Why streams handle event logs in Redis - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an event to a Redis stream named 'mystream'.

Redis

XADD mystream * event [1]

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

Astart

Bstart_time

Cstart_event

Dstart_value

Attempts:

3 left

💡 Hint

Common Mistakes

Using a complex or incorrect event name.

Forgetting the asterisk (*) for automatic ID generation.

The command XADD mystream * event start adds an event named 'start' to the stream 'mystream'. The asterisk * lets Redis generate the event ID automatically.

2fill in blank

medium

Complete the code to read the latest event from the Redis stream 'mystream'.

Redis

XREVRANGE mystream + - COUNT [1]

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

A5

B10

C0

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNT 0 which returns no events.

Using a large COUNT when only one event is needed.

The command XREVRANGE mystream + - COUNT 1 reads the most recent event from the stream.

3fill in blank

hard

Fix the error in the command to read events from 'mystream' starting from ID '0-0'.

Redis

XRANGE mystream [1] +

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

A0

B0-0

C-

D*

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' instead of '0-0' as the start ID.

Using '-' or '*' which are not valid start IDs.

The command XRANGE mystream 0-0 + reads all events from the beginning of the stream.

4fill in blank

hard

Fill both blanks to create a consumer group named 'mygroup' for the stream 'mystream' starting from the beginning.

Redis

XGROUP CREATE mystream [1] [2]

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

Amygroup

B0

C0-0

D$

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' instead of '0-0' as the starting ID.

Using '$' which starts from the latest event.

The command XGROUP CREATE mystream mygroup 0-0 creates a consumer group named 'mygroup' starting from the first event in the stream.

5fill in blank

hard

Fill all three blanks to read new messages from 'mystream' for consumer 'consumer1' in group 'mygroup'.

Redis

XREADGROUP GROUP [1] [2] COUNT 1 STREAMS mystream [3]

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

Amygroup

Bconsumer1

C>

D0-0

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0-0' instead of '>' which reads all messages from the start.

Mixing up group and consumer names.

The command XREADGROUP GROUP mygroup consumer1 COUNT 1 STREAMS mystream > reads new messages that have never been delivered to any consumer in the group.

Practice

1. Why are Redis streams well-suited for handling event logs?

easy

A. Because they only store one event at a time

B. Because they delete old events automatically

C. Because they store events in order with unique IDs

D. Because they require complex setup to use

Solution

1. Step 1: Understand Redis streams storage

   Redis streams store events sequentially with unique IDs, preserving order.

2. Step 2: Match this to event log needs

   Event logs need ordered, unique entries to track actions accurately.

3. Final Answer:

   Because they store events in order with unique IDs -> Option C

4. Quick Check:

   Streams = Ordered unique events [OK]

Hint: Streams keep events ordered with unique IDs [OK]

Common Mistakes:

• Thinking streams delete old events automatically
• Believing streams store only one event
• Assuming streams need complex setup

2. Which Redis command is used to add a new event to a stream?

easy

A. XREAD

B. XADD

C. XRANGE

D. XDEL

Solution

1. Step 1: Identify command to add events

   XADD is the Redis command designed to add new entries to a stream.

2. Step 2: Differentiate from other commands

   XREAD reads events, XRANGE queries a range, XDEL deletes entries.

3. Final Answer:

   XADD -> Option B

4. Quick Check:

   XADD = Add event [OK]

Hint: XADD adds events to streams [OK]

Common Mistakes:

• Confusing XADD with XREAD or XRANGE
• Thinking XDEL adds events
• Using XRANGE to add events

3. Given the Redis stream commands below, what will XRANGE mystream - + return?

XADD mystream * event "login" user "alice"
XADD mystream * event "logout" user "bob"

medium

A. All events in 'mystream' from the oldest to newest

B. Only the first event added to 'mystream'

C. An error because XRANGE needs a count parameter

D. No events because the stream is empty

Solution

1. Step 1: Understand XRANGE command

   XRANGE with '-' and '+' returns all entries from oldest to newest in the stream.

2. Step 2: Check stream content after XADD commands

   Two events were added: a login by alice and a logout by bob.

3. Final Answer:

   All events in 'mystream' from the oldest to newest -> Option A

4. Quick Check:

   XRANGE - + = all events [OK]

Hint: XRANGE - + returns all stream events [OK]

Common Mistakes:

• Thinking XRANGE returns only one event
• Believing XRANGE requires a count parameter
• Assuming stream is empty after XADD

4. You run the command XADD mystream * event but get an error. What is the likely cause?

medium

A. XRANGE command is required before XADD

B. Using * instead of a specific ID

C. Stream 'mystream' does not exist

D. Missing field-value pairs after the event name

Solution

1. Step 1: Check XADD syntax requirements

   XADD requires field-value pairs after the ID; 'event' lacks a value.

2. Step 2: Validate other options

   Using '*' is correct for auto ID, streams can be created automatically, and XRANGE is unrelated.

3. Final Answer:

   Missing field-value pairs after the event name -> Option D

4. Quick Check:

   XADD needs field-value pairs [OK]

Hint: XADD needs pairs: field and value [OK]

Common Mistakes:

• Thinking * is invalid for ID
• Assuming stream must exist before XADD
• Believing XRANGE must run first

5. You want to track user actions in a Redis stream but only keep the last 100 events to save memory. Which command option helps achieve this?

hard

A. XADD mystream MAXLEN ~ 100 * event "login" user "alice"

B. XADD mystream * event "login" user "alice" COUNT 100

C. XADD mystream * event "login" user "alice" MAXLEN 100

D. XADD mystream * event "login" user "alice" LIMIT 100

Solution

1. Step 1: Understand MAXLEN option in XADD

   MAXLEN limits stream length; '~' makes it approximate for better performance.

2. Step 2: Identify correct syntax for trimming

   XADD mystream MAXLEN ~ 100 * event "login" user "alice" uses 'MAXLEN ~ 100' which trims stream to about 100 entries efficiently.

3. Final Answer:

   XADD mystream MAXLEN ~ 100 * event "login" user "alice" -> Option A

4. Quick Check:

   MAXLEN ~ 100 trims stream length [OK]

Hint: Use MAXLEN ~ 100 to trim stream length efficiently [OK]

Common Mistakes:

• Using MAXLEN without ~ causes slower trimming
• Using COUNT or LIMIT which are invalid here
• Not trimming stream and causing memory issues