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106 lines
3.1 KiB
Markdown
106 lines
3.1 KiB
Markdown
## Modules
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etcd has a number of modules that are built on top of the core etcd API.
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These modules provide things like dashboards, locks and leader election.
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### Dashboard
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An HTML dashboard can be found at `http://127.0.0.1:4001/mod/dashboard/`.
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This dashboard is compiled into the etcd binary and uses the same API as regular etcd clients.
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Use the `-cors='*'` flag to allow your browser to request information from the current master as it changes.
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### Lock
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The Lock module implements a fair lock that can be used when lots of clients want access to a single resource.
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A lock can be associated with a value.
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The value is unique so if a lock tries to request a value that is already queued for a lock then it will find it and watch until that value obtains the lock.
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If you lock the same value on a key from two separate curl sessions they'll both return at the same time.
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Here's the API:
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**Acquire a lock (with no value) for "customer1"**
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```sh
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curl -X POST http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60
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```
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**Acquire a lock for "customer1" that is associated with the value "bar"**
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```sh
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curl -X POST http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d value=bar
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```
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**Renew the TTL on the "customer1" lock for index 2**
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```sh
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curl -X PUT http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d index=2
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```
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**Renew the TTL on the "customer1" lock for value "customer1"**
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```sh
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curl -X PUT http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d value=bar
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```
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**Retrieve the current value for the "customer1" lock.**
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```sh
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curl http://127.0.0.1:4001/mod/v2/lock/customer1
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```
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**Retrieve the current index for the "customer1" lock**
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```sh
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curl http://127.0.0.1:4001/mod/v2/lock/customer1?field=index
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```
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**Delete the "customer1" lock with the index 2**
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```sh
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curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?index=2
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```
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**Delete the "customer1" lock with the value "bar"**
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```sh
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curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?value=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 PUT 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 DELETE 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 `PUT` command that you used to set the value.
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