server might be able to still reach the server and renew the lease