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In order to optimise the connections opened by a mongos process when a nss.watch() request is received, the config server will have to persist in a new collection (called config.placementHistory) historical metadata about which shards of the cluster were hosting one or more chunks of nss at a specific point in time.
The objectives of this ticket are:
- The encoding of the schema for config.placementHistory, defined as
{
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namespace: string, // references db or collection
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timestamp: Timestamp,
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shards: array<ShardId> // may be empty
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}
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- The instantiation of config.placementHistory as part of the initialisation of the ShardingCatalogManager (with an index over namespace+timestamp).
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