expire cache task on rubygems repositories may lead to /api/v1/dependecies api performance degradation

Problem

Expiring Cache on a rubygems proxy repository can cause Nexus to make a single HEAD and GET request for every gem and dependency gem referenced by a single request listing gems to /api/v1/dependencies?gems=<list-of-gems>.

Before the Expire cache task is run, Nexus will not do this. Instead it passes the request to the remote as received.

For example:

• request /api/v1/dependencies?gems=foo,bar
• Nexus passes this single request as is, to any remote repository it is proxying
• Expire Cache on the Nexus repository
• sending the same request to Nexus causes Nexus to send both a HEAD and a GET as follows:
  HEAD /api/v1/dependencies?gems=foo
  GET /api/v1/dependencies?gems=foo
  HEAD /api/v1/dependencies?gems=bar
  GET /api/v1/dependencies?gems=bar

For long lists of gems, the many additional outbound requests can lead to a significant drop in response time as compared to when the single request is passed through to the remote 'as is'.

I've attached a jmeter test one can use the reproduce the issue. The attachment also contains a nexus log, conf directory, and csv format of performance data before and after expiring cache.

The key log lines in the attached nexus.log are as follows:

START
2016-01-20 16:52:30,566-0400 DEBUG [qtp699358581-561]  org.eclipse.jetty.server.Server - REQUEST /nexus/content/groups/rubygems.org-group/api/v1/dependencies on AsyncHttpConnection@4b6fa6fb,g=HttpGenerator{s=0,h=-1,b=-1,c=-1},p=HttpParser{s=-5,l=34,c=0},r=1

STOP
2016-01-20 16:56:17,246-0400 DEBUG [qtp699358581-556]  org.eclipse.jetty.server.Server - RESPONSE /nexus/content/groups/rubygems.org-group/api/v1/dependencies  200 handled=true

EXPIRE CACHE TASK RUN

TEST RUN 2 START
2016-01-20 16:59:38,230-0400 DEBUG [qtp699358581-561]  org.eclipse.jetty.server.Server - REQUEST /nexus/content/groups/rubygems.org-group/api/v1/dependencies on AsyncHttpConnection@25b9627a,g=HttpGenerator{s=0,h=-1,b=-1,c=-1},p=HttpParser{s=-5,l=34,c=0},r=1

TEST RUN 2 STOP
2016-01-20 17:06:11,864-0400 DEBUG [qtp699358581-265]  org.eclipse.jetty.server.Server - RESPONSE /nexus/content/groups/rubygems.org-group/api/v1/dependencies  200 handled=true

As you can see, it takes 3m47s for the first run, the second run takes 6m33 seconds for the same set of 20 requests repeated 10 times.