Hot versus cold restarts in Umbraco
When we talk about startup time there are different things folks are referring to. Let us cover all startup scenarios with comments.
- Production website: A site that is running live which means it has compilation
debug="false"in theweb.configfile and it is built in Release mode and without log4net Debug mode enabled (these are important factors) - Hot start: When a site is recycled in production without DLL, code or view file changes
- Cold start: When a site is recycled in production with DLL, code or view file changes
Startup scenarios
Hot start
A production website that gets restarted, possibly by a change in the web.config file or a scheduled IIS recycle.
There's an important thing to know about this scenario. ASP.NET has a built in CodeGen folder which it compiles all things to including App_Code, View files, DLLs and Resources.
On first run, ASP.NET will compile a lot and it's important to know that if you have debug="true" or have compiled in Debug mode. This compilation time and the amount of information included is exponentially larger. This means that it might take much longer on the first run.
A normal "hot start" in Umbraco will mean there is no assembly scanning taking place. This is do to the fact that Umbraco caches all assembly scans for its plugins unless any files in the /bin folder changes.
Hot start /w assembly scan
If a hot start occurs and the /App_Data/TEMP/PluginCache is cleared, then Umbraco will scan assemblies on startup for finding certain plugins.
This is rare and will generally only be caused on purpose by a developer.
Cold start
A production website that gets restarted by changing the global.asax or changes made in any of these folders: ~/Resource, ~/WebReferences, ~/App_Code, ~/bin.
This scenario can be caused by changes and it doesn't always come down to shipping new code. If you bump the global.asax, this will also cause a "cold start".
A good reference is the BuildManager source code that controls this. You can see what files are monitored which will force a cold start.
A cold start means that the CodeGen folder is cleared and all intermediate ASP.NET compilations will be gone and needs to be done again. It's important to understand the above comment with regards to debug="true" and Debug/Release modes.
Cold start w/ assembly rescan
If a cold start is triggered because of /bin folder changes then Umbraco will scan assemblies on startup for finding certain plugins.
Visual Studio cold start
This occurs during local development with Visual Studio or even VS Code if you are using App_Code. It also occurs anytime you are developing locally with assemblies built in Debug mode and with debug="true" in your web.config.
You will generally have log4net Debug or Info level enabled which can cause startup time increase.
This will always take a longer time than any of the above.
As for the Debug vs Release mode, there's a ton of info on this online. Depending on the type of project, it will vary on how much performance improvements Release mode is compared to Debug.
If you want to improve VS cold start times and you don't plan on debugging, change your web.config to debug="false" and try it out. Be aware that you will not get any detailed error information or be able to set breakpoints.