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Re: using memcached as a StateDB.. getting there

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  • Thanos Chatziathanassiou
    ... It would vaguely resemble ``StateDir . Each separate StateDir signifies a unique application. Thus far, using StateDir in shmfs/tmpfs I d use
    Message 1 of 5 , Mar 27, 2009
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      O/H Gregory S. Youngblood έγραψε:
      > I, for one, am interested, though I haven't been able to do much or get
      > involved.
      >
      > "Namespace" - I suggest making this an option that is user tunable, but pick
      > a good default value that hopefully won't need to be tweaked all the time.
      >
      It would vaguely resemble ``StateDir''. Each separate StateDir signifies
      a unique application.
      Thus far, using StateDir in shmfs/tmpfs I'd use
      /dev/shm/apache/website1 and /dev/shm/apache/website2 for two separate
      applications. Something similar would apply to using a single memcached
      for multiple applications, no ?
      Of course, the default StateDir ``.state'' would mix things up a bit.
      > "Lock/Unlock" - Each individual operation of memcached is atomic.. if two
      > processes attempt to write to one location at the same time, they will be
      > serialized and one will not corrupt the other. The problem is you don't
      > necessarily know which one will actually win. This means you can't guarantee
      > state if you don't do locking to ensure the right one wins.
      Well, the original point of Lock and UnLock was to avoid corrupting the
      on-disk files, but your point is valid..
      > Consider an
      > incrementing counter. Assuming you do a get to read the value, increment it
      > by one, and then do a set to save it; each individual operation is safe
      > (get, set), but not the combination (get+set). If two hit at the same time,
      > the number would increment by 1 and not by 2 (one for each). I'd worry that
      > making lock and unlock a no-op would create new problems, especially as site
      > volume increases and chances of simultaneous updates increase.
      >
      Each user gets his own key (session-id in Apache::ASP) to write, apart
      from the generic ``application''.
      The case you're describing isn't handled in Apache::ASP already and it's
      probably because the developer (end user in our case) should concern
      himself with that.
      But we can work on that one too if you feel so inclined.
      > "No list of keys" - Usually I know what keys I'm stuffing into memcacheb and
      > I don't need to walk through the keys, or if I do I have a place outside of
      > memcache that has the keys to lookup. This one could get tricky. How would
      > you keep the list of keys in a second entry intact, especially if two
      > processes wanted to add a key at the same time?
      >
      According to perltie, tying hashes pretty much expects ``FIRSTKEY'' and
      ``NEXTKEY'' to work. But this is an inadequacy of Cache::Memcached::Tie
      which we may or may not address.
      I was (indirectly) asking Josh if these are actually needed for
      Apache::ASP Sessions to work, seeing that they're defined in
      Apache::ASP::Session but never actually called - not from within
      Apache::ASP at least. But someone is bound to have used / wants to use
      keys(%$Session) already..
      > Other thoughts/suggestions:
      >
      > Having not looked at your code or design this may not be applicable, but
      > consider making this generic, something that memcache or another cache
      > engine could be plugged into. If you're interested, and my time permits, I'd
      > be interested in working on part of this with you.
      >
      The thing is, Apache::ASP::State is too tightly bound to on-disk dbm
      files and would require a major rewrite to facilitate other storage
      engines. I suspect that's what held Josh from implementing
      Apache::Session storage. Come to think of it, there already is an
      Apache::Session::Memcached thing out there, so perhaps we should focus
      on making Apache::ASP::State work with an Apache::Session and friends
      back-end instead of hacking around.
      I'd feel more comfortable if we had help from the original author of
      Apache::ASP for this (or at least his blessing ;)
      Let's think this through the weekend and decide on Monday.

      Best Regards,
      Thanos Chatziathanassiou



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    • Thanos Chatziathanassiou
      I had my svn repository disk die on me recently, but still have my working copy around and got some free time to hack it. It turned into a real Apache::Session
      Message 2 of 5 , Dec 17, 2009
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        I had my svn repository disk die on me recently, but still have my
        working copy around and got some free time to hack it.
        It turned into a real Apache::Session session store back-end for
        Apache::ASP and seems to be working fine so far, although it is a bit
        rough around the edges.

        Due to ``internal'' and ``application'' not being valid keys for most
        Apache::Session implementations, I had them hard-coded to
        ``00000000000000000000000000000000'' and
        ``ffffffffffffffffffffffffffffffff'' respectively. The ugly part is
        Apache::ASP::State::ApacheSession::ServerAutoKeys, which creates these
        automatically if they're needed when a request arrives.
        I'm afraid it is prone to deadlock if there are multiple simultaneous
        first requests and the session store is something heavier than memcached
        (memcached and redis seem to be fine with it by design of course). Due
        to this, it is off by default, but can be turned on in the server
        configuration.
        The hard-coded 1MB limit item size in memcached is somewhat problematic
        for internal and I had it run out without compression by creating about
        28000 sessions. The space each session occupies in internal is fixed,
        regardless of its contents, right ?
        I suppose having the session manager clean up on a tighter schedule is
        an option with a fast backend though.

        Some benchmarks look promising:
        memcached running on localhost
        $ perl multi_http.pl --requests=1000 --concurrency=5
        --url=http://127.0.0.1:8080/asp/benchmarks/memcached/index.asp
        Child 001: 200 requests success: 200, failure: 0 in 1.560713 sec
        Child 003: 200 requests success: 200, failure: 0 in 1.564235 sec
        Child 004: 200 requests success: 200, failure: 0 in 1.577891 sec
        Child 002: 200 requests success: 200, failure: 0 in 1.630787 sec
        Child 005: 200 requests success: 200, failure: 0 in 1.565008 sec
        Parent total time: 1.66203 sec

        classic sessions running on shmfs/tmpfs via MLDBM::Sync::SDBM_File
        $ perl multi_http.pl --requests=1000 --concurrency=5
        --url=http://127.0.0.1:8080/asp/benchmarks/classic/index.asp
        Child 001: 200 requests success: 200, failure: 0 in 1.849258 sec
        Child 005: 200 requests success: 200, failure: 0 in 1.973691 sec
        Child 004: 200 requests success: 200, failure: 0 in 2.004447 sec
        Child 002: 200 requests success: 200, failure: 0 in 2.064989 sec
        Child 003: 200 requests success: 200, failure: 0 in 2.094894 sec
        Parent total time: 2.106399 sec

        I did not yet try with MLDBM::Sync::SDBM_File on an NFS mounted
        filesystem (the server that would be hosting that died along with my svn
        repository), but these results look tempting.
        Since I'm afraid my disk might also die on me and I'd have to redo
        everything from the start, I'm attaching what I have so far.

        Once again, this has not been (properly) tested. This quite ugly. It is
        most definitely NOT production-stuff material. It is not even ready.
        Caveat emptor. Still, it shows Apache::Session back-end can be made to work.


        Configuration for the file served above:
        for memcached version:
        ---8<---
        PerlSetVar AllowSessionState 1
        PerlSetVar AllowApplicationState 1
        PerlSetVar StateDB Apache::Session::Memcached
        #Memcached
        PerlSetVar ApacheSessionParams Servers
        PerlAddVar ApacheSessionParams 127.0.0.1:11211
        PerlAddVar ApacheSessionParams Namespace
        PerlAddVar ApacheSessionParams testing
        PerlAddVar ApacheSessionParams CompressThreshold
        PerlAddVar ApacheSessionParams 10000
        PerlAddVar ApacheSessionParams AutoCreateApplicationInternalKeys
        PerlAddVar ApacheSessionParams 1
        ---8<---
        (The author of Apache::Session::Store::Memcached has not implemented
        ``Namespace'' and since I was already playing with his module, I cheated
        and changed it so that it uses Cache::Memcached::Fast instead of plain
        Cache::Memcached)

        for classic version:
        ---8<---
        PerlSetVar AllowSessionState 1
        PerlSetVar AllowApplicationState 1
        PerlSetVar StateDB MLDBM::Sync::SDBM_File
        PerlSetVar StateDir /dev/shm/apache
        ---8<---

        Other tested back-ends:
        #Sybase
        PerlSetVar StateDB Apache::Session::Sybase
        PerlSetVar ApacheSessionParams DataSource
        PerlAddVar ApacheSessionParams
        dbi:Sybase:database=session_db;server=dev_server
        PerlAddVar ApacheSessionParams UserName
        PerlAddVar ApacheSessionParams username
        PerlAddVar ApacheSessionParams Password
        PerlAddVar ApacheSessionParams password
        PerlAddVar ApacheSessionParams Commit
        PerlAddVar ApacheSessionParams 1

        #SQLite3
        PerlSetVar StateDB Apache::Session::SQLite3
        PerlSetVar ApacheSessionParams DataSource
        PerlAddVar ApacheSessionParams dbi:SQLite:dbname=/tmp/session.db

        #Postgres
        PerlSetVar StateDB Apache::Session::Postgres
        PerlSetVar ApacheSessionParams DataSource
        PerlAddVar ApacheSessionParams DBI:Pg:dbname=dev_server;host=192.168.100.46
        PerlAddVar ApacheSessionParams UserName
        PerlAddVar ApacheSessionParams username
        PerlAddVar ApacheSessionParams Password
        PerlAddVar ApacheSessionParams password
        PerlAddVar ApacheSessionParams Commit
        PerlAddVar ApacheSessionParams 1

        #Redis via self-made Apache::Session::Redis using Redis-0.08
        PerlSetVar StateDB Apache::Session::Redis
        PerlSetVar ApacheSessionParams Server
        PerlAddVar ApacheSessionParams 127.0.0.1:6379
        PerlAddVar ApacheSessionParams Namespace
        PerlAddVar ApacheSessionParams testing
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