On 7/20/21 3:18 PM, Ilya Maximets wrote:
> On 7/13/21 5:03 PM, Dumitru Ceara wrote:
>> On 7/12/21 10:36 PM, Ilya Maximets wrote:
>>> On 6/29/21 1:20 PM, Dumitru Ceara wrote:
>>>> Until now clients that needed to reconnect immediately could only use
>>>> reconnect_force_reconnect(). However, reconnect_force_reconnect()
>>>> doesn't reset the backoff for connections that were alive long enough
>>>> (more than backoff seconds).
>>>>
>>>> Moreover, the reconnect library cannot determine the exact reason why a
>>>> client wishes to initiate a reconnection. In most cases reconnection
>>>> happens because of a fatal error when communicating with the remote,
>>>> e.g., in the ovsdb-cs layer, when invalid messages are received from
>>>> ovsdb-server. In such cases it makes sense to not reset the backoff
>>>> because the remote seems to be unhealthy.
>>>>
>>>> There are however cases when reconnection is needed for other reasons.
>>>> One such example is when ovsdb-clients require "leader-only" connections
>>>> to clustered ovsdb-server databases. Whenever the client determines
>>>> that the remote is not a leader anymore, it decides to reconnect to a
>>>> new remote from its list, searching for the new leader. Using
>>>> jsonrpc_force_reconnect() (which calls reconnect_force_reconnect()) will
>>>> not reset backoff even though the former leader is still likely in good
>>>> shape.
>>>>
>>>> Since 3c2d6274bcee ("raft: Transfer leadership before creating
>>>> snapshots.") leadership changes inside the clustered database happen
>>>> more often and therefore "leader-only" clients need to reconnect more
>>>> often too. Not resetting the backoff every time a leadership change
>>>> happens will cause all reconnections to happen with the maximum backoff
>>>> (8 seconds) resulting in significant latency.
>>>>
>>>> This commit also updates the Python reconnect and IDL implementations
>>>> and adds tests for force-reconnect and graceful-reconnect.
>>>>
>>>> Reported-at: https://bugzilla.redhat.com/1977264
>>>> Signed-off-by: Dumitru Ceara <[email protected]>
>>>> ---
>>>
>>> Hi, Dumitru.
>>>
>>
>> Hi Ilya,
>>
>>> Thanks for working on this issue. I've seen it in practice while running
>>> OVN tests, but I still don't quiet understand why it happens. Could you,
>>> please, describe how state transitioning work here for the ovsdb-idl case?
>>>
>>
>> Without the patch, assuming a current backoff of X seconds, the sequence
>> of events (even for a connection that has seen activity after backoff)
>> is:
>>
>> - reconnect_force_reconnect() -> move to S_RECONNECT
>> - ovsdb_cs_run()
>> -> jsonrpc_session_run()
>> -> reconnect_run()
>> -> reconnect_disconnected()
>> -> because state is not S_ACTIVE | S_IDLE backoff is not
>> changed, stays X.
>
> Hmm, I see. Thanks for explanation!
>
>>
>>>> +# Forcefully reconnect.
>>>> +force-reconnect
>>>> + in RECONNECT for 0 ms (2000 ms backoff)
>>>> + 1 successful connections out of 3 attempts, seqno 2
>>>> + disconnected
>>>> +run
>>>> + should disconnect
>>>> +connecting
>>>> + in CONNECTING for 0 ms (2000 ms backoff)
>>>
>>> Especially this part seems wrong to me. Because after 'should disconnect'
>>> there should be 'disconnect' of 'connect-fail', but not 'connecting'. We
>>> literally should disconnect here, otherwise it's a violation of the
>>> reconnect
>>> API. And my concern is that ovsdb-cs or jsonrpc violates the API somewhere
>>> by not calling reconnect_disconnectd() when it is required, or there is some
>>> other bug that makes 'reconnect' module to jump over few states in a fsm.
>>>
>>> The logical workflow for the force-reconnect, from what I see in the code
>>> should be:
>>>
>>> 1. force-reconnect --> transition to S_RECONNECT
>>> 2. run -> in S_RECONNECT, so returning RECONNECT_DISCONNECT
>>> 3. disconnect -> check the state, update backoff and transition to S_BACKOFF
>>> 4. run -> in S_BACKOFF, so returning RECONNECT_CONNECT
>>> 5. connected ....
>>
>> This is just a bug in the test case I added, I need to issue
>> "disconnect" in the test case to trigger reconnect_disconnected()
>> to be called (like ovsdb-cs does).
>>
>>>
>>> Something is fishy here, because ovsdb-cs somehow jumps over step #3 and
>>> maybe also #4.
>>
>> As mentioned above, it's not the case for ovsdb-cs.
>>
>> However, while checking the test case I realized that the patch will
>> cause all "graceful" reconnects to happen with an initial backoff of 2
>> seconds for long lived sessions (instead of 1s).
>>
>> That's because:
>>
>> reconnect_graceful_reconnect()
>> -> reconnect_reset_backoff__()
>> -> session was active after the initial 'backoff' seconds, so
>> reset backoff to minimum.
>> -> reconnect_transition__(..., S_RECONNECT);
>> ovsdb_cs_run()
>> -> jsonrpc_session_run()
>> -> reconnect_run()
>> -> reconnect_disconnected():
>> -> if (!reconnect_reset_backoff__(..)) then double backoff.
>>
>> I see a couple of potential ways to fix that:
>> 1. reconnect_graceful_reconnect() could bump fsm->backoff_free_tries to
>> allow a single backoff free reconnect.
>> 2. reconnect_graceful_reconnect() could reset fsm->backoff to 0 if it
>> was active recently (instead of calling reconnect_reset_backoff__()).
>>
>> However, this is all under the assumption that we want to add support
>> for two types of reconnect "semantics":
>>
>> a. forced, e.g., when the client detects inconsistencies in the data
>> received from the server (the server is in "bad shape") in which we
>> should never reset the backoff.
>>
>> b. graceful, e.g., when the client reconnects because it needs a
>> leader-only connection and a leadership transfer happened on the
>> server side (the server is likely in "good shape" just not a leader
>> anymore).
>>
>> An alternative to all this is to change reconnect_disconnected() to
>> reset backoff for connections in states S_ACTIVE | S_IDLE | S_RECONNECT
>> (used to be just active and idle).
>>
>> I guess we could treat this as a bug fix and continue discussion for a
>> separate follow up patch to add the "graceful vs force" semantics if
>> they turn out to make sense for the future.
>>
>> In essence (excluding potential python and test changes) the patch would
>> become:
>>
>> diff --git a/lib/reconnect.c b/lib/reconnect.c
>> index a929ddfd2d..3a6d93f9d1 100644
>> --- a/lib/reconnect.c
>> +++ b/lib/reconnect.c
>> @@ -385,7 +385,7 @@ reconnect_disconnected(struct reconnect *fsm, long long
>> int now, int error)
>> if (fsm->backoff_free_tries > 1) {
>> fsm->backoff_free_tries--;
>> fsm->backoff = 0;
>> - } else if (fsm->state & (S_ACTIVE | S_IDLE)
>> + } else if (fsm->state & (S_ACTIVE | S_IDLE | S_RECONNECT)
>> && (fsm->last_activity - fsm->last_connected >=
>> fsm->backoff
>> || fsm->passive)) {
>> fsm->backoff = fsm->passive ? 0 : fsm->min_backoff;
>> ---
>>
>> What do you think?
>
> The problem I see with adding S_RECONNECT to this condition is that
> 'last_activity' and 'last_connected' doesn't belong to current
> connection in case we're forcefully interrupting connection in a
> S_CONNECTING state. So, we will re-set backoff based on outdated
> information. Also, I think, in this case, 'last_connected' might
> be larger than 'last_activity' and we will have a negative value here.
You're right, I missed this.
> All values are signed, so it should not be an issue, but it's not
> a clean solution.
It's not, I agree.
>
> Bumping the number of free tries seems like a better solution to me,
> because:
>
> 1. I'm not sure that we need 2 types of reconnect. I mean, backoff
> is intended to avoid overloading the already overloaded server with
> connection attempts. In case of forceful re-connection the server
> is not overloaded, so we should not increase a backoff.
> IMO, backoff should only be involved in cases where we have
> problems on the server side related to load, not the leader-only
> stuff or even database inconsistency problems.
>
> 2. force-reconnect should not consume free tries for exactly same
> reasons as in point 1. Free tries are to avoid backoff, but
> backoff exists only to avoid storming the remote server with
> connections while it's overloaded.
> No overload -> No need for backoff -> No need to consume free tries.
>
> 3. With bumping of free tries we're avoiding logical issues around
> using 'last_activity' and 'last_connected' from the old connection.
>
> 4. Faster reconnection, since backoff will be zero. And that is
> fine, because as far as we know, the server is not overloaded,
> it's just not suitable for our needs for some reason.
> If it is overloaded, we will backoff after the first failed
> connection attempt.
This is even better (instead of min_backoff, I mean).
>
> One potential problem I see with this solution is that if there
> are no servers on a list that are suitable for ovsdb-cs, we will
> have a log full of annoying reconnect logs. Backoff kind of
> rate-limits ovsdb-cs right now and we have no internal rate-limit
> inside ovsdb-cs. This looks more like an issue of ovsdb-cs and
> not the issue of reconnect module itself. But we need to have a
> solution for this.
>
> And actually, the problem here is that from the jsonrpc point of
> view the connection is perfectly fine and functional, but ovsdb-cs
> implements high-level logic on top of it and decides that
> connection is not good on a much later stage based on the actual
> received data. So, we need to somehow propagate this information
> from ovsdb-cs down to reconnect module or allow ovsdb-cs to control
> the state machine, otherwise we will need two separate backoffs:
That's what I was trying to implement with the "graceful reconnect:
allow ovsdb-cs to decide how to reconnect. But there are corner cases
like the ones pointed out during the review of this patch.
> one inside the reconnect for usual connection problems and one
> in ovsdb-cs for high-level data inconsistencies and leader changes.
>
> Thinking this way led me to a different solution. We could expose
> something like jsonrpc_session_set_backoff_free_tries() and allow
> ovsdb-cs to make a decision. E.g.:
>
> diff --git a/lib/ovsdb-cs.c b/lib/ovsdb-cs.c
> index f13065c6c..900597b96 100644
> --- a/lib/ovsdb-cs.c
> +++ b/lib/ovsdb-cs.c
> @@ -729,6 +729,17 @@ void
> ovsdb_cs_force_reconnect(struct ovsdb_cs *cs)
> {
> if (cs->session) {
> + if (cs->state == CS_S_MONITORING) {
> + /* The ovsdb-cs was in MONITORING state, so we either had data
> + * inconsistency on this server, or it stopped being the cluster
> + * leader, or the user requested to re-connect. Avoiding backoff
> + * in these cases, as we need to re-connect as soon as possible.
> + * Connections that are not in MONITORING state should have their
> + * backoff to avoid constant flood of re-connection attempts in
> + * case there is no suitable database server. */
> + jsonrpc_session_set_backoff_free_tries(
> + cs->session, jsonrpc_session_get_n_remotes(cs->session));
> + }
> jsonrpc_session_force_reconnect(cs->session);
> }
> }
> ---
>
> This way, if the server looses leadership or inconsistency detected,
> the client will have 3 free attempts to find a new suitable server.
> After that it will start to backoff as it does now. No changes in
> reconnect module required.
>
> Thoughts?
This works for me. I just have a question regarding the new API: should
we allow jsonrpc users to set the free tries to any value or shall we
make it more strict, e.g., jsonrpc_session_reset_backoff_free_tries(),
which would reset the number of free tries to 'n_remotes'?
Will you be sending a patch or shall I add your "Suggested-by"?
>
> Best regards, Ilya Maximets.
>
Thanks,
Dumitru
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