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# VCL-steps

## Built-in subroutines

Manual section:

  7

### DESCRIPTION

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

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

Built-in subroutines always terminate with a `return (<action>)`, where `<action>` determines how processing continues in the request processing state machine.

The behaviour of actions is identical or at least similar across subroutines, so differences are only documented where relevant.

Common actions are documented in VCL Actions in the next section. Actions specific to only one or some subroutines are documented in VCL Steps.

A default behavior is provided for all Vinyl Cache Processing States in the Built-in VCL code.

### VCL Actions

Actions are used with the `return(<action>)` keyword, which returns control from subroutines back to `vinyld`. The action determines how processing in `vinyld` continues as shown in Vinyl Cache Processing States.

Common actions are documented here, while additional actions specific to only one or some subroutines are documented in the next section VCL Steps as well as which action can be used from which built in subroutine.

#### Common actions for the client and backend side

##### `fail(err)`

> 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. The optional string argument err is logged to VSL under a VCL_Error tag.

> Returning fail from vcl_synth sends a minimal response with a 500 status.

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

#### Common actions for the client side

##### `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, making the current request not use the cache and not putting its response into it. 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.

#### Common actions for the backend side

##### `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.

### VCL Steps

#### 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(err)`
> see fail(err) section above
> `synth(status code, reason)`
> see synth(status code, reason) section above
> `restart`
> see restart section above
> `pass`
> see pass section above
> `pipe`
> see pipe section above
> `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 roll back the request as if `std.rollback(req)` was
> called and continue vcl processing in vcl_recv of the vcl
> labelled label as if it was the active vcl.
> The `vcl(label)` return is only valid while the `req.restarts`
> count is zero and if used from the active vcl.
> See the vcl.label <label> <configname> command in vinyl-cli.

##### 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, Vinyl Cache 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(err)`
> see fail(err) section above
> `synth(status code, reason)`
> see synth(status code, reason) section above
> `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(err)`
> see fail(err) section above
> `synth(status code, reason)`
> see synth(status code, reason) section above
> `restart`
> see restart section above
> `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 Vinyl Cache.

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

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

##### 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(err)`
> see fail(err) section above
> `synth(status code, reason)`
> see synth(status code, reason) section above
> `restart`
> see restart section above
> `pass`
> see pass section above
> `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(err)`
> see fail(err) section above
> `synth(status code, reason)`
> see synth(status code, reason) section above
> `restart`
> see restart section above
> `pass`
> see pass section above
> `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(err)`
> see fail(err) section above
> `synth(status code, reason)`
> see synth(status code, reason) section above
> `restart`
> see restart section above
> `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(err)`
> see fail(err) section above
> `restart`
> see restart section above
> `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(err)`
> see fail(err) section above
> `abandon`
> see abandon section above
> `fetch`
> Fetch the object from the backend.
> `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, Vinyl Cache 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-Vinyl` 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_refresh

This subroutine is called after a legitimate 304 response is received from the backend so that you can chose how to handle the object revalidation.

The default is to apply the RFC logic which is to merge the new headers with the stale ones.

The returned value only affects response headers, the body that is delivered is always the one from the stale object.

See vcl_refresh_* for additional explanations.

> `merge`
> Merge the headers we got from the backend response with
> the ones we had in the cached object.
> `obj_stale`
> Return the stale object’s headers without performing the RFC merge logic.
> `beresp`
> Return the beresp headers as they were at the end of this subroutine
> without executing the RFC merge logic. Any content-length and
> content-encoding headers present in the 304 beresp will be removed and
> replaced by the stale object ones if available. Etag and Last-Modified
> headers are also copied from the stale object to allow future revalidations.
> `fail`
> see fail(err) section above
> `abandon`
> see abandon section above
> `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.
> `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.

##### 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(err)`
> see fail(err) section above
> `abandon`
> see abandon section above
> `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.
> `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.

##### vcl_backend_error

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

Returning with abandon does not leave a cache object.

If returning with `deliver` and `beresp.uncacheable == false`, a synthetic cache object is generated in VCL, whose body may be constructed using the `synthetic()` function.

Since these synthetic objects are cached in these special circumstances, be cautious with putting private information there. If you really must, then you need to explicitly set `beresp.uncacheable` to `true` in `vcl_backend_error`.

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

> `fail(err)`
> see fail(err) section above
> `abandon`
> see abandon section above
> `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.

#### During 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(err)`
> 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.

### SEE ALSO

- vinyld

- VCL

### COPYRIGHT

This document is licensed under the same license as Vinyl Cache itself. See LICENSE for details.

- Copyright (c) 2006 Verdens Gang AS

- Copyright (c) 2006-2021 Varnish Software AS