How to Inject Latency into Your Flow with lueur

This guide shows how to delay traffic by a configurable amount, distribution, side (client or server), and direction (ingress or egress). You can simulate everything from a stable normal latency to heavy-tailed Pareto scenarios and selectively apply them to only client or server traffic.

Prerequisites

Install lueur

If you haven’t installed Lueur yet, follow the installation instructions.
Basic Proxy Setup Be familiar with running lueur run --with-[fault] commands from your terminal.

Normal Distribution

A normal (Gaussian) distribution around a mean of 300ms with a standard deviation of 40ms.

Most delays hover around 300ms, but some are quicker/slower based on the bell curve.

Start the proxy with a normal distribution latency

lueur run \
    --with-latency \ # (1)!
    --latency-distribution normal \ # (2)!
    --latency-mean 300 \ # (3)!
    --latency-stddev 40 # (4)!

Enable the latency fault support
Use the normal distribution
Introduce a latency of 300ms on average
Add 40ms standard deviation ±40 ms

Uniform Distribution

A uniform distribution means every delay in min..max is equally likely.

The added delay is anywhere between 300 / 500ms without bias around a middle value.

Start the proxy with a uniform distribution latency

lueur run \
    --with-latency \ # (1)!
    --latency-distribution uniform \ # (2)!
    --latency-min 300 \ # (3)!
    --latency-max 500 # (4)!

Enable the latency fault support
Use the uniform distribution
Introduce a latency of at least 300ms
Set the maximum latency to 500ms

Pareto Distribution

A Pareto distribution often creates a heavy‐tail, meaning most delays are small, but occasional extremely large spikes.

You’ll see frequent short delays (20ms or so) but occasionally large outliers.

Start the proxy with a Pareto distribution latency

lueur run \
    --with-latency \ # (1)!
    --latency-distribution pareto \ # (2)!
    --latency-scale 20 \ # (3)!
    --latency-shape 1.5 # (4)!

Enable the latency fault support
Use the pareto distribution
Set a scale of 20ms
Set the shape of the distribution to 1.5

Pareto + Normal Hybrid Distribution

Get a base normal offset of ~50±15ms, plus a heavy‐tailed portion from the Pareto factors.

Start the proxy with a Pareto + Normal distribution latency

lueur run \
    --with-latency \ # (1)!
    --latency-distribution paretonormal \ # (2)!
    --latency-scale 20 \ # (3)!
    --latency-shape 1.5 \ # (4)!
    --latency-mean 50 \ # (5)!
    --latency-stddev 15 # (6)!

Enable the latency fault support
Use the pareto distribution
Set a scale of 20ms
Set the shape of the distribution to 1.5
Set a mean of 50ms on average
Standard deviation of 15ms around that mean.

Latency On Ingress Only

Delay traffic from the server to the client.

Start the proxy with any distribution and set the direction to ingress.
```
lueur run \
    --with-latency \ # (1)!
    --latency-direction ingress \ # (2)!
    --latency-mean 50
```
1. Enable the latency fault support
2. Set the latency to take place in ingress

Latency On Egress Only

Delay traffic from the client to the server.

Start the proxy with any distribution and set the direction to egress.
```
lueur run \
    --with-latency \ # (1)!
    --latency-direction egress \ # (2)!
    --latency-mean 50
```
1. Enable the latency fault support
2. Set the latency to take place in egress

Latency On Client-Side Only

Start the proxy with any distribution and set the side to client.
```
lueur run \
    --with-latency \ # (1)!
    --latency-side client \ # (2)!
    --latency-mean 50
```
1. Enable the latency fault support
2. Set the latency to take place on client side

Latency On Server-Side Only

Start the proxy with any distribution and set the side to server.
```
lueur run \
    --with-latency \ # (1)!
    --latency-side server \ # (2)!
    --latency-mean 50
```
1. Enable the latency fault support
2. Set the latency to take place on server side

Latency On Ingress From Server-Side Only

Start the proxy with any distribution and set the direction to ingress and the side to server.

lueur run \
    --with-latency \
    --latency-direction ingress \
    --latency-side server \
    --latency-mean 50

Next Steps

Scheduled Delays: Use --latency-sched "start:20%,duration:30%" to enable high latency for part of the total run.
Stacking: Combine latency with jitter or bandwidth constraints for a more realistic environment.
Extreme Spikes: Increase standard deviation or shape to stress test how your application handles sudden bursts of slowness.