What is Azure Application Gateway
Azure Application Gateway is a Layer-7 Load Balancer. For a brief overview of the OSI Model and where Layer-7 fits, check out this fantastic Cloudflare Article.
In short, Azure Application Gateway is used to load balance HTTP/HTTPS traffic to a back-end pool of resources.
Azure Monitor and Log Analytics
Azure Monitor is a "one-stop-shop" for ingesting logs and running queries on logs for the Azure Application Gateway and other Azure resources. Utilizing this tool is the primary method to gain insight on what is happening inside of your load balancer at any given time.
These workspaces are where the logs are sent, and the Log Analytics Kusto language is used to query the logs.
The meat and potatoes of our logs is to work with the Kusto Language. Check out how to use the Kusto Language at this documentation link.
For the rest of this guide, we'll assume you have a service that exposes HTTP/HTTPS traffic.
Setting up the logs
You will need an active Azure Application Gateway already created for this. This article will not cover this process. You can check out how to do this here.
Creating a Log Analytics Workspace
Navigate to the Resource Group you need.
Create a new Log Analytics Workspace in the region you need.
Once the Workspace is created you are ready to move on.
Sending logs to the Log Analytics Workspace
Once the Application Gateway is created, navigate to the Application Gateway resource in the region you need (same as the Log Analytics workspace!)
Navigate to Monitoring -> Diagnostic Settings -> Add Diagnostic Setting
Select the appropriate log category. Below is a list of categories and their function:
ApplicationGatewayAccessLog - Access requests (such as GET requests)
ApplicationGatewayPerformanceLog - Performance data (such as latency)
ApplicationGatewayFirewallLog - Firewall logs, if you have enabled the Web Application Firewall (WAF) option
Select the Log Analytics workspace we created earlier as the destination.
Send some data to your web application, put some load on it and allow some time for Log Analytics to ingest your data.
Accessing the Logs and running queries
After some time ingesting data, you should be ready to query those logs!
Navigate to the Application Gateway resource.
On the resource, to the left scroll to Monitoring and select Logs.
Select Get Started.
By default, the Queries screen appears. Some basic example queries are listed. Close the window, and you are now ready to use the Kusto query language.
On to the queries!
Here's a list of the queries I've found useful and their associated task:
|Query requests per Minute by URI, displayed in a timechart|
|Total Requests by URI|
|Number of URIs by Client IP with Query Strings|
|HTTP Error count per hour by URI with error code greater than or equal to 500|
|HTTP 502 errors by backend VM rendered as a timechart|
|Failed requests by backend VM, by status code, rendered as a timechart|
|Failed requests by 500 status code|
|Sum of total error counts|
|Count total URI Hits in a 24-hour period|
|Average latency every hour (Performance of Application Gateway), rendered as a timechart|
|Number of Unique IP's by URI|
|Display hits by URI|