[GENERAL] bigserial continuity safety
Hi.
If I have a table created as:
CREATE TABLE xq_agr (
idBIGSERIAL PRIMARY KEY,
node text not null
);
and that multiple applications insert into. The applications never
explicitly specify the value for 'id'.
Is it safe to, on a single connection, do:
- open transaction (default transaction isolation)
- Open cursor for select * from xq_agr order by id asc
- do something with current record
- advance the cursor (and repeat something), but stop at some point (id =
LAST_ID), and
- delete from xq_agr where id = LAST_ID;
- commit
safe to means - whether the cursor will not miss any records that were
deleted at the end.
I'm suspecting that depending on the commit order, I may have situations
when:
- TX1 insert ID 1
- TX2 insert ID 2
- TX2 commits
- TX3 scans 2
- TX1 commits
- TX3 deletes = 2
- record ID1 is deleted, but never processed.
Is that sequence of events as listed above possible? If yes, is there a
transaction isolation I can use to avoid that?
Table and sequence definition, as present in the DB:
db= \d+ xq_agr_id_seq
Sequence public.xq_agr_id_seq
Column | Type |Value| Storage
---+-+-+-
sequence_name | name| xq_agr_id_seq | plain
last_value| bigint | 139898829 | plain
start_value | bigint | 1 | plain
increment_by | bigint | 1 | plain
max_value | bigint | 9223372036854775807 | plain
min_value | bigint | 1 | plain
cache_value | bigint | 1 | plain
log_cnt | bigint | 27 | plain
is_cycled | boolean | f | plain
is_called | boolean | t | plain
db= \d xq_agr
Table public.xq_agr
Column | Type | Modifiers
---+-+-
id| bigint | not null default
nextval('xq_agr_id_seq'::regclass)
node | text| not null
Indexes:
xq_agr_pkey PRIMARY KEY, btree (id)
Re: [GENERAL] bigserial continuity safety
On Mon, Apr 13, 2015 at 3:05 PM, Pawel Veselov pawel.vese...@gmail.com wrote: Hi. If I have a table created as: CREATE TABLE xq_agr ( idBIGSERIAL PRIMARY KEY, node text not null ); and that multiple applications insert into. The applications never explicitly specify the value for 'id'. Is it safe to, on a single connection, do: - open transaction (default transaction isolation) - Open cursor for select * from xq_agr order by id asc - do something with current record - advance the cursor (and repeat something), but stop at some point (id = LAST_ID), and - delete from xq_agr where id = LAST_ID; - commit safe to means - whether the cursor will not miss any records that were deleted at the end. I'm suspecting that depending on the commit order, I may have situations when: - TX1 insert ID 1 - TX2 insert ID 2 - TX2 commits - TX3 scans 2 - TX1 commits - TX3 deletes = 2 - record ID1 is deleted, but never processed. Going to ignore the MVC question for the moment and describe a better state transition mechanism to consider. pending - active - completed If you ensure you never delete (i.e., transition to completed) something that isn't active then you can never delete an item in pending. Limit the locking to the state transitions only. The downside is the need to deal with active items that have been abandoned by whatever process marked them active. Back to your question: you should probably not use = in your where clause. However, in READ COMMITTED TX3 cannot see ID1 since the snapshot it took out was created before TX1 committed. I am not fluent enough to work through the entire scenario in my head. I'd suggest you actually open up 3 psql sessions and play with them to see how things really behave. For me, a simply SELECT FOR UPDATE / UPDATE WHERE command in a function solves the problem as small scale with minimal performance degradation. The transition from pending to active is effectively serialized and the transition from active to completed only occurs when the process has been performed and it is not possible to have two client simultaneously processing the same work. David J.
Re: [GENERAL] bigserial continuity safety
On 4/13/15 7:45 PM, David G. Johnston wrote: On Mon, Apr 13, 2015 at 3:05 PM, Pawel Veselov pawel.vese...@gmail.com mailto:pawel.vese...@gmail.comwrote: Hi. If I have a table created as: CREATE TABLE xq_agr ( idBIGSERIAL PRIMARY KEY, node text not null ); and that multiple applications insert into. The applications never explicitly specify the value for 'id'. Is it safe to, on a single connection, do: - open transaction (default transaction isolation) - Open cursor for select * from xq_agr order by id asc - do something with current record - advance the cursor (and repeat something), but stop at some point (id = LAST_ID), and - delete from xq_agr where id = LAST_ID; - commit safe to means - whether the cursor will not miss any records that were deleted at the end. I'm suspecting that depending on the commit order, I may have situations when: - TX1 insert ID 1 - TX2 insert ID 2 - TX2 commits - TX3 scans 2 - TX1 commits - TX3 deletes = 2 - record ID1 is deleted, but never processed. Going to ignore the MVC question for the moment and describe a better state transition mechanism to consider. pending - active - completed If you ensure you never delete (i.e., transition to completed) something that isn't active then you can never delete an item in pending. Limit the locking to the state transitions only. The downside is the need to deal with active items that have been abandoned by whatever process marked them active. Another option is DELETE RETURNING. Instead of an initial SELECT to find records to work on, you would do DELETE FROM WHERE RETURNING * and deal with those records. I don't know if that's safe with a cursor though; I believe the DELETE fully materializes before records start coming back. So you need to handle all the rows from the SELECT or abort. Back to your question: you should probably not use = in your where clause. However, in READ COMMITTED TX3 cannot see ID1 since the snapshot it took out was created before TX1 committed. I am not fluent Actually, that's not necessarily true. It depends when TX3 actually takes it's snapshot, which is NOT when it runs BEGIN. I believe there's other problems you'd run into as well. Basically, READ COMMITTED does nothing to protect you from phantom reads. REPEATABLE READ should protect you from phantom reads, but it won't help you if someone changes the data. If you're going to try and go this route, SERIALIZABLE is your best bet. enough to work through the entire scenario in my head. I'd suggest you actually open up 3 psql sessions and play with them to see how things really behave. That's really not safe enough. There's just too many different race conditions you can encounter, and I'd bet that you couldn't even reproduce some of them from a client. For me, a simply SELECT FOR UPDATE / UPDATE WHERE command in a function solves the problem as small scale with minimal performance degradation. The transition from pending to active is effectively serialized and the transition from active to completed only occurs when the process has been performed and it is not possible to have two client simultaneously processing the same work. Note that that isn't safe from repeatable reads. What you're describing is only safe if the WHERE clause on the update is guaranteed to always find only one row (ie, in this example, by using xq_agr.id = something). Anything other than that is asking for trouble. BTW, since it looks like you're just looking for a queue, you should take a look at PgQ (https://wiki.postgresql.org/wiki/PGQ_Tutorial). It uses something more efficient than bulk deletes to handle it's queue, and you can set it up so it will handle a large amount of queued items rather well (increase the number of tables). The one downside you may run into is you MUST consume every event in a single batch. There used to be support for the concept of retrying an event, but that may have been removed. It would be safe for you to put failed events into a second queue. -- Jim Nasby, Data Architect, Blue Treble Consulting Data in Trouble? Get it in Treble! http://BlueTreble.com -- Sent via pgsql-general mailing list (pgsql-general@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
Re: [GENERAL] bigserial continuity safety
On Mon, Apr 13, 2015 at 7:01 PM, Jim Nasby jim.na...@bluetreble.com wrote: On 4/13/15 7:45 PM, David G. Johnston wrote: On Mon, Apr 13, 2015 at 3:05 PM, Pawel Veselov pawel.vese...@gmail.com mailto:pawel.vese...@gmail.comwrote: Hi. If I have a table created as: CREATE TABLE xq_agr ( idBIGSERIAL PRIMARY KEY, node text not null ); and that multiple applications insert into. The applications never explicitly specify the value for 'id'. Is it safe to, on a single connection, do: - open transaction (default transaction isolation) - Open cursor for select * from xq_agr order by id asc - do something with current record - advance the cursor (and repeat something), but stop at some point (id = LAST_ID), and - delete from xq_agr where id = LAST_ID; - commit safe to means - whether the cursor will not miss any records that were deleted at the end. I'm suspecting that depending on the commit order, I may have situations when: - TX1 insert ID 1 - TX2 insert ID 2 - TX2 commits - TX3 scans 2 - TX1 commits - TX3 deletes = 2 - record ID1 is deleted, but never processed. Going to ignore the MVC question for the moment and describe a better state transition mechanism to consider. pending - active - completed If you ensure you never delete (i.e., transition to completed) something that isn't active then you can never delete an item in pending. Limit the locking to the state transitions only. The downside is the need to deal with active items that have been abandoned by whatever process marked them active. Another option is DELETE RETURNING. Instead of an initial SELECT to find records to work on, you would do DELETE FROM WHERE RETURNING * and deal with those records. I don't know if that's safe with a cursor though; I believe the DELETE fully materializes before records start coming back. So you need to handle all the rows from the SELECT or abort. Back to your question: you should probably not use = in your where clause. However, in READ COMMITTED TX3 cannot see ID1 since the snapshot it took out was created before TX1 committed. I am not fluent Actually, that's not necessarily true. It depends when TX3 actually takes it's snapshot, which is NOT when it runs BEGIN. I believe there's other problems you'd run into as well. Basically, READ COMMITTED does nothing to protect you from phantom reads. I was basing that off of: - TX2 commits - TX3 scans 2 - TX1 commits The scanning causes the snapshot to be taken and occurs before TX1. As for the isolation levels I apparently got the two READ ones reversed in my head...my bad :( REPEATABLE READ should protect you from phantom reads, but it won't help you if someone changes the data. If you're going to try and go this route, SERIALIZABLE is your best bet. While this is likely true if there is no other use of SERIALIZABLE in the existing codebase then doing so requires learning/adding transaction retry to the necessary skills and tools. Its worth considering other approaches to avoid the cognitive overhead of serializable. enough to work through the entire scenario in my head. I'd suggest you actually open up 3 psql sessions and play with them to see how things really behave. That's really not safe enough. There's just too many different race conditions you can encounter, and I'd bet that you couldn't even reproduce some of them from a client. I guess there is a difference between knowing something is obviously wrong because this simple testing failed and not realizing that you still have a problem because there was no way to reasonably test the condition you are hitting. The question becomes whether you rephrase the solution to make it simpler and thus not as exposed to race conditions and the like (or fails gracefully if it is - no deadlocks and hopefully minimal waiting) or whether to simply prevent them outright (and quickly?) by failing with a serialization exception. For me, a simply SELECT FOR UPDATE / UPDATE WHERE command in a function solves the problem as small scale with minimal performance degradation. The transition from pending to active is effectively serialized and the transition from active to completed only occurs when the process has been performed and it is not possible to have two client simultaneously processing the same work. Note that that isn't safe from repeatable reads. What you're describing is only safe if the WHERE clause on the update is guaranteed to always find only one row (ie, in this example, by using xq_agr.id = something). Anything other than that is asking for trouble. Yes, the result of the select returns an ID of (one or more) pending jobs which has a LIMIT 1 applied to it and the first record is immediately updated to reflect its active status. At the scale I
