How to achieve active-active multidatacenter on mongo

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Stone Fang

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Oct 12, 2016, 7:23:33 PM10/12/16

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Mongo is a master-slave architecture.if need to deploy application on multi-datacenter.

I can do like this document described.

https://docs.mongodb.com/manual/core/replica-set-architecture-geographically-distributed/

and we can support datacenter down.

but how to support active-active.I mean write to both datacenter,but not write to primary datacenter,read from

all datacenter.active-active is meanful.for instance,

one of my user move from regionA to regionB.It would cause high latency while need to write data to local datacenter in regionA.

anyone have idea on this?

Lungang Fang

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Oct 23, 2016, 10:41:15 PM10/23/16

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Hi Stone

one of my user move from regionA to regionB.It would cause high latency while need to write data to local datacenter in regionA.

This seems to me a use case for sharding, particularly tag aware sharding. You may put one shard (or more) in each data center and ensure that all regionA users belong to the shard(s) in regionA data center, all regionB users belong to regionB shard(s).

Let’s assume we have two data centers, each of which correspond to a region. The following example shows how to write using a tag aware sharding:

Set up a two-shard cluster: “shard01” and “shard02”

Tag those two shards

  sh.addShardTag('shard01', 'regionA');
  sh.addShardTag('shard02', 'regionB');

Shard collection “users” in database “demo”

  sh.enableSharding('demo');
  sh.shardCollection('demo.users', {home_location:1, user_id: 1});

Attach home location to corresponding shard tags

  sh.addTagRange('demo.users',
                 {home_location:'regionA', user_id:MinKey},
                 {home_location:'regionA', user_id:MaxKey},
                 'regionA');

  sh.addTagRange('demo.users',
                 {home_location:'regionB', user_id:MinKey},
                 {home_location:'regionB', user_id:MaxKey},
                 'regionB');

Insert documents of different regions

  db.users.insert({home_location:'regionA', user_id:1, name:'adam'});
  db.users.insert({home_location:'regionB', user_id:2, name:'bob'});

Verify that documents are stored in shards as expected using either way:
- Run the following commands and check output.
```
  sh.status();
  db.users.find({user_id:1}).explain({verbose:1});
  db.users.find({user_id:2}).explain({verbose:1});
```
- Or, connect to and query on each shard directly.
NOTE: If there is already some data before sharding/tagging, we may need to wait for some time to allow chunks migrate to designated shards respectively.
Move one user to another region

Since values of shard keys cannot be changed, we have to delete the user and re-insert it with new location info.
```
  var adam = db.users.findOneAndDelete({home_location:'regionA', user_id:1});
  adam.home_location = 'regionB';
  db.users.insert(adam);
```

Make sure you perform tests under your specific environment and use cases first, before making changes to your production systems. Refer to Manage Shard Tags and Segmenting Data by Location for more information.