On 02/06/2013 06:15 PM, Yehuda Sadeh wrote:
On Wed, Feb 6, 2013 at 9:25 AM, Sage Weil <sage@xxxxxxxxxxx> wrote:
One of the goals for cuttlefish is to improvement manageability of the
system. This will invovle both cleaning up the CLI and adding a REST API
to do everything the CLI current does. There are a few implementation
choices to make.
Currenty the 'ceph' tool has a bunch of code to send messages to the
monitor and wait for replies. This is 90% of what users currently can do.
For the most part, the commands are interpreted by the monitor. A small
subset of commands (ceph tell ..., ceph pg <pgid> ...) will send commands
directory to OSDs.
There are two main options for a REST endpoint that we've discussed so
far:
1- Wrap the above in a clean library (probably integrating most of the
code into Objecter/MonClient.. see wip-monc for a start on this). Wrap
libcephadmin in python and make a simple HTTP/REST front-end. Users would
deploy mgmt endpoints in addition to/alongside monitors and everything
else. If they want the rest api at all.
2- Embed a web server in the ceph-mon daemons, and push the current admin
'client' functionality there. Come up with some basic authentication so
that this doesn't break the current security model.
I'm in favor of the more modular and flexible approach, #1.
Also in favour of #1 rather than #2. I believe the monitors should be
as light as possible, and embedding a web server is going on the
opposite direction. Also, relying on libraries to interface with the
monitors allows the users to employ them however they see fit.
Note that neither of these solves the HA issue directly; HTTP is a
client/server protocol, so whoever is using the API can specify only one
server endpoint. If it is a monitor, they'll need to be prepare to fail
over to another in their code, or set up a load balancer. Same goes for
the restful endpoint, if it fails. The difference is that a single
endpoint can proxy to whichever monitors are in quorum, so a much smaller
set of errors (endpoint machine crash, buggy endpoint) affect availability
of the API.
Right. However, that's really an orthogonal issue. It'll be easier
scaling the HTTP endpoints if they're decoupled from the monitors.
Again, I'm with Yehuda on this one. And I would also assume that by
relying on a library, instead of on a embedded web server, this could be
smoothed over by letting the library's monclient hunt for a new monitor,
thus freeing whatever is in the middle from taking on this responsibility.
The somewhat orthogonal question is how to clean up the CLI usage,
parsing, and get equivalence in the new REST API.
One option is to create a basic framework in the monitor so that there is
a table of api commands. The 'parsing' would be regularized and validated
in a generic way. The rest endpoint would pass the URI through in a
generic way (sanitized json?) that can be matched against the same table.
Another option is to support a single set of commands on the monitor side,
and do the REST->CLI or CLI->REST or CLI,REST->json translation on the
client side. The command registry framework above would live in the CLI
utility and REST endpoints instead (or libcephadmin). This means that the
monitor code is simpler, but also means that the libcephadmin or ceph tool
and REST endpoint need to be running the latest code to be able to send
the latest commands to the monitor. It also means multiple places where
the command set is defined (mon and endpoint and cli).
I'd rather keep the clients dumb, not involve them with the actual
configuration logic. Will make our lives easier in the long run.
I'm all for keeping clients dumb, but I believe that the responsibility
of outputting in a human-readable format should be theirs.
My take on this is to keep the current behaviour (client issues a
command and the monitor handles it as it sees fit), but all
communication should be done in json, either to or from the monitors.
This would allow us to provide more information on each result, getting
rid of all the annoying format on the reply messages and simplify a
great deal of code on the monitor end by removing the silly need of
returning on either plain-text or json. We would then let the
client-side libraries deal with converting it to whichever format the
user wants (plain-text, xml, w/e). As for new commands on the monitor
that are not present on the library, replies to said commands could then
be presented just in json, or we could come up with a standardized way
to always convert json into a human-readable format/any other format.
This would however mean to be able to parse json on the monitors (which
we do not currently do, although we do produce json output). I can't
say I have strong feeling for the client->monitor communication to be
done in json, but for sake of coherency I do think it would be best.
-Joao
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