Worker.cs

var dataService = new DataService();
dataService.AddWorker(new Worker(1));
dataService.AddWorker(new Worker(2));
dataService.AddWorker(new Worker(3));
dataService.AddWorker(new Worker(4));

var frontend1 = new FrontendInfo("tsushi01", "FE1");
var frontend2 = new FrontendInfo("tsushi02", "FE2");

// 適当に空いてるワーカーを返してもらう
Console.WriteLine(dataService.GetAvailableWorker(frontend1));
Console.WriteLine(dataService.GetAvailableWorker(frontend2));

// 再要求しても同じワーカーが返ってくるはず
Console.WriteLine(dataService.GetAvailableWorker(frontend1));
Console.WriteLine(dataService.GetAvailableWorker(frontend2));

record Worker(int Id);
record FrontendInfo(string HostName, string FrontEnd);
class DataService
{
    // ロック用
    private readonly object _syncRoot = new();
    // 誰も使ってないワーカー管理
    private readonly Queue<Worker> _availableWorkers= new();
    // フロントエンドに割り当て済みのワーカー管理
    private readonly Dictionary<FrontendInfo, Worker> _assignedWorkers = new();

    public Worker? GetAvailableWorker(FrontendInfo frontend)
    {
        lock(_syncRoot)
        {
            var worker = GetWorkerForFrontend(frontend);
            if (worker == null)
            {
                worker = AssignWorker(frontend);
            }

            return worker;
        }
    }

    // Frontend がワーカーが不要になったときに返却
    public void ReleaseWorker(FrontendInfo frontend)
    {
        lock(_syncRoot)
        {
            if (_assignedWorkers.TryGetValue(frontend, out var worker))
            {
                _assignedWorkers.Remove(frontend);
                _availableWorkers.Append(worker);
            }
        }
    }

    // データサービスが管理する Worker を追加
    public void AddWorker(Worker worker)
    {
        lock(_syncRoot)
        {
            _availableWorkers.Append(worker);
        }
    }

    // 誰も使ってない Worker を Frontend に割り当てる
    private Worker? AssignWorker(FrontendInfo frontend)
    {
        if (_availableWorkers.Count == 0)
        {
            return null;
        }

        var worker = _availableWorkers.Dequeue();
        _assignedWorkers.Add(frontend, worker);
        return worker;
    }

    // Frontend に対して既に割り当て済みの Worker があったらそれを返す
    private Worker? GetWorkerForFrontend(FrontendInfo frontend)
    {
        _ = _assignedWorkers.TryGetValue(frontend, out var worker);
        return worker;
    }
}