[Gnumed-devel] re: timeout instead of blocking

[catmat]

in postgresql.conf, if default_transaction_isolation and 
statement_timeout is uncommented
and default_transaction_isolation is 'serializable' instead of 'read 
committed',

the concurrent write timeout and  abort works as expected.

e.g. transaction A does read(x), write(x),   commit,  
and  transaction B does the same.

A commits first,
then B will report a concurrent write access error when it tries to 
write(x) after reading x's old value
and A has committed a new value for x.

If B doesn't read x's old value  before writing it, and A commits first, 
then B never saw an old value of
x so it can serialize the write to x, so it will succeed.

Interestingly, using listen "message" and notify  "message",  psql  gets 
the message
whenever any command is executed, unless it has started a transaction. 
Then it seems
to only get the message on that connection when the transaction has 
committed or aborted
and the next transaction started.  A solution might be to have 2 
connections per client,
one for doing  reading and writing, and one for message listening ( 
assuming the
pypgsql blocking listen command works).
    Then in the case above, when A commits , it issues a notify "message",
the listen channel on B's client gets the notification, and then B's 
client aborts its transaction,
begins a new transaction and re-reads, including A's changes. Now when 
it tries to write x
after reading it , it has no problems, because it has seen A's x.


BTW , the above isn't necessary to know  with the current  scheme to use 
xmin to detect
changes between read and write transactions  .

( it just seems untidy to have to do your own concurrency checking with 
timestamps , using a timestamping transactional database system).




> #---------------------------------------------------------------------------
> # CLIENT CONNECTION DEFAULTS
> #---------------------------------------------------------------------------
>
> # - Statement Behavior -
>
> #search_path = '$user,public'   # schema names
> #check_function_bodies = true
> default_transaction_isolation = 'serializable'
> default_transaction_read_only = false
> statement_timeout = 4000                # 0 is disabled, in milliseconds