can I set a value in redis from the redis server's current system time?

Question

that is, I'd like to append to a sorted-set something like:

"set1" -> [1371772258, 1371772265, 1371772299, etc.]

but I don't want to send in the time values from my server. I want to get a consistant timestamp from one server's clock, the server redis is running on.

Answer 1

This solution works since Redis version 2.6.0

There could be easy solution if Redis provide Lua os.time function, but it doesn't.

So I've invented some hack. We can set EXPIREAT in future, and then with TTL we will know current unix timestamp.

Here is proof of concept. Of course you can do at least 2 optimizations: set future key once in your redis database and then just read its ttl in lua script, and obviously use evalsha instead of eval.

zaddts.lua

local future_ts, cur_ts, zkey, zmember

future_ts = 2000000000
redis.call('setnx', 'future', 1)
redis.call('expireat', 'future', future_ts)
cur_ts = future_ts - redis.call('ttl', 'future')
zkey = KEYS[1]
zmember = KEYS[2]
return redis.call('zadd', zkey, cur_ts, zmember)

Example of oneliner with hardcoded timestamp:

return redis.call('zadd', KEYS[1], 2000000000 - redis.call('ttl', 'future'), KEYS[2])

example-client.php

<?php
ini_set('display_errors', true);

$r = new Redis();
$r->connect('localhost');

$script = file_get_contents('zaddts.lua');

$r->eval($script, array('events', 'event1'), 2);
sleep(1);
$r->eval($script, array('events', 'event2'), 2);
print_r($r->zrange('events', 0, -1, $withscore = true));
/* Output:
Array
(
    [event1] => 1371777755
    [event2] => 1371777756
)
*/

UPD: [facepalm] I didn't see TIME command. With it lua script became much simpler:

zaddts.lua

local time_full, time_sec, zkey, zmember

zkey = KEYS[1]
zmember = KEYS[2]

time_full = redis.call('time')
time_sec = tonumber(time_full[1])

return redis.call('zadd', zkey, time_sec, zmember)

UPD 2: Code with time call doesn't work due to the redis lua scripting restrictions (Scripts as pure functions):

redis 127.0.0.1:6379> eval "return redis.call('zadd', 'events', redis.call('time')[1], 'some-event')" 0
(error) ERR Error running script (call to f_426eeadf424497fc04eb8f06efac0553f3212660): Write commands not allowed after non deterministic commands 

So first version of code (with ttl) still makes sense.