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RE: using memcached as a StateDB.. getting there

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  • Gregory S. Youngblood
    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
    Message 1 of 5 , Mar 27, 2009
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      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.

      "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. 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.

      "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?

      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.

      Greg

      > -----Original Message-----
      > From: Thanos Chatziathanassiou [mailto:tchatzi@...]
      > Sent: Thursday, March 26, 2009 5:33 AM
      > Cc: 'asp@...'
      > Subject: Re: using memcached as a StateDB.. getting there
      >
      > (ignoring the fact that no-one seems interested)
      >
      > I came accross Cache::Memcached::Tie which pretty much does most of the
      > work, however I'm having some issues - actually design decisions:
      > - I suppose StateDir should equal ``namespace'' in memcached parlance.
      > Do we want to make this transparent to the end user or add a
      > configuration option ?
      > - memcached has no concept of Lock(), UnLock() and some stuff I haven't
      > figured out yet. Add no-ops for these to Cache::Memcached::Tie or is
      > there some more elegant way to bypass them inside Apache::ASP ?
      > - It seems there's no obvious way to enumerate keys in memcached
      > (FIRSTKEY NEXTKEY). Perhaps keep a separate index of keys inside
      > memcached and add methods that use those (?)
      > - memcached needs a few extra configuration options. Most important is
      > obviously ``servers'', but also ``compress_threshold'' and ``debug''
      >
      >
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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 2 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 3 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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