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

Thanks to Melvin for your remarks. My reply is posted below.

Steve

From: "Melvin Davidson"
Sent: Sunday, January 03, 2016 4:32 PM
> More to the point, if you 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
>

Sorry for not having defined the term "session" in my original posting. Here
are quotes from the draft design document, for the proposed session
operations system:

"In this document, the term "session" does not refer to connections with a
postgres server through a postgres client, which connections are referred to
as "sessions" in postgres docs. In this document, the term "session" means a
dialog between an app and the consumer of that app, where the app may or may
not use postgres, for other than its session context data.

"The app that motivates this present proposed design for a postgres-based
session operations facility, is an Internet website application (HTML / HTTP
/ PHP / PostgreSQL) that provides services to a visitor using a web browser.

The "session" under discussion therefore, is used to maintain the
contextual data of the (visitor <=> app) dialog, during which dialog the app
will open and close numerous brief connections with the postgres client,
such connections also being referred to as "sessions" within postgres
docs.

So a TEMPORARY table will not suffice, but using a tablespace in SSHD for
the session operations tables, might yield performance gains like
TEMPORARY..

>
> On Sun, Jan 3, 2016 at 3:43 PM, Pavel Stehule <pavel.stehule@gmail.com>
> wrote:
>
>> Hi
>>
>> 2016-01-03 20:46 GMT+01:00 Steve Petrie, P.Eng. <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. I
>>> believe the last posting to that thread was on 1 December 2015 by George
>>> Neuner <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>.
>>>
>>
>> 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*
>>>
>>>
>>>
>>> --
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>>>
>>>
>>
>
>
> --
> *Melvin Davidson*
> I reserve the right to fantasize.  Whether or not you
> wish to share my fantasy is entirely up to you.
>