PHPRedis Scan doesn't return the next cursor

Question

In my Laravel application, I used to use the predis/predis package to be able to use Redis for caching. Recently as recommended in the docs I have switched to PhpRedis but part of my code that was using SCAN is broken. In predis when I was using scan like this:

Redis::scan(0,'match',$pattern //Test:*);

I used to get a result like this:

[
     "253952",
     [
       "Test::296589",
       "Test::299112",
       "Test::332487",
       "Test::320358",
     ],
   ]

in this case 253952 was the next cursor that I could pass to scan again to get the next batch like this Redis::scan(253952,'match',$pattern //Test:*); and I could run this in a while loop until I reach the end:

$all_keys = [];
$keys = RedisManager::scan(0, 'match', $pattern);
while ($keys[0] !== "0") {
   foreach($keys[1] as $key) {
      $all_keys[] = $key;
   }
 $keys = RedisManager::scan($keys[0], 'match', $pattern);
}

Since I switched to Phpredis I can no longer iterate over the list. First of all the syntax seem to be different:

$it=null;
$keys = RedisManager::scan($it,$pattern);

I'm not sure what is the point of passing null as iterator to the first parameter. It also just returns:

[
 "Test::296589",
 "Test::299112"
]

which is just part of the result. It doesn't give me everything and it doesn't give me the next cursor. Can someone please guide me in the right direction?

score 1 · Accepted Answer · answered Oct 21 '20 at 01:05

I switched to Phpredis on my local and made it work with the following snippet. The documentation states that there are different usages of the scan.

$redis = RedisManager::connection(); // initialize

$redis->setOption(\Redis::OPT_SCAN, \Redis::SCAN_RETRY); // please check documentation for other option
$keyList = [];
$iterator = null;

// you may update the count or match options according to your needs
while ($keys = $redis->scan($iterator, ['match' => 'Test:*', 'count' => 20])) {
    $iterator = $keys[0];
    foreach ($keys[1] as $key) {
        $keyList[] = $key;
    }
}

return $keyList;

While debugging or developing it would be useful to use monitor command. While that snippet is working it prints something like this;

1603241935.261667 [0 127.0.0.1:52536] "SCAN" "0" "COUNT" "20" "MATCH" "Test:*"
1603241935.262241 [0 127.0.0.1:52536] "SCAN" "48" "COUNT" "20" "MATCH" "Test:*"
1603241935.263421 [0 127.0.0.1:52536] "SCAN" "424" "COUNT" "20" "MATCH" "Test:*"
1603241935.264476 [0 127.0.0.1:52536] "SCAN" "260" "COUNT" "20" "MATCH" "Test:*"
1603241935.265508 [0 127.0.0.1:52536] "SCAN" "100" "COUNT" "20" "MATCH" "Test:*"
1603241935.266405 [0 127.0.0.1:52536] "SCAN" "460" "COUNT" "20" "MATCH" "Test:*"
1603241935.267137 [0 127.0.0.1:52536] "SCAN" "258" "COUNT" "20" "MATCH" "Test:*"
1603241935.268271 [0 127.0.0.1:52536] "SCAN" "402" "COUNT" "20" "MATCH" "Test:*"
1603241935.269344 [0 127.0.0.1:52536] "SCAN" "42" "COUNT" "20" "MATCH" "Test:*"
1603241935.270328 [0 127.0.0.1:52536] "SCAN" "198" "COUNT" "20" "MATCH" "Test:*"
1603241935.275193 [0 127.0.0.1:52536] "SCAN" "342" "COUNT" "20" "MATCH" "Test:*"
1603241935.276690 [0 127.0.0.1:52536] "SCAN" "286" "COUNT" "20" "MATCH" "Test:*"
1603241935.279386 [0 127.0.0.1:52536] "SCAN" "510" "COUNT" "20" "MATCH" "Test:*"
1603241935.281084 [0 127.0.0.1:52536] "SCAN" "81" "COUNT" "20" "MATCH" "Test:*"
1603241935.282568 [0 127.0.0.1:52536] "SCAN" "41" "COUNT" "20" "MATCH" "Test:*"
1603241935.284090 [0 127.0.0.1:52536] "SCAN" "185" "COUNT" "20" "MATCH" "Test:*"
1603241935.285949 [0 127.0.0.1:52536] "SCAN" "117" "COUNT" "20" "MATCH" "Test:*"
1603241935.288443 [0 127.0.0.1:52536] "SCAN" "253" "COUNT" "20" "MATCH" "Test:*"
1603241935.293034 [0 127.0.0.1:52536] "SCAN" "147" "COUNT" "20" "MATCH" "Test:*"
1603241935.294957 [0 127.0.0.1:52536] "SCAN" "155" "COUNT" "20" "MATCH" "Test:*"
1603241935.296113 [0 127.0.0.1:52536] "SCAN" "199" "COUNT" "20" "MATCH" "Test:*"
1603241935.296456 [0 127.0.0.1:52536] "SCAN" "247" "COUNT" "20" "MATCH" "Test:*"
1603241935.296723 [0 127.0.0.1:52536] "SCAN" "223" "COUNT" "20" "MATCH" "Test:*"

Thank you for your response. When I run you code I get this error `PHP Warning: Redis::scan() expects parameter 2 to be string, array given in /var/www/api/vendor/laravel/framework/src/Illuminate/Redis/Connections/Connection.php on line 114` — Hirad Roshandel, Oct 21 '20 at 01:15
@HiradRoshandel probably different versions, please replace `scan($iterator, ['match' => 'Test:*', 'count' => 20]` with `scan($iterator, 'Test:*', 20)` https://prnt.sc/v3gkjc the documentation of the method in PhpRedisConnection class may give some hints. — Ersoy, Oct 21 '20 at 01:20
`$keys = $redis->scan($iterator, 'Test:*', 20)` just returns an array of string `["Test::296589","Test::299112"]` which makes that forloop break. I wonder why we are getting different results. Your code is written for the way Predis outputs Scan. — Hirad Roshandel, Oct 21 '20 at 01:24
forgot to mention I'm using laravel 5.8. Not sure if that makes a difference. In PhpRedisConnection.php I don't have any `scan` function — Hirad Roshandel, Oct 21 '20 at 01:29
@HiradRoshandel Maybe you can try the example code in the documentation. https://github.com/phpredis/phpredis#example-27 - this code didn't work on my development environment (it doesn't re-assign the value of iterator(cause infinite loop), also returns a single array instead of multidimensional) but the key thing is setOption i think. — Ersoy, Oct 21 '20 at 01:32
`$redis->setOption(\Redis::OPT_SCAN, \Redis::SCAN_RETRY);` is useful if you use a \Generator. — David DIVERRES, Apr 17 '21 at 23:51

PHPRedis Scan doesn't return the next cursor

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