State Replication in ORMultiMap

[Mushtaq Ahmed]

Suppose there is an ORMultiMap with 5 keys, each pointing to an ORSet of size 1000 items. Given that ORSet is a delta-CRDT, which one of the following happens on ORMultiMap.addBinding("a", 4) operation:

1. Entire map with 5000 entries will be replicated.

2. Only the delta of the ORSet (against key "a") is replicated.

Thanks.

[Michal Borowiecki]

ORMap and ORMultiMap don't mention delta support. On the contrary, docs for Maps say:

When a data entry is changed the full state of that entry is replicated to other nodes, i.e. when you update a map the whole map is replicated. Therefore, instead of using one ORMap with 1000 elements it is more efficient to split that up in 10 top level ORMap entries with 100 elements each. Top level entries are replicated individually, which has the trade-off that different entries may not be replicated at the same time and you may see inconsistencies between related entries. Separate top level entries cannot be updated atomically together.

The only way I can read it is that in your case the entire map with 5000 entries will be replicated.

Thanks,
Michał

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[Patrik Nordwall]

ORMap and ORMultiMap have support for deltas. Nested deltas can be difficult so the current implementation is sending the full state of the changed entry of the ORMap, i.e. the 1000 items of the "a" ORSet in your example. There is variant of ORMultiMap that supports nested deltas but it has some other drawbacks as described in the (to be released) documentation: http://doc.akka.io/docs/akka/snapshot/scala/distributed-data.html#maps

Cheers,

Patrik

On Sun, Jun 18, 2017 at 7:27 PM, 'Michal Borowiecki' via Akka User List <akka...@googlegroups.com> wrote:

ORMap and ORMultiMap don't mention delta support. On the contrary, docs for Maps say:

When a data entry is changed the full state of that entry is replicated to other nodes, i.e. when you update a map the whole map is replicated. Therefore, instead of using one ORMap with 1000 elements it is more efficient to split that up in 10 top level ORMap entries with 100 elements each. Top level entries are replicated individually, which has the trade-off that different entries may not be replicated at the same time and you may see inconsistencies between related entries. Separate top level entries cannot be updated atomically together.

The only way I can read it is that in your case the entire map with 5000 entries will be replicated.

Thanks,
Michał

On 18/06/17 18:00, Mushtaq Ahmed wrote:

Suppose there is an ORMultiMap with 5 keys, each pointing to an ORSet of size 1000 items. Given that ORSet is a delta-CRDT, which one of the following happens on ORMultiMap.addBinding("a", 4) operation:

1. Entire map with 5000 entries will be replicated.

2. Only the delta of the ORSet (against key "a") is replicated.

Thanks.

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[Mushtaq Ahmed]

Thanks Patrik! I will read though the snapshot docs.

Good to know the answer was none of the above :)