Understanding System Models and Properties

1. Understanding CAP Theorem

Property	Meaning	Sacrifice Under Partition
Consistency (C)	Every read sees the latest write (linearizable)	—
Availability (A)	Every request gets a non-error response	—
Partition Tolerance (P)	System continues despite network partitions	Mandatory in real systems
CP Systems	HBase, MongoDB (default), etcd, ZooKeeper, Spanner	Reject writes on minority side
AP Systems	Cassandra, DynamoDB, Riak, Couchbase	Accept writes; reconcile later

2. Understanding PACELC Theorem

Condition	Choice	Examples
P (partition)	A vs C	Same as CAP
E (else, normal ops)	L (latency) vs C (consistency)	—
PA/EL	Available + low latency	Cassandra, DynamoDB, Riak
PC/EC	Always consistent	HBase, BigTable, Spanner (mostly)
PA/EC	Available on partition, consistent normally	MongoDB

3. Understanding ACID vs BASE Properties

4. Understanding Fault Models

Model	Assumption	Tolerance Cost
Crash-Stop	Failed nodes halt forever	f+1 nodes for f failures
Crash-Recovery	Nodes may recover with persistent state	2f+1 (Paxos/Raft)
Omission	Messages may be dropped	Retransmission protocols
Byzantine	Arbitrary/malicious behavior	3f+1 (PBFT)

5. Understanding Timing Models

Model	Bound Assumption	Practicality
Synchronous	Known bounds on delay & clock drift	Real-time/embedded only
Asynchronous	No timing assumptions	FLP impossibility for consensus
Partial Synchrony	Bounds exist but unknown, eventually hold	Real-world model (Raft, Paxos)

6. Understanding System Guarantees

Guarantee	Meaning
Safety	"Bad things never happen" (e.g., no two leaders)
Liveness	"Good things eventually happen" (e.g., progress)
At-most-once	Message delivered 0 or 1 times
At-least-once	Message delivered ≥ 1 time (duplicates possible)
Exactly-once	Effectively once via dedup + idempotency

7. Understanding Availability Metrics (SLA, SLO, SLI)

Term	Definition	Example
SLI	Service Level Indicator — measured metric	p99 latency = 120ms
SLO	Service Level Objective — internal target	99.9% requests < 200ms
SLA	Service Level Agreement — contractual + penalty	99.95% uptime or refund
Error Budget	1 − SLO; allowed failure quota	0.1% = 43.2 min/month

8. Understanding Durability Guarantees

Level	Mechanism	Risk
Memory only	RAM	Lost on crash
Single disk fsync	WAL + fsync	Disk failure
RAID	Multiple disks	Host failure
Replicated (sync)	Quorum write	Region failure
Cross-region	Geo-replication	Multi-region outage
11 nines (S3)	Erasure coding + multi-AZ	~10⁻¹¹/yr loss

9. Understanding Failure Modes

Mode	Symptom	Detection Strategy
Fail-stop	Process exits	TCP RST, heartbeat loss
Fail-slow / gray	Degraded throughput/latency	SLI monitoring, outlier detection
Network partition	Subset unreachable	Quorum loss, gossip view
Resource exhaustion	OOM, FD limit, disk full	USE metrics, alerts
Data corruption	Checksum mismatch	CRC, Merkle trees
Configuration drift	Inconsistent behavior	Audit, IaC reconciliation

10. Understanding Performance Guarantees

Metric	Common Target
p50 latency	10-50ms
p99 latency	100-500ms
p999 latency	< 1-2s
Throughput	RPS sustained at SLO
Saturation	< 70% utilization for headroom