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Data change operations on temporary spaces are not written to the WAL and are not replicated.
Data change operations on **replication-local** spaces (:doc:`created </reference/reference_lua/box_schema/space_create>` with ``is_local = true``) are written to the WAL but are not replicated.
To learn how to enable replication, check the :ref:`Bootstrapping a replica set <replication-setup>` guide.
Replication stages
To create a valid initial state, to which WAL changes can be applied, every instance of a replica set requires a start set of :ref:`checkpoint files <index-box_persistence>`, such as ``.snap`` files for memtx and ``.run`` files for vinyl. A replica goes through the following stages:
**Bootstrap** (optional)
When an entire replica set is bootstrapped for the first time, there is no master that could provide the initial checkpoint. In such a case, replicas connect to each other and elect a master. The master creates the starting set of checkpoint files and distributes them to all the other replicas. This is called an **automatic bootstrap** of a replica set.
**Join**
At this stage, a replica downloads the initial state from the master. The master register this replica in the :ref:`box.space._cluster <box_space-cluster>` space. If join fails with a non-critical :ref:`error <error_codes>`, for example, ``ER_READONLY``, ``ER_ACCESS_DENIED``, or a network-related issue, an instance tries to find a new master to join.
On subsequent connections, a replica downloads all changes happened after the latest local LSN (there can be many LSNs – each master has its own LSN).
**Follow**
At this stage, a replica fetches and applies updates from the master's write-ahead log.
You can use the :ref:`box.info.replication[n].upstream.status <box_info_replication>` property to monitor the status of a replica.
Replica set and instance UUIDs
Each replica set is identified by a globally unique identifier, called the **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 the :ref:`box.space._schema <box_space-schema>` system space, 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. The instance UUID is checked 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, to 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 a 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 the system space :ref:`box.space._cluster <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 instance IDs is also handy for tracking the state of the entire replica set. For example, :ref:`box.info.vclock <box_introspection-box_info>` describes the state of replication in regard to each connected peer.
tarantool> box.info.vclock
---
- {1: 827, 2: 584}
...
Here ``vclock`` contains log sequence numbers (827 and 584) for instances with instance IDs ``1`` and ``2``.
If required, you can explicitly specify the instance and the replica set UUID values rather than letting Tarantool generate them. To learn more, see the :ref:`replicaset_uuid <cfg_replication-replicaset_uuid>` configuration parameter description.
Replication roles: master and replica
The replication role (master or replica) is set by the :ref:`read_only <cfg_basic-read_only>` configuration parameter. The recommended role is "read_only" (replica) for all but one instance in the replica set.
In a master-replica configuration, every change that happens on the master will be visible on the replicas, but not vice versa.
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.
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.

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testing_project / Documentation: concepts/replication/repl_architecture — English

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