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MXS-2655 Document session command limitation

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Markus Mäkelä
2019-09-04 07:28:00 +03:00
parent 46a3c9f61c
commit 2a93d86f1a
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Documentation/Routers/ReadWriteSplit.md
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@ -477,11 +477,48 @@ failure of a master node without any visible effects to the client.
If no replacement node becomes available before the timeout controlled by If no replacement node becomes available before the timeout controlled by
`delayed_retry_timeout` is exceeded, the client connection is closed. `delayed_retry_timeout` is exceeded, the client connection is closed.
#### Transaction Replay Limitations
If the results from the replacement server are not identical when the If the results from the replacement server are not identical when the
transaction is replayed, the client connection is closed. This means that any transaction is replayed, the client connection is closed. This means that any
transaction with a server specific result (e.g. `NOW()`, `@@server_id`) cannot transaction with a server specific result (e.g. `NOW()`, `@@server_id`) cannot
be replayed successfully but it will still be attempted. be replayed successfully but it will still be attempted.
Any changes to the session state (e.g. autocommit state, SQL mode) done inside a
transaction will remain in effect even if the connection to the server where the
transaction is being executed fails. When readwritesplit creates a new
connection to a server to replay the transaction, it will first restore the
session state by executing all session commands that were executed. This means
that if the session state is changed mid-transaction in a way that affects the
results, transaction replay will fail.
The following partial transaction demonstrates the problem by using
[`SQL_MODE`](https://mariadb.com/kb/en/library/sql-mode/) inside a transaction.
```
SET SQL_MODE=''; -- A session command
BEGIN;
SELECT "hello world"; -- Returns the string "hello world"
SET SQL_MODE='ANSI_QUOTES'; -- A session command
SELECT 'hello world'; -- Returns the string "hello world"
```
If this transaction has to be replayed the actual SQL that gets executed is the
following.
```
SET SQL_MODE=''; -- Replayed session command
SET SQL_MODE='ANSI_QUOTES'; -- Replayed session command
BEGIN;
SELECT "hello world"; -- Returns an error
SELECT 'hello world'; -- Returns the string "hello world"
```
First the session state is restored by executing all commands that changed the
state after which the actual transaction is replayed. Due to the fact that the
SQL_MODE was changed mid-transaction, one of the queries will now return an
error instead of the result we expected leading to a transaction replay failure.
### `transaction_replay_max_size` ### `transaction_replay_max_size`
The limit on transaction size for transaction replay in bytes. Any transaction The limit on transaction size for transaction replay in bytes. Any transaction