Stream entry IDs in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this XADD command?

Consider the Redis command:
XADD mystream * sensor-id 1234 temperature 19.8
What does the command return?

Redis

XADD mystream * sensor-id 1234 temperature 19.8

A An error message about missing stream key

B The string "OK"

C The number of fields added, e.g., 2

D A generated stream entry ID in the format <millisecondsTime>-<sequenceNumber>, e.g., "1687000000000-0"

💡 Hint

XADD returns the ID of the added entry, which is a timestamp plus a sequence number.

The XADD command adds an entry to a Redis stream and returns the entry ID generated automatically when using '*'. This ID looks like a timestamp in milliseconds followed by a sequence number separated by a dash.

🧠 Conceptual

intermediate

What does the stream entry ID '1687000000000-5' represent?

In Redis streams, what do the two parts of the entry ID '1687000000000-5' mean?

A The first part is a Unix timestamp in milliseconds; the second part is a sequence number for entries with the same timestamp

B The first part is the stream length; the second part is the entry's position

C The first part is the server's process ID; the second part is the entry's priority

D The first part is the entry's hash; the second part is a random number

💡 Hint

Think about how Redis ensures unique IDs for entries added at the same millisecond.

Redis stream entry IDs consist of a millisecond timestamp and a sequence number to differentiate entries added in the same millisecond.

📝 Syntax

advanced

Which XADD command syntax is correct to add an entry with a custom ID?

You want to add an entry to 'mystream' with ID '1687000000000-0' and fields 'name'='sensor1' and 'value'='42'. Which command is correct?

A XADD mystream 1687000000000-0 name sensor1 value 42

B XADD mystream * 1687000000000-0 name sensor1 value 42

C XADD mystream name sensor1 value 42 1687000000000-0

D XADD mystream ID 1687000000000-0 name sensor1 value 42

💡 Hint

The ID comes immediately after the stream key in XADD syntax.

The XADD command syntax is: XADD key ID field value [field value ...]. To specify a custom ID, place it right after the stream key.

❓ optimization

advanced

How to efficiently trim a Redis stream to keep only the last 1000 entries?

Which command efficiently trims the stream 'mystream' to keep only the latest 1000 entries?

A XTRIM mystream MAXLEN 1000

B XTRIM mystream MAXLEN ~ 1000

C XTRIM mystream MINLEN 1000

D XTRIM mystream COUNT 1000

💡 Hint

Use approximate trimming for better performance.

Using XTRIM with MAXLEN ~ 1000 trims the stream approximately to 1000 entries, which is faster than exact trimming.

🔧 Debug

expert

Why does this XREAD command return no entries?

You run:
XREAD COUNT 2 STREAMS mystream 1687000000000-5
But get no results, even though the stream has entries. Why?

A Because the stream key 'mystream' does not exist

B Because the COUNT parameter must be after STREAMS keyword

C Because XREAD returns entries with IDs strictly greater than the given ID, so if no newer entries exist, it returns nothing

D Because the ID '1687000000000-5' is invalid format

💡 Hint

XREAD returns entries with IDs greater than the one specified, not equal or less.

XREAD returns entries with IDs strictly greater than the ID specified. If no such entries exist, it returns no results.