Rakesh Emmadi e32f5a1fb1 sync metadata cache across multiple instances connected to same db (closes #1182) (#1574) ... 1. Haskel library `pg-client-hs` has been updated to expose a function that helps listen to `postgres` notifications over a `channel` in this [PR](https://github.com/hasura/pg-client-hs/pull/5) 2. The server records an event in a table `hdb_catalog.hdb_cache_update_event` whenever any `/v1/query` (that changes metadata) is requested. A trigger notifies a `cache update` event via `hasura_cache_update` channel 3. The server runs two concurrent threads namely `listener` and `processor`. The `listener` thread listens to events on `hasura_cache_update` channel and pushed into a `Queue`. The `processor` thread fetches events from that `Queue` and processes it. Thus server rebuilds schema cache from database and updates.		2019-03-12 11:16:27 +05:30
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basic	sync metadata cache across multiple instances connected to same db (closes #1182) (#1574)	2019-03-12 11:16:27 +05:30