Consul 1.15 Key Value Store Practice Test 3 — 15 Questions

Question 1

You are using Consul KV to store application configuration. You need to retrieve all keys under the prefix 'app/config/' and also see the metadata for each key, such as the number of times it has been modified. Which TWO of the following approaches will provide both the keys and their metadata? (Choose two.)

Choose an answer to begin — your selection is scored in the full session.

Accepted Answer

Use the HTTP API endpoint GET /v1/kv/app/config/?recurse=true and parse the JSON response.. The HTTP API GET with 'recurse=true' returns a JSON array with full key objects including metadata. The CLI command 'consul kv get -detailed -recurse' also returns keys with detailed metadata. Both approaches provide the keys and their metadata in one operation. The other options either omit metadata or require multiple steps.

Answer

Run 'consul kv get -recurse app/config/' and use the output as is.

Answer

Use the Consul UI to browse to the 'app/config/' prefix; it displays keys but not metadata.

Answer

Use the HTTP API endpoint GET /v1/kv/app/config/?recurse=true&keys=true to list only the keys, then query each key individually for metadata.

Question 2

You are using Consul KV to store application configuration. You need to update a key's value and ensure that the update only succeeds if the key's current value matches a specific value you provide. Which Consul KV operation should you use?

Accepted Answer

consul kv put -cas=<modify_index> config/key newvalue. Consul KV supports check-and-set (CAS) operations via the -cas flag, which takes a modify index. To conditionally update based on the current value, you must first read the key to obtain its value and modify index. If the value matches your expectation, you use that modify index in the put command. This guarantees that the update only succeeds if the key has not changed since the read, effectively achieving value-based conditional updates.

Answer

consul kv put -cas=1 config/key newvalue

Answer

consul kv put -cas=value config/key newvalue

Answer

consul kv put -cas=0 config/key newvalue

Question 3

You are designing a leader-election mechanism using Consul KV and sessions for a cluster of workers. You need to ensure that only one worker holds leadership at a time and that leadership is automatically relinquished if a worker dies. Which two statements about Consul session-based locking are true? (Choose two.)

Accepted Answer

A key can be acquired by a session only if it is currently unlocked or already held by that same session, and the `LockIndex` increments on each successful acquisition.. Session-based locks provide mutual exclusion because `acquire` succeeds only on an unlocked key or one already owned by the caller, and the lock is a lease tied to session validity. When a worker dies or is partitioned and stops renewing, the TTL expires, the session is invalidated, and Consul releases the lock so another worker can take leadership. Together these give safe, automatic leader failover.

Answer

A session created with `-behavior=delete` deletes its keys on invalidation, while the default behavior releases locks but leaves key values intact.

Answer

A single session can hold locks on multiple keys simultaneously, and destroying the session releases all of those locks at once.

Answer

Locks acquired through sessions are enforced by Consul's Raft consensus, so a partitioned client retains its lock until it explicitly calls the release endpoint.

Question 4

A developer is using the Consul KV store to coordinate a distributed batch processing system. They need to atomically acquire a lock on a key to ensure only one worker processes a batch at a time. They have created a Consul session and want to use the KV API to acquire the lock. Which two HTTP API endpoints should they use to acquire and release the lock? (Choose two.)

Accepted Answer

`PUT /v1/kv/?release=`. Lock acquisition and release in Consul KV are performed using the `acquire` and `release` query parameters on the `PUT /v1/kv/` endpoint. The `acquire` parameter attempts to lock the key with the given session, and `release` releases it. These operations are atomic and form the basis of distributed locking.

Answer

`DELETE /v1/kv/?session=`

Answer

`GET /v1/kv/?lock=`

Answer

`PUT /v1/session/renew/<session>`

Question 5

When using the Consul KV store, what is the maximum size allowed for a single value in a key?

Accepted Answer

512 KB. The Consul KV store enforces a maximum value size of 512 KB per key. This limit is a hard constraint imposed by the Raft consensus protocol, which replicates all writes across the cluster. Values larger than 512 KB are rejected by the server to prevent excessive memory usage and network overhead during replication. This is documented in the official Consul limits and is consistent across versions, including 1.15.

Answer

There is no limit.

Answer

128 KB

Answer

1 MB

Question 6

Which Consul API endpoint is used to create a new session that can be used for locking KV keys?

Accepted Answer

/v1/session/create. The correct endpoint is /v1/session/create because it is specifically designed to create a new session in Consul. Sessions are the foundation for distributed locks and KV key acquisition. When you create a session, Consul returns a session ID that can be used with the KV acquire endpoint to obtain a lock on a key. This endpoint accepts parameters like name, TTL, and behavior on invalidation, allowing fine-grained control over lock semantics.

Answer

/v1/kv/acquire

Answer

/v1/lock/init

Answer

/v1/session/renew

Question 7

You are configuring a Consul Watch to trigger a script whenever a specific KV key changes. Which type of watch should you define in your configuration file?

Accepted Answer

key. The 'key' watch type in Consul is specifically designed to monitor a single KV pair for changes. When the value of the specified key changes, the watch triggers the configured handler (e.g., a script). This matches the requirement of triggering a script whenever a specific KV key changes. Other watch types monitor different Consul resources such as events, services, or nodes, and are not suited for tracking a single KV key.

Answer

event

Answer

service

Answer

nodes

Question 8

You have implemented a leader election mechanism using Consul KV sessions. The session has a TTL of 15 seconds. If the application crashes, how long will it take for the lock to be automatically released?

Accepted Answer

15 seconds.. Consul sessions are invalidated when their TTL expires without a renewal. With a 15-second TTL, if the application crashes and stops calling session renew, the session becomes invalid after 15 seconds, and any locks or KV entries tied to that session are released at that point. The lock release is therefore bounded by the session TTL, not by any manual intervention.

Answer

Immediately.

Answer

60 seconds.

Answer

The lock is never released until manually deleted.

Question 9

You observe that a session lock is being released unexpectedly. What is the most likely cause?

Accepted Answer

The session TTL is too low, causing premature expiration.. Consul sessions are tied to a TTL (Time-To-Live) that must be periodically renewed by the owning agent. If the TTL is set too low and the agent fails to renew in time—due to network latency, agent overload, or misconfiguration—the session expires, and any locks held by that session are automatically released. This is the most common cause of unexpected lock releases in Consul.

Answer

The Consul agent is in leader mode.

Answer

The index was not incremented.

Answer

The KV key was deleted.

Question 10

You want to perform a blocking query on a KV key to watch for updates. Which parameter must you provide in your request based on the last observed index?

Accepted Answer

index. To perform a blocking query on a KV key, you must provide the 'index' parameter, which specifies the last observed ModifyIndex. Consul will hold the request until the key's index exceeds this value or the wait time expires.

Answer

consistency

Answer

wait

Answer

timeout

Question 11

You need to perform a write operation to the Consul KV store that only succeeds if the current ModifyIndex of the key is zero. Which parameter should you include in your API call?

Accepted Answer

cas=0. The Consul KV write API supports a 'cas' (Check-And-Set) parameter that performs an atomic compare-and-swap against the key's ModifyIndex. Setting cas=0 means the write only succeeds if the key does not currently exist (ModifyIndex is zero), which is the standard way to implement create-if-not-exists semantics.

Answer

acquire=0

Answer

flags=0

Answer

index=0

Question 12

Which command-line interface command is used to set a value in the Consul KV store?

Accepted Answer

consul kv put. The correct Consul CLI command to set a value in the KV store is 'consul kv put'. This command writes a value to the specified key path. For example, 'consul kv put mykey myvalue' stores 'myvalue' at the key 'mykey'. The other commands do not exist in the Consul CLI.

Answer

consul kv write

Answer

consul kv set

Answer

consul kv update

Question 13

When a session is associated with a lock, what happens to the lock if the session is deleted?

Accepted Answer

The lock is released.. Consul locks are bound to the session that acquired them. When a session is deleted — whether explicitly via the session destroy API or implicitly through TTL expiry — Consul automatically releases every lock and KV entry associated with that session. This is the core safety property that makes session-based locking reliable for leader election.

Answer

The lock remains held by the server.

Answer

The lock is held until the key is deleted.

Answer

The lock transfers to a new session.

Question 14

Which HTTP method is used to create or update a key in the Consul KV store?

Accepted Answer

PUT. The Consul KV store uses the PUT HTTP method to create or update a key. A PUT to /v1/kv/<key> with the value in the request body either creates the key if it does not exist or overwrites it if it does, making PUT the idempotent write operation for KV entries.

Answer

GET

Answer

DELETE

Answer

POST

Question 15

You need to ensure that a read operation from the KV store is strongly consistent. Which consistency mode should you specify?

Accepted Answer

consistent. The 'consistent' mode forces the Consul server handling the request to forward it to the leader, which then verifies it still holds leadership via a quorum of followers before serving the read. This guarantees linearizable (strongly consistent) reads, ensuring you never see stale data. In contrast, 'default' and 'stale' allow reads from any server and may return outdated values, while 'quorum' is not a valid consistency mode for KV reads.

Answer

default

Answer

quorum

Answer

stale