Add leader module to README.
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README.md
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README.md
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@ -926,6 +926,41 @@ curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?index=customer1
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curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?name=bar
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```
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### Leader Election
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The Leader Election module wraps the Lock module to allow clients to come to consensus on a single value.
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This is useful when you want one server to process at a time but allow other servers to fail over.
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The API is similar to the Lock module but is limited to simple strings values.
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Here's the API:
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**Attempt to set a value for the "order_processing" leader key:**
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```sh
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curl -X POST http://127.0.0.1:4001/mod/v2/leader/order_processing?ttl=60 -d name=myserver1.foo.com
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```
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**Retrieve the current value for the "order_processing" leader key:**
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```sh
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curl http://127.0.0.1:4001/mod/v2/leader/order_processing
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myserver1.foo.com
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```
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**Remove a value from the "order_processing" leader key:**
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```sh
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curl -X POST http://127.0.0.1:4001/mod/v2/leader/order_processing?name=myserver1.foo.com
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```
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If multiple clients attempt to set the value for a key then only one will succeed.
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The other clients will hang until the current value is removed because of TTL or because of a `DELETE` operation.
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Multiple clients can submit the same value and will all be notified when that value succeeds.
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To update the TTL of a value simply reissue the same `POST` command that you used to set the value.
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## Contributing
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See [CONTRIBUTING](https://github.com/coreos/etcd/blob/master/CONTRIBUTING.md) for details on submitting patches and contacting developers via IRC and mailing lists.
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