Wcol-E supports hierarchical cache. This page describes what is supported in Wcol-E.
Keywords: Parent, Sibling, Query, Notify

Family

figure of family Hierarchical cache is made to reduce traffic. It consists of parent proxy server and child proxy server. `Parent' means `proxy of proxy'. Parent access remote resource by other proxy's requests. `Child' means `client of parent'. In this page, we call it `family'.

Relationship

figure of relation The relationship among proxies are only two type which are called parent and sibling.
parent
'Parent' specify parent proxy of the proxy. The proxy retrieves resources via parent.

sibling
'Sibling' specify to query existence of cache on other proxies called sibling. The proxy query existence of cache on sibling proxies via ICP (ICP_QUERY) before resource retrieval.

If cache exists on siblings, they reply ICP_HIT. The proxy retrieves the resource from one of siblings.

Fetching and prefetching

Type of resource retrievals in Wcol are fetching and prefetching. FetchParent and PrefetchParent specify parent in fetching and prefetching. If FetchParent is disable, Wcol accesses original server directly. If PrefetchParent is disable, Wcol use FetchParent as PrefetchParent.

You can specify parent via editing of wcol.conf. 

    FetchParent http://high-response.no-disk.net:3128/
    PrefetchParent http://low-response.large-disk.net:3128/

Query and notify

Type of sibling instruction among proxies in Wcol are query and notify. If query is enable, the proxy sent ICP_QUERY to that sibling at each retrievals. If notify is enable, the proxy sent ICP_NOTIFY to that sibling at each retrievals. 

    #       hostname        type         HTTP ICP
    Sibling sibling.com     query        3128 3130  # query only
    Sibling listener.com    query,notify 3128 3130  # query and notify

Query

Query, checking of cache existence in siblings is designed reducing of HTTP traffic. If cache of requested resource is exist in near proxies, the system can retrieve quickly without backbone traffic. The checking consists of ICP_QUERY (ICP opcode=1), ICP_HIT (opcode=2) and ICP_MISS (opcode=3).

The proxy query with ICP_QUERY. Queried system replys ICP_HIT when cache of specified resource is exist, otherwise ICP_MISS.

Notify

Notify, send handling URL is designed extention of the hierarchical cache. Wcol use it to hints of prefetching.

ICP_NOTIFY (ICP opcode=12) carries URL like as ICP_QUERY.