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

# Degraded State Handling

## Overview

Node degradation refers to the condition in which a node cannot perform most queries. If Oxla is misconfigured or faces a startup issue, it will enter a degraded state, return an error and reject all requests. This state can be temporary or permanent, affecting a single node or the entire cluster. This guide explains when degradation occurs and its impact on the node or cluster.

## Cluster State

In Oxla, most errors that would crash a server should instead put it into a degraded state. Below are the key terms related to the node / cluster state:

* **Liveness**: node serves incoming client connections, e.g. via psql. It does not have to allow the user to connect to the database - returning an error on connection attempt still meets liveness condition.
* **Readiness**: cluster can execute queries. It requires leader node to be in a proper state. If the leader node is degraded, the cluster is not ready to execute queries.

<Warning>**Exception** <br /> Invalid `postgresql_port` is an exception to the degraded state. Without it being properly set, even liveness condidtion is not met.</Warning>

## Degradation State Period

The degradation state of a node can be either **permanent** or **temporary**.

### Permanent Degradation

Permanent degradation occurs when a node encounters an error from which it cannot recover. The server logs the reason for this error and each query returns the error reason. As a result, the node goes into a degraded state. In order to resolve the issue, the node requires a reboot. Here are a few error examples that can put an Oxla node in a permanently degraded state:

* Invalid configuration file
* Invalid `OXLA_HOME` layout or version
* An error occurred while reading the database state on the leader node

### Temporary Degradation

Temporary degradation occurs when a node cannot perform queries because it waits for specific conditions. Below you can find errors that are related to a temporary degraded state:

* Unelected Leader (default starting state of each node)
* The node is the Leader, but it has not been initialized yet

## Effects of Degraded State

| Effects             | Details                                                                                                                                                                                                                                                              |
| ------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Database connection | If the Leader is degraded, user cannot connect to the database and all connection attempts will return degradation error.                                                                                                                                            |
| Query Handling      | - When a degraded node receives a query, it responds with a degradation error and cannot process it. - If the Leader is degraded, the whole cluster is considered degraded and most queries are not processed.                                                       |
| Degradation Types   | - **Permanent Degradation**: Nodes permanently degraded are excluded from query planning. <br /> - **Temporary Degradation**: Nodes temporarily degraded are assumed to recover and are not considered in query planning.                                            |
| Query Execution     | The `SHOW NODES` query requires the cluster to be ready and the scheduling node to not be degraded. It allows you to check the degradation status of each node in the cluster. A non-degraded leader collects data on every connected node, including degraded ones. |