I cant seem to find any documentation for it.
If connection draining is not available how is one supposed to do zero-downtime deployments?
Rick Rainey answered essentially the same question on Server Fault. He states:
The recommended way to do this is to have a custom health probe in
your load balanced set. For example, you could have a simple
healthcheck.html page on each of your VM's (in wwwroot for example)
and direct the probe from your load balanced set to this page. As long
as the probe can retrieve that page (HTTP 200), the Azure load
balancer will keep sending user requests to the VM.
When you need to update a VM, then you can simply rename the
healthcheck.html to a different name such as _healthcheck.html. This
will cause the probe to start receiving HTTP 404 errors and will take
that machine out of the load balanced rotation because it is not
getting HTTP 200. Existing connections will continue to be serviced
but the Azure LB will stop sending new requests to the VM.
After your updates on the VM have been completed, rename
_healthcheck.html back to healthcheck.html. The Azure LB probe will start getting HTTP 200 responses and as a result start sending
requests to this VM again.
Repeat this for each VM in the load balanced set.
Note, however, that Kevin Williamson from Microsoft states in his MSDN blog post Heartbeats, Recovery, and the Load Balancer, "Make sure your probe path is not a simple HTML page, but actually includes logic to determine your service health (eg. Try to connect to your SQL database)." So you may actually want an aspx page that can check several factors, including a custom "drain" flag you put somewhere.
Your clients need to simply retry.
The load balancer only forwards a request to an instance that is alive (determined by pings), it doesn't keep track of the connections. So if you have long-standing connections, it is your responsibility to clean them up on restart events or leave it to the OS to clean them up on restarts (which is obviously not gracefully in most of the cases).
Zero-downtime means that you'll always be able to reach an instance that is alive, nothing more- it gives you no guarantees on long running requests.
Note that when a probe is down, only new connections will go to other VMs
Existing connections are not impacted.
Related
I'm currently researching and experimenting with Kubernetes in Azure. I'm playing with AKS and the Application Gateway ingress. As I understand it, when a pod is added to a service, the endpoints are updated and the ingress controller continuously polls this information. As new endpoints are added AG is updated. As they're removed AG is also updated.
As pods are added there will be a small delay whilst that pod is added to the AG before it receives requests. However, when pods are removed, does that delay in update result in requests being forwarded to a pod that no longer exists?
If not, how does AG/K8S guarantee this? What behaviour could the end client potentially experience in this scenario?
Azure Application gateway ingress is an ingress controller for your kubernetes deployment which allows you to use native Azure Application gateway to expose your application to the internet. Its purpose is to route the traffic to pods directly. At the same moment all questions about pods availability, scheduling and generally speaking management is on kubernetes itself.
When a pod receives a command to be terminated it doesn't happen instantly. Right after kube-proxies will update iptables to stop directing traffic to the pod. Also there may be ingress controllers or load balancers forwarding connections directly to the pod (which is the case with an application gateway). It's impossible to solve this issue completely, while adding 5-10 seconds delay can significantly improve users experience.
If you need to terminate or scale down your application, you should consider following steps:
Wait for a few seconds and then stop accepting connections
Close all keep-alive connections not in the middle of request
Wait for all active requests to finish
Shut down the application completely
Here are exact kubernetes mechanics which will help you to resolve your questions:
preStop hook - this hook is called immediately before a container is terminated. This is very helpful for graceful shutdowns of an application. For example simple sh command with "sleep 5" command in a preStop hook can prevent users to see "Connection refused errors". After the pod receives an API request to be terminated, it takes some time to update iptables and let an application gateway know that this pod is out of service. Since preStop hook is executed prior SIGTERM signal, it will help to resolve this issue.
(example can be found in attach lifecycle event)
readiness probe - this type of probe always runs on the container and defines whether pod is ready to accept and serve requests or not. When container's readiness probe returns success, it means the container can handle requests and it will be added to the endpoints. If a readiness probe fails, a pod is not capable to handle requests and it will be removed from endpoints object. It works very well with newly created pods when an application takes some time to load as well as for already running pods if an application takes some time for processing.
Before removing from the endpoints readiness probe should fail several times. It's possible to lower this amount to only one fail using failureTreshold field, however it still needs to detect one failed check.
(additional information on how to set it up can be found in configure liveness readiness startup probes)
startup probe - for some applications which require additional time on their first initialisation it can be tricky to set up a readiness probe parameters correctly and not compromise a fast response from the application.
Using failureThreshold * periodSecondsfields will provide this flexibility.
terminationGracePeriod - is also may be considered if an application requires more than default 30 seconds delay to gracefully shut down (e.g. this is important for stateful applications)
Has anyone successfully done rolling updates with Azure Front Door? We have an application in 2 regions, and we want to disable the backend in region 1 while it gets updated and same for the backend in region 2. However, there seems to be a ridiculous amount of lag time between when you disable or remove a backend from a pool, making this basically impossible.
We've tried:
Disabling/totally removing backends
Setting high/low backend priorities/weights
Modifying health probe intervals
Changing sample size/successful samples/latency to 1/1/100
I have an endpoint that I watch during the deployment process which tells me which region it's in, and it never changes during the operation, and becomes unavailable when the region is being updated. There's gotta be a way to do this, right?
I have a suggestion,
Reduce the Health Probe Interval.
Reduce the sample size and successful sample required. (Make sure you are probing to a simple HTTP page so your backend resource can handle the loads. You will start receiving probes from all the POP servers with the interval you specified.)
3.For the sever which you need to do maintenance, stop the service or make the probe fail, so that all traffic will switch to the healthy server. Then do the maintenance and start the service again. This will make sure your service is not disrupted.
We have a setup with several RESTful APIs on the same VM in Azure.
The websites run in Kestrel on IIS.
They are protected by the azure application gateway with firewall.
We now have requests that would run for at least 20 minutes.
The request run the full length uninterrupted on Kestrel (Visible in the logs) but the sender either get "socket hang up" after exactly 5 minutes or run forever even if the request finished in kestrel. The request continue in Kestrel even if the connection was interrupted for the sender.
What I have done:
Wrote a small example application that returns after a set amount of
seconds to exclude our websites being the problem.
Ran the request in the VM (to localhost): No problems, response was received.
Ran the request within Azure from one to another VM: Request ran forever.
Ran the request from outside of Azure: Request terminates after 5 minutes
with "socket hang up".
Checked set timeouts: Kestrel: 50m , IIS: 4000s, ApplicationGateway-HttpSettings: 3600
Request were tested with Postman,
Is there another request or connection timeout hidden somewhere in Azure?
We now have requests that would run for at least 20 minutes.
This is a horrible architecture and it should be rewritten to be async. Don't take this personally, it is what it is. Consider returning a 202 Accepted with a Location header to poll for the result.
You're most probably hitting the Azure SNAT layer timeout —
Change it under the Configuration blade for the Public IP.
So I ran into something like this a little while back:
For us the issue was probably the timeout like the other answer suggests but the solution was (instead of increasing timeout) to add PGbouncer in front of our postgres database to manage the connections and make sure a new one is started before the timeout fires.
Not sure what your backend connection looks like but something similar (backend db proxy) could work to give you more ability to tune connection / reconnection on your side.
For us we were running AKS (azure Kubernetes service) but all azure public ips obey the same rules that cause issues similar to this one.
While it isn't an answer I know there are also two types of public IP addresses, one of them is considered 'basic' and doesn't have the same configurability, could be something related to the difference between basic and standard public ips / load balancers?
I am running a load test using JMeter on my Azure web services.
I scale my services on S2 with 4 instances and run JMeter 4 instances with 500 threads on each.
It starts perfectly fine but after a while calls start failing and giving Timeout error (HTTP status:500).
I have checked HTTP request queue on azure and found that on 2nd instance it is very high and two instances it is very low.
Please help me to success my load test.
I assume you are using Azure App Service. If you check the settings of your App, you will notice ARR’s Instance Affinity will be enabled by default. A brief explanation:
ARR cleverly keeps track of connecting users by giving them a special cookie (known as an affinity cookie), which allows it to know, upon subsequent requests, to which server instance they were talking to. This way, we can be sure that once a client establishes a session with a specific server instance, it will keep talking to the same server as long as his session is active.
This is an important feature for session-sensitive applications, but if it's not your case then you can safely disable it to improve the load balance between your instances and avoid situations like the one you've described.
Disabling ARR’s Instance Affinity in Windows Azure Web Sites
It might be due to caching of network names resolution on JVM or OS level so all your requests are hitting only one server. If it is the case - add DNS Cache Manager to your Test Plan and it should resolve your issue.
See The DNS Cache Manager: The Right Way To Test Load Balanced Apps article for more detailed explanation and configuration instructions.
We have a stateless (with shared Azure Redis Cache) WebApp that we would like to automatically scale via the Azure auto-scale service. When I activate the auto-scale-out, or even when I activate 3 fixed instances for the WebApp, I get the opposite effect: response times increase exponentially or I get Http 502 errors.
This happens whether I use our configured traffic manager url (which worked fine for months with single instances) or the native url (.azurewebsites.net). Could this have something to do with the traffic manager? If so, where can I find info on this combination (having searched)? And how do I properly leverage auto-scale with traffic-manager failovers/perf? I have tried putting the traffic manager in both failover and performance mode with no evident effect. I can gladdly provide links via private channels.
UPDATE: We have reproduced the situation now the "other way around": On the account where we were getting the frequent 5XX errors, we have removed all load balanced servers (only one server per app now) and the problem disappeared. And, on the other account, we started to balance across 3 servers (no traffic manager configured) and soon got the frequent 502 and 503 show stoppers.
Related hypothesis here: https://ask.auth0.com/t/health-checks-response-with-500-http-status/446/8
Possibly the cause? Any takers?
UPDATE
After reverting all WebApps to single instances to rule out any relationship to load balancing, things ran fine for a while. Then the same "502" behavior reappeared across all servers for a period of approx. 15 min on 04.Jan.16 , then disappeared again.
UPDATE
Problem reoccurred for a period of 10 min at 12.55 UTC/GMT on 08.Jan.16 and then disappeared again after a few min. Checking logfiles now for more info.
UPDATE
Problem reoccurred for a period of 90 min at roughly 11.00 UTC/GMT on 19.Jan.16 also on .scm. page. This is the "reference-client" Web App on the account with a Web App named "dummy1015". "502 - Web server received an invalid response while acting as a gateway or proxy server."
I don't think Traffic Manager is the issue here. Since Traffic Manager works at the DNS level, it cannot be the source of the 5XX errors you are seeing. To confirm, I suggest the following:
Check if the increased response times are coming from the DNS lookup or from the web request.
Introduce Traffic Manager whilst keeping your single instance / non-load-balanced set up, and confirm that the problem does not re-appear
This will help confirm if the issue relates to Traffic Manager or some other aspect of the load-balancing.
Regards,
Jonathan Tuliani
Program Manager
Azure Networking - DNS and Traffic Manager