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Modules
etcd has a number of modules that are built on top of the core etcd API. These modules provide things like dashboards, locks and leader election.
Dashboard
An HTML dashboard can be found at http://127.0.0.1:4001/mod/dashboard/.
This dashboard is compiled into the etcd binary and uses the same API as regular etcd clients.
Use the -cors='*' flag to allow your browser to request information from the current master as it changes.
Lock
The Lock module implements a fair lock that can be used when lots of clients want access to a single resource. A lock can be associated with a value. 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. If you lock the same value on a key from two separate curl sessions they'll both return at the same time.
Here's the API:
Acquire a lock (with no value) for "customer1"
curl -X POST http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60
Acquire a lock for "customer1" that is associated with the value "bar"
curl -X POST http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d value=bar
Renew the TTL on the "customer1" lock for index 2
curl -X PUT http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d index=2
Renew the TTL on the "customer1" lock for value "customer1"
curl -X PUT http://127.0.0.1:4001/mod/v2/lock/customer1?ttl=60 -d value=bar
Retrieve the current value for the "customer1" lock.
curl http://127.0.0.1:4001/mod/v2/lock/customer1
Retrieve the current index for the "customer1" lock
curl http://127.0.0.1:4001/mod/v2/lock/customer1?field=index
Delete the "customer1" lock with the index 2
curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?index=2
Delete the "customer1" lock with the value "bar"
curl -X DELETE http://127.0.0.1:4001/mod/v2/lock/customer1?value=bar
Leader Election
The Leader Election module wraps the Lock module to allow clients to come to consensus on a single value. This is useful when you want one server to process at a time but allow other servers to fail over. The API is similar to the Lock module but is limited to simple strings values.
Here's the API:
Attempt to set a value for the "order_processing" leader key:
curl -X PUT http://127.0.0.1:4001/mod/v2/leader/order_processing?ttl=60 -d name=myserver1.foo.com
Retrieve the current value for the "order_processing" leader key:
curl http://127.0.0.1:4001/mod/v2/leader/order_processing
myserver1.foo.com
Remove a value from the "order_processing" leader key:
curl -X DELETE http://127.0.0.1:4001/mod/v2/leader/order_processing?name=myserver1.foo.com
If multiple clients attempt to set the value for a key then only one will succeed.
The other clients will hang until the current value is removed because of TTL or because of a DELETE operation.
Multiple clients can submit the same value and will all be notified when that value succeeds.
To update the TTL of a value simply reissue the same PUT command that you used to set the value.