Hi,
I'm working on implementing a MariaDB resource-agent based on the mysql one.
The idea is to take advantage of new features in MariaDB, especially
semi-synchronous replication and GTID.
GTID (Global Transaction ID) means that there is a counter that applies
to the replicated databases, which is unique within the cluster (there
can be multiple replication clusters with overlapping ID's).
Semi-synchronous replication means that the master will replicate
synchronously to AT LEAST ONE slave, before actually performing the
transaction. In theory there can be no data-loss due to a single node
failure, a big improvement compared to the normal async replication in
MariaDB.
These two sets of technologies should allow for quite a straightforward
set of semantics in the resource-agent.
On master failure, the node with the highest GTID must be the one that
was replicating synchronously, and should be promoted to be the new
master. The question is how to relay the information to crmd.
My current working hypothesis is that I can place the GTID as a
crm-attribute both when starting the resource-agent and in a post-demote
notify. During the subsequent monitor operation the resource-agents can
then scan the the crm-attributes from other nodes and simply prioritise
themselves in relation to others (some relative scoring?).
This requires a few things though:
- If there is no master when the resource agent starts we need to wait
for all nodes to come online (i.e) the cluster is just starting before
promoting any to master, so they can read GTID from the attributes.
- There must be a monitor step after start and demote and before the
promotion of any resource to master, and this must execute on all nodes
so they can set their priority for promotion.
- The post-demote notifier must complete execution before a node can
start the monitor operation. I THINK that it is ok for not all nodes to
have completed the post-demote notifier before the monitor operation
starts, probably this can work by creating a sparse priority
distribution, i.e. First node to execute monitor sets a priority of 100
- the next one down 90 - the next one in the middle at 95, based on the
number of nodes etc.
I hope this doesn't sound too tangled, I will try this out, but I can't
find any clear documentation on the ordering and completion of start,
notifiers, monitor and promote operations as well as master selection,
so all pointers are very much welcome.
And completely alternative suggestions also very much welcome.
Thanks for any and all assistance,
Nils
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