Delivery agent assignment in LLD - Architecture Diagram

System Overview - Delivery agent assignment

This system assigns delivery agents to customer orders efficiently. It ensures orders are matched with available agents nearby, considering agent capacity and location to optimize delivery speed and resource use.

Architecture Diagram

User
  |
  v
Load Balancer
  |
  v
API Gateway
  |
  v
Assignment Service <--> Agent Location Service
  |                     |
  v                     v
Order Database       Agent Database
  |
  v
Cache (for recent assignments)

Components

User

client

Customer placing delivery orders

Load Balancer

load_balancer

Distributes incoming requests evenly to API Gateway instances

API Gateway

api_gateway

Handles client requests, routes them to appropriate services

Assignment Service

service

Assigns delivery agents to orders based on availability and location

Agent Location Service

service

Tracks real-time locations and statuses of delivery agents

Order Database

database

Stores order details and assignment records

Agent Database

database

Stores delivery agent profiles and availability

Cache

cache

Stores recent assignment data for quick access

Request Flow - 10 Hops

User → Load Balancer

Load Balancer → API Gateway

API Gateway → Assignment Service

Assignment Service → Cache

Assignment Service → Agent Location Service

Assignment Service → Agent Database

Assignment Service → Order Database

Assignment Service → Cache

Assignment Service → API Gateway

API Gateway → User

Failure Scenario

Component Fails:Agent Location Service

Impact:Assignment Service cannot get real-time agent locations, leading to possible suboptimal agent assignments or delays

Mitigation:Fallback to cached agent locations in Cache; alert system operators; degrade to less precise assignment logic temporarily

Architecture Quiz - 3 Questions

Which component is responsible for distributing incoming user requests evenly?

A API Gateway

B Assignment Service

C Load Balancer

D Cache

Design Principle

This design uses caching to reduce database load and improve response times. It separates concerns by using dedicated services for assignment logic and agent location tracking. Load balancing and API gateway ensure scalability and security. Failure handling includes graceful degradation using cache fallback.

Practice

1.

What is the primary goal of a delivery agent assignment system?

easy

A. Match orders to available delivery agents nearby

B. Track payment details of customers

C. Manage inventory in warehouses

D. Calculate delivery charges for orders

Solution

1. Step 1: Understand the system purpose

   The delivery agent assignment system focuses on connecting orders with delivery agents who can fulfill them.

2. Step 2: Identify the main function

   Matching orders to free agents nearby ensures timely delivery and efficient resource use.

3. Final Answer:

   Match orders to available delivery agents nearby -> Option A

4. Quick Check:

   Delivery agent assignment = Matching orders to agents [OK]

Hint: Focus on matching orders to agents, not payments or inventory [OK]

Common Mistakes:

• Confusing delivery assignment with payment processing
• Thinking inventory management is part of agent assignment
• Assuming delivery charges calculation is the main goal

2.

Which data structure is best to quickly find the nearest free delivery agent for an order?

easy

A. Priority queue sorted by distance from order location

B. Stack of agents in order of registration

C. Hash map keyed by agent ID

D. Linked list of all agents

Solution

1. Step 1: Identify the need for sorting by distance

   To find the nearest free agent, sorting agents by their distance to the order location is essential.

2. Step 2: Choose a data structure supporting efficient nearest retrieval

   A priority queue can efficiently provide the closest agent by always giving the smallest distance first.

3. Final Answer:

   Priority queue sorted by distance from order location -> Option A

4. Quick Check:

   Nearest agent search = Priority queue [OK]

Hint: Use priority queue for nearest-first retrieval [OK]

Common Mistakes:

• Using hash map which doesn't sort by distance
• Using stack or linked list which are inefficient for nearest search
• Ignoring the need to sort by distance

3.

Consider this pseudocode for assigning an agent:
for agent in agents:
  if agent.status == 'free' and distance(agent, order) < min_distance:
    min_distance = distance(agent, order)
    assigned_agent = agent
return assigned_agent
What will this code return if all agents are busy?

medium

A. The closest free agent

B. None or null

C. The first agent in the list

D. An error due to undefined variable

Solution

1. Step 1: Check variable initializations

   The code does not initialize assigned_agent or min_distance before the loop.

2. Step 2: Trace execution when all agents are busy

   The if condition's first part (agent.status == 'free') fails for all agents, so due to short-circuit evaluation of 'and', the second part (distance < min_distance) is never evaluated. The loop ends without ever setting assigned_agent. Returning an uninitialized assigned_agent causes an error due to undefined variable.

3. Final Answer:

   An error due to undefined variable -> Option D

4. Quick Check:

   No initialization + all busy = undefined variable error [OK]

Hint: No variable initialization leads to undefined variable error [OK]

Common Mistakes:

• Thinking assigned_agent defaults to None or null
• Assuming it returns the first agent regardless of status
• Believing the code handles no free agents gracefully

4.

Given this snippet:
assigned_agent = None
for agent in agents:
  if agent.status = 'free':
    assigned_agent = agent
return assigned_agent
What is the main error in this code?

medium

A. Returning assigned_agent inside the loop

B. Not initializing assigned_agent before loop

C. Using assignment operator '=' instead of comparison '==' in if condition

D. Using 'free' instead of 'available' as status

Solution

1. Step 1: Check the if condition syntax

   The condition uses '=' which assigns value instead of '==' which compares values.

2. Step 2: Understand impact of wrong operator

   This causes a syntax error or unintended behavior because '=' cannot be used in conditions.

3. Final Answer:

   Using assignment operator '=' instead of comparison '==' in if condition -> Option C

4. Quick Check:

   Use '==' for comparison, not '=' [OK]

Hint: Use '==' for comparisons in conditions [OK]

Common Mistakes:

• Confusing '=' with '==' in if statements
• Thinking assigned_agent must be initialized inside loop
• Assuming return inside loop is the error

5.

You want to design a scalable delivery agent assignment system for a city with thousands of agents and orders per minute. Which approach best improves scalability?

hard

A. Use a centralized server to check all agents for every order

B. Partition the city into zones and assign agents within zones only

C. Assign agents randomly without considering location

D. Assign the oldest registered agent to every order

Solution

1. Step 1: Understand scalability challenges

   Checking all agents for every order is slow and resource-heavy at large scale.

2. Step 2: Choose a partitioning strategy

   Dividing the city into zones limits search space, making assignment faster and scalable.

3. Step 3: Evaluate other options

   Random or oldest agent assignment ignores location, causing delays and inefficiency.

4. Final Answer:

   Partition the city into zones and assign agents within zones only -> Option B

5. Quick Check:

   Zone partitioning = scalable assignment [OK]

Hint: Divide city into zones to limit search scope [OK]

Common Mistakes:

• Using centralized server causing bottlenecks
• Ignoring agent location in assignment
• Assigning agents randomly or by registration time