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config: support anonymous replicas #9432

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Totktonada opened this issue Nov 29, 2023 · 0 comments · Fixed by #9418
Closed

config: support anonymous replicas #9432

Totktonada opened this issue Nov 29, 2023 · 0 comments · Fixed by #9418
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@Totktonada
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3.0 Target is 3.0 and all newer release/master branches config

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@Totktonada
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Totktonada commented Nov 29, 2023 •
edited

There is a couple of problems we need to resolve regarding anonymous replicas in the declarative configuration code.

  • An anonymous replica shouldn't attempt to persist its name in _cluster.
  • An anonymous replica shouldn't be used as an upstream for non-anonymous replica.
  • (I would also don't use anonymous replicas as upstreams for other anonymous replicas by default.)
  • An anonymous replica shouldn't be chosen as a boostrap leader in replication.failover: supervised mode.
  • Verify that a replicaset contains at least one non-anonymous instance.
  • Verify that an anonymous replica is not configured in read-write mode (using database.mode or <repicaset>.leader options).
  • Verify that an anonymous replica is not configured with replication.election_mode != off.
  • An anonymous replica should be able to be bootstrapped from a replicaset, where all the instances are in read-only mode (because it doesn't need a write to _cluster to join the replicaset).
@Totktonada Totktonada added 3.0 Target is 3.0 and all newer release/master branches config labels Nov 29, 2023
@Totktonada Totktonada self-assigned this Nov 29, 2023
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: don't try to persist anon replica name
05784df 
The commit effectively enables support of anonymous replicas in the
declarative configuration. It has several caveats (see the changelog
entry), which will be resolved in the following commits of the patchset.

An attempt to persist an instance name of an anonymous replica can't be
successful, because it has no entry in `_cluster` system space. Such an
attempt leads to ER_INSTANCE_NAME_MISMATCH error.

This commit patches the configuration applying logic to skip attempt to
set `box.cfg({instance_name = <...>})` if the instance is configured as
an anonymous replica using `replication.anon: true` option.

Part of tarantool#9432

NO_DOC=replication.anon option is already documented in the scope of
       tarantool/doc#3851. The bugfix
       shouldn't affect the documentation pages much, however related
       constraints are summarized in a documentation request in the last
       commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: don't use an anon replica as an upstream
e2facb0 
This commit effectively allows to set `replication.anon: true` without
specifying `replication.peers`.

Without filtering out anonymous replicas from the list of upstreams we
get an error regarding attempt to use an anonymous replica as an
upstream for a non-anonymous instance.

Also, anonymous replicas are excluded from autogenerated upstream list
for other anonymous replicas. It makes the list the same on all the
peers.

A user can configure a custom data flow using `replication.peers`
option.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: supervised replicaset is now anon aware
7d820fe 
Filter out anonymous replicas when choosing a bootstrap leader in
`replication.failover: supervised` mode. An anonymous replica can't be
in read-write mode, so it can't be a replicaset bootstrap leader.

Part of tarantool#9432

NO_DOC=It is bugfix. However, this detail is mentioned in the
       documentation request is in the last commit of the series just in
       case.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: forbid a replicaset of only anon replicas
b66e9aa 
A replicaset that contains only anonymous replicas can't be
bootstrapped, because all the instances must in read-only mode.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: forbid attempt make an anon replica RW
52ffd46 
This commit adds several checks that are specific for
`replication.failover` mode.

* `replication.failover: off`: an anonymous replica shouldn't be set to
  read-write mode using `database.mode` option.
* `replication.failover: manual`: an anonymous replica shouldn't be
  configured as a replicaset leader using `<replicaset>.leader` option.
* `replication.failover: election`: an anonymous replica can't be
  configured with `replication.election_mode` other than `off`.

This commit also adjusts default `replication.election_mode` to `off`
for an anonymous replica if it is part of a `replication.failover:
election` replicaset (the default for a non-anonymous instance is
`candidate`).

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: allow to bootstrap anon replica from RO rs
63030ce 
This commit allows to bootstrap an anonymous replica from a replicaset,
where all the instances are in read-only mode.

The reason of the change is that there are no technical reasons to
forbid this action. An anonymous replica is not registered in `_cluster`
system space, so it can join a replicaset even if there are no writable
instances.

Fixes tarantool#9432

@TarantoolBot document
Title: config: anonymous replica is now supported

`replication.anon: true` option is now working.

There are configuration constraints that are related to anonymous
replicas.

* A replicaset must contain at least one non-anonymous instance.
* An anonymous replica can't be configured as writable instance using
  `database.mode` or `<replicaset>.leader` options.
* An anonymous replica can't be configured with
  `replication.election_mode` equals to `candidate`, `voter` or `manual`
  (only `off` is allowed).

A few more nuances about anonymous replicas:

* Anonymous replicas are filtered out from default upstream list.
* A `replication.failover: election` replicaset can contain anonymous
  replicas, but `replication.election_mode` defaults to `off` for them
  (unlike non-anonymous instances, where the default is `candidate`).
* `replication.failover: supervised` skips anonymous replicas, when
  choosing a bootstrap leader.
* A anonymous replica can joined a replicaset, which has all the
  instances in read-only mode (ulike a non-anonymous instance).

See details in [1] and [2].

[1]: tarantool#9432
[2]: tarantool#9418
@Totktonada Totktonada mentioned this issue Dec 5, 2023
Merged
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: allow to bootstrap anon replica from RO rs
4711e53 
This commit allows to bootstrap an anonymous replica from a replicaset,
where all the instances are in read-only mode.

The reason of the change is that there are no technical reasons to
forbid this action. An anonymous replica is not registered in `_cluster`
system space, so it can join a replicaset even if there are no writable
instances.

Fixes tarantool#9432

@TarantoolBot document
Title: config: anonymous replica is now supported

`replication.anon: true` option is now working.

There are configuration constraints that are related to anonymous
replicas.

* A replicaset must contain at least one non-anonymous instance.
* An anonymous replica can't be configured as writable instance using
  `database.mode` or `<replicaset>.leader` options.
* An anonymous replica can't be configured with
  `replication.election_mode` equals to `candidate`, `voter` or `manual`
  (only `off` is allowed).

A few more nuances about anonymous replicas:

* Anonymous replicas are filtered out from default upstream list.
* A `replication.failover: election` replicaset can contain anonymous
  replicas, but `replication.election_mode` defaults to `off` for them
  (unlike non-anonymous instances, where the default is `candidate`).
* `replication.failover: supervised` skips anonymous replicas, when
  choosing a bootstrap leader.
* A anonymous replica can joined a replicaset, which has all the
  instances in read-only mode (unlike a non-anonymous instance).

See details in [1] and [2].

[1]: tarantool#9432
[2]: tarantool#9418
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: don't try to persist anon replica name
965e5a0 
The commit effectively enables support of anonymous replicas in the
declarative configuration. It has several caveats (see the changelog
entry), which will be resolved in the following commits of the patchset.

An attempt to persist an instance name of an anonymous replica can't be
successful, because it has no entry in `_cluster` system space. Such an
attempt leads to ER_INSTANCE_NAME_MISMATCH error.

This commit patches the configuration applying logic to skip attempt to
set `box.cfg({instance_name = <...>})` if the instance is configured as
an anonymous replica using `replication.anon: true` option.

Part of tarantool#9432

NO_DOC=replication.anon option is already documented in the scope of
       tarantool/doc#3851. The bugfix
       shouldn't affect the documentation pages much, however related
       constraints are summarized in a documentation request in the last
       commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: don't use an anon replica as an upstream
6817aab 
This commit effectively allows to set `replication.anon: true` without
specifying `replication.peers`.

Without filtering out anonymous replicas from the list of upstreams we
get an error regarding attempt to use an anonymous replica as an
upstream for a non-anonymous instance.

Also, anonymous replicas are excluded from autogenerated upstream list
for other anonymous replicas. It makes the list the same on all the
peers.

A user can configure a custom data flow using `replication.peers`
option.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: supervised replicaset is now anon aware
2fc3882 
Filter out anonymous replicas when choosing a bootstrap leader in
`replication.failover: supervised` mode. An anonymous replica can't be
in read-write mode, so it can't be a replicaset bootstrap leader.

Part of tarantool#9432

NO_DOC=It is bugfix. However, this detail is mentioned in the
       documentation request is in the last commit of the series just in
       case.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: forbid a replicaset of only anon replicas
835e4d1 
A replicaset that contains only anonymous replicas can't be
bootstrapped, because all the instances must in read-only mode.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: forbid attempt make an anon replica RW
63534c0 
This commit adds several checks that are specific for
`replication.failover` mode.

* `replication.failover: off`: an anonymous replica shouldn't be set to
  read-write mode using `database.mode` option.
* `replication.failover: manual`: an anonymous replica shouldn't be
  configured as a replicaset leader using `<replicaset>.leader` option.
* `replication.failover: election`: an anonymous replica can't be
  configured with `replication.election_mode` other than `off`.

This commit also adjusts default `replication.election_mode` to `off`
for an anonymous replica if it is part of a `replication.failover:
election` replicaset (the default for a non-anonymous instance is
`candidate`).

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 5, 2023
@Totktonada
config: allow to bootstrap anon replica from RO rs
603162d 
This commit allows to bootstrap an anonymous replica from a replicaset,
where all the instances are in read-only mode.

The reason of the change is that there are no technical reasons to
forbid this action. An anonymous replica is not registered in `_cluster`
system space, so it can join a replicaset even if there are no writable
instances.

Fixes tarantool#9432

@TarantoolBot document
Title: config: anonymous replica is now supported

`replication.anon: true` option is now working.

There are configuration constraints that are related to anonymous
replicas.

* A replicaset must contain at least one non-anonymous instance.
* An anonymous replica can't be configured as writable instance using
  `database.mode` or `<replicaset>.leader` options.
* An anonymous replica can't be configured with
  `replication.election_mode` equals to `candidate`, `voter` or `manual`
  (only `off` is allowed).

A few more nuances about anonymous replicas:

* Anonymous replicas are filtered out from default upstream list.
* A `replication.failover: election` replicaset can contain anonymous
  replicas, but `replication.election_mode` defaults to `off` for them
  (unlike non-anonymous instances, where the default is `candidate`).
* `replication.failover: supervised` skips anonymous replicas, when
  choosing a bootstrap leader.
* A anonymous replica can joined a replicaset, which has all the
  instances in read-only mode (unlike a non-anonymous instance).

See details in [1] and [2].

[1]: tarantool#9432
[2]: tarantool#9418
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: don't try to persist anon replica name
dbdb80f 
The commit effectively enables support of anonymous replicas in the
declarative configuration. It has several caveats (see the changelog
entry), which will be resolved in the following commits of the patchset.

An attempt to persist an instance name of an anonymous replica can't be
successful, because it has no entry in `_cluster` system space. Such an
attempt leads to ER_INSTANCE_NAME_MISMATCH error.

This commit patches the configuration applying logic to skip attempt to
set `box.cfg({instance_name = <...>})` if the instance is configured as
an anonymous replica using `replication.anon: true` option.

Part of tarantool#9432

NO_DOC=replication.anon option is already documented in the scope of
       tarantool/doc#3851. The bugfix
       shouldn't affect the documentation pages much, however related
       constraints are summarized in a documentation request in the last
       commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: don't use an anon replica as an upstream
1664a53 
This commit effectively allows to set `replication.anon: true` without
specifying `replication.peers`.

Without filtering out anonymous replicas from the list of upstreams we
get an error regarding attempt to use an anonymous replica as an
upstream for a non-anonymous instance.

Also, anonymous replicas are excluded from autogenerated upstream list
for other anonymous replicas. It makes the list the same on all the
peers.

A user can configure a custom data flow using `replication.peers`
option.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: supervised replicaset is now anon aware
0b0d137 
Filter out anonymous replicas when choosing a bootstrap leader in
`replication.failover: supervised` mode. An anonymous replica can't be
in read-write mode, so it can't be a replicaset bootstrap leader.

Part of tarantool#9432

NO_DOC=It is bugfix. However, this detail is mentioned in the
       documentation request is in the last commit of the series just in
       case.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: forbid a replicaset of only anon replicas
248c107 
A replicaset that contains only anonymous replicas can't be
bootstrapped, because all the instances must in read-only mode.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: forbid attempt make an anon replica RW
00f1be6 
This commit adds several checks that are specific for
`replication.failover` mode.

* `replication.failover: off`: an anonymous replica shouldn't be set to
  read-write mode using `database.mode` option.
* `replication.failover: manual`: an anonymous replica shouldn't be
  configured as a replicaset leader using `<replicaset>.leader` option.
* `replication.failover: election`: an anonymous replica can't be
  configured with `replication.election_mode` other than `off`.

This commit also adjusts default `replication.election_mode` to `off`
for an anonymous replica if it is part of a `replication.failover:
election` replicaset (the default for a non-anonymous instance is
`candidate`).

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: allow to bootstrap anon replica from RO rs
89c3b82 
This commit allows to bootstrap an anonymous replica from a replicaset,
where all the instances are in read-only mode.

The reason of the change is that there are no technical reasons to
forbid this action. An anonymous replica is not registered in `_cluster`
system space, so it can join a replicaset even if there are no writable
instances.

Fixes tarantool#9432

@TarantoolBot document
Title: config: anonymous replica is now supported

`replication.anon: true` option is now working.

There are configuration constraints that are related to anonymous
replicas.

* A replicaset must contain at least one non-anonymous instance.
* An anonymous replica can't be configured as writable instance using
  `database.mode` or `<replicaset>.leader` options.
* An anonymous replica can't be configured with
  `replication.election_mode` equals to `candidate`, `voter` or `manual`
  (only `off` is allowed).

A few more nuances about anonymous replicas:

* Anonymous replicas are filtered out from default upstream list.
* A `replication.failover: election` replicaset can contain anonymous
  replicas, but `replication.election_mode` defaults to `off` for them
  (unlike non-anonymous instances, where the default is `candidate`).
* `replication.failover: supervised` skips anonymous replicas, when
  choosing a bootstrap leader.
* A anonymous replica can joined a replicaset, which has all the
  instances in read-only mode (unlike a non-anonymous instance).

See details in [1] and [2].

[1]: tarantool#9432
[2]: tarantool#9418
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: don't try to persist anon replica name
2b0f33a 
The commit effectively enables support of anonymous replicas in the
declarative configuration. It has several caveats (see the changelog
entry), which will be resolved in the following commits of the patchset.

An attempt to persist an instance name of an anonymous replica can't be
successful, because it has no entry in `_cluster` system space. Such an
attempt leads to ER_INSTANCE_NAME_MISMATCH error.

This commit patches the configuration applying logic to skip attempt to
set `box.cfg({instance_name = <...>})` if the instance is configured as
an anonymous replica using `replication.anon: true` option.

Part of tarantool#9432

NO_DOC=replication.anon option is already documented in the scope of
       tarantool/doc#3851. The bugfix
       shouldn't affect the documentation pages much, however related
       constraints are summarized in a documentation request in the last
       commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: don't use an anon replica as an upstream
84ac8ed 
This commit effectively allows to set `replication.anon: true` without
specifying `replication.peers`.

Without filtering out anonymous replicas from the list of upstreams we
get an error regarding attempt to use an anonymous replica as an
upstream for a non-anonymous instance.

Also, anonymous replicas are excluded from autogenerated upstream list
for other anonymous replicas. It makes the list the same on all the
peers.

A user can configure a custom data flow using `replication.peers`
option.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: supervised replicaset is now anon aware
f5d2963 
Filter out anonymous replicas when choosing a bootstrap leader in
`replication.failover: supervised` mode. An anonymous replica can't be
in read-write mode, so it can't be a replicaset bootstrap leader.

Part of tarantool#9432

NO_DOC=It is bugfix. However, this detail is mentioned in the
       documentation request is in the last commit of the series just in
       case.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: forbid a replicaset of only anon replicas
151edc0 
A replicaset that contains only anonymous replicas can't be
bootstrapped, because all the instances must be in read-only mode.

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: forbid attempt make an anon replica RW
f732a86 
This commit adds several checks that are specific for
`replication.failover` mode.

* `replication.failover: off`: an anonymous replica shouldn't be set to
  read-write mode using `database.mode` option.
* `replication.failover: manual`: an anonymous replica shouldn't be
  configured as a replicaset leader using `<replicaset>.leader` option.
* `replication.failover: election`: an anonymous replica can't be
  configured with `replication.election_mode` other than `off`.

This commit also adjusts default `replication.election_mode` to `off`
for an anonymous replica if it is part of a `replication.failover:
election` replicaset (the default for a non-anonymous instance is
`candidate`).

Part of tarantool#9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit to Totktonada/tarantool that referenced this issue Dec 6, 2023
@Totktonada
config: allow to bootstrap anon replica from RO rs
e068c48 
This commit allows to bootstrap an anonymous replica from a replicaset,
where all the instances are in read-only mode.

The reason of the change is that there are no technical reasons to
forbid this action. An anonymous replica is not registered in `_cluster`
system space, so it can join a replicaset even if there are no writable
instances.

Fixes tarantool#9432

@TarantoolBot document
Title: config: anonymous replica is now supported

`replication.anon: true` option is now working.

There are configuration constraints that are related to anonymous
replicas.

* A replicaset must contain at least one non-anonymous instance.
* An anonymous replica can't be configured as writable instance using
  `database.mode` or `<replicaset>.leader` options.
* An anonymous replica can't be configured with
  `replication.election_mode` equals to `candidate`, `voter` or `manual`
  (only `off` is allowed).

A few more nuances about anonymous replicas:

* Anonymous replicas are filtered out from default upstream list.
* A `replication.failover: election` replicaset can contain anonymous
  replicas, but `replication.election_mode` defaults to `off` for them
  (unlike non-anonymous instances, where the default is `candidate`).
* `replication.failover: supervised` skips anonymous replicas, when
  choosing a bootstrap leader.
* A anonymous replica can joined a replicaset, which has all the
  instances in read-only mode (unlike a non-anonymous instance).

See details in [1] and [2].

[1]: tarantool#9432
[2]: tarantool#9418
Totktonada added a commit that referenced this issue Dec 6, 2023
@Totktonada
config: don't try to persist anon replica name
1f533fc 
The commit effectively enables support of anonymous replicas in the
declarative configuration. It has several caveats (see the changelog
entry), which will be resolved in the following commits of the patchset.

An attempt to persist an instance name of an anonymous replica can't be
successful, because it has no entry in `_cluster` system space. Such an
attempt leads to ER_INSTANCE_NAME_MISMATCH error.

This commit patches the configuration applying logic to skip attempt to
set `box.cfg({instance_name = <...>})` if the instance is configured as
an anonymous replica using `replication.anon: true` option.

Part of #9432

NO_DOC=replication.anon option is already documented in the scope of
       tarantool/doc#3851. The bugfix
       shouldn't affect the documentation pages much, however related
       constraints are summarized in a documentation request in the last
       commit of the series.
Totktonada added a commit that referenced this issue Dec 6, 2023
@Totktonada
config: don't use an anon replica as an upstream
71b69c4 
This commit effectively allows to set `replication.anon: true` without
specifying `replication.peers`.

Without filtering out anonymous replicas from the list of upstreams we
get an error regarding attempt to use an anonymous replica as an
upstream for a non-anonymous instance.

Also, anonymous replicas are excluded from autogenerated upstream list
for other anonymous replicas. It makes the list the same on all the
peers.

A user can configure a custom data flow using `replication.peers`
option.

Part of #9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit that referenced this issue Dec 6, 2023
@Totktonada
config: supervised replicaset is now anon aware
e60494f 
Filter out anonymous replicas when choosing a bootstrap leader in
`replication.failover: supervised` mode. An anonymous replica can't be
in read-write mode, so it can't be a replicaset bootstrap leader.

Part of #9432

NO_DOC=It is bugfix. However, this detail is mentioned in the
       documentation request is in the last commit of the series just in
       case.
Totktonada added a commit that referenced this issue Dec 6, 2023
@Totktonada
config: forbid a replicaset of only anon replicas
a1580a5 
A replicaset that contains only anonymous replicas can't be
bootstrapped, because all the instances must be in read-only mode.

Part of #9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit that referenced this issue Dec 6, 2023
@Totktonada
config: forbid attempt make an anon replica RW
acaccb0 
This commit adds several checks that are specific for
`replication.failover` mode.

* `replication.failover: off`: an anonymous replica shouldn't be set to
  read-write mode using `database.mode` option.
* `replication.failover: manual`: an anonymous replica shouldn't be
  configured as a replicaset leader using `<replicaset>.leader` option.
* `replication.failover: election`: an anonymous replica can't be
  configured with `replication.election_mode` other than `off`.

This commit also adjusts default `replication.election_mode` to `off`
for an anonymous replica if it is part of a `replication.failover:
election` replicaset (the default for a non-anonymous instance is
`candidate`).

Part of #9432

NO_DOC=The documentation request is in the last commit of the series.
Totktonada added a commit that referenced this issue Dec 6, 2023
@Totktonada
config: allow to bootstrap anon replica from RO rs
5ba3ace 
This commit allows to bootstrap an anonymous replica from a replicaset,
where all the instances are in read-only mode.

The reason of the change is that there are no technical reasons to
forbid this action. An anonymous replica is not registered in `_cluster`
system space, so it can join a replicaset even if there are no writable
instances.

Fixes #9432

@TarantoolBot document
Title: config: anonymous replica is now supported

`replication.anon: true` option is now working.

There are configuration constraints that are related to anonymous
replicas.

* A replicaset must contain at least one non-anonymous instance.
* An anonymous replica can't be configured as writable instance using
  `database.mode` or `<replicaset>.leader` options.
* An anonymous replica can't be configured with
  `replication.election_mode` equals to `candidate`, `voter` or `manual`
  (only `off` is allowed).

A few more nuances about anonymous replicas:

* Anonymous replicas are filtered out from default upstream list.
* A `replication.failover: election` replicaset can contain anonymous
  replicas, but `replication.election_mode` defaults to `off` for them
  (unlike non-anonymous instances, where the default is `candidate`).
* `replication.failover: supervised` skips anonymous replicas, when
  choosing a bootstrap leader.
* A anonymous replica can joined a replicaset, which has all the
  instances in read-only mode (unlike a non-anonymous instance).

See details in [1] and [2].

[1]: #9432
[2]: #9418
@TarantoolBot TarantoolBot mentioned this issue Dec 6, 2023
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config: allow to configure an anonymous replica Totktonada/tarantool
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