# Built in subroutines

Various built-in subroutines are called during processing of client- and backend requests as well as upon `vcl.load` and `vcl.discard`.

See Varnish Processing States for a detailed graphical overview of the states and how they relate to core code functions and VCL subroutines.

Subroutines always terminate with a `return()` on a keyword, which determines how processing continues in the request processing state machine.

The behaviour for `return()` keywords is identical or at least similar across subroutines, so differences are only documented where relevant.

## common return keywords

> `fail`

>   Transition to vcl_synth on the client side as for `return(synth(503, "VCL Failed"))`, but with any request state changes undone as if `std.rollback()` was called and forcing a connection close.

>   Intended for fatal errors, for which only minimal error handling is possible.

> `synth(status code, reason)`
>   Transition to vcl_synth with `resp.status` and `resp.reason` being preset to the arguments of `synth()`.

> `pass`
>   Switch to pass mode. Control will eventually pass to vcl_pass.

> `pipe`
>   Switch to pipe mode. Control will eventually pass to vcl_pipe.

> `restart`

>   Restart the transaction. Increases the `req.restarts` counter.

>   If the number of restarts is higher than the max_restarts parameter, control is passed to vcl_synth as for `return(synth(503, "Too many restarts"))`

>   For a restart, all modifications to `req` attributes are preserved except for `req.restarts` and `req.xid`, which need to change by design.

## client side

### vcl_recv

Called at the beginning of a request, after the complete request has been received and parsed, after a restart or as the result of an ESI include.

Its purpose is to decide whether or not to serve the request, possibly modify it and decide on how to process it further. A backend hint may be set as a default for the backend processing side.

The vcl_recv subroutine may terminate with calling `return()` on one of the following keywords:

> `fail`
>   see fail
> `synth(status code, reason)`
>   see synth
> `restart`
>   see restart
> `pass`
>   see pass
> `pipe`
>   see 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.
> `vcl(label)`

>   Switch to vcl labelled label. This will continue vcl processing in this vcl’s vcl_recv as if it was the active vcl.

>   See the varnishadm `vcl.label` command.

### vcl_pipe

Called upon entering pipe mode. In this mode, the request is passed on to the backend, and any further data from both the client and 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. For a connection in pipe mode, no other VCL subroutine will ever get called after vcl_pipe.

The vcl_pipe subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `synth(status code, reason)`
>   see synth
> `pipe`
>   Proceed with pipe mode.

### vcl_pass

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:

> `fail`
>   see fail
> `synth(status code, reason)`
>   see synth
> `restart`
>   see restart
> `fetch`
>   Proceed with pass mode - initiate a backend request.

### vcl_hash

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:

> `fail`
>   see fail
> `lookup`

>   Look up the object in cache.

>   Control passes to vcl_purge when coming from a `purge` return in vcl_recv.

>   Otherwise control passes to the next subroutine depending on the result of the cache lookup:

>   - a hit: pass to vcl_hit
>   - a miss or a hit on a hit-for-miss object (an object with `obj.uncacheable == true`): pass to vcl_miss
>   - a hit on a hit-for-pass object (for which `pass(DURATION)` had been previously returned from `vcl_backend_response`): pass to vcl_pass

### vcl_purge

Called after the purge has been executed and all its variants have been evicted.

The vcl_purge subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `synth(status code, reason)`
>   see synth
> `restart`
>   see restart

### vcl_miss

Called after a cache lookup if the requested document was not found in the cache or if vcl_hit returned `fetch`.

Its purpose is to decide whether or not to attempt to retrieve the document from the backend. A backend hint may be set as a default for the backend processing side.

The vcl_miss subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `synth(status code, reason)`
>   see synth
> `restart`
>   see restart
> `pass`
>   see pass
> `fetch`
>   Retrieve the requested object from the backend. Control will eventually pass to vcl_backend_fetch.

### vcl_hit

Called when a cache lookup is successful. The object being hit may be stale: It can have a zero or negative ttl with only grace or keep time left.

The vcl_hit subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `synth(status code, reason)`
>   see synth
> `restart`
>   see restart
> `pass`
>   see pass
> `miss`
>   Synchronously refresh the object from the backend despite the cache hit. Control will eventually pass to vcl_miss.
> `deliver`
>   Deliver the object. If it is stale, a background fetch to refresh it is triggered.

### vcl_deliver

Called before any object except a vcl_synth result is delivered to the client.

The vcl_deliver subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `synth(status code, reason)`
>   see synth
> `restart`
>   see restart
> `deliver`
>   Deliver the object to the client.

### vcl_synth

Called to deliver a synthetic object. A synthetic object is generated in VCL, not fetched from the backend. Its body may be constructed using the `synthetic()` function.

A vcl_synth defined object never enters the cache, contrary to a vcl_backend_error defined object, which may end up in cache.

The subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `restart`
>   see restart
> `deliver`
>   Directly deliver the object defined by vcl_synth to the client without calling vcl_deliver.

## Backend Side

### vcl_backend_fetch

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:

> `fail`
>   see fail
> `fetch`
>   Fetch the object from the backend.
> `abandon`
>   Abandon the backend request. Unless the backend request was a background fetch, control is passed to vcl_synth on the client side with `resp.status` preset to 503.
> `error(status code, reason)`
>   Transition to vcl_backend_error with `beresp.status` and `beresp.reason` being preset to the arguments of `error()` if arguments are provided.

Before calling vcl_backend_fetch, Varnish core prepares the bereq backend request as follows:

- Unless the request is a pass,
  - set `bereq.method` to `GET` and `bereq.proto` to `HTTP/1.1` and
  - set `bereq.http.Accept_Encoding` to `gzip` if http_gzip_support is enabled.
- If there is an existing cache object to be revalidated, set `bereq.http.If-Modified-Since` from its `Last-Modified` header and/or set `bereq.http.If-None-Match` from its `Etag` header
- Set `bereq.http.X-Varnish` to the current transaction id (vxid)

These changes can be undone or modified in vcl_backend_fetch before the backend request is issued.

In particular, to cache non-GET requests, `req.method` needs to be saved to a header or variable in vcl_recv and restored to `bereq.method`. Notice that caching non-GET requests typically also requires changing the cache key in vcl_hash e.g. by also hashing the request method and/or request body.

HEAD request can be satisfied from cached GET responses.

### vcl_backend_response

Called after the response headers have been successfully retrieved from the backend.

The vcl_backend_response subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `deliver`
>   For a 304 response, create an updated cache object. Otherwise, fetch the object body from the backend and initiate delivery to any waiting client requests, possibly in parallel (streaming).
> `retry`
>   Retry the backend transaction. Increases the retries counter. If the number of retries is higher than max_retries, control will be passed to vcl_backend_error.
> `abandon`
>   Abandon the backend request. Unless the backend request was a background fetch, control is passed to vcl_synth on the client side with `resp.status` preset to 503.
> `pass(duration)`
>   Mark the object as a hit-for-pass for the given duration. Subsequent lookups hitting this object will be turned into passed transactions, as if `vcl_recv` had returned `pass`.
> `error(status code, reason)`
>   Transition to vcl_backend_error with `beresp.status` and `beresp.reason` being preset to the arguments of `error()` if arguments are provided.

### 304 handling

For a 304 response, varnish core code amends `beresp` before calling vcl_backend_response:

- If the gzip status changed, `Content-Encoding` is unset and any `Etag` is weakened
- Any headers not present in the 304 response are copied from the existing cache object. `Content-Length` is copied if present in the existing cache object and discarded otherwise.
- The status gets set to 200.

beresp.was_304 marks that this conditional response processing has happened.

Note: Backend conditional requests are independent of client conditional requests, so clients may receive 304 responses no matter if a backend request was conditional.

### beresp.ttl / beresp.grace / beresp.keep

Before calling vcl_backend_response, core code sets `beresp.ttl` based on the response status and the response headers `Age`, `Cache-Control` or `Expires` and `Date` as follows:

- If present and valid, the value of the `Age` header is effectively deduced from all ttl calculations.
- For status codes 200, 203, 204, 300, 301, 304, 404, 410 and 414:
  - If `Cache-Control` contains an `s-maxage` or `max-age` field (in that order of preference), the ttl is set to the respective non-negative value or 0 if negative.
  - Otherwise, if no `Expires` header exists, the default ttl is used.
  - Otherwise, if `Expires` contains a time stamp before `Date`, the ttl is set to 0.
  - Otherwise, if no `Date` header is present or the `Date` header timestamp differs from the local clock by no more than the clock_skew parameter, the ttl is set to
    - 0 if `Expires` denotes a past timestamp or
    - the difference between the local clock and the `Expires` header otherwise.
  - Otherwise, the ttl is set to the difference between `Expires` and `Date`
- For status codes 302 and 307, the calculation is identical except that the default ttl is not used and -1 is returned if neither `Cache-Control` nor `Expires` exists.
- For all other status codes, ttl -1 is returned.

`beresp.grace` defaults to the default_grace parameter.

For a non-negative ttl, if `Cache-Control` contains a `stale-while-revalidate` field value, `beresp.grace` is set to that value if non-negative or 0 otherwise.

`beresp.keep` defaults to the default_keep parameter.

### vcl_backend_error

This subroutine is called if we fail the backend fetch or if max_retries has been exceeded.

A synthetic object is generated in VCL, whose body may be constructed using the `synthetic()` function.

The vcl_backend_error subroutine may terminate with calling `return()` with one of the following keywords:

> `fail`
>   see fail
> `deliver`
>   Deliver and possibly cache the object defined in vcl_backend_error as if it was fetched from the backend, also referred to as a “backend synth”.
> `retry`
>   Retry the backend transaction. Increases the retries counter. If the number of retries is higher than max_retries, vcl_synth on the client side is called with `resp.status` preset to 503.

## vcl.load / vcl.discard

### vcl_init

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.
> `fail`
>   Abort loading of this VCL.

### vcl_fini

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.