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Руководство пользователя / Репликация / Архитектура механизма репликации
Руководство пользователя / Репликация / Архитектура механизма репликации

Архитектура механизма репликации

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Документация находится в процессе перевода и может отставать от английской версии.

Архитектура механизма репликации

Replication mechanism

A pack of instances which operate on copies of the same databases make up a replica set. Each instance in a replica set has a role, master or replica.

A replica gets all updates from the master by continuously fetching and applying its write ahead log (WAL). Each record in the WAL represents a single Tarantool data-change request such as INSERT, UPDATE or DELETE, and is assigned a monotonically growing log sequence number (LSN). In essence, Tarantool replication is row-based: each data-change request is fully deterministic and operates on a single tuple. However, unlike a classical row-based log, which contains entire copies of the changed rows, Tarantool’s WAL contains copies of the requests. For example, for UPDATE requests, Tarantool only stores the primary key of the row and the update operations, to save space.

Invocations of stored programs are not written to the WAL. Instead, records of the actual data-change requests, performed by the Lua code, are written to the WAL. This ensures that possible non-determinism of Lua does not cause replication to go out of sync.

Data definition operations on temporary spaces, such as creating/dropping, adding indexes, truncating, etc., are written to the WAL, since information about temporary spaces is stored in non-temporary system spaces, such as box.space._space. Data change operations on temporary spaces are not written to the WAL and are not replicated.

To create a valid initial state, to which WAL changes can be applied, every instance of a replica set requires a start set of checkpoint files, such as .snap files for memtx and .run files for vinyl. A replica joining an existing replica set, chooses an existing master and automatically downloads the initial state from it. This is called an initial join.

When an entire replica set is bootstrapped for the first time, there is no master which could provide the initial checkpoint. In such case, replicas connect to each other, elect a master, which then creates the starting set of checkpoint files, and distributes it across all other replicas. This is called an automatic bootstrap of a replica set.

When a replica contacts a master (there can be many masters) for the first time, it becomes part of a replica set. On subsequent occasions, it should always contact a master in the same replica set. Once connected to the master, the replica requests all changes that happened after the latest local LSN (there can be many LSNs – each master has its own LSN).

Each replica set is identified by a globally unique identifier, called replica set UUID. The identifier is created by the master which creates the very first checkpoint, and is part of the checkpoint file. It is stored in system space box.space._schema. For example:

tarantool> box.space._schema:select{'cluster'}
---
- - ['cluster', '6308acb9-9788-42fa-8101-2e0cb9d3c9a0']
...

Additionally, each instance in a replica set is assigned its own UUID, when it joins the replica set. It is called an instance UUID and is a globally unique identifier. This UUID is used to ensure that instances do not join a different replica set, e.g. because of a configuration error. A unique instance identifier is also necessary to apply rows originating from different masters only once, that is, implement multi-master replication. This is why each row in the write ahead log, in addition to its log sequence number, stores the instance identifier of the instance on which it was created. But using UUID as such an identifier would take too much space in the write ahead log, thus a shorter integer number is assigned to the instance when it joins a replica set. This number is then used to refer to the instance in the write ahead log. It is called instance id. All identifiers are stored in system space box.space._cluster. For example:

tarantool> box.space._cluster:select{}
---
- - [1, '88580b5c-4474-43ab-bd2b-2409a9af80d2']
...

Here the instance ID is 1 (unique within the replica set), and the instance UUID is 88580b5c-4474-43ab-bd2b-2409a9af80d2 (globally unique).

Using shorter numeric identifiers is also handy to track the state of the entire replica set. For example, box.info.vclock describes the state of replication in regard to each connected peer.

box.info.vclock
---
- {1: 827, 2: 584}
...

Here vclock contains log sequence numbers (827 and 584) for instances with short identifiers 1 and 2.

Replication setup

To enable replication, you need to specify two parameters in a box.cfg{} request:

  • replication parameter which defines the replication source(s), and
  • read_only parameter which is true for a replica and false for a master.

Both these parameters are «dynamic». This allows a replica to become a master and vice versa on the fly with the help of a box.cfg{} request.

Further we’re giving a detailed example of bootstrapping a replica set.

Replication roles: master and replica

Replication role (master or replica) is set in read_only configuration parameter. The recommended role for all-but-one instances in a replica set is «read-only» (replica).

In a master-replica configuration, every change that happens on the master will be visible on the replicas, but not vice versa.

../../../../_images/mr-1m-2r-oneway.svg

A simple two-instance replica set with the master on one machine and the replica on a different machine provides two benefits:

  • failover, because if the master goes down then the replica can take over, and
  • load balancing, because clients can connect to either the master or the replica for read requests.

In a master-master configuration (also called «multi-master»), every change that happens on either instance will be visible on the other one.

../../../../_images/mm-3m-mesh.svg

The failover benefit in this case is still present, and the load-balancing benefit is enhanced, because any instance can handle both read and write requests. Meanwhile, for multi-master configurations, it is necessary to understand the replication guarantees provided by the asynchronous protocol that Tarantool implements.

Tarantool multi-master replication guarantees that each change on each master is propagated to all instances and is applied only once. Changes from the same instance are applied in the same order as on the originating instance. Changes from different instances, however, can mix and apply in a different order on different instances. This may lead to replication going out of sync in certain cases.

For example, assuming the database is only appended to (i.e. it contains only insertions), it is safe to set each instance to a master. If there are also deletions, but it is not mission critical that deletion happens in the same order on all replicas (e.g. the DELETE is used to prune expired data), a master-master configuration is also safe.

UPDATE operations, however, can easily go out of sync. For example, assignment and increment are not commutative, and may yield different results if applied in different order on different instances.

More generally, it is only safe to use Tarantool master-master replication if all database changes are commutative: the end result does not depend on the order in which the changes are applied. You can start learning more about conflict-free replicated data types here.

Replication topologies: cascade, ring and full mesh

Replication topology is set in replication configuration parameter. The recommended topology is a full mesh, because it makes potential failover easy.

Some database products offer cascading replication topologies: creating a replica on a replica. Tarantool does not recommend such setup.

../../../../_images/no-cascade.svg

The problem with a cascading replica set is that some instances have no connection to other instances and may not receive changes from them. One essential change that must be propagated across all instances in a replica set is an entry in box.space._cluster system space with replica set UUID. Without knowing a replica set UUID, a master refuses to accept connections from such instances when replication topology changes. Here is how this can happen:

../../../../_images/cascade-problem-1.svg

We have a chain of three instances. Instance #1 contains entries for instances #1 and #2 in its _cluster space. Instances #2 and #3 contain entries for instances #1, #2 and #3 in their _cluster spaces.

../../../../_images/cascade-problem-2.svg

Now instance #2 is faulty. Instance #3 tries connecting to instance #1 as its new master, but the master refuses the connection since it has no entry for instance #3.

Ring replication topology is, however, supported:

../../../../_images/cascade-to-ring.svg

So, if you need a cascading topology, you may first create a ring to ensure all instances know each other’s UUID, and then disconnect the chain in the place you desire.

A stock recommendation for a master-master replication topology, however, is a full mesh:

../../../../_images/mm-3m-mesh.svg

You then can decide where to locate instances of the mesh – within the same data center, or spread across a few data centers. Tarantool will automatically ensure that each row is applied only once on each instance. To remove a degraded instance from a mesh, simply change replication configuration parameter.

This ensures full cluster availability in case of a local failure, e.g. one of the instances failing in one of the data centers, as well as in case of an entire data center failure.

The maximal number of replicas in a mesh is 32.