XADD for adding entries in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does this XADD command return?

Consider the Redis command:
XADD mystream * sensor-id 1234 temperature 19.8
What is the output of this command?

Redis

XADD mystream * sensor-id 1234 temperature 19.8

A An error message because * is invalid here

B An integer representing the number of fields added, e.g., 2

C The string 'OK' indicating success

D A string representing the ID of the added entry, e.g., '1657891234567-0'

💡 Hint

XADD returns the ID of the new entry added to the stream.

The XADD command adds an entry to the stream and returns the entry's ID, which is a string with a timestamp and sequence number.

📝 Syntax

intermediate

Which XADD command is syntactically correct?

Choose the syntactically valid Redis XADD command to add an entry with fields 'user'='alice' and 'score'='42' to stream 'game_scores'.

A XADD game_scores * user alice score 42

B XADD game_scores * user=alice score=42

C XADD game_scores user alice score 42 *

D XADD game_scores * (user alice) (score 42)

💡 Hint

The * must come immediately after the stream name, followed by field-value pairs without extra symbols.

The correct syntax is: XADD [ ...]. Option A follows this exactly.

❓ optimization

advanced

How to limit stream length efficiently with XADD?

You want to add entries to a Redis stream 'logs' but keep only the latest 1000 entries to save memory. Which XADD option achieves this efficiently?

A XADD logs MAXLEN=1000 * sensor temp

B XADD logs MAXLEN ~ 1000 * sensor temp

C XADD logs MAXLEN 1000~ * sensor temp

D XADD logs MAXLEN 1000 * sensor temp

💡 Hint

Using ~ after MAXLEN allows approximate trimming which is faster.

Option B uses 'MAXLEN ~ 1000' which trims the stream approximately to 1000 entries, improving performance over exact trimming.

🔧 Debug

advanced

Why does this XADD command cause an error?

Given the command:
XADD mystream 1657891234567-0 sensor 12
Why might this cause an error?

Redis

XADD mystream 1657891234567-0 sensor 12

A The ID 1657891234567-0 already exists in the stream, causing a duplicate ID error

B The field 'sensor' is missing a value

C The ID format is invalid because it must be '*' for auto-generated IDs

D The stream 'mystream' does not exist yet

💡 Hint

Manually specifying an ID that already exists causes an error.

XADD with a manual ID fails if that ID already exists in the stream, causing a duplicate ID error.

🧠 Conceptual

expert

What is the purpose of the '*' ID in XADD?

In Redis Streams, what does using '*' as the ID in the XADD command do?

A It causes Redis to reject the command with a syntax error

B It deletes the oldest entry in the stream before adding the new one

C It tells Redis to auto-generate a unique ID based on the current timestamp and sequence number

D It sets the ID of the new entry to zero

💡 Hint

The * is a special placeholder for IDs in Redis Streams.

Using '*' lets Redis create a unique ID automatically, combining the current time and a sequence number to avoid collisions.

Practice

1. What does the Redis command XADD do?

easy

A. Adds a new entry to a Redis stream with a unique ID

B. Deletes an entry from a Redis stream

C. Reads entries from a Redis stream

D. Creates a new Redis database

Solution

1. Step 1: Understand the purpose of XADD

   XADD is used to add new entries to a Redis stream, each with a unique ID generated by Redis or specified by the user.

2. Step 2: Compare with other commands

   Deleting entries is done by XDEL, reading by XRANGE or XREAD, and creating databases is unrelated to XADD.

3. Final Answer:

   Adds a new entry to a Redis stream with a unique ID -> Option A

4. Quick Check:

   XADD adds entries = A [OK]

Hint: XADD always adds entries to streams, not deletes or reads [OK]

Common Mistakes:

• Confusing XADD with XDEL or XRANGE
• Thinking XADD creates databases
• Assuming XADD reads data instead of adding

2. Which of the following is the correct syntax to add an entry with fields name and age to a Redis stream mystream using XADD?

easy

A. XADD mystream name John age 30 *

B. XADD mystream name=John age=30 *

C. XADD mystream * name John age 30

D. XADD mystream * (name John) (age 30)

Solution

1. Step 1: Recall XADD syntax

   The correct syntax is: XADD stream_name * field1 value1 field2 value2 ... The asterisk (*) tells Redis to generate the ID automatically.

2. Step 2: Check each option

   XADD mystream * name John age 30 matches the syntax exactly. Options A, C, and D have incorrect order or formatting.

3. Final Answer:

   XADD mystream * name John age 30 -> Option C

4. Quick Check:

   XADD syntax = XADD stream * fields [OK]

Hint: Use * right after stream name to auto-generate ID [OK]

Common Mistakes:

• Placing * after fields instead of immediately after stream name
• Using equal signs (=) in field-value pairs
• Adding parentheses around fields

3. Given the Redis command: XADD mystream MAXLEN 2 * temperature 22 humidity 60 executed three times in a row, how many entries will mystream contain afterwards?

medium

A. 2 entries

B. 0 entries

C. 1 entry

D. 3 entries

Solution

1. Step 1: Understand MAXLEN usage

   The MAXLEN 2 option limits the stream length to 2 entries by trimming older entries when new ones are added.

2. Step 2: Apply the command three times

   After the first two adds, the stream has 2 entries. The third add triggers trimming, keeping only the latest 2 entries.

3. Final Answer:

   2 entries -> Option A

4. Quick Check:

   MAXLEN 2 limits stream to 2 entries [OK]

Hint: MAXLEN limits entries; stream never exceeds that number [OK]

Common Mistakes:

• Assuming all 3 entries remain without trimming
• Confusing MAXLEN with deleting all entries
• Thinking MAXLEN sets minimum length

4. What is wrong with this command? XADD mystream MAXLEN 5 name John age

medium

A. Stream name cannot be 'mystream'

B. MAXLEN must be after the fields

C. XADD does not support MAXLEN option

D. Missing value for field 'age'

Solution

1. Step 1: Check field-value pairs

   Fields and values must come in pairs. Here, 'age' has no value following it.

2. Step 2: Verify MAXLEN position and support

   MAXLEN is correctly placed after stream name and before fields. XADD supports MAXLEN.

3. Final Answer:

   Missing value for field 'age' -> Option D

4. Quick Check:

   Fields need values; 'age' missing one [OK]

Hint: Every field must have a value; check pairs carefully [OK]

Common Mistakes:

• Forgetting to provide value for last field
• Misplacing MAXLEN option
• Thinking stream name is invalid

5. You want to add multiple entries to a Redis stream sensor_data but keep only the latest 100 entries to save memory. Which command correctly adds a new entry with fields temp and humidity and enforces this limit?

hard

A. XADD sensor_data * MAXLEN 100 temp 25 humidity 40

B. XADD sensor_data MAXLEN 100 * temp 25 humidity 40

C. XADD sensor_data MAXLEN ~ 100 * temp 25 humidity 40

D. XADD sensor_data * temp 25 humidity 40 MAXLEN 100

Solution

1. Step 1: Understand MAXLEN usage and placement

   MAXLEN must come immediately after the stream name, optionally with ~ for approximate trimming. The * must come after MAXLEN to auto-generate ID.

2. Step 2: Analyze options

   XADD sensor_data MAXLEN 100 * temp 25 humidity 40 correctly places MAXLEN 100 after stream name and before *, then fields. XADD sensor_data MAXLEN ~ 100 * temp 25 humidity 40 uses ~ (approximate) which is optional but not required. Options B and D have wrong order.

3. Final Answer:

   XADD sensor_data MAXLEN 100 * temp 25 humidity 40 -> Option B

4. Quick Check:

   MAXLEN 100 before * enforces limit [OK]

Hint: Put MAXLEN right after stream name, then * for ID [OK]

Common Mistakes:

• Placing MAXLEN after * or fields
• Using ~ when exact trimming is needed
• Adding MAXLEN at the end of command