Key-value stores

The key-value model on Redis: basic commands, five value types, key naming, expiry and eviction, and what the model forbids: finding data by its content.

24 cards
5 topics
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Key-value store: what is it?
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A database that files each piece of data under a unique name and can only fetch it by that name; the stored content stays opaque to it.
This very simple model makes reads and writes extremely fast, but you cannot ask for "every value that contains this word".
What does the server answer to the last command?

```text
SET compteur 10
INCR compteur
INCR compteur
GET compteur
`
- "12"
- "10"
- (integer) 12
- (nil)
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"12"
INCR adds 1 on each call: 10, then 11, then 12. GET always returns the value's text, which redis-cli shows in quotes; only INCR answers with an integer.
To store a token that must vanish by itself after 30 seconds, you give it a … with the … command.
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To store a token that must vanish by itself after 30 seconds, you give it a time to live with the EXPIRE command.
SET jeton abc EX 30 does both in a single atomic command: writing the value and setting the time to live.
Time to live (TTL): what is it?
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A delay, in seconds, after which the server deletes by itself an entry it was attached to.
EXPIRE session 60 sets it, TTL session gives the time left: -1 if the key never expires, -2 if it does not exist (any more).
A Redis cache refuses writes with "OOM command not allowed when used memory > 'maxmemory'". The keys have no time to live. Which fix is right?
- Set maxmemory to 0 so that memory is no longer limited
- Give the keys a time to live and pick the allkeys-lru eviction
- Add an index on the values so that they are found faster
- Restart the server every night so that its memory is emptied
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Give the keys a time to live and pick the allkeys-lru eviction
With the default noeviction policy, a full server refuses to write rather than delete anything. Times to live let stale entries leave on their own, and allkeys-lru evicts the rest when needed. With no limit, a cache that never drops anything grows until it exhausts the machine's memory.

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