Compression
Ktor provides the capability to compress outgoing content by using the Compression plugin. You can use different compression algorithms, including gzip
and deflate
, specify the required conditions for compressing data (such as a content type or response size), or even compress data based on specific request parameters. Ktor provides the capability to compress response body and decompress request body by using the Compression plugin. You can use different compression algorithms, including gzip
and deflate
, specify the required conditions for compressing data (such as a content type or response size), or even compress data based on specific request parameters.
Add dependencies
To use Compression
, you need to include the ktor-server-compression
artifact in the build script:
Install Compression
To install the Compression
plugin to the application, pass it to the install
function in the specified module. The code snippets below show how to install Compression
...
... inside the
embeddedServer
function call.... inside the explicitly defined
module
, which is an extension function of theApplication
class.
This enables the gzip
, deflate
, and identity
encoders on a server. In the next chapter, we'll see how to enable only specific encoders and configure conditions for compressing data. Note that every added encoder will be used to decompress request body if needed.
Configure compression settings
You can configure compression in multiple ways: enable only specific encoders, specify their priorities, compress only specific content types, and so on.
Add specific encoders
To enable only specific encoders, call the corresponding extension functions, for example:
You can specify the priority for each compression algorithm by establishing the priority
property:
In the example above, deflate
has a higher priority value and takes precedence over gzip
. Note that the server first looks at the quality values within the Accept-Encoding header and then takes into account the specified priorities.
Configure content type
By default, Ktor doesn't compress specific content types, such as audio
, video
, image
, and text/event-stream
. You can choose the content types to compress by calling matchContentType
or exclude the desired media types from compression by using excludeContentType
. The code snippet below shows how to compress JavaScript code using gzip
and all text subtypes using deflate
:
You can find the full example here: compression.
Configure response size
The Compression
plugin allows you to disable compression for responses whose size doesn't exceed the specified value. To do this, pass the desired value (in bytes) to the minimumSize
function:
Specify custom conditions
If necessary, you can provide a custom condition using the condition
function and compress data depending on the specific request parameters. The code snippet below shows how to compress requests for the specified URI:
HTTPS security
HTTPS with the enabled compression is vulnerable to the BREACH attack. You can use various ways to mitigate this attack. For example, you can disable compression whenever the referrer header indicates a cross-site request. In Ktor, this can be done by checking the referrer header value:
Implement custom encoder
If necessary, you can provide your own encoder by implementing the ContentEncoder interface. See GzipEncoder as an example of implementation.