Transaction Retry¶

When using serializable transactions, it's often necessary to employ a retry strategy in case of serialization failure. The algorthms are fairly typical, so repeating them manually doesn't always make a lot of sense. omni_txn provides retry procedure to handle such typical cases.

Parameter	Type	Description
stmts	text	Statement(s) to execute. Multiple statements separated by semicolon.
max_attempts	int	Max number of times to retry. 0 means no retries. 10 by default.
repeatable_read	boolean	Use `REPEATABLE READ` instead of `SERIALIZABLE`. False by default.
collect_backoff_values	boolean	Collect actual backoff values for inspection. False by default.
params	record	A record of parameters to pass to the statement. NULL by default
linearize	boolean	If a transaction should be linearized (experimental). False by default.

Retry attempt¶

There is a helper function omni_txn.current_retry_attempt() that provides retry attempt during the retry() call. 0 stands for the first run, 1 for the first retry, etc.

Example¶

Let's consider the following schema:

create table inventory
(
    id           serial primary key,
    product_name text,
    quantity     int
);
insert into inventory (product_name, quantity)
values ('Widget', 100);

Now, if we have these two simultaneous transactions happening, the second one may have committed first:

--- Transaction (1)
begin;

select quantity
from inventory
where product_name = 'Widget';

--- and here (2) will happen
update inventory
set quantity = quantity + 20
where product_name = 'Widget';

commit;
-- ERROR: could not serialize access due to read/write dependencies among transactions

--- Transaction (2)
begin;

update inventory
set quantity = quantity - 10
where product_name = 'Widgert';

commit;

If we use omni_txn.retry, the failed transaction can be driven to completion:

--- (1)
call omni_txn.retry($$
select quantity from inventory where product_name = 'Widget';
update inventory set quantity = quantity + 20
       where product_name = 'Widget'
$$);
--- (2)
call omni_txn.retry($$
update inventory set quantity = quantity - 10
       where product_name = 'Widgert'
$$);

Parameterized statements¶

Statement(s) passed to omni_txn.retry can be parameterized with the params argument:

call omni_txn.retry($$ insert into tab values ($1) $$, params => row (1));

Debugging¶

omni_txn.retry will cache prepared statement plans for every new statement provided. To see the list of currently cached planned statements, query omni_txn.retry_prepared_statements view. If you want to reset the cache, query select omni_txn.reset_retry_prepared_statements().