herodb/specs/backgroundinfo/redis_basics.md

Got it 👍 — let’s break this down properly.

Redis has two broad classes you’re asking about:

1. Basic key-space functions (SET, GET, DEL, EXISTS, etc.)
2. Iteration commands (SCAN, SSCAN, HSCAN, ZSCAN)

And for each I’ll show:

• What it does
• How it works at a high level
• Its RESP protocol implementation (the actual wire format).

---

1. Basic Key-Space Commands

SET key value

• Stores a string value at a key.
• Overwrites if the key already exists.

Protocol (RESP2):

*3
$3
SET
$3
foo
$3
bar

(client sends: array of 3 bulk strings: ["SET", "foo", "bar"])

Reply:

+OK

---

GET key

• Retrieves the string value stored at the key.
• Returns nil if key doesn’t exist.

Protocol:

*2
$3
GET
$3
foo

Reply:

$3
bar

(or $-1 for nil)

---

DEL key [key ...]

• Removes one or more keys.
• Returns number of keys actually removed.

Protocol:

*2
$3
DEL
$3
foo

Reply:

:1

(integer reply = number of deleted keys)

---

EXISTS key [key ...]

• Checks if one or more keys exist.
• Returns count of existing keys.

Protocol:

*2
$6
EXISTS
$3
foo

Reply:

:1

---

KEYS pattern

• Returns all keys matching a glob-style pattern. ⚠️ Not efficient in production (O(N)), better to use SCAN.

Protocol:

*2
$4
KEYS
$1
*

Reply:

*2
$3
foo
$3
bar

(array of bulk strings with key names)

---

2. Iteration Commands (SCAN family)

SCAN cursor [MATCH pattern] [COUNT n]

• Iterates the keyspace incrementally.
• Client keeps sending back the cursor from previous call until it returns 0.

Protocol example:

*2
$4
SCAN
$1
0

Reply:

*2
$1
0
*2
$3
foo
$3
bar

Explanation:

• First element = new cursor ("0" means iteration finished).
• Second element = array of keys returned in this batch.

---

HSCAN key cursor [MATCH pattern] [COUNT n]

• Like SCAN, but iterates fields of a hash.

Protocol:

*3
$5
HSCAN
$3
myh
$1
0

Reply:

*2
$1
0
*4
$5
field
$5
value
$5
age
$2
42

(Array of alternating field/value pairs)

---

SSCAN key cursor [MATCH pattern] [COUNT n]

• Iterates members of a set.

Protocol and reply structure same as SCAN.

---

ZSCAN key cursor [MATCH pattern] [COUNT n]

• Iterates members of a sorted set with scores.
• Returns alternating member, score.

---

Quick Comparison

Command	Purpose	Return Type
SET	Store a string value	Simple string +OK
GET	Retrieve a string value	Bulk string / nil
DEL	Delete keys	Integer (count)
EXISTS	Check existence	Integer (count)
KEYS	List all matching keys (slow)	Array of bulk strings
SCAN	Iterate over keys (safe)	[cursor, array]
HSCAN	Iterate over hash fields	[cursor, array]
SSCAN	Iterate over set members	[cursor, array]
ZSCAN	Iterate over sorted set	[cursor, array]