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diff --git a/doc/Load_Balancing.md b/doc/Load_Balancing.md index ead7eb92..808bb4b3 100644 --- a/doc/Load_Balancing.md +++ b/doc/Load_Balancing.md @@ -2,30 +2,37 @@ I have conducted a quick study of different methods to load-balance data over di ### Requirements -- good balancing: two nodes that have the same announced capacity should receive close to the same number of items -- multi-datacenter: the replicas of a partition should be distributed over as many datacenters as possible -- minimal disruption: when adding or removing a node, as few partitions as possible should have to move around +- *good balancing*: two nodes that have the same announced capacity should receive close to the same number of items + +- *multi-datacenter*: the replicas of a partition should be distributed over as many datacenters as possible + +- *minimal disruption*: when adding or removing a node, as few partitions as possible should have to move around + +- *order-agnostic*: the same set of nodes (associated with a datacenter name + and a capacity) should always return the same distribution of partition + replicas, independently of the order in which nodes were added/removed (this + is to keep the implementation simple) ### Methods #### Naive multi-DC ring walking strategy -This strategy can be used with any ring-linke algorithm to make it aware of the *multi-datacenter* requirement: +This strategy can be used with any ring-like algorithm to make it aware of the *multi-datacenter* requirement: - the ring is a list of positions, each associated with a single node in the cluster - look up position of item on ring - select the node for that position - go clockwise, skipping nodes that: - we halve already selected - - are in a datacenter of a node we have selected, except if we already have nodes from all available datacenters + - are in a datacenter of a node we have selected, except if we already have nodes from all possible datacenters In this way the selected nodes will always be distributed over `min(n_datacenters, n_replicas)` different datacenters, which is the best we can do. -This method was implemented in the first iteration of Garage, with the basic -ring construction that consists in associating `n_token` random positions to -each node. +This method was implemented in the first version of Garage, with the basic +ring construction from Dynamo DB that consists in associating `n_token` random positions to +each node (I know it's not optimal, the Dynamo paper already studies this). #### Better rings @@ -43,7 +50,7 @@ To solve this, we want to apply a second method for partitionning our dataset: I have studied two ways to do the attribution, in a way that is deterministic: -- Custom: take `argmin_node(hash(node, partition_number))` +- Min-hash: for each partition, select node that minimizes `hash(node, partition_number)` - MagLev: see [here](https://blog.acolyer.org/2016/03/21/maglev-a-fast-and-reliable-software-network-load-balancer/) MagLev provided significantly better balancing, as it guarantees that the exact |