On 7/20/21 3:39 PM, Dumitru Ceara wrote:
> 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 <dce...@redhat.com>
>>>>> ---
>>>>
>>>> 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'?
jsonrpc_session_reset_backoff_free_tries() seems better, because ovsdb-cs
doesn't actually set number of free tries for jsonrpc from the start.
Maybe we can name it just jsonrpc_session_reset_backoff() ? I'm not
sure, but I just don't like this very long name.
>
> Will you be sending a patch or shall I add your "Suggested-by"?
I'm OK with "Suggested-by".
Would be also great to have some type of a test for this functionality.
>
>>
>> Best regards, Ilya Maximets.
>>
>
> Thanks,
> Dumitru
>
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