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one or more :ref:`router <vshard-architecture-router>` instances.
The number of ``storage`` instances in a replica set defines the redundancy factor of the data. The recommended value is 3 or more. The number of ``router`` instances is not limited, because routers are completely stateless. We recommend increasing the number of routers when an existing ``router`` instance becomes CPU or I/O bound.
``vshard`` supports multiple ``router`` instances on a single Tarantool instance. Each ``router`` can be connected to any ``vshard`` cluster. Multiple ``router`` instances can be connected to the same cluster.
As the ``router`` and ``storage`` applications perform completely different sets of functions, they should be deployed to different Tarantool instances. Although it is technically possible to place the router application on every ``storage`` node, this approach is highly discouraged and should be avoided on production deployments.
All ``storage`` instances can be deployed using identical instance (configuration) files.
All ``router`` instances can also be deployed using identical instance (configuration) files.
All cluster nodes must share a common topology. An administrator must ensure that the configurations are identical. We suggest using a configuration management tool like Ansible or Puppet to deploy the cluster.
Sharding is not integrated into any system for centralized configuration management. It is expected that the application itself is responsible for interacting with such a system and passing the sharding parameters.
The configuration example of a simple sharded cluster is available :ref:`here <vshard-config-cluster-example>`.
Replica weights
The ``router`` sends all read-write requests to the master instance only. Setting replica weights allows sending read-only requests not only to the master instance, but to any available replica that is the 'nearest' to the ``router``. Weights are used to define distances between replicas within a replica set.
Weights can be used, for example, to define the physical distance between the ``router`` and each replica in each replica set. In this case read requests are sent to the nearest replica (with the lowest weight).
Setting weights can also help to define the most powerful replicas: the ones that can process the largest number of requests per second.
The idea is to specify the zone for every ``router`` and every replica, therefore filling a matrix of relative zone weights. This approach allows setting different weights in different zones for the same replica set.
To set weights, use the zone attribute for each replica during configuration:
local cfg = {
sharding = {
['...replicaset_uuid...'] = {
replicas = {
['...replica_uuid...'] = {
...,
zone = <number or string>
}
}
}
}
}
Then, specify relative weights for each zone pair in the ``weights`` parameter of ``vshard.router.cfg``. For example:
weights = {
[1] = {
[2] = 1, -- Routers of the 1st zone see the weight of the 2nd zone as 1.
[3] = 2, -- Routers of the 1st zone see the weight of the 3rd zone as 2.
[4] = 3, -- ...
},
[2] = {
[1] = 10,
[2] = 0,
[3] = 10,
[4] = 20,
},
[3] = {
[1] = 100,
[2] = 200, -- Routers of the 3rd zone see the weight of the 2nd zone as 200.
-- Mind that it is not equal to the weight of the 2nd zone visible
-- from the 1st zone (= 1).
[4] = 1000,
}
}

local cfg = vshard.router.cfg({weights = weights, sharding = ...})
Replica set weights
A replica set weight is not the same as the replica weight. The weight of a replica set defines the capacity of the replica set: the larger the weight, the more buckets the replica set can store. The total size of all sharded spaces in the replica set is also its capacity metric.
You can consider replica set weights as the relative amount of data within a replica set. For example, if ``replicaset_1 = 100``, and ``replicaset_2 = 200``, the second replica set stores twice as many buckets as the first one. By default, all weights of all replica sets are equal.
You can use weights, for example, to store the prevailing amount of data on a replica set with more memory space.
Rebalancing process
There is an **etalon number** of buckets for a replica set. (Etalon in this context means "ideal".) If there is no deviation from this number in the whole replica set, then the buckets are distributed evenly.
The etalon number is calculated automatically considering the number of buckets in the cluster and weights of the replica sets.
Rebalancing starts if the **disbalance threshold of a replica set** exceeds the disbalance threshold :ref:`specified in the configuration <cfg_basic-rebalancer_disbalance_threshold>`.
The disbalance threshold of a replica set is calculated as follows:
|etalon_bucket_number - real_bucket_number| / etalon_bucket_number * 100
For example: The user specified the number of buckets is 3000, and weights of 3 replica sets are 1, 0.5, and 1.5. The resulting etalon numbers of buckets for the replica sets are: 1st replica set – 1000, 2nd replica set – 500, 3rd replica set – 1500.
This approach allows assigning a zero weight to a replica set, which initiates migration of its buckets to the remaining cluster nodes. It also allows adding a new zero-load replica set, which initiates migration of the buckets from the loaded replica sets to the zero-load replica set.
A new zero-load replica set should be assigned a weight for rebalancing to start.

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testing_project / Documentation: book/admin/vshard_admin — English

local cfg = {
sharding = {
['...replicaset_uuid...'] = {
replicas = {
['...replica_uuid...'] = {
...,
zone = <number or string>
}
}
}
}
}
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