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It is difficult to implement a generic mechanism in the client library to address these problems. There are many options for external resource (HTTP, DNS, NFS, LDAP, virtual IP, etc). From security mechanisms view point; there are many solutions as well and some of them could require tedious setup. Even if we resolve this problemFurther, the process of updating a resource is not simple. The process that updates the external resources must ensure that only the most recent server list gets recorded at the resource. We could add the capability in the client library to record that records a union of the old and new server configuration at the URL, perform performs the reconfiguration, and then update updates the URL to with the most modified server list. However, the client library would also need to handle failures that may occur in this 3-step process. Further, if the application performs concurrent reconfigurations (which is supported by the server), then the client library would need to resolve race conditions to ensure that the result of last successful configuration is correctly recorded. In order to synchronize updates, the client library would need the external resource to support some atomic primitives (e.g., for locking and fencing).

Considering the complexity of the problem, in the first version, we propose to not implement any mechanisms to update the external resource. The ZooKeeper application would need to implement its own mechanism to update the URL. We expect that in most cases the change in ZooKeeper configuration will be an administrative process. Therefore, the Administrator can easily ensure that the URL is kept up-to-date. When we gain more experience in the future, we could consider adding this part the client library.

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1. A new optional property “version” will added to the configuration file. The version number indicates the reconfiguration version number and should match with the version number of the current server configuration returned by ZooKeeper.getConfig() (see [1],[2] for further details). The version number, when specified, will be used to ensure that the client will be choose the new server list only if the version number of the list is higher than the current version number.
2. The “server.x=host:serverPort:electionPort” property will be changed to “server.x=host:serverPort:electionPort:clientPort”.

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<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="45431ae0cca4e7a6-5eb7f3e7-46d74702-85bd8af1-2f31c1d18e39a7b6b5d81c32"><ac:parameter ac:name="">107</ac:parameter></ac:structured-macro> \[1\]  JIRA for dynamic cluster membership. [https://issues.apache.org/jira/browse/ZOOKEEPER-107]

<ac:structured-macro ac:name="anchor" ac:schema-version="1" ac:macro-id="b421e136c98e1199-80957ff7-42fd4b92-b399a96b-dbbe49615a2f41c3057e04c0"><ac:parameter ac:name="">107wiki</ac:parameter></ac:structured-macro> \[2\] Cluster Membership. [https://cwiki.apache.org/confluence/display/ZOOKEEPER/ClusterMembership]