Issue #6135 has been updated by Luke Kanies.

This is why I mentioned the possibility of the clients queueing the requests 
themselves - then there's no http connect or anything.

And yeah, it'd be great to have it set up so that clients have a back-off 
algorithm:  If the connection gets reset after 10s or whatever, check back in 
5s, then 10s, then etc.
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Feature #6135: Elastic Compiler Pool
https://projects.puppetlabs.com/issues/6135#change-74344

Author: Luke Kanies
Status: Accepted
Priority: Normal
Assignee: eric sorenson
Category: 
Target version: 
Affected Puppet version: 
Keywords: backlog
Branch: 


Puppet deals poorly with the 'thundering herd' problem today, where a given 
system has sufficient capacity to respond to typical usage but falls down when 
usage goes above that (e.g., 10% of machines check in every minute normally, 
but 100% check in sometimes).

The architecture should be switched to a model where client requests are 
queued, and then a pool of compilers respond to those requests as they're able 
to.  The compiler pool could grow and shrink as needed, and at worst the queue 
would just be longer than one might like, rather than too many clients 
destroying a server.

This has the added benefit of providing visibility of how busy the pool is, by 
assessing queue length.

There are two basic models that could be used:

* Clients use the message bus directly - they queue requests and wait for a 
response to get sent back to them.  This is preferred, but won't always be 
possible because of client architectural limitations (e.g., firewalls).  The 
benefit of this is that there aren't even open sockets on the server as 
requests come in.

* Clients still connect with HTTP to the server, and the server handles 
queueing requests and returning compiled catalogs.  This provides nearly as 
much scalability, and in some ways is preferable because it is fully compatible 
with existing Puppet installations.


This should be accomplishable with just indirector plugins - a new terminus for 
queueing requests and returning catalogs, and a 


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