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‎doc/code_snippets/snippets/replication/instances.enabled/box_info_synchro/README.md‎

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# Using box.info.synchro
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A sample application demonstrating how to work with [box.info.synchro](https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_info/synchro/).
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## Running
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To start all instances, execute the following command in the [replication](../../../replication) directory:
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```console
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$ tt start box_info_synchro
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```
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To check the instance status, run:
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```console
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$ tt status box_info_synchro
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```
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To connect to the ``instance001`` instance, run:
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```console
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$ tt connect box_info_synchro:instance001
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```

‎doc/code_snippets/snippets/replication/instances.enabled/box_info_synchro/config.yaml‎

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credentials:
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users:
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replicator:
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password: 'topsecret'
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roles: [replication]
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iproto:
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advertise:
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peer:
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login: replicator
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groups:
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group001:
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replicasets:
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replicaset001:
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instances:
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instance001:
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database:
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mode: rw
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replication:
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synchro_quorum: 2
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synchro_timeout: 1000
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iproto:
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listen:
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- uri: '127.0.0.1:3301'
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instance002:
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database:
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mode: ro
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iproto:
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listen:
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- uri: '127.0.0.1:3302'

‎doc/code_snippets/snippets/replication/instances.enabled/box_info_synchro/instances.yml‎

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instance001:
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instance002:

‎doc/reference/reference_lua/box_info/synchro.rst‎

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confirmed entry waited for the quorum to collect.
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* ``quorum`` -- the resulting value of the
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:ref:`replication_synchro_quorum <cfg_replication-replication_synchro_quorum>` configuration option.
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:ref:`replication.synchro_quorum <configuration_reference_replication_synchro_quorum>` configuration option.
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Since version :doc:`2.5.3 </release/2.5.3>`, the option can be set as a dynamic formula.
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In this case, the value of the ``quorum`` member depends on the current number of replicas.
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That is, synchronous transactions work like asynchronous ones.
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`1` means that a successful WAL writing to the master is enough to commit.
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.. code-block:: console
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.. code-block:: tarantoolsession
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instance001> box.info.synchro
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box_info_synchro:instance001> box.info.synchro
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---
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- queue:
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owner: 1
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**Example 2:**
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First, set a quorum number and a timeout for synchronous replication in the configuration file (``config.yaml``):
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Example on GitHub: `box_info_synchro <https://github.com/tarantool/doc/tree/latest/doc/code_snippets/snippets/replication/instances.enabled/box_info_synchro>`_
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.. code-block:: yaml
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In this example, there are two instances:
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replication:
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synchro_quorum: 2
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synchro_timeout: 1000
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- ``instance001`` is going to be the leader.
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- ``instance002`` is a follower instance.
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.. literalinclude:: /code_snippets/snippets/replication/instances.enabled/box_info_synchro/config.yaml
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:language: yaml
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:start-at: groups
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:end-at: 3302
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:dedent:
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On the **first** instance, grant the user with the ``super`` role:
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.. code-block:: tarantoolsession
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box_info_synchro:instance001> box.schema.user.grant('guest', 'super')
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After that, use ``box.ctl.promote()`` function to claim the queue:
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.. code-block:: tarantoolsession
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box_info_synchro:instance001> box.ctl.promote()
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Create a space called ``sync`` and enable synchronous replication on this space:
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.. code-block:: tarantoolsession
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box_info_synchro:instance001> s = box.schema.space.create("sync", {is_sync=true})
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Then, create an index:
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.. code-block:: tarantoolsession
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box_info_synchro:instance001> _ = s:create_index('pk')
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Check the current state of synchronous replication:
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.. code-block:: console
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.. code-block:: tarantoolsession
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app:instance001> box.info.synchro
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box_info_synchro:instance001> box.info.synchro
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---
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- queue:
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owner: 1
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quorum: 2
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...
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Create a space called ``sync`` and enable synchronous replication on this space:
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On the **second** instance, simulate failure like if this instance would crash or go out of the network:
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.. code-block:: console
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.. code-block:: tarantoolsession
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app:instance001> s = box.schema.space.create("sync", {is_sync=true})
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---
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...
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Then, create an index:
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.. code-block:: console
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app:instance001> _ = s:create_index('pk')
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---
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...
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After that, use ``box.ctl.promote()`` function to claim a queue:
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box_info_synchro:instance002> os.exit(0)
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⨯ Connection was closed. Probably instance process isn't running anymore
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.. code-block:: console
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On the **first** instance, try to perform some synchronous transactions.
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The transactions would hang, because the :ref:`replication.synchro_quorum <configuration_reference_replication_synchro_quorum>`
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option is set to `2`, and the second instance is not available:
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app:instance001> box.ctl.promote()
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.. code-block:: tarantoolsession
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Next, perform data manipulations:
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.. code-block:: console
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app:instance001> require('fiber').new(function() box.space.sync:replace{1} end)
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---
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- status: suspended
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name: lua
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id: 130
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...
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app:instance001> require('fiber').new(function() box.space.sync:replace{1} end)
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box_info_synchro:instance001> fiber = require('fiber')
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---
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- status: suspended
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name: lua
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id: 131
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...
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app:instance001> require('fiber').new(function() box.space.sync:replace{1} end)
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---
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- status: suspended
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name: lua
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id: 132
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box_info_synchro:instance001> for i = 1, 3 do fiber.new(function() box.space.sync:replace{i} end) end
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--- end
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...
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If you call the ``box.info.synchro`` command again,
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you will see that now there are 3 transactions waiting in the queue:
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Call the ``box.info.synchro`` command on the first instance again:
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.. code-block:: console
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.. code-block:: tarantoolsession
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app:instance001> box.info.synchro
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box_info_synchro:instance001> box.info.synchro
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---
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- queue:
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owner: 1
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confirm_lag: 0
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term: 2
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age: 0
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len: 0
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age: 5.2658250015229
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len: 3
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busy: false
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quorum: 2
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...
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The ``len`` field is now equal to 3. It means that there are 3 transactions waiting in the queue.

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