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Fix transaction_replay documentation

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The documentation stated that MVCC reads weren't safe with
transaction_replay when in reality they are not safe in general and
transaction_replay has no effect on it.
This commit is contained in:
Markus Mäkelä 2019-03-28 16:28:43 +02:00
parent 514dd96301
commit 4f8d6d1853
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1 changed files with 7 additions and 9 deletions
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16
Documentation/Routers/ReadWriteSplit.md
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@ -438,11 +438,8 @@ failure of a master node without any visible effects to the client.
If no replacement node becomes available before the timeout controlled by
`delayed_retry_timeout` is exceeded, the client connection is closed.
Not all transactions can be safely replayed. Only when the following criteria
are met, the transaction can be safely replayed.
* Transaction contains only data modification (`INSERT`, `UPDATE`, `DELETE`
etc.) or `SELECT ... FOR UPDATE` statements.
Only when the following criteria are met, the transaction can be safely
replayed.
* The replacement server where the transaction is applied returns results
identical to the original partial transaction.
@ -451,10 +448,11 @@ If the results from the replacement server are not identical when the transactio
replayed, the client connection is closed. This means that any transaction with a server
specific result (e.g. `NOW()`, `@@server_id`) cannot be replayed successfully.
Performing MVCC reads (`SELECT` queries without `FOR UPDATE` or `LOCK IN SHARE MODE`)
with transaction replay is discouraged. If such statements are executed
but the results of each reply are identical, the transaction is replayed but the results
are not guaranteed to be consistent on the database level.
It is recommended that the transaction contains only data modification
(`INSERT`, `UPDATE`, `DELETE` etc.) or `SELECT ... FOR UPDATE`
statements. Performing MVCC reads (`SELECT` queries without `FOR UPDATE` or
`LOCK IN SHARE MODE`) is discouraged as it doesn't guarantee strict ordering of
events.
### `transaction_replay_max_size`