Reg: global variable in vcl

Geoff Simmons geoff at
Fri May 17 11:09:43 UTC 2019

On 5/16/19 10:41, Hardik wrote:
> Does I have to take care something while using this vmod ? Because first
> time going to use and also not getting any information about this.

That's a very generic question. Do you have something specific in mind?

objvar is actually a bundle of four VMODs, and they each have
documentation as .rst files in the src/ directory. These get converted
and installed as man pages on installation. So you might want to start

To take a guess at possible answers to the generic question, the main
thing to decide is the scope and mutability of the variables you want to
use. That's what distinguishes the four VMODs:

- constant: global constants (immutable)

- globalvar: global variables (mutable)

- taskvar: variables with task scope - scoped to the client or backend
context of a single request/response transaction

- topvar: ESI top-level scope - these are variables accessible in client
context for an ESI include and all of its sub-includes

You should be aware that the global variables use locking and memory
barriers to prevent simultaneous updates. So if you have a lot of
threads accessing a variable at the same time, you could run into lock

Conceivably you might want to think about how memory is allocated for
the different scopes. If you only need a couple of variables in VCL, you
won't notice the difference. But if you're using a lot of them, if
you're trying to minimize RAM usage, trying to minimize workspace size
configurations to reduce Varnish's footprint, if you're using a lot of
workspace for other purposes (large ESI trees, other VMODs that consume
workspace ...) ... then it could matter.

The global constants and variables are allocated from the system heap,
for the lifetime of the VCL instance. Task scoped variables are
allocated from the workspace of the client or backend task. ESI
top-level variables are allocated from client workspace, which is
re-used for the entire ESI include tree. Workspaces are reset and
re-used for new request/response transactions.

> what is difference between below this ?
> and
> I am familiar with first link flow wise. Generation of *.vcc file in second
> link is totally different then previous one which I am not familiar with.
> Someone please clarify confusion that which one should use for global
> variable vmod ?

VMOD var supports global variables for the STRING, INT and REAL types,
and task-scoped variables for STRING, INT, REAL, DURATION, IP and BACKEND.

The objvar VMODs cover all of the VCL data types, and have the different
options for scope and mutability described above. They also have methods
to undefine or protect a variable (a protected variable may not be
changed), and check if a variable is defined or protected. See the .rst
files for documentation, or the manpage if you install any of the VMODs.

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