# RAIC.Extensions.Configuration.EntityFrameworkCore.PostgreSQL

This library enhances `RAIC.Extensions.Configuration.EntityFrameworkCore` with support for reload on update via PostgreSQL's `LISTEN`/`NOTIFY` 
[capabilities](https://www.postgresql.org/docs/current/sql-listen.html) as exposed via [npgsql](https://www.npgsql.org/doc/wait.html).

## Goals
1. No polling!
1. Updates happen in background via worker service (`IHostedService`)
1. Only update settings which change rather than reloading all of them

## Requirements
* .NET 8

## Gotchas
* Setting values cannot be `null` (as signified by the `RequiredAttribute` on `ISetting.Value`)
* Setting keys must not contain the `=` character (similar to `CommandLineConfigurationProvider` & `EnvironmentVariablesConfigurationProvider`)
* Small window of opportunity for updates to be missed during reconnection process
* Consider adding `Keepalive` to your conenction string (https://www.npgsql.org/doc/keepalive.html) if its not already present

## Known Issues
* Not tested under load

## Configuration Options
There are four properties which can be configured (cf. the `PostgreSQLNotificationConfigurationBackgroundServiceOptions` POCO)
1. `ChannelName` - the name of the notification channel to listen to for updates
1. `DebounceInterval` - the time interval to wait after a notification before signaling the `IEntityFrameworkCoreDbSetConfigurationProvider` to reload.
1. `MaxReconnectionAttempts` - Maximum number of times to retry connecting after a connection dropout (eg. PostgreSQL restart, network connectivity issue)
1. `InitialReconnectionDelay` - How long to wait after connection dropout before attempting to reconnection.

## Setup
For `PostgreSQLNotificationConfigurationReloader` to work it requires triggers on the `Settings` table to be established which send notifications to the channel
corresponding to that specified by the `ChannelName` option (defaults to `settings_channel`). SQL similar to below is one (but not only) way this can be acheived:

```sql
CREATE OR REPLACE FUNCTION notify_setting_change()
RETURNS trigger AS $$
BEGIN
    PERFORM pg_notify('settings_channel', concat(NEW.key,'=',NEW.value));
    RETURN NEW;
END;
$$ LANGUAGE plpgsql;

CREATE OR REPLACE FUNCTION notify_setting_remove()
RETURNS trigger AS $$
BEGIN
    PERFORM pg_notify('settings_channel', OLD.key);
    RETURN OLD;
END;
$$ LANGUAGE plpgsql;

CREATE TRIGGER setting_insert_trigger
AFTER INSERT ON settings
FOR EACH ROW EXECUTE FUNCTION notify_setting_change();

CREATE TRIGGER setting_update_trigger
AFTER UPDATE ON settings
FOR EACH ROW WHEN (NEW.value <> OLD.value) EXECUTE FUNCTION notify_setting_change();

CREATE TRIGGER setting_key_update_trigger
AFTER UPDATE ON settings
FOR EACH ROW WHEN (NEW.key <> OLD.key) EXECUTE FUNCTION notify_setting_remove();

CREATE TRIGGER setting_delete_trigger
AFTER DELETE ON settings
FOR EACH ROW EXECUTE FUNCTION notify_setting_remove();
```

Reccommend adding your SQL to the migration which adds the `Settings` table/view (or a new migration if that table/view already exists).

## Usage Example

```csharp

using RAIC.Extensions.Configuration.EntityFrameworkCore;
using RAIC.Extensions.Configuration.EntityFrameworkCore.Extensions;
using RAIC.Extensions.Configuration.EntityFrameworkCore.PostgreSQL.Extensions;

public record Setting : ISetting
{
    [Key]
    public required string Key { get; set; }

    [Required]
    public required string Value { get; set; }
}

public class MyDbContext(DbContextOptions<MyDbContext> options) : DbContext(options), ISettingsDbContext<DbSet<Setting>, Setting>
{
    public DbSet<Setting> Settings { get; set; }
}


var builder = Host.CreateApplicationBuilder(args); // or  WebApplication.CreateBuilder(args); 

// build an initial configuration
builder.Configuration.AddJsonFile("appsettings.json")
                     ...
                     .AddUserSecrets<Program>(); // or wherever your connection string lives

builder.Configuration.AddDbSet<MyDbContext, Setting>(dbContextOptions => dbContextOptions.UseNpgsql(builder.Configuration.GetConnectionString("Default")));

...
// Add the PostgreSQLNotificationConfigurationReloader background service and supporting services to obtain setting reloading functionalty
builder.Services.AddPostgreSQLNotificationConfigurationReloadService<SettingsDbContext>(); // uses default settings, other overrides exist - see code docs

await builder.Build().RunAsync(); // use config as normal

```

Read more about [Configuration](https://docs.microsoft.com/en-us/dotnet/core/extensions/configuration) and [Options](https://docs.microsoft.com/en-us/dotnet/core/extensions/options) on the Microsoft Docs site.