Event sourcing pattern in Microservices - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define the main storage in event sourcing.

Microservices

events = []  # This list stores all [1]

Drag options to blanks, or click blank then click option'

Astates

Blogs

Cevents

Dsnapshots

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing events with snapshots or current states.

Using logs as a generic term instead of events.

In event sourcing, all changes are stored as events in an append-only log.

2fill in blank

medium

Complete the code to rebuild the current state from events.

Microservices

state = initial_state
for event in events:
    state = [1](state, event)

Drag options to blanks, or click blank then click option'

Aapply_event

Bsave_event

Cdelete_event

Dlog_event

Attempts:

3 left

💡 Hint

Common Mistakes

Using functions that do not modify the state.

Confusing event application with event storage.

The apply_event function updates the state by applying each event in order.

3fill in blank

hard

Fix the error in the event storage function to append new events.

Microservices

def store_event(event):
    [1].append(event)

Drag options to blanks, or click blank then click option'

Astate

Bevents

Csnapshot

Dlog

Attempts:

3 left

💡 Hint

Common Mistakes

Appending events to the wrong variable.

Using a variable that does not exist.

New events must be appended to the events list to keep the event history.

4fill in blank

hard

Fill both blanks to create a snapshot after every 100 events.

Microservices

if len(events) [1] 100 == 0:
    snapshot = [2](state)

Drag options to blanks, or click blank then click option'

A%

B==

C>

Dcreate_snapshot

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operators.

Confusing snapshot creation with event storage.

The modulo operator % checks if 100 events have occurred, then create_snapshot saves the current state.

5fill in blank

hard

Fill both blanks to implement event replay with snapshot optimization.

Microservices

def rebuild_state():
    state = [1]
    for event in events[[2]:]:
        state = apply_event(state, event)  # Apply remaining events
    return state  # Return the latest state

Drag options to blanks, or click blank then click option'

Asnapshot

B0

Clen(snapshot_events)

Dinitial_state

Attempts:

3 left

💡 Hint

Common Mistakes

Starting replay from the beginning every time.

Ignoring snapshots and replaying all events.

Start from the latest snapshot, replay events after the snapshot index len(snapshot_events).

Practice

1. What is the main idea behind the Event sourcing pattern in microservices?

easy

A. Store all changes as a sequence of events instead of only the current state

B. Store only the latest snapshot of data for faster access

C. Use events only for communication between services, not for storage

D. Store events only when errors occur in the system

Solution

1. Step 1: Understand event sourcing concept

   Event sourcing means saving every change as an event, not just the final state.

2. Step 2: Compare options with definition

   Only Store all changes as a sequence of events instead of only the current state correctly describes storing all changes as events, others focus on snapshots or partial use.

3. Final Answer:

   Store all changes as a sequence of events instead of only the current state -> Option A

4. Quick Check:

   Event sourcing = storing all changes as events [OK]

Hint: Event sourcing saves every change, not just the final state [OK]

Common Mistakes:

• Confusing event sourcing with snapshot storage
• Thinking events are only for communication
• Believing events are stored only on errors

2. Which of the following is the correct sequence in event sourcing when a user updates their profile?

easy

A. Update state directly, then create an event for the change

B. Send event to other services before updating state

C. Create an event representing the change, then update the state from the event

D. Store the new state without creating any event

Solution

1. Step 1: Recall event sourcing flow

   In event sourcing, changes are first recorded as events, then state is updated from those events.

2. Step 2: Match options to flow

   Create an event representing the change, then update the state from the event correctly shows event creation before state update; others do not follow this order.

3. Final Answer:

   Create an event representing the change, then update the state from the event -> Option C

4. Quick Check:

   Event first, then state update = Create an event representing the change, then update the state from the event [OK]

Hint: Events come before state updates in event sourcing [OK]

Common Mistakes:

• Updating state before creating events
• Skipping event creation
• Sending events before state is updated

3. Given this simplified event sourcing code snippet, what will be the final user balance?

events = [
  {"type": "Deposit", "amount": 100},
  {"type": "Withdraw", "amount": 30},
  {"type": "Deposit", "amount": 50}
]

balance = 0
for event in events:
    if event["type"] == "Deposit":
        balance += event["amount"]
    elif event["type"] == "Withdraw":
        balance -= event["amount"]
print(balance)

medium

A. 80

B. 150

C. 100

D. 120

Solution

1. Step 1: Calculate deposits and withdrawals

   Deposits: 100 + 50 = 150; Withdrawals: 30

2. Step 2: Compute final balance

   Balance = 0 + 150 - 30 = 120

3. Final Answer:

   120 -> Option D

4. Quick Check:

   Deposits minus withdrawals = 120 [OK]

Hint: Add deposits, subtract withdrawals to find balance [OK]

Common Mistakes:

• Adding withdrawals instead of subtracting
• Ignoring last deposit event
• Starting balance not zero

4. In an event sourcing system, which issue is caused by missing an event during state reconstruction?

medium

A. System crashes immediately

B. State becomes inconsistent or incorrect

C. Events are duplicated in the event store

D. Commands fail to generate new events

Solution

1. Step 1: Understand event replay in event sourcing

   State is rebuilt by replaying all events in order.

2. Step 2: Effect of missing an event

   If an event is missing, the rebuilt state will not reflect all changes, causing inconsistency.

3. Final Answer:

   State becomes inconsistent or incorrect -> Option B

4. Quick Check:

   Missing event = incorrect state [OK]

Hint: Missing events cause wrong state, not crashes [OK]

Common Mistakes:

• Assuming system crashes on missing event
• Confusing missing event with duplicate events
• Thinking commands stop working due to missing event

5. You are designing a microservice using event sourcing. To improve performance, you want to avoid replaying all events every time. Which approach is best to achieve this?

hard

A. Use snapshots to save periodic state and replay only recent events

B. Store only the latest event and discard older ones

C. Send all events to other services for processing

D. Update state directly and ignore events after initial load

Solution

1. Step 1: Identify performance issue in event sourcing

   Replaying all events from the start can be slow as event count grows.

2. Step 2: Choose solution to reduce replay time

   Snapshots save the current state at intervals, so only recent events need replaying.

3. Final Answer:

   Use snapshots to save periodic state and replay only recent events -> Option A

4. Quick Check:

   Snapshots improve replay speed [OK]

Hint: Snapshots speed up state rebuild by reducing event replay [OK]

Common Mistakes:

• Discarding old events breaks event sourcing
• Sending all events elsewhere doesn't reduce replay
• Ignoring events after initial load loses audit trail