Splitting the query and job execution paths¶
- Created
2012-May-07
- Status
Implemented
- Ganeti-Version
2.7.0, 2.8.0, 2.9.0
Introduction¶
Currently, the master daemon does two main roles:
execute jobs that change the cluster state
respond to queries
Due to the technical details of the implementation, the job execution and query paths interact with each other, and for example the “masterd hang” issue that we had late in the 2.5 release cycle was due to the interaction between job queries and job execution.
Furthermore, also because technical implementations (Python lacking read-only variables being one example), we can’t share internal data structures for jobs; instead, in the query path, we read them from disk in order to not block job execution due to locks.
All these point to the fact that the integration of both queries and job execution in the same process (multi-threaded) creates more problems than advantages, and hence we should look into separating them.
Proposed design¶
In Ganeti 2.7, we will introduce a separate, optional daemon to handle queries (note: whether this is an actual “new” daemon, or its functionality is folded into confd, remains to be seen).
This daemon will expose exactly the same Luxi interface as masterd, except that job submission will be disabled. If so configured (at build time), clients will be changed to:
keep sending REQ_SUBMIT_JOB, REQ_SUBMIT_MANY_JOBS, and all requests except REQ_QUERY_* to the masterd socket (but also QR_LOCK)
redirect all REQ_QUERY_* requests to the new Luxi socket of the new daemon (except generic query with QR_LOCK)
This new daemon will serve both pure configuration queries (which confd can already serve), and run-time queries (which currently only masterd can serve). Since the RPC can be done from any node to any node, the new daemon can run on all master candidates, not only on the master node. This means that all gnt-* list options can be now run on other nodes than the master node. If we implement this as a separate daemon that talks to confd, then we could actually run this on all nodes of the cluster (to be decided).
During the 2.7 release, masterd will still respond to queries itself, but it will log all such queries for identification of “misbehaving” clients.
Advantages¶
As far as I can see, this will bring some significant advantages.
First, we remove any interaction between the job execution and cluster query state. This means that bugs in the locking code (job execution) will not impact the query of the cluster state, nor the query of the job execution itself. Furthermore, we will be able to have different tuning parameters between job execution (e.g. 25 threads for job execution) versus query (since these are transient, we could practically have unlimited numbers of query threads).
As a result of the above split, we move from the current model, where shutdown of the master daemon practically “breaks” the entire Ganeti functionality (no job execution nor queries, not even connecting to the instance console), to a split model:
if just masterd is stopped, then other cluster functionality remains available: listing instances, connecting to the console of an instance, etc.
if just “luxid” is stopped, masterd can still process jobs, and one can furthermore run queries from other nodes (MCs)
only if both are stopped, we end up with the previous state
This will help, for example, in the case where the master node has crashed and we haven’t failed it over yet: querying and investigating the cluster state will still be possible from other master candidates (on small clusters, this will mean from all nodes).
A last advantage is that we finally will be able to reduce the footprint of masterd; instead of previous discussion of splitting individual jobs, which requires duplication of all the base functionality, this will just split the queries, a more trivial piece of code than job execution. This should be a reasonable work effort, with a much smaller impact in case of failure (we can still run masterd as before).
Disadvantages¶
We might get increased inconsistency during queries, as there will be a delay between masterd saving an updated configuration and confd/query loading and parsing it. However, this could be compensated by the fact that queries will only look at “snapshots” of the configuration, whereas before it could also look at “in-progress” modifications (due to the non-atomic updates). I think these will cancel each other out, we will have to see in practice how it works.
Another disadvantage might be that we have a more complex setup, due to the introduction of a new daemon. However, the query path will be much simpler, and when we remove the query functionality from masterd we should have a more robust system.
Finally, we have QR_LOCK, which is an internal query related to the master daemon, using the same infrastructure as the other queries (related to cluster state). This is unfortunate, and will require untangling in order to keep code duplication low.
Long-term plans¶
If this works well, the plan would be (tentatively) to disable the query functionality in masterd completely in Ganeti 2.8, in order to remove the duplication. This might change based on how/if we split the configuration/locking daemon out, or not.
Once we split this out, there is not technical reason why we can’t execute any query from any node; except maybe practical reasons (network topology, remote nodes, etc.) or security reasons (if/whether we want to change the cluster security model). In any case, it should be possible to do this in a reliable way from all master candidates.
Update: We decided to keep the restriction to run queries on the master node. The reason is that it is confusing from a usability point of view that querying will work on any node and suddenly, when the user tries to submit a job, it won’t work.
Some implementation details¶
We will fold this in confd, at least initially, to reduce the proliferation of daemons. Haskell will limit (if used properly) any too deep integration between the old “confd” functionality and the new query one. As advantages, we’ll have a single daemons that handles configuration queries.
The redirection of Luxi requests can be easily done based on the request type, if we have both sockets open, or if we open on demand.
We don’t want the masterd to talk to the luxid itself (hidden redirection), since we want to be able to run queries while masterd is down.
During the 2.7 release cycle, we can test all queries against both masterd and luxid in QA, so we know we have exactly the same interface and it is consistent.