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Included note on auto_eject behavior #73

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commit 60466e2ba3d82b8ca8ec716d6a9dcf5ee0cf346f 1 parent 84c8f08
@matschaffer matschaffer authored
Showing with 3 additions and 1 deletion.
  1. +1 −1  README.md
  2. +2 −0  notes/recommendation.md
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2  README.md
@@ -95,7 +95,7 @@ nutcracker can be configured through a YAML file specified by the -c or --conf-f
+ **preconnect**: A boolean value that controls if nutcracker should preconnect to all the servers in this pool on process start. Defaults to false.
+ **redis**: A boolean value that controls if a server pool speaks redis or memcached protocol. Defaults to false.
+ **server_connections**: The maximum number of connections that can be opened to each server. By default, we open at most 1 server connection.
-+ **auto_eject_hosts**: A boolean value that controls if server should be ejected temporarily when it fails consecutively server_failure_limit times. Defaults to false.
++ **auto_eject_hosts**: A boolean value that controls if server should be ejected temporarily when it fails consecutively server_failure_limit times. See [liveness recommendations](notes/recommendation.md#liveness) for information. Defaults to false.
+ **server_retry_timeout**: The timeout value in msec to wait for before retrying on a temporarily ejected server, when auto_eject_host is set to true. Defaults to 30000 msec.
+ **server_failure_limit**: The number of conseutive failures on a server that would leads to it being temporarily ejected when auto_eject_host is set to true. Defaults to 2.
+ **servers**: A list of server address, port and weight (name:port:weight or ip:port:weight) for this server pool.
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2  notes/recommendation.md
@@ -34,6 +34,8 @@ Failures are a fact of life, especially when things are distributed. To be resil
Enabling `auto_eject_hosts:` ensures that a dead server can be ejected out of the hash ring after `server_failure_limit:` consecutive failures have been encountered on that said server. A non-zero `server_retry_timeout:` ensures that we don't incorrectly mark a server as dead forever especially when the failures were really transient. The combination of `server_retry_timeout:` and `server_failure_limit:` controls the tradeoff between resiliency to permanent and transient failures.
+Note that an ejected server will not be included in the hash ring for any requests until the retry timeout passes. This will lead to data partitioning as keys originally on the ejected server will now be written to a server still in the pool.
+
To ensure that requests always succeed in the face of server ejections (`auto_eject_hosts:` is enabled), some form of retry must be implemented at the client layer since nutcracker itself does not retry a request. This client-side retry count must be greater than `server_failure_limit:` value, which ensures that the original request has a chance to make it to a live server.
## Timeout
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