Resource download worker daemon

[pudo]

Superticket: #1397

Write a worker daemon to download all resources from a CKAN instance to a local repository.

Questions

• Do we only want to download openly licensed information? ANS: no, we do everything (though do need to think about this re. IP issues)
• Should we have clever ways to dump APIs? ANS: no.
• Do we respect robots.txt even for openly licensed information? ANS: No (we're not crawling we're archiving)
• Use HTTP/1.1 Caching headers? ANS: if not changed since we last updated don't bother to recache.
  • Complete support for ETags
  • Expires, Max-Age etc.
• Check

Functionality

• Download files via HTTP, HTTPS (will not do FTP)

Process:

1. [Archiver.Update checks queue (automated as part of celery)]
2. Open url and get any info from resource on cache / content-length etc
   1. If FAILURE status: update task_status table (could retry if not more than 3 failures so far). Report task failure in celery
   2. Check headers for content-length and content-type ...
      • IF: content-length > max_content_length: EXIT (store outcomes on task_status, and update resource with size and content-type and any other info we get?)
      • ELSE: check content-type.
        • IF: NOT data stuff (e.g. text/html) then EXIT. (store outcomes and info on resource)
        • ELSE: archive it (compute md5 hash etc)
      • IF: get content-length and content-length unchanged GOTO step 4
3. Archive it: connect to storage system and store it. Bucket: from config, Key: /archive/{timestamp}/{resourceid}/filename.ext
   • Add cache url to resource and updated date
   • Add other relevant info to resource such as md5, content-type etc
4. Update task_status

Optional functionality

• If result object is HTML, search for references to "proper data" (CSV download pages etc.)
• Download from POST forms (accepting licenses or weird proprietary systems)
• Support running on Google Apps Engine to save traffic costs.

Existing work

• ​https://bitbucket.org/okfn/ckanext-qa/overview
• out of date: ​https://bitbucket.org/pudo/ckanextarchive - Old archiver extension, largely experimental.
• out of date: ​https://bitbucket.org/ollyc/ckan/changeset/1b16fbe9aa65 - Openness scores by ollyc

[rgrp]

May only do link-checker and not do full storage in this sprint.

[johnglover]

Almost finished (see ​http://github.com/okfn/ckanext-archiver).

Still to address:

• check headers to see if hash / cache / max-age / expires indicates that the resource does not need to be downloaded.
• add cache url to resource

[johnglover]

Added cache_url and cache_last_updated to resources after archiving.

Not checking for hash value in headers. This process will generally only run when a new resource is added or someone updates a URL, so we don't expect to be regularly downloading the same resource.

We will need something along these lines if this is running as a regular cron job, but in that case the logic will be added to the cron job itself (probably a paster command).