[Tarantool-patches] [PATCH 3/4] replication: retry in case of XlogGapError

[Cyrill Gorcunov]

On Sat, Sep 12, 2020 at 07:25:55PM +0200, Vladislav Shpilevoy wrote:
> Previously XlogGapError was considered a critical error stopping
> the replication. That may be not so good as it looks.
> 
> XlogGapError is a perfectly fine error, which should not kill the
> replication connection. It should be retried instead.
> 
> Because here is an example, when the gap can be recovered on its
> own. Consider the case: node1 is a leader, it is booted with
> vclock {1: 3}. Node2 connects and fetches snapshot of node1, it
> also gets vclock {1: 3}. Then node1 writes something and its
> vclock becomes {1: 4}. Now node3 boots from node1, and gets the
> same vclock. Vclocks now look like this:
> 
>   - node1: {1: 4}, leader, has {1: 3} snap.
>   - node2: {1: 3}, booted from node1, has only snap.
>   - node3: {1: 4}, booted from node1, has only snap.
> 
> If the cluster is a fullmesh, node2 will send subscribe requests
> with vclock {1: 3}. If node3 receives it, it will respond with
> xlog gap error, because it only has a snap with {1: 4}, nothing
> else. In that case node2 should retry connecting to node3, and in
> the meantime try to get newer changes from node1.
> 
> The example is totally valid. However it is unreachable now
> because master registers all replicas in _cluster before allowing
> them to make a join. So they all bootstrap from a snapshot
> containing all their IDs. This is a bug, because such
> auto-registration leads to registration of anonymous replicas, if
> they are present during bootstrap. Also it blocks Raft, which
> can't work if there are registered, but not yet joined nodes.
> 
> Once the registration problem will be solved in a next commit, the
> XlogGapError will strike quite often during bootstrap. This patch
> won't allow that happen.
> 
> Needed for #5287

While indeed this looks like a sane thing to do (ie try to reconnect
in a cycle) when such error happens it breaks backward compatibility
because state of the node is getting changed.

For me the new behaviour is more sane and valid but I'm a bit nervious
if it could cause problems on third party tools which might rely on
the former behaviour...