How to retrieve info about sessions when they are stored in Redis?

[Lisandro]

Recently I moved the sessions from the database to Redis, and I'm wondering: is there a way to retrieve info about sessions when they are stored in Redis? 

For example, when sessions are stored in the database, you have the option to use SQL to do some stuff like counting or deleting sessions. How to do it when sessions are stored in Redis?

I also use Redis to cache HTML responses from web2py and any other stuff that can be cached (lists, dictionaries, etc). In order to be able to list the keys cached by one specific web2py application, I have written this custom function to retrieve those keys. 

I've read that it's not a good idea to use cache.redis.r_server.keys() method on production, so I written this code based on what I saw in the clear() method at gluon.contrib.redis_cache:

def get_cache_keys(application, prefix=''):

    import re

    result = []

    regex = ':%s*' % prefix

    prefix = 'w2p:%s' % application

    cache_set = 'w2p:%s:___cache_set' % application

    r = re.compile(regex)

    buckets = current.cache.redis.r_server.smembers(cache_set)  # get all buckets

    if buckets:  # get all keys in buckets

        keys = current.cache.redis.r_server.sunion(buckets)

    else:

        return result

    for a in keys:

        if r.match(str(a).replace(prefix, '', 1)):

            result.append(a)

    return result

With that code, I'm able to list all the keys cached by a web2py application.

As I'm also using Redis to store sessions, I want to be able to list all the session keys.

I've tried a similar code to the one showed above, replacing this:

    prefix = 'w2p:sess:%s' % application

    cache_set = 'w2p:sess:%s:id_idx' % application

But that doesn't work. Is it possible to achieve what I want? Any suggestion will be much appreciated.

Regards,

Lisandro.

[Richard Vézina]

I gave a look directly in the redis shell

redis-cli -h localhost -p 6379 -a PASSWORD_IF_YOU_SET_ONE

It appears that each session store will have a different key which goes like that : w2p:sess:APP_NAME:SOME_ID

And w2p:sess:APP_NAME:id_idx will contains a set of each unique session existings, so you would have to access this list of session id then access the actual session.

You can list keys in redis with :

SCAN 0

Better then KEYS that could cause lock and lot of memory usage 

To list the set of existing sessions in ...:id_idx you need to use

smembers w2p:sess:APP_NAME:id_idx

It migth help you figure out how to manage you redis sessions form python and better understand session contrib : https://github.com/web2py/web2py/blob/0d646fa5e7c731cb5c392adf6a885351e77e4903/gluon/contrib/redis_session.py

Good luck

Richard

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[Lisandro]

Thank you very much Richard!

With your help I was able to retrieve the keys of the sessions stored in Redis with this simple line of code:

session_keys = cache.redis.r_server.smembers('w2p:sess:%s:id_idx' % application_name)

That line returns a set with all the session keys stored in Redis.

Notice that the keys retrieved may be expired, so if you need to check how many of those keys are still valid, you would have to iterate over the set checking the ttl of each key, like this:

valid_session_keys = [key for key in session_keys if cache.redis.r_server.ttl(key) > 0]

I'm not sure why the deleted keys remain in Redis with a negative TTL, but I presume this is because redis server would be doing some automatically cleaning once every time, deleting definitively those keys with negative TTL.

Thanks again!

Regards,

Lisandro.

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