# Response Compression

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Asp.Net Core provides a response compression middleware, which used to reduce response size to improve responsiveness.

Use response middleware when you are:

1. Unable to use the IIS Dynamic Compression module, Apache mod_deflate module, or Nginx Compression and Decompression because of those have a built-in response compression support.
2. Response size smaller than about 150-1000 bytes(depending on the file’s content and the efficiency of compression). The overhead of compressing small files may produce a compressed file larger than the uncompressed file about 150-1000 bytes.
3. Compress MIME types not belong to the default compression MIME types, like image/svg+xml.

The following code shows how to use response compression middleware in the Startup file

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{
app.UseResponseCompression();
}


Use the UseResponseCompression method to achieve response compression. As our previous article mentioned, the execution sequence is critical. So, the app.UseResponseCompression must be called before any middleware that compresses responses.

In most cases, a website contains a static file serving short-circuiting middleware and an endpoint middleware. We need to be careful that the position of response compression should be before any of them. Like the following code illustrates:

public void Configure(IApplicationBuilder app,
IWebHostEnvironment env)
{
app.UseResponseCompression();

app.UseStaticFiles();

app.UseRouting();
app.UseEndpoints(endpoints =>
{
endpoints.MapRazorPages();
});
}


You can specify a response compression service by calling the AddResponseCompression extension method in ConfigureServices, which accepts a Action<ResponseCompressionOptions> parameter.

services.AddResponseCompression(options => {
options.MimeTypes =
ResponseCompressionDefaults.MimeTypes.Concat(
new[] { "image/jpeg", "image/png", "application/font-woff2", "image/svg+xml" });
options.EnableForHttps = true;
});


The BrotliCompressionProvider is for Brotli encoding, and the GzipCompressionProvider is for Gzip encoding. You can even provide your custom compression provider, which needs to implement the ICompressionProvider.

It’s not all MIMEs are supported for default response compression. The following table illustrates a default set of MIME types for compression.

MIME
application/javascript
application/json
application/xml
text/css
text/html
text/json
text/plain
text/xml

We can note that the above list mimes don’t contain image related MIME types. We can replace or append with the Response Compression Middleware options to make it support our custom MIME types. Note that wildcard MIME types, such as text/* aren’t supported.

//Concat our custom MIME types to the default mime
options.MimeTypes =
ResponseCompressionDefaults.MimeTypes.Concat(
new[] { "image/jpeg", "image/png", "application/font-woff2", "image/svg+xml" });

//wildcard MIME types aren't supported
// options.MimeTypes =
//     ResponseCompressionDefaults.MimeTypes.Concat(
//         new[] { "image/*" });


By default, the compression doesn’t support the Https connection. If you want to the response over HTTPS connection should be compression, then assign with true value.

options.EnableForHttps = true;