Question -- Session Operations - Feasibility Of Proposed Synchronization Method? - Mailing list pgsql-general
| From | Steve Petrie, P.Eng. |
|---|---|
| Subject | Question -- Session Operations - Feasibility Of Proposed Synchronization Method? |
| Date | |
| Msg-id | 094497F865B9461EBF217698845D773E@Dell Whole thread Raw |
| Responses |
Re: Question -- Session Operations - Feasibility Of
Proposed Synchronization Method?
(Pavel Stehule <pavel.stehule@gmail.com>)
|
| List | pgsql-general |
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>.
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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.
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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.)
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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).
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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 */
|
---------------
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 */
|
-------------------------------------------------------------
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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 */
|
-------------------------------------------------------------
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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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