# MaxScale Optimistic Transaction Execution Filter

## Overview

The `optimistictrx` filter implements optimistic transaction execution. The filter is designed for a use-case where most of the transactions are read-only and writes happen rarely but each set of read-only statements is still grouped into a read-write transaction (i.e. `START TRANSACTION`, `BEGIN` or`SET autocommit=0`).

This filter will replace the `BEGIN` and `START TRANSACTION` SQL commands with`START TRANSACTION READ ONLY`. If the transaction is fully read-only, the transaction completes normally. However, if a write happens in the middle of a transaction, the filter issues a `ROLLBACK` command and then replays the read-only part of the transaction, including the original `BEGIN` statement. If the results of the replayed read-only part of the transaction is identical to the one that was returned to the client, the transaction proceeds normally. If the result checksum does not match, the connection is closed to prevent a write with the wrong transaction state from happening.

## Configuration

To add the filter to a service, define an instance of the filter and then add it to a service's `filters` list:

```
[OptimisticTrx]
type=filter
module=optimistictrx

[MyService]
...
filters=OptimisticTrx
```

This can also be done at runtime with:

```
maxctrl create filter OptimisticTrx optimistictrx
maxctrl alter service-filter MyService OptimisticTrx
```

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