Protecting Database Centric Applications
In the same way that some application require the ability to persist records to disk, for some applications the loss of access to the database means game over - more so than disconnection from the storage.
Cinder-volume is one such application and as it moves towards an active/active model, it is important that a failure in one peer does not represent a SPoF. In the Cinder architecture, the API server has no way to know if the cinder- volume process is fully functional - so they will still recieve new requests to execute.
A cinder-volume process that has lost access to the storage will naturally be unable to complete requests. Worse though is loosing access to the database, as this will means the result of an action cannot be recorded.
For some operations this is ok, if wasteful, because the operation will fail and be retried. Deletion of something that was already deleted is usually treated as a success and re-attempted operations for creating volume will return a new volume. However performing the same resize operation twice is highly problematic since the recorded old size no longer matches the actual size.
Even the safe operations may never complete because the bad cinder-volume process may end up being asked to perform the cleanup operations from its own failures, which would result in additional failures.
Additionally, despite not being recommended, some Cinder drivers make use of locking. For those drivers it is just as crucial that any locks held by a faulty or hung peer can be recovered within a finite period of time. Hence the need for fencing.
Since power-based fencing is so dependant on node hardware and there is always some kind of storage involved, the idea of leveraging the SBD ( Storage Based Death ) project’s capabilities to do disk based heartbeating and poison-pills is attractive. When combined with a hardware watchdog, it is an extremely reliable way to ensure safe access to shared resources.
However in Cinder’s case, not all vendors can provide raw access to a small block device on the storage. Additionally, it is really access to the database that needs protecting not the storage. So while useful, it is still relatively easy to construct scenarios that would defeat SBD.
A New Type of Death
Where SBD uses storage APIs to protect applications persisting data to disk, we could also have one based on SQL calls that did the same for Cinder-volume and other database centric applications.
I therefor propose TBD - “Table Based Death” (or “To Be Decided” depending on how you’re wired).
Instead of heartbeating to a designated slot on a block device, the slots become rows in a small table in the database that this new daemon would interact with via SQL.
When a peer is connected to the database, a cluster manager like Pacemaker can use a poison pill to fence the peer in the event of a network, node, or resource level failure. Should the peer ever loose quorum or its connection to the database, surviving peers can assume with a degree of confidence that it will self terminate via the watchdog after a known interval.
The desired behaviour can be derived from the following properties:
Quorum is required to write poison pills into a peer’s slot
A peer that finds a poison pill in its slot triggers its watchdog and reboots
A peer that looses connection to the database won’t be able to write status information to its slot which will trigger the watchdog
A peer that looses connection to the database won’t be able to write a poison pill into another peer’s slot
If the underlying database looses too many peers and reverts to read-only, we won’t be able to write to our slot which triggers the watchdog
When a peer that looses connection to its peers, the survivors would maintain quorum(1) and write a poison pill to the lost node (1) ensuring the peer will terminate due to scenario (2) or (3)
N seconds is the worst case time a peer would need to either notice a poison pill, or disconnection from the database, and trigger the watchdog. Then we can arrange for services to be recovered after some multiple of
N has elasped in the same way that Pacemaker does for SBD.
While TBD would be a valuable addition to a traditional cluster architecture, it is also concievable that it could be useful in a stand-alone configuration. Consideration should therefor be given during the design phase as to how best consume membership, quorum, and fencing requests from multiple sources - not just a particular application or cluster manager.
Just as in the SBD architecture, we need TBD to be configured to use the same persistent store (database) as is being consumed by the applications it is protecting. This is crucial as it means the same criteria that enables the application to function, also results in the node self-terminating if it cannot be satisfied.
However for security reasons, the table would ideally live in a different namespace and with different access permissions.
It is also important to note that significant design challenges would need to be faced in order to protect applications managed by the same cluster that was providing the highly available database being consumed by TBD. Consideration would particularly need to be given to the behaviour of TBD and the applications it was protecting during shudown and cold-start scenarios. Care would need to be taken in order to avoid unnecessary self-fencing operations and that failure responses are not impacted by correctly handling these scenarios.
 SBD lives under the ClusterLabs banner but can operate without a traditional corosync/pacemaker stack.