MULTI & EXEC — Transactions

Redis transactions group commands to execute atomically — no other client can interleave.

How Transactions Work

1. MULTI → enters transaction mode, returns +OK
2. All subsequent commands return +QUEUED instead of executing
3. EXEC → executes all queued commands, returns results as RESP array
4. DISCARD → cancels the transaction, returns +OK

Example

MULTI           → +OK
SET name Alice  → +QUEUED
GET name        → +QUEUED
EXEC            → *2\r\n+OK\r\n$5\r\nAlice\r\n

Error Handling

• EXEC without MULTI → -ERR EXEC without MULTI
• MULTI inside MULTI → -ERR MULTI calls can not be nested
• Errors in queued commands appear in the EXEC result array but don't abort the transaction

Implementation

Use a boolean flag in_transaction and a list queue. In transaction mode, append commands to the queue instead of executing. On EXEC, execute all queued commands and collect results.

Congratulations — you've built Redis from scratch!