Re: Question -- Session Operations - Feasibility Of Proposed Synchronization Method? - Mailing list pgsql-general

On 01/03/2016 01:32 PM, Melvin Davidson wrote:
> As others have pointed out, storing session data in a table is not a
> good idea. Even if you use TRUNCATE, you will still not reclaim all the
> space used unless you use vacuum full. More to the point, if you

Actually:

http://www.postgresql.org/docs/9.4/interactive/sql-truncate.html

"TRUNCATE quickly removes all rows from a set of tables. It has the same
effect as an unqualified DELETE on each table, but since it does not
actually scan the tables it is faster. Furthermore, it reclaims disk
space immediately, rather than requiring a subsequent VACUUM operation.
This is most useful on large tables."

the rub is:

"TRUNCATE acquires an ACCESS EXCLUSIVE lock on each table it operates
on, which blocks all other concurrent operations on the table. When
RESTART IDENTITY is specified, any sequences that are to be restarted
are likewise locked exclusively. If concurrent access to a table is
required, then the DELETE command should be used instead."


> absolutely must store session data, then why not just do it in a
> TEMPORARY table, which will reside in memory and clean itself up when
> the session ends?
>
> http://www.postgresql.org/docs/9.4/static/sql-createtable.html
>
>
> On Sun, Jan 3, 2016 at 3:43 PM, Pavel Stehule <pavel.stehule@gmail.com
> <mailto:pavel.stehule@gmail.com>> wrote:
>
>     Hi
>
>     2016-01-03 20:46 GMT+01:00 Steve Petrie, P.Eng.
>     <apetrie@aspetrie.net <mailto:apetrie@aspetrie.net>>:
>
>         __
>         *Greetings To Postgres Forum,*
>         This posting is further to a prior forum thread -- subject
>         "[/GENERAL] using a postgres table as a multi-writer
>         multi-updater queue/", that was started on 23 November 2015 by
>         Chris Withers chris@simplistix.co.uk
>         <mailto:chris@simplistix.co.uk>. I believe the last posting to
>         that thread was on 1 December 2015 by George Neuner
>         <gneuner2@comcast.net <mailto:gneuner2@comcast.net>>.
>         A related thread of interest, was started earlier -- subject
>         /"[GENERAL] using postgresql for session/",  on 7 October 2015
>         by John Tiger <john.tigernassau@gmail.com
>         <mailto:john.tigernassau@gmail.com>>.
>
>
>     I am sorry for off topic. But are you sure, so using Postgres for
>     session data is a good idea? Using Postgres for short living data is
>     can enforce a performance problems when a load will be higher.
>
>     Regards
>
>     Pavel
>
>         * * *
>         * * *
>         I made some postings to the first above-mentioned thread, as I
>         am working to migrate a php website application from mysql to
>         postgres. An important objective of this migration is to find a
>         good way to use a postgres table to store session context data
>         rows, one row for each active website visitor.
>         One item of advice (among much other helpful advice) I took away
>         from the first thread mentioned above, was to avoid use of
>         DELETE commands as a means to recycle session context table row
>         image storage, when a session is terminated.
>         To use instead, a TRUNCATE command on an entire session context
>         table, to quickly and efficiently recycle session context row
>         image storage space, back to the filesystem, so the space is
>         immediately available for reuse.
>         * * *
>         * * *
>         Since then, I have been working to design a way to use postgres
>         table(s) as a session context store, for a simple, reliable and
>         high-performance "session operations system" (SOS).
>         A design for a postgres-based SOS, that follows two key
>         principles to ensure maximum session workload throughput capacity:
>         *PRINCIPLE #1*: *1.1* Use only the TRUNCATE TABLE command, to
>         recycle frequently, rapidly and efficiently back to the
>         filesystem, session context table storage space occupied by
>         obsolete images of session context rows; and *1.2* do not use
>         DELETE / AUTOVACUUM / VACUUM commands at all, for this recycling.
>         *PRINCIPLE #2*: *2.1* Use sequence generators for various
>         globally-addressable fast-access "iterators"**, that provide the
>         php website app (and its PL/pgSQL functions), with e.g. access
>         to an appropriate individual session context table; *2.2* Access
>         granted to a table from a pool of session context tables, each
>         pool having its tables all in the same operational state.
>         The downside of Principle #1 is the considerable added
>         complexity of having to manage multiple tables, to store session
>         context data rows.
>         The downside of Principle #2 is that the sequence generator has
>         no role in sql transaction / savepoint semantics. So explicit
>         provision for synchronization is required, adding further
>         complexity.
>           (** An "iterator" is derived from a sequence generator,
>         by using excess unneeded precision in high-order bits of the
>         sequence integer value, to encode "iterator" metadata -- as an
>         efficient way to make this metadata available to multiple
>         concurrently executing app execution control flow paths.)
>         * * *
>         * * *
>         *The purpose of this present email, is to present (in
>         pseudocode) for critque by forum members, a proposed approach to
>         synchronizing use of the "iterators" (sequence generators)
>         described above, among multiple concurrent actors, in the
>         website php app session operations scenario.*
>         Since I am a postgres novice, I am hoping that members of this
>         postgres forum, will be kind enough to examine and critique the
>         (boiled-down, simplified) pseudocode for the proposed approach
>         to synchronization.
>         (In this discussion, the term "process" does not refer
>         specifically to a "process" as implemented in operating systems,
>         as one form of program execution control, that is contrasted
>         with "thread" as another form of program execution control. In
>         this discussion, the term "process" means the general sense of
>         any program execution path that can occur in parallel
>         concurrently with other program execution paths.)
>         In the pseudocode example provided below, two concurrent
>         processes (session process, supervisory process) operate on the
>         same same table *sql_table_01*, and they use sequence generator
>         *sql_sequence_01* as a "version" number for the operational
>         state of table *sql_table_01*.
>
>             *QUESTION: In _supervisory process_ step sup.2 (below), will
>             the command:*
>             **
>             */LOCK TABLE sql_table_01 IN ACCESS EXCLUSIVE MODE;/*
>             **
>             *ensure that the _session process_, having read a value from
>             sequence generator sql_sequence_01 in step ses.1, will
>             _never ever begin to execute step ses.6_:*
>             **
>             */SELECT currval('sql_sequence_01');/*
>             **
>             *so long as the supervisory process, has _completed step
>             sup.2:_*
>             **
>             */LOCK TABLE sql_table_01 IN ACCESS EXCLUSIVE MODE;/*
>             *__*
>             *_but has not yet completed step sup.4_:*
>             **
>             */COMMIT TRANSACTION;/*
>             **
>             *???*
>
>         Essentially, the idea is to piggyback, synchronization of the
>         use of sequence generator *sql_sequence_01*, on the suprvisory
>         process' LOCK TABLE */sql_table_01/* command, assuming that the
>         session process has some INSERT / SELECT / UPDATE command to
>         perform on the same table (a command that will be blocked by the
>         LOCK TABLE command).
>         * * *
>         * * *
>         Here is pseudocode for the _session process_ (use a wide viewing
>         window to avoid line wrap):
>         *Session Process
>         * ---------------
>                 INSERT / SELECT / UPDATE row in table *sql_table_01
>         *-------------------------------------------------------------
>         |
>         *ses.0* |(Decide to update a row in table *sql_table_01*).
>         |
>         *ses.1* | /SELECT currval(*'sql_sequence_01*');/
>         /ses.2/ | $save_seq1 = (value of sequence obtained in *ses.1*);
>         |
>         *ses.3* | /SAVEPOINT *session_savepoint*;
>         / |
>         *ses.4* | /SELECT ... FROM *sql_table_01* FOR UPDATE;
>         /      |
>         *ses.5* | /UPDATE *sql_table_01* ...;
>         /      |
>         *ses.6* | /SELECT currval(*'sql_sequence_01*');
>         /*ses.7* |  $save_seq2 = (value of seq obtained in ses.6);
>                |
>         |  /*
>                |     IS IT SAFE TO COMMIT THE UNIT OF WORK ?
>         |      (i.e. is operational state of table
>         | *sql_table_01* unchanged?)
>                | */
>         *ses.8* |  if ($save_seq1 == $save_seq2)
>                | /*
>                |     YES -- SAFE TO COMMIT
>                |      ( sequence *sql_sequence_01* is unchanged).
>                | */
>                |  {
>         *ses.9* | /RELEASE SAVEPOINT *session_savepoint*;
>         / |  }
>                | else
>                | /*
>                |     NO -- NOT SAFE TO COMMIT
>                | (sequence *sql_sequence_01* has changed
>         |         abandon unit of work and retry).
>                | */
>                | {
>         *ses.10*| /ROLLBACK TO SAVEPOINT *session_savepoint*;
>         / |  }
>                |
>         | /* DONE */
>                |
>         -------------------------------------------------------------
>         * * *
>         * * *
>         Here is pseudocode for the _supervisoty process_ (use a wide
>         viewing window to avoid line wrap):
>
>         *Supervisory Process
>         * -------------------
>             Change operational state of table sql_table_01
>         -------------------------------------------------------------
>         |
>         *sup.0* | (Decide to change operational state of table
>                 | *sql_table_01*).
>         |
>         *sup.1* | /BEGIN TRANSACTION;/
>         |
>                 | /*
>                 |    Block all other access to table sql_table_01.
>                 | */
>         *sup.2* | /LOCK TABLE sql_table_01 IN ACCESS EXCLUSIVE MODE;
>         /       | ...
>                 | ... (change operational state of table sql_table_01
>                 | ...
>                 | ...  e.g. /TRUNCATE ONLY TABLE sql_table_01;/)
>                 | ...
>         |
>                 | /*
>                 |   Advance sequence
>                 | *sql_sequence_01
>         * |    to indicate that the operational state of table
>                 | *sql_table_01
>         * |     has changed.
>                 | */
>                 |
>         *sup.3* | /SELECT nextval('*sql_sequence_01'*);
>         / |
>                 | /*
>                 |    Release the EXCLUSIVE MODE lock on table
>         | sql_table_01.
>                 | */
>         *sup.4* | /COMMIT TRANSACTION;
>         / |
>                 | /* DONE */
>         |
>         -------------------------------------------------------------
>         * * *
>         * * *
>         I attach a PDF with the pseudocode given above.
>
>           * Attachment <eto_sql_pg - Session Context Storage - 8.1
>             Synchronize Process Access To Table - 20160103.odt>
>
>         The design document for the session operations system (SOS) is
>         well advanced, but  not yet ready for general distribution. If
>         a forum member would like to see a copy of the design document
>         in its present draft state, please feel free to email me offline
>         to request a PDF copy.
>         Thanks and Regards,
>         *Steve*
>
>
>         --
>         Sent via pgsql-general mailing list
>         (pgsql-general@postgresql.org <mailto:pgsql-general@postgresql.org>)
>         To make changes to your subscription:
>         http://www.postgresql.org/mailpref/pgsql-general
>
>
>
>
>
> --
> *Melvin Davidson*
> I reserve the right to fantasize.  Whether or not you
> wish to share my fantasy is entirely up to you.


--
Adrian Klaver
adrian.klaver@aklaver.com