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Failover architecture
Failover architecture

Failover architecture

Failover architecture

An important concept in cluster topology is appointing a leader. Leader is an instance which is responsible for performing key operations. To keep things simple, you can think of a leader as of the only writable master. Every replica set has its own leader, and there’s usually not more than one.

Which instance will become a leader depends on topology settings and failover configuration.

An important topology parameter is the failover priority within a replica set. This is an ordered list of instances. By default, the first instance in the list becomes a leader, but with the failover enabled it may be changed automatically if the first one is malfunctioning.

When Cartridge configures roles, it takes into account the leadership map (consolidated in the failover.lua module). The leadership map is composed when the instance enters the ConfiguringRoles state for the first time. Later the map is updated according to the failover mode.

Every change in the leadership map is accompanied by instance re-configuration. When the map changes, Cartridge updates the read_only setting and calls the apply_config callback for every role. It also specifies the is_master flag (which actually means is_leader, but hasn’t been renamed yet due to historical reasons).

It’s important to say that we discuss a distributed system where every instance has its own opinion. Even if all opinions coincide, there still may be races between instances, and you (as an application developer) should take them into account when designing roles and their interaction.

The logic behind leader election depends on the failover mode: disabled, eventual, or stateful.

This is the simplest case. The leader is always the first instance in the failover priority. No automatic switching is performed. When it’s dead, it’s dead.

In the eventual mode, the leader isn’t elected consistently. Instead, every instance in the cluster thinks that the leader is the first healthy instance in the failover priority list, while instance health is determined according to the membership status (the SWIM protocol).

The member is considered healthy if both are true:

  1. It reports either ConfiguringRoles or RolesConfigured state;
  2. Its SWIM status is either alive or suspect.

A suspect member becomes dead after the failover_timout expires.

Leader election is done as follows. Suppose there are two replica sets in the cluster:

  • a single router «R»,
  • two storages, «S1» and «S2».

Then we can say: all the three instances (R, S1, S2) agree that S1 is the leader.

The SWIM protocol guarantees that eventually all instances will find a common ground, but it’s not guaranteed for every intermediate moment of time. So we may get a conflict.

For example, soon after S1 goes down, R is already informed and thinks that S2 is the leader, but S2 hasn’t received the gossip yet and still thinks he’s not. This is a conflict.

Similarly, when S1 recovers and takes the leadership, S2 may be unaware of that yet. So, both S1 and S2 consider themselves as leaders.

Moreover, SWIM protocol isn’t perfect and still can produce false-negative gossips (announce the instance is dead when it’s not).

Similarly to the eventual mode, every instance composes its own leadership map, but now the map is fetched from an external state provider (that’s why this failover mode called «stateful»). Nowadays there are two state providers supported – etcd and stateboard (standalone Tarantool instance). State provider serves as a domain-specific key-value storage (simply replicaset_uuid -> leader_uuid) and a locking mechanism.

Changes in the leadership map are obtained from the state provider with the long polling technique.

All decisions are made by the coordinator – the one that holds the lock. The coordinator is implemented as a built-in Cartridge role. There may be many instances with the coordinator role enabled, but only one of them can acquire the lock at the same time. We call this coordinator the «active» one.

The lock is released automatically when the TCP connection is closed, or it may expire if the coordinator becomes unresponsive (in stateboard it’s set by the stateboard’s --lock_delay option, for etcd it’s a part of clusterwide configuration), so the coordinator renews the lock from time to time in order to be considered alive.

The coordinator makes a decision based on the SWIM data, but the decision algorithm is slightly different from that in case of eventual failover:

  • Right after acquiring the lock from the state provider, the coordinator fetches the leadership map.
  • If there is no leader appointed for the replica set, the coordinator appoints the first leader according to the failover priority, regardless of the SWIM status.
  • If a leader becomes dead, the coordinator makes a decision. A new leader is the first healthy instance from the failover priority list. If an old leader recovers, no leader change is made until the current leader down. Changing failover priority doesn’t affect this.
  • Every appointment (self-made or fetched) is immune for a while (controlled by the IMMUNITY_TIMEOUT option).

In this case instances do nothing: the leader remains a leader, read-only instances remain read-only. If any instance restarts during an external state provider outage, it composes an empty leadership map: it doesn’t know who actually is a leader and thinks there is none.

An active coordinator may be absent in a cluster either because of a failure or due to disabling the role everywhere. Just like in the previous case, instances do nothing about it: they keep fetching the leadership map from the state provider. But it will remain the same until a coordinator appears.

It differs a lot depending on the failover mode.

In the disabled and eventual modes, you can only promote a leader by changing the failover priority (and applying a new clusterwide configuration).

In the stateful mode, the failover priority doesn’t make much sense (except for the first appointment). Instead, you should use the promotion API (the Lua cartridge.failover_promote or the GraphQL mutation {cluster{failover_promote()}}) which pushes manual appointments to the state provider.

The stateful failover mode implies consistent promotion: before becoming writable, each instance performs the wait_lsn operation to sync up with the previous one.

Information about the previous leader (we call it a vclockkeeper) is also stored on the external storage. Even when the old leader is demoted, it remains the vclockkeeper until the new leader successfully awaits and persists its vclock on the external storage.

If replication is stuck and consistent promotion isn’t possible, a user has two options: to revert promotion (to re-promote the old leader) or to force it inconsistently (all kinds of failover_promote API has force_inconsistency flag).

Consistent promotion doesn’t work for replicasets with all_rw flag enabled and for single-instance replicasets. In these two cases an instance doesn’t even try to query vclockkeeper and to perform wait_lsn. But the coordinator still appoints a new leader if the current one dies.

Neither eventual nor stateful failover modes don’t protect a replicaset from the presence of multiple leaders when the network is partitioned. But fencing does. It enforces at-most-one leader policy in a replicaset.

Fencing operates as a fiber that occasionally checks connectivity with the state provider and with replicas. Fencing fiber runs on vclockkeepers; it starts right after consistent promotion succeeds. Replicasets which don’t need consistency (single-instance and all_rw) don’t defense, though.

The condition for fencing actuation is the loss of both the state provider quorum and at least one replica. Otherwise, if either state provider is healthy or all replicas are alive, the fencing fiber waits and doesn’t intervene.

When fencing is actuated, it generates a fake appointment locally and sets the leader to nil. Consequently, the instance becomes read-only. Subsequent recovery is only possible when the quorum reestablishes; replica connection isn’t a must for recovery. Recovery is performed according to the rules of consistent switchover unless some other instance has already been promoted to a new leader.

These are clusterwide parameters:

  • mode: «disabled» / «eventual» / «stateful».
  • state_provider: «tarantool» / «etcd».
  • failover_timeout – time (in seconds) to mark suspect members as dead and trigger failover (default: 20).
  • tarantool_params: {uri = "...", password = "..."}.
  • etcd2_params: {endpoints = {...}, prefix = "/", lock_delay = 10, username = "", password = ""}.
  • fencing_enabled: true / false (default: false).
  • fencing_timeout – time to actuate fencing after the check fails (default: 10).
  • fencing_pause – the period of performing the check (default: 2).

It’s required that failover_timeout > fencing_timeout >= fencing_pause.

Use your favorite GraphQL client (e.g. Altair) for requests introspection:

  • query {cluster{failover_params{}}},
  • mutation {cluster{failover_params(){}}},
  • mutation {cluster{failover_promote()}}.

Like other Cartridge instances, the stateboard supports cartridge.argprase options:

  • listen
  • workdir
  • password
  • lock_delay

Similarly to other argparse options, they can be passed via command-line arguments or via environment variables, e.g.:

.rocks/bin/stateboard --workdir ./dev/stateboard --listen 4401 --password qwerty

Besides failover priority and mode, there are some other private options that influence failover operation:

  • LONGPOLL_TIMEOUT (failover) – the long polling timeout (in seconds) to fetch new appointments (default: 30);
  • NETBOX_CALL_TIMEOUT (failover/coordinator) – stateboard client’s connection timeout (in seconds) applied to all communications (default: 1);
  • RECONNECT_PERIOD (coordinator) – time (in seconds) to reconnect to the state provider if it’s unreachable (default: 5);
  • IMMUNITY_TIMEOUT (coordinator) – minimal amount of time (in seconds) to wait before overriding an appointment (default: 15).