251 lines
3.2 KiB
Markdown
251 lines
3.2 KiB
Markdown
Got it 👍 — let’s break this down properly.
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Redis has two broad classes you’re asking about:
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1. **Basic key-space functions** (SET, GET, DEL, EXISTS, etc.)
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2. **Iteration commands** (`SCAN`, `SSCAN`, `HSCAN`, `ZSCAN`)
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And for each I’ll show:
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* What it does
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* How it works at a high level
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* Its **RESP protocol implementation** (the actual wire format).
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---
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# 1. Basic Key-Space Commands
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### `SET key value`
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* Stores a string value at a key.
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* Overwrites if the key already exists.
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**Protocol (RESP2):**
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```
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*3
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$3
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SET
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$3
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foo
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$3
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bar
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```
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(client sends: array of 3 bulk strings: `["SET", "foo", "bar"]`)
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**Reply:**
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```
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+OK
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```
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---
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### `GET key`
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* Retrieves the string value stored at the key.
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* Returns `nil` if key doesn’t exist.
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**Protocol:**
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```
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*2
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$3
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GET
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$3
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foo
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```
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**Reply:**
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```
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$3
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bar
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```
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(or `$-1` for nil)
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---
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### `DEL key [key ...]`
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* Removes one or more keys.
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* Returns number of keys actually removed.
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**Protocol:**
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```
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*2
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$3
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DEL
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$3
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foo
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```
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**Reply:**
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```
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:1
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```
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(integer reply = number of deleted keys)
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---
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### `EXISTS key [key ...]`
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* Checks if one or more keys exist.
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* Returns count of existing keys.
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**Protocol:**
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```
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*2
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$6
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EXISTS
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$3
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foo
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```
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**Reply:**
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```
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:1
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```
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---
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### `KEYS pattern`
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* Returns all keys matching a glob-style pattern.
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⚠️ Not efficient in production (O(N)), better to use `SCAN`.
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**Protocol:**
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```
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*2
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$4
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KEYS
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$1
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*
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```
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**Reply:**
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```
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*2
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$3
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foo
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$3
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bar
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```
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(array of bulk strings with key names)
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---
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# 2. Iteration Commands (`SCAN` family)
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### `SCAN cursor [MATCH pattern] [COUNT n]`
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* Iterates the keyspace incrementally.
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* Client keeps sending back the cursor from previous call until it returns `0`.
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**Protocol example:**
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```
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*2
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$4
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SCAN
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$1
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0
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```
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**Reply:**
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```
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*2
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$1
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0
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*2
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$3
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foo
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$3
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bar
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```
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Explanation:
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* First element = new cursor (`"0"` means iteration finished).
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* Second element = array of keys returned in this batch.
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---
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### `HSCAN key cursor [MATCH pattern] [COUNT n]`
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* Like `SCAN`, but iterates fields of a hash.
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**Protocol:**
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```
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*3
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$5
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HSCAN
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$3
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myh
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$1
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0
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```
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**Reply:**
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```
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*2
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$1
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0
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*4
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$5
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field
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$5
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value
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$5
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age
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$2
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42
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```
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(Array of alternating field/value pairs)
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---
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### `SSCAN key cursor [MATCH pattern] [COUNT n]`
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* Iterates members of a set.
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Protocol and reply structure same as SCAN.
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---
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### `ZSCAN key cursor [MATCH pattern] [COUNT n]`
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* Iterates members of a sorted set with scores.
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* Returns alternating `member`, `score`.
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---
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# Quick Comparison
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| Command | Purpose | Return Type |
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| -------- | ----------------------------- | --------------------- |
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| `SET` | Store a string value | Simple string `+OK` |
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| `GET` | Retrieve a string value | Bulk string / nil |
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| `DEL` | Delete keys | Integer (count) |
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| `EXISTS` | Check existence | Integer (count) |
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| `KEYS` | List all matching keys (slow) | Array of bulk strings |
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| `SCAN` | Iterate over keys (safe) | `[cursor, array]` |
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| `HSCAN` | Iterate over hash fields | `[cursor, array]` |
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| `SSCAN` | Iterate over set members | `[cursor, array]` |
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| `ZSCAN` | Iterate over sorted set | `[cursor, array]` |
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## |