XADD for adding entries in Redis - Step-by-Step Execution

Concept Flow - XADD for adding entries

Start XADD command

↓

Check stream exists?

No→Create new stream

Yes↓

Generate entry ID

↓

Add entry with fields

↓

Return entry ID

↓

End

This flow shows how Redis XADD adds an entry: it checks the stream, creates if missing, generates an ID, adds the entry, then returns the ID.

Execution Sample

Redis

XADD mystream * sensor-id 1234 temperature 19.8
XADD mystream * sensor-id 1235 temperature 20.1

Adds two entries to 'mystream' with auto-generated IDs and fields for sensor-id and temperature.

Execution Table

Step	Command	Stream Exists?	Entry ID Generated	Entry Added	Output
1	XADD mystream * sensor-id 1234 temperature 19.8	No (stream created)	1609459200000-0	Yes	1609459200000-0
2	XADD mystream * sensor-id 1235 temperature 20.1	Yes	1609459200001-0	Yes	1609459200001-0
3	End	-	-	-	No more commands

💡 All entries added; no more commands to process.

Variable Tracker

Variable	Start	After 1	After 2	Final
mystream entries	empty	[1609459200000-0: {sensor-id:1234, temperature:19.8}]	[1609459200000-0: {...}, 1609459200001-0: {sensor-id:1235, temperature:20.1}]	2 entries

Key Moments - 3 Insights

Why does XADD create the stream if it doesn't exist?

XADD automatically creates the stream to simplify adding entries, so you don't need to create it manually first. See execution_table step 1 where the stream was created.

What does the '*' mean in the XADD command?

The '*' tells Redis to auto-generate a unique entry ID based on the current time. This is shown in execution_table where IDs like '1609459200000-0' are generated.

Can the entry ID be specified manually instead of '*'?

Yes, you can specify your own ID, but using '*' is easier and ensures uniqueness. This example uses '*', as shown in the execution_sample.

Visual Quiz - 3 Questions

Look at the execution_table, what is the entry ID generated for the first XADD command?

A *

B 1609459200001-0

C 1609459200000-0

D mystream

💡 Hint

Check the 'Entry ID Generated' column in row 1 of the execution_table.

At which step does the stream get created?

A Step 2

B Step 1

C Step 3

D It is never created

💡 Hint

Look at the 'Stream Exists?' column in the execution_table for step 1.

If you replace '*' with a manual ID, how would the execution_table change?

A Entry ID Generated would show the manual ID instead of auto-generated

B Stream would not be created

C Entry would not be added

D Output would be empty

💡 Hint

Consider the 'Entry ID Generated' column and how IDs are assigned.

Concept Snapshot

XADD key * field value [field value ...]
- Adds an entry to a Redis stream
- '*' auto-generates a unique ID
- Creates stream if missing
- Returns the entry ID
- Fields are key-value pairs for the entry

Full Transcript

The XADD command in Redis adds entries to a stream. If the stream does not exist, XADD creates it automatically. The '*' symbol tells Redis to generate a unique entry ID based on the current time. Each entry consists of fields as key-value pairs. After adding, XADD returns the entry ID. This process is shown step-by-step in the execution table, tracking stream creation, ID generation, and entry addition.