Vitrage HA and History Vision

Vitrage HA and History Vision

Overview

In order to support some of the main future use cases of Vitrage, including full HA support, alarm history and RCA history, we will need to make some architectural changes.

This document contains the required use cases and a high level design for supporting them.

Use Cases

Full HA Support for Vitrage

Vitrage should have full HA support. There are different aspects that should be considered:

  • Vitrage should be able to recover and return to a consistent state after one of its instances failed.
  • Notifications from external datasources (like Nova, Neutron, Zabbix, etc.) should not be lost, even if Vitrage is down.
  • Notifications from Vitrage to external components (like Nova, SNMP) should be sent once Vitrage is recovered.
  • The Sub-graph matching algorithm should recover in case Vitrage was down in the middle of the calculation.
  • In order to support VNF use cases (like OPNFV Doctor), a notification from Vitrage should be sent within one second.

RCA History

Note: This use case covers only the RCA information. See also the next use case of ‘Alarm History’.

At the moment, Vitrage shows Root Cause Analysis only for alarms that are currently triggered. We would like Vitrage to include also information for alarms that were already disabled.

An example: If the host is down, then the instance is down, as well as an application running on it. Later on, the problem of the host might be fixed, but the application might not recover automatically. The cloud operator should be aware of the fact that the alarm on the application resulted from the alarm on the host (even though this alarm no longer exists).

Alarm History

Vitrage should keep alarm history for a specified period of time. This history can be used for analytics or machine learning purposes, as well as to show the user statistics about the alarms in the cloud.

Note: This use case is of a lower priority, and is not answered by the current design. It can be implemented in the future by storing new tables with alarms information in a relational database.

Vitrage Graph Performance

Vitrage should perform well under load. In order to support it, we might want to introduce a persistent graph database as an alternative to the current in-memory implementation with NetworkX.

There are several aspects to this decision:

  • An in-memory implementation is usually faster than working against a persistent database
  • A persistent database, on the other hand, allows multi processing

For now we believe that an in-memory graph database will be faster, so this use case does not require introducing a persistent graph database.

Huge Entity Graph

The in-memory NetworkX graph can work well with XXX number of vertices. In order to support a bigger entity graph, we will have to switch to a persistent graph database.

Vitrage Consistency

The Vitrage entity graph must remain consistent even if Vitrage is down. Note that this is usually the case with the current implementation, since the entity graph is recalculated after every restart. The only exception is that the collectd datasource does not have a ‘get all’ implementation and works only by notifications, so after Vitrage recovers we won’t have the alarms that were previously reported by collectd.

Suggested Architecture

../_images/vitrage-ha-vision.png

The datasource drivers will be responsible for periodically querying the external datasources for all of their resources/alarms. They are already separated from the vitrage-graph process, and run in their own processes. Upon failure of a datasource driver, another driver process will take over calling the ‘get all’ method. A certain delay in the call is not crucial (as by default this method is called every 10 minutes).

The service listeners will be responsible to get notifications from the OpenStack message bus (RabbitMQ1), enrich them and pass them on to the processors. Upon failure, the notifications will remain in the message bus until another service listener gets them.

The current multi-processing queue between the datasource drivers and the processor will be replaced with a RabbitMQ. That way, in case of failure in a processor, the events will be kept in the RabbitMQ until they are processed by another processor.

Events will arrive to the RabbitMQ2 after the filter/enrich phase (done either by the datasource driver or by the service listener). The processor will pass the events to the transformer, as done today.

The persister process will also listen to the RabbitMQ2 (on a different topic) and will asynchronously write the events to a relational database. All events will be stored after the filter/enrich phase. In the first version we will support MariaDB, and we can support other databases if needed in the future.

The processor will be responsible, when it is convenient (i.e. when it is not busy handling events), to export the NetworkX graph as a snapshot into MariaDB. The snapshot frequency should be determined by a combination of the time that passed and the number of events that arrived since the last snapshot.

Reconstructing the graph from the historic data will be controlled by the processor, and will be used in two cases:

  • Upon failure, in order to initiate the standby processor
  • For RCA history

The first phase of the graph reconstruction will be to identify the relevant snapshot in MariaDB and import it. The second phase will be to replay all of the events that happened from the time of the snapshot until the wanted time for the graph reconstruction. Replaying the graph will be done by pushing the relevant events to the RabbitMQ2, as if they arrived from the datasources drivers or from the service listeners.

In order to support the RCA history use case, we will have to reconstruct the graph on a separate graph instance and use a different RabbitMQ, while keeping the current active graph intact.

How The architecture supports the different use cases

Full HA Support for Vitrage

In general, each component will manage its own HA. Specific implementation is required for the processor process. If it fails, a standby will take over. The standby will not be initialized from scratch; instead, it will be initialized in the following way:

  • Start with an empty graph
  • Import the latest stored snapshot
  • Replay all of the events from the time of the latest snapshot and on
  • Start handling the newest events in the queue

TBD: While the processor was down, the persister kept storing events to the database. When the standby processor takes over, the wanted behavior is:

  • Do not send notifications on events that were already processed by the previously-active processor
  • Send notifications on events that were not processed by the previously-active processor yet

We need a way to determine which events were processed and which were not. This is relevant for the Reliable Notification feature that has been discussed in the past, and will be handled as part of the implementation of this feature.

RCA History

Short-term RCA history (~1 day long) can be implemented with the current architecture.

Implementation tasks:

  • In the Consistency process (that is responsible for deleting old vertices), do not delete ‘causes’ vertices that are connected to non-deleted alarms.
  • In the API, return also the disabled alarms with an indication about it.
  • In the UI, display also the disabled alarms with an indication about it.

In order to query RCA for a longer period in the history, we will do the following:

  • Build a separate graph for that purpose
  • Import to that graph the relevant snapshot, e.g. of Sep 14, 17:00
  • Replay all of the events from the time of the snapshot until the wanted time in the past, e.g. Sep 14, 17:26
  • Check the RCA information based on this graph

Alarm History

Will be implemented in the future, probably based on new information that will be stored in the database.

Vitrage Graph Performance

Not affected by this architectural change. Whether a persistent graph DB should be used will be discussed in a different document.

Huge Entity Graph

Will require a persistent and distributed graph DB. Replacing the graph DB should have no effect on the overall architectural change.

Vitrage Consistency

A full consistency will be achieved by the new architecture, since every un-processed notification will be stored in the RabbitMQ, and every processed notification will be stored as an event in MariaDB.

Alternatives

Move the Service Listeners inside the Processor process

The service listeners do very little, they call a single enrich method and pass the event on to the RabbitMQ2. They do not have to run on separate processes. The problem is that if we move the code inside the processor processes, we will have two different sources of information to the processor:

  • RabbitMQ1, before the driver processing
  • RabbitMQ2, after the driver processing

The processor can handle this situation, the problem is with the persister. We would like the persister to store only events after the driver processing, and the easiest way to do so is by having all of the events pushed to RabbitMQ2.

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