[GitHub] [couchdb] kocolosk edited a comment on issue #2037: Add "Dreyfus" (Erlang side of Search) to CouchDB

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kocolosk edited a comment on issue #2037: Add "Dreyfus" (Erlang side of Search) to CouchDB
URL: https://github.com/apache/couchdb/pull/2037#issuecomment-502330265
 
 
   I made a small change to replace the check for the `dreyfus_index` module with a test message to the Clouseau node. Here's a summary how CouchDB behaves when Clouseau is not connected:
   
   ### Search
   
   * Updates to a Search design document (i.e., one with a top-level `"indexes"` field) **succeed**. I doubt we want to change this as it would have significant implications for replication.
   * Search requests fail after ~30 seconds with a 500 status code and `{"error":"timeout","reason":"The request could not be processed in a reasonable amount of time."}`. This could be improved.
   
   ### Mango
   
   * Attempts to create a Mango text index via the `_index` endpoint fail ~immediately with a 503 status and the following response: `{"error":"required index service unavailable","reason":"text"}`. I think this is the right behavior.
   * The Mango query optimizer will skip any existing "text" indexes if the Clouseau subsystem is not available at query time. This could result in a previously fast query falling back to a full database scan. I'm OK with this.
   
   I think there are three followups:
   
   1. Reject the Search request immediately instead of timing out with a generic error
   1. Consider some retry logic to mask Clouseau reboots during upgrades as suggested by @rnewson. I could see doing a short bounded exponential backoff for a couple of seconds.
   1. Quantify the cost of the RPC-based check for Clouseau's presence; both the amount of time to do the check, and the number of times the check is invoked in the request path for a healthy system.

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