Internal fault-tolerant storage

The following steps assume that an instance of MITIGATOR has already been installed. Otherwise, perform installation using one of the following methods.

Before configuring a cluster, you must configure Virtual network(VPN). It needs network connectivity between instances to work. Detailed information on setting up and the necessary access are described at the link.

For fault tolerance, synchronized copies of the database must be physically stored on different servers (replicated). With this scheme, database replicas are stored on the same servers where MITIGATOR instances are running. This saves resources and does not require knowledge of PostgreSQL configuration.

For correct system operation all packet processors must have the same amount of system resources available.

If the cluster is assembled from MITIGATOR instances that previously worked independently, then conflicts may arise during the integration. Therefore, on all instances except the future leader, you must execute the command:

docker-compose down -v

PostgreSQL instances run in a streaming replication scheme active — hot standby. Instead of connecting directly to PostgreSQL, each MITIGATOR connects to a local running program pgfailover that redirects connections to the PostgreSQL replication leader.

If no leader is available, pgfailover makes the leader out of one of the slave replicas according to the given order. The MITIGATOR cluster leader and PostgreSQL replication leader do not have to match.

It is assumed that there is reliable communication between the nodes. If a group of instances is cut off from the replication leader, a new leader (split-brain) will be selected among them. After the connection is restored, you will have to manually delete the data on the cut off part of the instances and re-enter them into the cluster (#recovery).

Metrics for graphs are written by the MITIGATOR leader for all instances for safety. This is given by the list of servers FWSTATS_GRAPHITE_ADDRESS.

Setup

The configuration process is described for two instances and is the same on all instances, except for the specific values of MITIGATOR_OWN_INDEX. For more instances, you need to expand the pgfailover and pgfailover и FWSTATS_GRAPHITE_ADDRESS.

1. Set the MITIGATOR_HOST_ADDRESS=192.0.2.1 variable in the .env file. Where 192.0.2.1 is the host address for this instance.

2. In the .env file, set the variable MITIGATOR_OWN_INDEX=0. Where 0 is the queue ID of this instance to become PostgreSQL Active-server. Must be unique and sequentially increasing on each instance.

   Info

   `MITIGATOR_OWN_INDEX` queue IDs do not map to `own_id` cluster instance IDs.
   

3. In the .env file, set the variables SERVER1=10.8.3.1 and SERVER2=10.8.3.2. Where 10.8.3.1 and 10.8.3.2 are the addresses of the servers inside the VPN that are running the instances.

4. In the MITIGATOR web interface set real IP addresses of packet processors.

   

5. Create docker-compose.failover.yml based on template:

   wget https://docs.mitigator.ru/v24.04/dist/multi/docker-compose.failover.yml

   Editing is needed if more than two instances are used. The extension is done by analogy with the two available in the template.

6. In the .env file, set the COMPOSE_FILE variable like this:

   COMPOSE_FILE=docker-compose.yml:docker-compose.failover.yml

Usage

• Stand with Active base starts as usual:

  docker-compose up -d

• A stand with Standby is initialized with a replica:

  docker-compose run --rm -e PGPORT=15432 postgres standby

  after which it starts as usual:

  docker-compose up -d

If the connection to the leader instance (base in Active mode) is broken, the instance with databases in Standby mode takes its place and becomes the leader. The mechanism for switching the bases of the former leader to Standby mode is not provided by regular PostgreSQL replication. For a two-database scheme, this means stopping replication to the other server until the scheme is manually reconfigured.

Restoring Standby from Active

To switch the former Active and return it to the replication scheme, you must stop the PostgreSQL service and delete the local database data.

1. Stop the PostgreSQL service:

   docker-compose rm -fsv postgres

2. Standby initialization is similar to the first initialization:

   docker-compose run --rm -e PGPORT=15432 postgres standby

   then run as usual:

   docker-compose up -d

Leadership conflict

In the case of split brain, each isolated part of the cluster will have its own replication leader, that is, the cluster will split into several smaller ones (possibly from a single machine).

After pgfailover connectivity is restored, all smaller clusters will find that there are multiple PostgreSQL servers acting as replication leaders. n each of the clusters, an alert will be triggered about this situation, log event «Instance leadership conflict occurred» will be generated.

In the logs of each backend-leader (docker-compose logs backend) there will be a message like this:

time="2021-03-03T19:32:47+03:00" level=error msg=multi-conflict data="{\"primary\":0,\"rivals\":[1],\"sender\":0}" hook=on-multi-conflict

In the data field:

• sender is the index of the instance that notifies on what had happened (MITIGATOR_OWN_INDEX, -index у pgfailover).
• primary is the index of the instance that sender reads as a valid replication leader.
• rivals is a list of instance indexes on which PostgreSQL is running as a replication leader besides primary.

It is necessary to analyze such records in the logs of all instances, choose which one will be the leader, and make standby the rest.