Is `get key` the only way to check if a key exists?
Looking at stats output:
STAT cmd_get 51
STAT get_hits 32
STAT get_misses 19
19+32=51, so the stats do make perfect sense.
In my (very early stage) code I'm doing something very basic:
if ( $cache = $mc-get($key) {
// use cached data
die($cache);
}else{
// use output buffering with a callback function to
// render the page normally, and then
// cache it under $key then serve it
$mc-set($key, $html);
die( $html );
}
The main reason I ask is, I was wondering if there is any smarter way
to check if a key exists, because, I'm just thinking that it would be
advantageous to be able to differentiate a get_miss for a reason
different than the key wasn't cached yet, like application logic
error, versus everything working correctly but my key isn't cached yet
or it expired. In my mind, it seems like it would be cleaner from
client-side code, to check if a key exists, then issue a get only if
the key exists, and be able to see 100% get_hits, but this may not be
possible.
Re: Is `get key` the only way to check if a key exists?
The main reason I ask is, I was wondering if there is any smarter way
to check if a key exists, because, I'm just thinking that it would be
advantageous to be able to differentiate a get_miss for a reason
different than the key wasn't cached yet, like application logic
error, versus everything working correctly but my key isn't cached yet
or it expired. In my mind, it seems like it would be cleaner from
client-side code, to check if a key exists, then issue a get only if
the key exists, and be able to see 100% get_hits, but this may not be
possible.
Not sure how you'd expect memcached to know the difference. If your
application logic fails, you wouldn't have stored anything in memcached,
so there's no way to tell the difference.
The only way you *could* tell the difference is if you issue a get and the
key existed but was expired for some reason, but it doesn't return that
info.
If you're worried about the application never setting the keys, you could
set a canary key at the very top of your app logic.
ie:
if ($cache = $mc-get($page_blah)) {
# use $cache
} else {
$canary = $mc-get($page_canary);
if ($canary) {
die(Something *might* have gone wrong);
}
$mc-set($page_canary, chirp);
# Do real logic, fetch from database, render page.
$mc-set($page_blah, $page_data);
$mc-delete($page_canary);
}
So if your page successfully works, the canary should't exist, but the
cache entry will. I don't see that as being necessary though... if your
app errors that should go to log somewhere, which you should really be
watching anyway.
-Dormando
Re: Is `get key` the only way to check if a key exists?
On Sat, Aug 20, 2011 at 11:56 AM, dormando dorma...@rydia.net wrote:
The main reason I ask is, I was wondering if there is any smarter way
to check if a key exists, because, I'm just thinking that it would be
advantageous to be able to differentiate a get_miss for a reason
different than the key wasn't cached yet, like application logic
error, versus everything working correctly but my key isn't cached yet
or it expired. In my mind, it seems like it would be cleaner from
client-side code, to check if a key exists, then issue a get only if
the key exists, and be able to see 100% get_hits, but this may not be
possible.
Not sure how you'd expect memcached to know the difference. If your
application logic fails, you wouldn't have stored anything in memcached,
so there's no way to tell the difference.
The only way you *could* tell the difference is if you issue a get and the
key existed but was expired for some reason, but it doesn't return that
info.
If you're worried about the application never setting the keys, you could
set a canary key at the very top of your app logic.
ie:
if ($cache = $mc-get($page_blah)) {
# use $cache
} else {
$canary = $mc-get($page_canary);
if ($canary) {
die(Something *might* have gone wrong);
}
$mc-set($page_canary, chirp);
# Do real logic, fetch from database, render page.
$mc-set($page_blah, $page_data);
$mc-delete($page_canary);
}
So if your page successfully works, the canary should't exist, but the
cache entry will. I don't see that as being necessary though... if your
app errors that should go to log somewhere, which you should really be
watching anyway.
-Dormando
Thanks Dormando, the canary example makes sense, and I can see why it's
unnecessary. I think I'm just hung up on get_misses terminology seeming like
an error occurred, but in reality it's a measure of how many requested cache
items weren't there yet.
