Details about HTTP Headers Caching module

Marcus Clyne eugaia at
Wed Mar 18 16:40:50 MSK 2009

Hi Latesh,

Thanks for the information.

I have some further questions, mostly to give me a clearer idea of your 

1) Are you only looking at caching redirection (i.e. 3xx) responses?
2) Are you looking to cache the responses in memory, on disk, or both?
3) When you say 'invalidate', do you mean purge (i.e. remove the cached 
entry from the cache)?
4) How are you intending on purging the caches?  Here are some possible 
ways with pros/cons

- using ttl's (expiry times) for each cached object individually
    Pros : provides the most granularity and will probably give the 
smoothest load to backends
    Cons : adds slight overhead to the request, and complexity in 
writing it (if the caching part is done from scratch)

- purging all objects at once whenever you wanted
    Pros : very simple to implement
    Cons : might cause a temporary rush on the backends when objects are 

- purging a subsection of the objects on a regular basis (e.g. 1/32 of 
the objects every 30 mins)
    Pros : very simple to implement, much less of a rush on backends 
after purges
    Cons : limited granularity

5) How many objects are we looking at caching roughly?  thousands, 
millions, tens/hundreds of millions?

6) How many worker processes do you/will you use on the Nginx 
installation (if you use more than 1, then this adds complexity to the 
caching process, and makes memory caching a pain - to the extent that 
you might as well use memcached for it).

7) Are you definitely looking at having an internal cache, or would you 
be happy with using memcached?  One idea I thought of is to check for 
cached pages  memcached first for a cached header, then serving from the 
backend if no header-only object is found.  A header-only response would 
then put the object in memcached.
    Pros : you could put objects/control the cache separately to the 
Nginx installation, perhaps saving some overhead
    Cons : for non-header-only objects, there would be the slight extra 
overhead of checking memcached on each request (which would obviously be 
slower                 than using an in-memory cache)

8) What kind of load does your server get (average req/s etc)?

I will probably have a few more questions later, but the answers to the 
above should give me a better idea of what you want, and give me a 
better idea of a good way to implement your task.



Latesh Galia wrote:
> Here are some details about the module that we want to sponsor 
> development for:
> 1. We want to use nginx as a reverse proxy for a Domain Forwarding 
> Service
> 2. This would require nginx to cache responses that contain only 
> headers and no body (301 HTTP responses)
> 3. We also need a way to invalidate caches. This should ideally be a 
> command line tool.
> Regards,
> Latesh Galia
> Software Engineer,
> Directi

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