Accessing keys and arguments in Redis - Step-by-Step Execution

Concept Flow - Accessing keys and arguments

Start command

↓

Parse input

↓

Identify keys

↓

Identify arguments

↓

Execute command using keys and args

↓

Return result

The flow shows how a Redis command input is parsed to separate keys and arguments, then executed using those parts.

Execution Sample

Redis

EVAL "return {KEYS[1], ARGV[1]}" 1 mykey myarg

This Redis Lua script returns the first key and the first argument passed to the script.

Execution Table

Step	Input Part	Action	Value Extracted	Notes
1	EVAL	Start script execution	-	Begin processing the EVAL command
2	"return {KEYS[1], ARGV[1]}"	Parse Lua script	Script code	Script to return first key and argument
3	1	Number of keys	1	Indicates one key is passed
4	mykey	Extract KEYS[1]	mykey	First key identified
5	myarg	Extract ARGV[1]	myarg	First argument identified
6	Execute script	Return values	{mykey, myarg}	Script returns key and argument
7	-	End	-	Execution complete

💡 Script execution ends after returning the first key and argument.

Variable Tracker

Variable	Start	After Step 4	After Step 5	Final
KEYS[1]	-	mykey	mykey	mykey
ARGV[1]	-	-	myarg	myarg

Key Moments - 2 Insights

Why do we specify the number '1' after the script in the command?

The number '1' tells Redis how many keys are being passed. This is important because Redis separates keys from arguments based on this number, as shown in execution_table step 3.

What is the difference between KEYS and ARGV in the script?

KEYS holds the keys passed to the script, while ARGV holds the additional arguments. This distinction is clear in execution_table steps 4 and 5 where KEYS[1] and ARGV[1] are extracted separately.

Visual Quiz - 3 Questions

Look at the execution_table, what value is assigned to KEYS[1] at step 4?

A mykey

B myarg

C 1

D return {KEYS[1], ARGV[1]}

💡 Hint

Refer to execution_table row with Step 4 under 'Value Extracted' column.

At which step does the script extract the first argument (ARGV[1])?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Check execution_table rows for the step mentioning ARGV[1].

If the number of keys was changed from 1 to 2, what would change in the execution?

A Arguments would be ignored

B Two keys would be extracted as KEYS[1] and KEYS[2]

C Only one key would be extracted

D Script would fail to run

💡 Hint

Look at the role of the number after the script in execution_table step 3.

Concept Snapshot

Redis scripts receive keys and arguments separately.
Syntax: EVAL <script> <numkeys> <key1> ... <keyN> <arg1> ... <argM>
KEYS array holds keys; ARGV array holds arguments.
The number after script tells how many keys are passed.
Scripts access keys as KEYS[1], KEYS[2], etc., and args as ARGV[1], ARGV[2], etc.

Full Transcript

This visual execution trace shows how Redis processes a Lua script command that accesses keys and arguments. The command EVAL "return {KEYS[1], ARGV[1]}" 1 mykey myarg is parsed step-by-step. First, Redis reads the script and the number of keys (1). Then it extracts the first key 'mykey' as KEYS[1] and the first argument 'myarg' as ARGV[1]. The script executes and returns both values. The variable tracker shows how KEYS[1] and ARGV[1] are assigned. Key moments clarify why the number of keys is important and the difference between keys and arguments. The quiz tests understanding of these steps and the effect of changing the number of keys.