Objects are things referenced by URL. pages, images, and so on - requests, cached items, clients, servers - these are all objects. Please note that a URL isn’t a unique identifier, as it’s possible to cache variants of a URL, such as by language, encoding, or the value of the ‘vary’ header set by the backend. Certainly, in many cases the cookies in the request can make it unique, as can the cookies in the response.
Objects in Varnish are not objects as in object-oriented - there’s no inheritance or behaviour assigned to them. They are data structures that can be accessed and modified by the sub-routines, which is where all the cache logic is implemented. It’s not possible to extend objects in Varnish, but you can set important built-in values, and you can arbitrarily set HTTP headers so it is possible to store state for use later in the same execution or to influence how Varnish behaves when a cached object is retrieved.
Some important definitions relating to objects in varnish include TTL, Grace, and Stale objects:
The TTL is how long an object will be cached. Varnish won’t ask the backend for an update during this period.
is how long a stale object will be served for while varnish tries to get an updated version. When this time expires then the object will no longer be served from cache and a fresh copy must be used.
The definition of a stale object is ‘objects that are past their TTL’. They are still in the cache and may be served while a fresh request is being made to the backend.
This is the request received from the client. This is basically a HTTP method, a URL, request body if present, and and headers. Varnish treats cookies as headers, without any special treatment.
This is the response about to be delivered back to the client. This is headers and response body.
This is the request to the backend. This is not necessarily the same as the client request, as it’s possible to modify it in vcl_recv and other sub-routines before the backend request is issued.
This is the data returned from the backend. We don’t have to return this verbatim to the client; it can have headers changed in vcl_deliver.
This is the object from the cache, and is read-only.
This is an ‘origin’ server, the thing that is servicing the requests. We will have at least one of these, unless we have some insane setup where all the responses from Varnish are synthetic (unlikely, but theoretically possible).
‘req_top’ is the top level request when dealing with ESI included page fragments that are being assembled by Varnish into a single output.
Other object variables
Most of these are of limited use beyond special logic or diagnostic purposes but it’s good to be aware that they exist and can be used by the forces of good.
This is the local IP.
This the time since the epoch in seconds. This is in seconds, not in milliseconds as is the norm for most languages.
The client object has the IP address for the requesting client, and an identity record used for load balancing.
Some basic information on the Varnish server, including hostname and IP address.
Information about used and free space for storage.