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Resolving replication conflicts
Solving problems with master-master replication
Tarantool guarantees that every update is applied only once on every replica. However, due to the asynchronous nature of replication, the order of updates is not guaranteed. We now analyze this problem with more details, provide examples of replication going out of sync, and suggest solutions.
Replacing the same primary key
**Case 1:** You have two instances of Tarantool. For example, you try to make a ``replace`` operation with the same primary key on both instances in the same time. This will cause a conflict over which tuple to save and which one to discard.
Tarantool :ref:`trigger functions <triggers>` can help here to implement the rules of conflict resolution on some condition. For example, if you have a timestamp, you can declare saving the tuple with the bigger one.
First, you need a :ref:`before_replace() <box_space-before_replace>` trigger on the space which may have conflicts. In this trigger you can compare the old and new replica records and choose which one to use (or skip the update entirely, or merge two records together).
Then you need to set the trigger at the right time, before the space starts to receive any updates. The way you usually set the ``before_replace`` trigger is right when the space is created, so you need a trigger to set another trigger on the system space ``_space``, to capture the moment when your space is created and set the trigger there. This can be an :ref:`on_replace() <box_space-on_replace>` trigger.
The difference between ``before_replace`` and ``on_replace`` is that ``on_replace`` is called after a row is inserted into the space, and ``before_replace`` is called before that.
To set a ``_space:on_replace()`` trigger correctly, you also need the right timing. The best timing to use it is when ``_space`` is just created, which is the :ref:`box.ctl.on_schema_init() <box_ctl-on_schema_init>` trigger.
You will also need to utilize ``box.on_commit`` to get access to the space being created. The resulting snippet would be the following:
local my_space_name = 'my_space'
local my_trigger = function(old, new) ... end -- your function resolving a conflict
box.ctl.on_schema_init(function()
box.space._space:on_replace(function(old_space, new_space)
if not old_space and new_space and new_space.name == my_space_name then
box.on_commit(function()
box.space[my_space_name]:before_replace(my_trigger)
end
end
end)
end)
Preventing duplicate insert
**Case 2:** In a replica set of two masters, suppose master #1 tries to ``insert`` a tuple with the same unique key:
tarantool> box.space.tester:insert{1, 'data'}
This would cause an error saying ``Duplicate key exists in unique index 'primary' in space 'tester'`` and the replication would be stopped. (This is the behavior when the :ref:`replication_skip_conflict <cfg_replication-replication_skip_conflict>` configuration parameter has its default recommended value, ``false``.)
$ # error messages from master #1
2017-06-26 21:17:03.233 [30444] main/104/applier/rep_user@100.96.166.1 I> can't read row
2017-06-26 21:17:03.233 [30444] main/104/applier/rep_user@100.96.166.1 memtx_hash.cc:226 E> ER_TUPLE_FOUND:
Duplicate key exists in unique index 'primary' in space 'tester'
2017-06-26 21:17:03.233 [30444] relay/[::ffff:100.96.166.178]/101/main I> the replica has closed its socket, exiting
2017-06-26 21:17:03.233 [30444] relay/[::ffff:100.96.166.178]/101/main C> exiting the relay loop

$ # error messages from master #2
2017-06-26 21:17:03.233 [30445] main/104/applier/rep_user@100.96.166.1 I> can't read row
2017-06-26 21:17:03.233 [30445] main/104/applier/rep_user@100.96.166.1 memtx_hash.cc:226 E> ER_TUPLE_FOUND:
Duplicate key exists in unique index 'primary' in space 'tester'
2017-06-26 21:17:03.234 [30445] relay/[::ffff:100.96.166.178]/101/main I> the replica has closed its socket, exiting
2017-06-26 21:17:03.234 [30445] relay/[::ffff:100.96.166.178]/101/main C> exiting the relay loop
If we check replication statuses with ``box.info``, we will see that replication at master #1 is stopped (``1.upstream.status = stopped``). Additionally, no data is replicated from that master (section ``1.downstream`` is missing in the report), because the downstream has encountered the same error:
# replication statuses (report from master #3)
tarantool> box.info
---
- version: 1.7.4-52-g980d30092
id: 3
ro: false
vclock: {1: 9, 2: 1000000, 3: 3}
uptime: 557
lsn: 3
vinyl: []
cluster:
uuid: 34d13b1a-f851-45bb-8f57-57489d3b3c8b
pid: 30445
status: running
signature: 1000012
replication:
1:
id: 1
uuid: 7ab6dee7-dc0f-4477-af2b-0e63452573cf
lsn: 9
upstream:
peer: replicator@192.168.0.101:3301
lag: 0.00050592422485352
status: stopped
idle: 445.8626639843
message: Duplicate key exists in unique index 'primary' in space 'tester'
2:
id: 2
uuid: 9afbe2d9-db84-4d05-9a7b-e0cbbf861e28
lsn: 1000000
upstream:
status: follow
idle: 201.99915885925
peer: replicator@192.168.0.102:3301
lag: 0.0015020370483398
downstream:
vclock: {1: 8, 2: 1000000, 3: 3}
3:
id: 3
uuid: e826a667-eed7-48d5-a290-64299b159571
lsn: 3
uuid: e826a667-eed7-48d5-a290-64299b159571
...
When replication is later manually resumed:
# resuming stopped replication (at all masters)
tarantool> original_value = box.cfg.replication
tarantool> box.cfg{replication={}}
tarantool> box.cfg{replication=original_value}
... the faulty row in the write-ahead-log files is skipped.
**Solution #1: replication runs out of sync**
In a master-master cluster of two instances, suppose we make the following operation:
tarantool> box.space.tester:upsert({1}, {{'=', 2, box.info.uuid}})
When this operation is applied on both instances in the replica set:
# at master #1
tarantool> box.space.tester:upsert({1}, {{'=', 2, box.info.uuid}})
# at master #2
tarantool> box.space.tester:upsert({1}, {{'=', 2, box.info.uuid}})

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testing_project / Documentation: book/admin/replication/repl_problem_solving — English

local my_space_name = 'my_space'
local my_trigger = function(old, new) ... end -- your function resolving a conflict
box.ctl.on_schema_init(function()
box.space._space:on_replace(function(old_space, new_space)
if not old_space and new_space and new_space.name == my_space_name then
box.on_commit(function()
box.space[my_space_name]:before_replace(my_trigger)
end
end
end)
end)
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