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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
`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
transaction is 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 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`
The limit on transaction size for transaction replay in bytes. Any transaction