[Hawkular-dev] RfC: Availability

Hey, there was apparently some watercooler discussion yesterday without any minutes, so the following will not be able to refer to it in any way. Hawkular needs to have a way to store, retrieve and display availability of a resource or a bunch of them [1]. While we have some short term goals, for the longer run we need to better identify what needs to be done. I think we need to separately look at the following concerns: * availability reporting * api *values * availability computation * availability storage * availability retrieval * alerting on availability * computed resource state The basic assumption here is that availability is something relatively stable. Meaning that usually the same state (hopefully "UP") is reported each time in a row for a veeery long period of time (there are some servers with uptimes >> 1 year). == reporting Feeds report availability to Hawkular, where the data may be further processed and stored. The reported values are probably in the range of "UP", "DOWN". I can also imagine that e.g. an application server that starts shutting down could send a "GOING_DOWN" value. On the API side, we need to be able to receive (a list of) tuples `< resource id, report time, state >` In case of full Hawkular, the _resource id_ needs to be a valid one from Inventory. _Report time_ is the local time on the resource / agent when that state was retrieved, represented in ms since the epoch UTC and then finally the _state_ which would be an Enum of "UP", "DOWN" and potentially some other values. While I have described them as string here, the representation on the wire may be implemented differently like 1 and 0 or true and false. == computed availability In addition to above reporting we may have feeds that either are not able to deliver availability or where the availability is delivered as a numeric value - see e.g. the pinger, where a <rid>.status.code is delivered as metric value representing the http status codes. Here we need to be apply a mapping from return code -> availability. f(code) -> code < 400 ? "UP" : "DOWN" and then further proceed with that computed availability value. See also [2] and [3] === "Backfill" As feeds may not report back all the time, we may want to have a watchdog which adds a transition into "UNKNOWN" state. === Admin-down A feed may discover resources that report their state as DOWN but where this is not an issue and rather an administrative decision. Take a network card as example where the card as 8 ports, but only 4 of them are connected. So the other 4 will be reported as DOWN, but in fact they are DOWN on purpose. The admin may mark those interfaces as ADMIN_DOWN, which also implies that further incoming DOWN-reports (what about UP, UNKNOWN?) reports can be ignored until the admin re-enables the interface. This admin-down probably also needs to be marked in inventory. === Maintenance mode On top of the availability we also have maintenance mode which is orthogonal to availability and is more meant for alert suppression and SLA computation. Maintenance mode should not overwrite the recorded or computed availability. We still want to record the original state no matter how maintenance mode is. == Storage As I wrote earlier, the base assumption is that availability is supposed to stay the same for long periods of time. For that reason run-length encoded storage is advised < resource id, state, from , to > The fields are more or less self-explanatory - to would be "null" if the current state continues. This is also sort of what we have done in RHQ, where we have also been running into some issues, (especially as we had a very db-bound approach). One issue is that if you have a transition from UP to DOWN the DB situation looks like this: Start: <rid , UP, from1 , null > up-> Down at time = from2 find tuple <rid, ??, ??, null > and update to <rid, UP, from1, from2> append new tuple <rid, DOWN, from2, null> The other issue is to get the current availability (for display in UI and/or in the previous transition) find tuple <rid, ??, ??, null> which are expensive. The retrieval of the current availability for a resource can be improved by introducing a cache that stores as minimal information <rid, last state>. Another issue that Yak pointed out is that if availability is recorded infrequently and at random points in time, just recording when a transition from UP to DOWN or even UNKNOWN happened may be not enough, as there are scenarios when it is still important to know when we heard the last UP report. So above storage (and cache) tuple needs to be extended to contain the _last heard_ time: < resource id, state, from , to, last_head > In this case, as we do not want to update that record for each incoming availability report, we need to really cache this information and have either some periodic write back to the store or at least when a shutdown listener indicates that Hawkular is going down. In case that we have multiple API endpoints that receive alert reports , this may need to be a distributed cache. == Retrieval Retrieval of availability information may actually a bit more tricky as returning the current availability state, as there will be more information to convey: We have two basic cases * return current availability / resource state : this can probably be answered directly from above mentioned cache * return a timeline between some arbitrary start and end times. Here we need to go out and return all records that satisfy something like ( start_time < requested start && end_time > requested start ) || (start_time > requested start && end_time <= requested_end ) === application / group of resources For applications the situation becomes more complicated as we need to retrieve the state (records) for each involved resource and then compute the total state of the application. Take an app with a load balancer, 3 app servers and a DB then this computation may go like avail ( app ) := UP if all resources are UP MIXED if one app server is not UP DOWN otherwise Actually this may even contain a time component avail ( app , time of day ) := if (business_hours (time of day) ) UP if all resources are UP MIXED if one app server is not UP DOWN otherwise else UP if all resources are UP MIXED if two app servers are not UP DOWN otherwise It may be a good idea to not compute that on the fly at retrieval time, but to add the result as synthetic availability records for the computation into the normal availability processing stream as indicated earlier in the "computed availability" section. This way, the computed information is also available for alerting as input == Alerting on availability Alerting will need to see the (computed) availability data and also the maintenance mode information to be able to alert on * is UP/DOWN/... ( for X time ) * goes UP/DOWN/... With the above I think that alerting should not need to do complex availability calculations on its own, but rather work on the stream of incoming (compute [1] https://issues.jboss.org/browse/HWKMETRICS-35 [2] http://lists.jboss.org/pipermail/hawkular-dev/2015-March/000413.html [3] http://lists.jboss.org/pipermail/hawkular-dev/2015-March/000402.html