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Developer Tasks

Step-by-step guides for developers using etcd as a key-value store in their applications.

1: Reading from etcd
2: Writing to etcd
3: How to get keys by prefix
4: How to delete keys
5: How to make multiple writes in a transaction
6: How to watch keys
7: How to create lease
8: How to create locks

1 - Reading from etcd

Reading a value in an etcd cluster

Prerequisites

Install etcdctl

Procedure

Use the get subcommand to read from etcd:

$ etcdctl --endpoints=$ENDPOINTS get foo
foo
Hello World!
$

where:

foo is the requested key
Hello World! is the retrieved value

Or, for formatted output:

$ etcdctl --endpoints=$ENDPOINTS --write-out="json" get foo
{"header":{"cluster_id":289318470931837780,"member_id":14947050114012957595,"revision":3,"raft_term":4,
"kvs":[{"key":"Zm9v","create_revision":2,"mod_revision":3,"version":2,"value":"SGVsbG8gV29ybGQh"}]}}
$

where write-out="json" causes the value to be output in JSON format (note that the key is not returned).

2 - Writing to etcd

Adding a KV pair to an etcd cluster

Prerequisites

Install etcdctl

Procedure

Use the put subcommand to write a key-value pair:

etcdctl --endpoints=$ENDPOINTS put foo "Hello World!"

where:

foo is the key name
"Hello World!" is the quote-delimited value

3 - How to get keys by prefix

Guide to extracting etcd keys by their prefix

Pre-requisites

Get keys by prefix

$ etcdctl --endpoints=$ENDPOINTS get PREFIX --prefix

Global Options

--endpoints=[127.0.0.1:2379], gRPC endpoints

Options

--prefix, get a range of keys with matching prefix

Example

etcdctl --endpoints=$ENDPOINTS put web1 value1
etcdctl --endpoints=$ENDPOINTS put web2 value2
etcdctl --endpoints=$ENDPOINTS put web3 value3

etcdctl --endpoints=$ENDPOINTS get web --prefix

4 - How to delete keys

Describes a way to delete etcd keys

Prerequisites

Install etcd and etcdctl

Add or delete keys

del to remove the specified key or range of keys:

etcdctl del $KEY [$END_KEY]

Options

--prefix[=false]: delete keys with matching prefix
--prev-kv[=false]: return deleted key-value pairs
--from-key[=false]: delete keys that are greater than or equal to the given key using byte compare
--range[=false]: delete range of keys without delay

Options inherited from parent commands

--endpoints="127.0.0.1:2379": gRPC endpoints

Examples

etcdctl --endpoints=$ENDPOINTS put key myvalue
etcdctl --endpoints=$ENDPOINTS del key

etcdctl --endpoints=$ENDPOINTS put k1 value1
etcdctl --endpoints=$ENDPOINTS put k2 value2
etcdctl --endpoints=$ENDPOINTS del k --prefix

5 - How to make multiple writes in a transaction

Guide to making transactional writes

Prerequisites

Install etcd and etcdctl .
A running etcd cluster.

Terminology

Here are definitions of some key terms used in the Example below.

Terms	Definition
etcdctl	The command line tool for interacting with the etcd server.
`txn` command	`txn` command is an abbreviation for “transaction”. It reads multiple etcd requests from standard input and applies them as a single atomic transaction. A transaction consists of list of conditions, a list of requests to apply if all the conditions are true, and a list of requests to apply if any condition is false. View etcdctl key-value commands for more information.
`compare`	The `compare` clause within a transaction (`txn`) serves as a conditional check that determines whether the transaction’s operations should proceed. It ensures changes are only applied if the current state of the key-value store matches expected conditions, thereby maintaining data consistency and preventing conflicts in concurrent environments. To see how the command is structured, view Perform a transaction section below.

Transactions

txn to process all the requests in one transaction:

etcdctl txn --help

Transactions in etcd allow you to execute multiple operations atomically, ensuring that either all operations are applied or none are. This is crucial for maintaining data consistency when performing related updates. Learn more about transactions in the API documentation .

Example

Let’s consider a scenario where you want to update a user’s email and phone number in a single transaction. This ensures that both updates are applied together.

0. Variables and Flags used

Variables
`/users/{<user_id>/email` : etcd key representing a user’s email address.
`/users/<user_id>/phone` : etcd key representing a user’s phone number.
Flags
`--interactive` : A flag to allow inputting transaction data manually

1. Set up initial data

First, create a user with some initial data.

etcdctl put /users/12345/email "old.address@johndoe.com"
etcdctl put /users/12345/phone "123-456-7890"

2. Perform a transaction

Update the user’s email and phone number in a single transaction.

etcdctl txn --interactive

compares:
value("/users/12345/email") = "old.address@johndoe.com"

success requests (get, put, delete):
put /users/12345/email "new.address@johndoe.com"
put /users/12345/phone "098-765-4321"

failure requests (get, put, delete):
get /users/12345/email

Compare: Check if the current email is “old.address@johndoe.com ”. This ensures the transaction only proceeds if the data is as expected.
Success: If the comparison is true, update both the email and phone number.
Failure: If the comparison fails, retrieve the current email to understand why the transaction didn’t proceed.

Important considerations

Atomicity: The transaction ensures that both the email and phone number are updated together. If the initial condition (comparison) is not met, neither update is applied.
Consistency: Using transactions maintains data consistency, especially when dealing with multiple related updates.
Avoid multiple puts on the same key: Do not put multiple values for the same key within a single transaction, as this can lead to unexpected results. Each key should be updated only once per transaction.

6 - How to watch keys

Guide to watching etcd keys

Prerequisites

Install etcd and etcdctl

Watching keys

watch to get notified of future changes:

etcdctl watch $KEY [$END_KEY]

Options

-i, --interactive[=false]: interactive mode
--prefix[=false]: watch on a prefix if prefix is set
--rev=0: Revision to start watching
--prev-kv[=false]: get the previous key-value pair before the event happens
--progress-notify[=false]: get periodic watch progress notification from server

Options inherited from parent commands

--endpoints="127.0.0.1:2379": gRPC endpoints

Examples

etcdctl --endpoints=$ENDPOINTS watch stock1
etcdctl --endpoints=$ENDPOINTS put stock1 1000

etcdctl --endpoints=$ENDPOINTS watch stock --prefix
etcdctl --endpoints=$ENDPOINTS put stock1 10
etcdctl --endpoints=$ENDPOINTS put stock2 20

7 - How to create lease

Guide to creating a lease in etcd

lease to write with TTL:

etcdctl --endpoints=$ENDPOINTS lease grant 300
# lease 2be7547fbc6a5afa granted with TTL(300s)

etcdctl --endpoints=$ENDPOINTS put sample value --lease=2be7547fbc6a5afa
etcdctl --endpoints=$ENDPOINTS get sample

etcdctl --endpoints=$ENDPOINTS lease keep-alive 2be7547fbc6a5afa
etcdctl --endpoints=$ENDPOINTS lease revoke 2be7547fbc6a5afa
# or after 300 seconds
etcdctl --endpoints=$ENDPOINTS get sample

8 - How to create locks

Guide to creating distributed locks in etcd

LOCK acquires a distributed mutex with a given name. Once the lock is acquired, it will be held until etcdctl is terminated.

Prerequisites

Install etcd and etcdctl

Creating a lock

lock for distributed lock:

etcdctl --endpoints=$ENDPOINTS lock mutex1

Options

endpoints - defines a comma-delimited list of machine addresses in the cluster.
ttl - time out in seconds of lock session.