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esi

Available inall subroutines.

Flags a request for parsing edge side includes. Parsing occurs after the vcl_deliver subroutine has finished, so this statement has no effect if executed in vcl_log.

Fastly only supports the following ESI directives:

• include
• comment
• remove

When specifying the src URL with <esi:include src=... />, the URL can be either an absolute or relative URL. If you specify an absolute URL, you must prefix it with http:// or https://.

<esi:include src="https://example.com/index.html" />

ESI requests are made to your Fastly service as new client requests, with a copy of the HTTP header information from the original client request. If a hostname is specified in the ESI src attribute, the Host header of the new request will be set to the value of that hostname, otherwise the Host header of the ESI request will be the same as for the parent request.

In the above example, the ESI parser takes example.com in the URL as the Host request header value (i.e., req.http.host) and creates a new request to your service. Although you can specify any URL in an ESI include, and the hostname does not have to be one of your service's configured domains you can only forward that request to one of your configured backends. In general, it is often better to use relative links for ESI includes to avoid confusion.

In ESI child requests, the req.topurl variable is set to the value of req.url from the parent request.

Limitations and constraints

Performance

ESI tags are processed sequentially, not concurrently, because when the parser encounters an ESI include tag, it will make the child request, wait for the HTTP response from the child request to complete, splice that response into the response stream, and then continue parsing the parent response.

Resource limits

• The maximum depth of esi:include processing is 5
• The maximum total esi:include processing fragments is 256

Communication and co-ordination

It is not possible for edge logic within a child request to pass data back to the parent, except by embedding the response body of the child request within the parent. However, it is possible to know whether a request is an ESI child request by checking the value of req.topurl.

ESI tags within CDATA

Parsing ESI tags within CDATA is not enabled by default. You can set esi.allow_inside_cdata variable to true if needed.

ESI with origin compression

In order for the ESI parser to detect <esi:... tags in the response stream, the response cannot be compressed. Compressed responses will obfuscate the ESI tags and they will be delivered unprocessed to the client.

Try it out

esi is used in the following code examples. Examples apply VCL to real-world use cases and can be deployed as they are, or adapted for your own service. See the full list of code examples for more inspiration.

Click RUN on a sample below to provision a Fastly service, execute the code on Fastly, and see how the function behaves.