> ## Documentation Index
> Fetch the complete documentation index at: https://docs.ditto.live/llms.txt
> Use this file to discover all available pages before exploring further.

# Customizing System Settings

> This article provides a high-level overview of the `ALTER SYSTEM` Ditto Query Language (DQL) statement, as well as instructions on how to use it, once offered, to set, reset, and query peer-to-peer system settings.

`ALTER SYSTEM` is an advanced system performance tuning feature for configuring and retrieving *system settings* for your peer-to-peer mesh network.

<Info>
  For information on specific configs supported via `ALTER SYSTEM` reach out to the Ditto team member.
</Info>

System settings consist of values the Ditto library uses to configure specific behaviors, such as timeouts, resource limits, and so on.

This article provides a high-level overview of the `ALTER SYSTEM` Ditto Query Language (DQL) statement, as well as instructions on how to use it, once offered, to set, reset, and query peer-to-peer system settings.

## Tasks Overview

The following table provides an overview of the various tasks you can perform with the `ALTER SYSTEM` statement:

| **Task**                                                                                                       | **Query**            |
| -------------------------------------------------------------------------------------------------------------- | -------------------- |
| Retrieve current system settings for all configurations. ([Retrieving Values](#retrieving-values))             | `SHOW ALL`           |
| Retrieve current system settings for a specific configuration. ([Retrieving Values](#retrieving-values))       | `SHOW`               |
| Change a specific system setting. ([Modifying Values](#modifying-values))                                      | `ALTER SYSTEM SET`   |
| Reset system settings to default configurations. ([Resetting Values to Default](#resetting-values-to-default)) | `ALTER SYSTEM RESET` |

## Retrieving Values

Fetch current setting values for either the entire mesh network or a specific setting:

* To return system-wide configurations, use a `SHOW ALL` statement:

<CodeGroup>
  ```mysql theme={null}
  SHOW ALL
  ```
</CodeGroup>

* To return only the value for a specific setting, pass the setting you want to retrieve a value for in a `SHOW` statement. For example:

<CodeGroup>
  ```mysql theme={null}
  SHOW REPLICATION_GC_STARTUP_DELAY_SECONDS
  ```
</CodeGroup>

### Executing SHOW and SHOW ALL

To invoke your statement, call the Execute API method on the `ditto.store` namespace:

<CodeGroup>
  ```swift Swift theme={null}
  let result = try await ditto.store.execute(query: "SHOW ALL")
  ```

  ```java Java theme={null}
  CompletableFuture<DittoQueryResult> result = ditto.getStore().execute("SHOW ALL");
  ```
</CodeGroup>

### Modifying Values

To modify a system setting, use `ALTER SYSTEM SET`:

<Info>
  Similar to PostgresSQL, DQL supports both the `=` and `TO` syntax for setting system configurations.

  Regardless of the syntax you use in your `ALTER SYSTEM SET` query, the documents Ditto returns contain the current value of the setting, identified by the setting name.
</Info>

<CodeGroup>
  ```mysql theme={null}
  ALTER SYSTEM SET <setting> = <value>
  ```
</CodeGroup>

For example:

<CodeGroup>
  ```mysql theme={null}
  ALTER SYSTEM SET REPLICATION_GC_STARTUP_DELAY_SECONDS = 10
  ```
</CodeGroup>

### Executing SET

Similar to the execution pattern recommended when configuring transports in your app, invoke your statement modifying system configurations immediately after initializing the `ditto` object. (See [Customizing Transports Configurations](./customizing-transport-configurations))

<Warning>
  Ditto stores configuration settings modified by `ALTER SYSTEM` in memory rather than persisting them to disk.

  So, while you can execute your statement to modify a setting at any point during your app's lifecycle, changing certain settings within your app may not result in immediate effects.
</Warning>

To invoke the `SET` statement, add a call to the Execute API method on the `ditto.store` namespace.

<CodeGroup>
  ```swift Swift theme={null}
  var result = try await ditto.store.execute(query: "ALTER SYSTEM SET example_parameter = 321")
  result.items[0].value["example_parameter"] as? Int // 321
  ```

  ```java Java theme={null}
  CompletableFuture<DittoQueryResult> result = ditto.getStore()
      .execute("ALTER SYSTEM SET example_parameter = 321");

  result.thenAccept(queryResult -> {
      int value = (Integer) queryResult.getItems().get(0).getValue().get("example_parameter"); // 321
  });
  ```
</CodeGroup>

## Resetting Values to Default

To return peer-to-peer system configuration settings to default values, do either of the following:

* To reset all configuration settings:

  For example:

<CodeGroup>
  ```mysql theme={null}
  ALTER SYSTEM RESET ALL
  ```
</CodeGroup>

* To reset a specific configuration setting:

<CodeGroup>
  ```mysql theme={null}
  ALTER SYSTEM RESET <setting>
  ```
</CodeGroup>

Once executed, Ditto returns a document containing current values, identified by their setting name.

## Troubleshooting

Ditto throws an error when an `ALTER SYSTEM` query operation fails to execute due to various issues, including:

* Syntax mistakes, such as a typo, causing parsing of the DQL statement to fail.
* Using an unrecognized setting name in the query (often due to a typo).
* Attempting to set a value outside the acceptable bounds for a setting in an `ALTER SYSTEM SET` query.
* An internal error occurred while processing the query.