On Fri, May 18, 2018 at 02:33:32PM +0200, Pavel Březina wrote:
I sent a mail about new sbus implementation (I'll refer to it as sbus2) .
Now, I'm integrating it into SSSD. The work is quite difficult since it
touches all parts of SSSD and the changes are usually interconnected but I'm
slowly moving towards the goal .
At this moment, I'm trying to take "miminum changes" paths so the code can
be built and function with sbus2, however to take full advantage of it, it
will take further improvements (that will not be very difficult).
There is one big change that I would like to take though, that needs to be
discussed. It is about how we currently handle sbus connections.
In current state, monitor and each backend creates a private sbus server.
The current implementation of a private sbus server is not a message bus, it
only serves as an address to create point to point nameless connection. Thus
each client must maintain several connections:
- each responder is connected to monitor and to all backends
- each backend is connected to monitor
- we have monitor + number of backends private servers
- each private server maintains about 10 active connections
This has several disadvantages - there are many connections, we cannot
broadcast signals, if a process wants to talk to other process it needs to
connect to its server and maintain the connection. Since responders do not
currently provider a server, they cannot talk between each other.
sbus2 implements proper private message bus. So it can work in the same way
as session or system bus. It is a server that maintains the connections,
keep tracks of their names and then routes messages from one connection to
My idea is to have only one sbus server managed by monitor. Other processes
will connect to this server with a named connection (e.g. sssd.nss,
sssd.backend.dom1, sssd.backend.dom2). We can then send message to this
message bus (only one connection) and set destination to name (e.g. sssd.nss
to invalidate memcache). We can also send signals to this bus and it will
broadcast it to all connections that listens to this signals. So, it is
proper way how to do it. It will simplify things and allow us to send
signals and have better IPC in general.
I know we want to eventually get rid of the monitor, the process would stay
as an sbus server. It would become a single point of failure, but the
process can be restarted automatically by systemd in case of crash.
Also here is a bonus question - do any of you remember why we use private
server at all?
In the very original design there was a "switch-board" process which
received a request from one component and forwarded it to the right
target. I guess at this time we didn't know a lot about DBus to
implement this properly. In the end we thought it was a useless overhead
and removed it. I think we didn't thought about signals to all components
or the backend sending requests to the frontends.
Why don't we connect to system message bus?
Mainly because we do not trust it to handle plain text passwords and
other credentials with the needed care.
I do not see any benefit in having a private server.
If I understood you correctly we not only have 'a' private server but 4
for a typically minimal setup (monitor, pam, nss, backend).
Given your arguments above I think using a private message bus would
have benefits. Currently two questions came to my mind. First, what
happens to ongoing requests if the monitor dies and is restarted. E.g.
If the backend is processing a user lookup request and the monitor is
restarted can the backend just send the reply to the freshly stared
instance and the nss responder will finally get it? Or is there some
state lost which would force the nss responder to resend the request?
The second is about the overhead. Do you have any numbers how much
longer e.g. the nss responder has to wait e.g. for a backend if offline
reply? I would expect that we loose more time at other places,
nevertheless it would be good to have some basic understanding about the
Thank you for your hard work on sbus2.
>  https://github.com/pbrezina/sbus
>  https://github.com/pbrezina/sssd/tree/sbus
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