Azure webapp to elasticsearch, sometimes "A connection attempt failed" on azure - azure

Question

I have a web app and a elasticsearch cluster inside a virtiual network.
The web app is on one azure catalog and the elasticsearch cluster on another catalog/subscription. I cannot have them in the same catalog because of bizspark subscription rules (5 accounts $150 each).
Due to this I cannot have the web app connected to the virtual network with Point to site as I can understand.
Therefore I have opened traffic in the virtual network firewall for port 9200. And it works great to index and search in the web app which is connected to elasticsearch cluster. But only sometimes?!
Now and then I get this error:
A connection attempt failed because the connected party did not
properly respond after a period of time, or established connection
failed because connected host has failed to respond x.x.x.x:9200
I also get this error sometimes from my web job which is in the same webb app.
Are there something that is blocking my connection to elasticsearch?
If I change plan for my web app (e.g. Standard S1 to S1 or vice versa) it starts to work again. But then after a while I get blocked gain.

Answer 1

As Martin Laarman wrote. The SetTcpKeepAlive was the problem.
NEST has implemented this in their ConnnectionConfiguration with the method EnableTcpKeepAlive().
I don't know what parameters call EnableTcpKeepAlive. The values i set for now seems to work ok.
public static ElasticClient ElasticClient
{
get
{
var uri = new Uri(ConfigurationManager.AppSettings["ElasticSearchUrl"]);
var setting = new ConnectionSettings(uri);
setting.SetDefaultIndex("myIndex");
var connectionConfiguration = new ConnectionConfiguration(uri);
connectionConfiguration.EnableTcpKeepAlive(200000, 200000);
var client = new ElasticClient(setting, new HttpConnection(connectionConfiguration));
return client;
}
}
In version 7.13.0 of NEST, EnableTcpKeepAlive is on the settings object:
var settings = new ConnectionSettings(url);
settings.EnableTcpKeepAlive(TimeSpan.FromMinutes(1), TimeSpan.FromSeconds(1));
client = new ElasticClient(settings);