Image handling in edge environment
I have a few questions (or rather just thoughts) for the "new synchronization service" discussed earlier in [0]:
... and some concerns for the database replication capabilities to support always available autonomy (AAA requirement, further on) of cloud instances, which is (IIUC) an ability to make progress (i.e. DB records in the local cloud DB), when it is managed locally and is disconnected from the central cloud/control plane. Here it is. We *will* need that new sync service, for any service that is expected to sync its state from edge cloud instances to the central cloud of clouds and support that AAA requirement. And highly likely a new or additional data storage solution for that purpose as well. At very least in order to replicate state changes right and resolve conflicts, inevitable for the described AAA case. The issue is mysql/mariadb/galera database transactional consitency models [1], like read committed or repeatable read, do not support that AAA scenario well. Repeatable read transaction isolation level (TI), and stronger consistency models, are "Unavailable" by design (see [1] for the terminology I'm using here). That means it may require the progress to be stopped (no writes accepted and, sometimes, no reads) to maintain DB consistency for the corner cases, like being partitioned/disconnected off the central cloud. But that is a totally accepted situation for AAA, and it expects writes to proceed for edge cloud instances! On the other hand, the read commited transaction isolation level, while "Totally available", provides really poor consistency guarantees IMO, comparing to the better options (see "Sticky Available"). So I believe that new sync service, backed with another data/KV storage solution, must support the best of existing AA consistency models, like "Sticky Available" causal/RTC [2],[3]. While galera et al inter-sites replication of DB, like it was proposed [4] for Keystone federation might not fit the aforementioned consistency requirements/constraints. Its time to consider tools and options for AAA requirement. WDYT? [0] http://lists.openstack.org/pipermail/edge-computing/2018-July/000322.html [1] http://jepsen.io/consistency [2] http://jepsen.io/consistency/models/causal [3] http://www.cs.cornell.edu/lorenzo/papers/cac-tr.pdf [4] https://wiki.openstack.org/wiki/Keystone_edge_architectures#Keystone_databas...
-- Best regards, Bogdan Dobrelya, Irc #bogdando
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Bogdan Dobrelya