# The Distributed Systems Tax — Monolith vs Microservices

Source: https://www.skillbyai.com/en/monolith-microservices/t-costs

> List the costs microservices introduce, including the fallacies of distributed computing.

## What you pay for the network

Splitting a process into services turns function calls into **network calls**, and the network is unreliable. The classic **fallacies of distributed computing** (attributed to L. Peter Deutsch and others at Sun Microsystems) are false assumptions: the network is reliable; latency is zero; bandwidth is infinite; the network is secure; topology does not change; there is one administrator; transport cost is zero; the network is homogeneous. Concrete costs follow. **Latency** adds up across call chains. **Partial failures** need timeouts, retries and circuit breakers. **Data consistency** across services needs sagas and eventual consistency instead of transactions. **Testing** and **debugging** span many services, requiring contract tests and distributed tracing. **Operations** multiply: many pipelines, deployments, dashboards, alerts and on-call rotations. **Infrastructure** costs more: more instances, a gateway, a mesh, brokers and observability tools.

## Latency adds up in a call chain

Five sequential calls at a modest p99 add up quickly.

```python
chain_p50_ms = [3, 4, 5, 3, 6]          # each hop, typical
chain_p99_ms = [25, 40, 60, 30, 80]     # each hop, slow case

print("typical", sum(chain_p50_ms), "ms")    # same work in-process: well under 1 ms of call overhead
print("worst-ish", sum(chain_p99_ms), "ms")

# chance that at least one of five hops is at its p99 or slower
print(round(1 - 0.99 ** 5, 3))            # about 5% of requests
```

## Phone calls instead of shouting across the room

In a small office you shout a question across the room and get an answer instantly. Move everyone to different buildings and every question becomes a phone call: sometimes busy, sometimes dropped, always slower, and someone has to pay the phone bill.

**Quiz:** Which is one of the fallacies of distributed computing?

- [ ] Databases are always relational
- [x] The network is reliable
- [ ] Monoliths cannot use caching
- [ ] Containers are secure by default

*Answer:* The network is reliable. "The network is reliable" is the first of the classic fallacies.