Called at the beginning of a request, after the complete request has been received and parsed. Its purpose is to decide whether or not to serve the request, how to do it, and, if applicable, which backend to use.
It is also used to modify the request, something you’ll probably find yourself doing frequently.
The vcl_recv subroutine may terminate with calling return() on one of the following keywords:
- synth(status code, reason)
- Return a synthetic object with the specified status code to the client and abandon the request.
- pass
- Switch to pass mode. Control will eventually pass to vcl_pass.
- pipe
- Switch to pipe mode. Control will eventually pass to vcl_pipe.
- hash
- Continue processing the object as a potential candidate for caching. Passes the control over to vcl_hash.
- purge
- Purge the object and it’s variants. Control passes through vcl_hash to vcl_purge.
Called upon entering pipe mode. In this mode, the request is passed on to the backend, and any further data from either client or backend is passed on unaltered until either end closes the connection. Basically, Varnish will degrade into a simple TCP proxy, shuffling bytes back and forth.
The vcl_pipe subroutine may terminate with calling return() with one of the following keywords:
- synth(status code, reason)
- Return the specified status code to the client and abandon the request.
- pipe
- Proceed with pipe mode.
Called upon entering pass mode. In this mode, the request is passed on to the backend, and the backend’s response is passed on to the client, but is not entered into the cache. Subsequent requests submitted over the same client connection are handled normally.
The vcl_pass subroutine may terminate with calling return() with one of the following keywords:
- synth(status code, reason)
- Return the specified status code to the client and abandon the request.
- pass
- Proceed with pass mode.
- restart
- Restart the transaction. Increases the restart counter. If the number of restarts is higher than max_restarts Varnish emits a guru meditation error.
Called when a cache lookup is successful.
The vcl_hit subroutine may terminate with calling return() with one of the following keywords:
- restart
- Restart the transaction. Increases the restart counter. If the number of restarts is higher than max_restarts Varnish emits a guru meditation error.
- deliver
- Deliver the object. Control passes to vcl_deliver.
- synth(status code, reason)
- Return the specified status code to the client and abandon the request.
Called after a cache lookup if the requested document was not found in the cache. Its purpose is to decide whether or not to attempt to retrieve the document from the backend, and which backend to use.
The vcl_miss subroutine may terminate with calling return() with one of the following keywords:
- synth(status code, reason)
- Return the specified status code to the client and abandon the request.
- pass
- Switch to pass mode. Control will eventually pass to vcl_pass.
- fetch
- Retrieve the requested object from the backend. Control will eventually pass to vcl_backend_fetch.
- restart
- Restart the transaction. Increases the restart counter. If the number of restarts is higher than max_restarts Varnish emits a guru meditation error.
Called after vcl_recv to create a hash value for the request. This is used as a key to look up the object in Varnish.
The vcl_hash subroutine may terminate with calling return() with one of the following keywords:
- lookup
- Look up the object in cache. Control passes to vcl_miss, vcl_hit or vcl_purge.
Called after the purge has been executed and all its variants have been evited.
The vcl_purge subroutine may terminate with calling return() with one of the following keywords:
- synth
- Produce a response.
- restart
- Restart the transaction. Increases the restart counter. If the number of restarts is higher than max_restarts Varnish emits a guru meditation error.
Called before a cached object is delivered to the client.
The vcl_deliver subroutine may terminate with calling return() with one of the following keywords:
- deliver
- Deliver the object to the client.
- restart
- Restart the transaction. Increases the restart counter. If the number of restarts is higher than max_restarts Varnish emits a guru meditation error.
Called before sending the backend request. In this subroutine you typically alter the request before it gets to the backend.
The vcl_backend_fetch subroutine may terminate with calling return() with one of the following keywords:
- fetch
- Fetch the object from the backend.
- abandon
- Abandon the backend request and generates an error.
Called after the response headers has been successfully retrieved from the backend.
The vcl_backend_response subroutine may terminate with calling return() with one of the following keywords:
- deliver
- Possibly insert the object into the cache, then deliver it to the Control will eventually pass to vcl_deliver.
- abandon
- Abandon the backend request and generates an error.
- retry
- Retry the backend transaction. Increases the retries counter. If the number of retries is higher than max_retries Varnish emits a guru meditation error.
This subroutine is called if we fail the backend fetch.
The vcl_backend_error subroutine may terminate with calling return() with one of the following keywords:
- deliver
- Deliver the error.
- retry
- Retry the backend transaction. Increases the retries counter. If the number of retries is higher than max_retries Varnish emits a guru meditation error.
Called when VCL is loaded, before any requests pass through it. Typically used to initialize VMODs.
The vcl_init subroutine may terminate with calling return() with one of the following keywords:
- ok
- Normal return, VCL continues loading.
Called when VCL is discarded only after all requests have exited the VCL. Typically used to clean up VMODs.
The vcl_fini subroutine may terminate with calling return() with one of the following keywords:
- ok
- Normal return, VCL will be discarded.