# Consistent Critical Read Filter

This filter was introduced in MariaDB MaxScale 2.1.

## Overview

The Consistent Critical Read (CCR) filter allows consistent critical reads to be
done through MaxScale while still allowing scaleout of non-critical reads.

When the filter detects a statement that would modify the database, it attaches
a routing hint to all following statements. This routing hint guides the routing
module to route the statement to the master server where data is guaranteed to
be in an up-to-date state.

### Controlling the Filter with SQL Comments

The triggering of the filter can be limited further by adding MaxScale supported
comments to queries and/or by using regular expressions. The query comments take
precedence: if a comment is found it is obayed even if a regular expression
parameter might give a different result. Even a comment cannot cause a
SELECT-query to trigger the filter. Such a comment is considered an error and
ignored.

The comments must follow the [MaxScale hint syntax](../Reference/Hint-Syntax.md)
and the *HintFilter* needs to be in the filter chain before the CCR-filter. If a
query has a MaxScale supported comment line which defines the parameter `ccr`,
that comment is caught by the  CCR-filter. Parameter values `match` and `ignore`
are supported, causing the filter to trigger (`match`) or not trigger (`ignore`)
on receiving the write query. For example, the query
```
INSERT INTO departments VALUES ('d1234', 'NewDepartment'); -- maxscale ccr=ignore
```
would normally cause the filter to trigger, but does not because of the
comment. The `match`-comment typically has no effect, since write queries by
default trigger the filter anyway. It can be used to override an ignore-type
regular expression that would othewise prevent triggering.

## Filter Parameters

The CCR filter has no mandatory parameters.

### `time`

The time window in seconds during which queries are routed to the master. The
default value for this parameter is 60 seconds.

When a data modifying SQL statement is processed, a timer is set to the value of
_time_. Once the timer has elapsed, all statements are routed normally. If a new
data modifying SQL statement is processed within the time window, the timer is
reset to the value of _time_.

Enabling this parameter in combination with the _count_ parameter causes both
the time window and number of queries to be inspected. If either of the two
conditions are met, the query is re-routed to the master.

### `count`

The number of SQL statements to route to master after detecting a data modifying
SQL statement. This feature is disabled by default.

After processing a data modifying SQL statement, a counter is set to the value
of _count_ and all statements are routed to the master. Each executed statement
after a data modifying SQL statement cause the counter to be decremented. Once
the counter reaches zero, the statements are routed normally. If a new data
modifying SQL statement is processed, the counter is reset to the value of
_count_.

### `match`, `ignore` and `options`

These [regular expression settings](../Getting-Started/Configuration-Guide.md#standard-regular-expression-settings-for-filters)
control which statements trigger statement re-routing. Only non-SELECT statements are
inspected. For CCRFilter, the *exclude*-parameter is instead named *ignore*, yet works
similarly.

```
match=.*INSERT.*
ignore=.*UPDATE.*
options=case,extended
```

## Example Configuration

Here is a minimal filter configuration for the CCRFilter which should solve most
problems with critical reads after writes.

```
[CCRFilter]
type=filter
module=ccrfilter
time=5
```

This configuration will force all read queries after a write to the master for 5
seconds, preventing read scaling until the modifications have been replicated to
the slaves.

For best performance, the value of _time_ should be slightly greater than the
actual replication lag between the master and its slaves. If the number of
critical read statements is known, the _count_ parameter could be used to
control the number reads that are sent to the master.
