Stream entry IDs in Redis

Introduction

Stream entry IDs help you identify and order messages in a Redis stream easily.

When you want to add messages to a stream and keep track of their order.

When you need to read messages from a stream starting from a specific point.

When you want to acknowledge processing of messages using their IDs.

When you want to trim or manage stream length based on entry IDs.

When you want to query or filter stream entries by their IDs.

Syntax

Redis

stream_entry_id := <millisecondsTime>-<sequenceNumber>
Example: 1526985058136-0

The ID has two parts separated by a dash: the time in milliseconds and a sequence number.

If multiple entries are added in the same millisecond, the sequence number increases to keep IDs unique.

Examples

An entry ID with timestamp 1526985058136 milliseconds and sequence number 0.

Redis

1526985058136-0

Another entry added in the same millisecond but with sequence number 1.

Redis

1526985058136-1

Use * to let Redis auto-generate the next stream entry ID.

Redis

*

Sample Program

Add two entries to 'mystream' with auto-generated IDs, then list all entries.

Redis

XADD mystream * name Alice
XADD mystream * name Bob
XRANGE mystream - +

Important Notes

Stream entry IDs are always unique and sorted by time.

You can specify your own ID but it must be greater than the last ID in the stream.

Using '*' lets Redis handle ID generation automatically.

Summary

Stream entry IDs uniquely identify each message in a Redis stream.

They consist of a timestamp and a sequence number separated by a dash.

You can use '*' to auto-generate IDs when adding entries.

Practice

1. What does a Redis stream entry ID consist of?

easy

A. Only a timestamp

B. A timestamp and a sequence number separated by a dash

C. Only a sequence number

D. A random string generated by Redis

Solution

1. Step 1: Understand Redis stream entry ID format

   Redis stream entry IDs are made of two parts: a timestamp and a sequence number.

2. Step 2: Identify the separator

   These two parts are separated by a dash (-), for example, 1526985058136-0.

3. Final Answer:

   A timestamp and a sequence number separated by a dash -> Option B

4. Quick Check:

   Stream entry ID = timestamp-sequence [OK]

Hint: Remember: ID = timestamp + dash + sequence number [OK]

Common Mistakes:

• Thinking ID is only timestamp
• Confusing sequence number as random string
• Ignoring the dash separator

2. Which of the following is the correct way to add a new entry with an auto-generated ID to a Redis stream named mystream?

easy

A. XADD mystream # field1 value1

B. XADD mystream 1234567890-0 field1 value1

C. XADD mystream * field1 value1

D. XADD mystream $ field1 value1

Solution

1. Step 1: Recall how to auto-generate IDs in Redis streams

   Using '*' as the ID in the XADD command tells Redis to generate the ID automatically.

2. Step 2: Check the options for correct syntax

   XADD mystream * field1 value1 uses '*', which is the correct way to auto-generate the ID.

3. Final Answer:

   XADD mystream * field1 value1 -> Option C

4. Quick Check:

   Use '*' to auto-generate stream entry ID [OK]

Hint: Use '*' in XADD to auto-generate IDs [OK]

Common Mistakes:

• Using a fixed ID instead of '*'
• Using '#' or '$' which are invalid here
• Forgetting to include field-value pairs

3. Given the following Redis stream entries:
1526985058136-0: field1=value1
1526985058136-1: field1=value2
What will be the ID of the next entry if you add it with XADD mystream * field1 value3 immediately after?

medium

A. 1526985058136-2

B. 1526985058137-0

C. 1526985058136-0

D. 1526985058135-0

Solution

1. Step 1: Understand sequence number increment for same timestamp

   When multiple entries share the same timestamp, Redis increments the sequence number.

2. Step 2: Identify the next sequence number

   Existing entries have sequence numbers 0 and 1 for timestamp 1526985058136, so the next will be 2.

3. Final Answer:

   1526985058136-2 -> Option A

4. Quick Check:

   Next sequence number increments by 1 for same timestamp [OK]

Hint: Next ID increments sequence if timestamp same [OK]

Common Mistakes:

• Assuming timestamp always increments
• Repeating an existing ID
• Using a lower timestamp than existing

4. You run the command XADD mystream 1526985058136-1 field1 value1 but get an error. What is the most likely cause?

medium

A. The ID 1526985058136-1 already exists in the stream

B. The timestamp part is not a valid number

C. The field name is missing

D. The sequence number is negative

Solution

1. Step 1: Understand Redis stream ID uniqueness

   Each stream entry ID must be unique; duplicate IDs cause errors.

2. Step 2: Check the given ID

   If 1526985058136-1 already exists, adding it again causes an error.

3. Final Answer:

   The ID 1526985058136-1 already exists in the stream -> Option A

4. Quick Check:

   Duplicate stream entry IDs cause errors [OK]

Hint: Duplicate IDs cause errors in XADD [OK]

Common Mistakes:

• Assuming timestamp format error
• Forgetting field-value pairs
• Thinking negative sequence numbers are allowed

5. You want to add multiple entries to a Redis stream with IDs that must be strictly increasing, but your system clock is not reliable and may go backward. Which approach ensures correct stream entry IDs?

hard

A. Use fixed sequence numbers with zero timestamps

B. Manually specify IDs with timestamps from your system clock

C. Generate random IDs and add them to the stream

D. Use XADD mystream * to let Redis generate IDs automatically

Solution

1. Step 1: Understand the problem with unreliable clocks

   Manually specifying timestamps can cause IDs to go backward if the clock is wrong.

2. Step 2: Use Redis auto-generated IDs

   Using '*' lets Redis generate strictly increasing IDs based on its own clock and sequence logic.

3. Final Answer:

   Use XADD mystream * to let Redis generate IDs automatically -> Option D

4. Quick Check:

   Auto-generated IDs ensure increasing order despite clock issues [OK]

Hint: Let Redis generate IDs with '*' for reliable ordering [OK]

Common Mistakes:

• Trusting unreliable system clock for IDs
• Using fixed or random IDs causing duplicates
• Ignoring Redis auto-generation feature