A Comprehensive Guide to Using varnishlog (with examples)

Introduction

Varnish is a high-performance HTTP accelerator that helps improve the speed and scalability of websites. One useful tool in the Varnish suite is varnishlog, which allows you to display Varnish logs. This article will guide you through the different use cases of the varnishlog command, providing code examples and explanations for each scenario.

1. Displaying Logs in Real Time

To display Varnish logs in real time, simply execute the varnishlog command without any additional arguments:

varnishlog

Motivation: Real-time log monitoring is essential for troubleshooting and analyzing server performance. By using varnishlog, you can monitor Varnish logs in real time and quickly identify any issues that may arise. This can be particularly helpful during peak traffic periods or when observing the effects of configuration changes.

Output Example:

   0 CLI          - Rd ping
   0 CLI          - Wr 200 19 PONG 1627989796 1.0
   0 CLI          - Rd start
   0 CLI          - Rd open
...

2. Displaying Requests to a Specific Domain

To filter log entries and display only requests to a specific domain, you can use the -q flag along with a VCL expression. For example, to view logs related to requests made to the domain “example.com”, use the following command:

varnishlog -q 'ReqHeader eq "Host: example.com"'

Motivation: Filtering logs by domain is useful when troubleshooting issues specific to a particular website or application. By focusing on logs related to the desired domain, you can quickly identify any errors or anomalies that may occur.

Output Example:

   0 VCL_call     - RxRequest         c GET
   0 VCL_return   -           deliver
   0 TxProtocol   - HTTP/1.1
   0 TxStatus     - 200
   0 TxResponse   - OK
...

3. Displaying Only POST Requests

To filter logs and display only POST requests, you can utilize the -q option with a VCL expression. The following command will show only logs for POST requests:

varnishlog -q 'ReqMethod eq "POST"'

Motivation: Monitoring only POST requests is valuable when troubleshooting issues related to form submissions, API calls, or other POST-specific actions. It allows you to focus on the relevant logs and quickly identify any errors or performance concerns.

Output Example:

   0 VCL_call     - RxRequest         c POST
   0 VCL_return   -           deliver
   0 TxProtocol   - HTTP/1.1
   0 TxStatus     - 200
   0 TxResponse   - OK
...

4. Displaying Requests to a Specific Path

Sometimes, it is necessary to filter logs based on the requested path. You can achieve this by using the -q flag with a VCL expression that matches a specific path. Here’s an example:

varnishlog -q 'ReqURL eq "/path"'

Motivation: Focusing on requests to a specific path is helpful when troubleshooting issues related to a particular URL or endpoint. By filtering logs based on the requested path, you can quickly identify any errors or performance problems specific to that path.

Output Example:

   0 VCL_call     - RxRequest         c GET
   0 VCL_return   -           deliver
   0 TxProtocol   - HTTP/1.1
   0 TxStatus     - 200
   0 TxResponse   - OK
...

5. Displaying Requests to Paths Matching a Regular Expression

For more advanced filtering, you can use regular expressions to match the requested paths. The -q flag allows you to filter logs based on a VCL expression that utilizes regular expressions. Here’s an example:

varnishlog -q 'ReqURL ~ "regex"'

Motivation: Regular expressions provide powerful pattern matching capabilities, allowing you to filter logs based on complex criteria. This is useful when you need to analyze requests matching specific patterns or when troubleshooting issues related to dynamic URLs.

Output Example:

   0 VCL_call     - RxRequest         c GET
   0 VCL_return   -           deliver
   0 TxProtocol   - HTTP/1.1
   0 TxStatus     - 200
   0 TxResponse   - OK
...

Conclusion

In this article, we have explored the various use cases of the varnishlog command. By following the examples provided, you can effectively monitor and filter Varnish logs based on different criteria such as real-time display, specific domains, request methods, paths, and regular expressions. Leveraging varnishlog can greatly assist in troubleshooting and optimizing the performance of your Varnish cache.