Consul 1.15 Service Discovery — 20 Questions

Question 1

You are troubleshooting why a service registered via HTTP API is not showing up in the catalog. Which tool provides the most immediate insight into the registration status?

Accepted Answer

consul catalog services. The 'consul catalog' CLI command allows direct inspection of the catalog state.

Answer

consul kv get

Answer

consul reload

Answer

consul members

Question 2

Which Consul feature allows you to perform health checks against a service that does not have an exposed HTTP endpoint but resides on the same machine?

Accepted Answer

Script health check.. Script-based checks run locally on the agent node, allowing monitoring of processes or system resources.

Answer

TCP health check.

Answer

HTTP health check.

Answer

gRPC health check.

Question 3

An administrator wants to register a service using a JSON file. Where should this file be placed on a standard Consul agent installation to ensure automatic loading?

Accepted Answer

/etc/consul.d/. The Consul agent watches the directory defined by the '-config-dir' flag.

Answer

/var/lib/consul/

Answer

/etc/consul/

Answer

/usr/local/bin/

Question 4

You need to register a service that runs on a dynamic port. How should the service definition be configured to allow Consul to detect the port?

Accepted Answer

Set 'port' to 0 in the service definition.. Consul allows the use of 0 as a port, and the registration process typically involves discovering the port via an external process or environment variable.

Answer

Set 'port' to 'auto' in the service definition.

Answer

Omit the 'port' field entirely.

Answer

Use the 'address' field for the port.

Question 5

You have a service that requires specific routing logic based on the environment. How can you distinguish between 'production' and 'staging' instances of the same service name using metadata?

Accepted Answer

Apply 'production' or 'staging' tags to the service definition.. Service tags are the standard way to categorize and filter services within the Consul catalog.

Answer

Use different service names for every environment.

Answer

Modify the service's port number.

Answer

Assign different Consul datacenters.

Question 6

An administrator needs to query service information. Which DNS record type is commonly used by Consul to return service addresses?

Accepted Answer

SRV. Consul maps service names to SRV or A records for discovery.

Answer

TXT

Answer

MX

Answer

CNAME

Question 7

A service is intermittently failing to resolve via DNS because it has multiple health checks, some of which are 'warning'. How do you ensure it is only returned when all checks are 'passing'?

Accepted Answer

Use the 'passing' query parameter in the DNS request.. Consul DNS returns services if they are not in a 'critical' state by default; enforcing strict 'passing' requires query parameters.

Answer

Configure 'service_prefix' in DNS settings.

Answer

Use the 'stale' query parameter.

Answer

Set 'enable_tag_override' to true.

Question 8

You need to ensure that a service is only discoverable via DNS if it passes its health checks. Which configuration setting in the service definition achieves this?

Accepted Answer

Set the 'passing' parameter to true in the DNS query.. Consul allows filtering services based on health checks via the 'passing' parameter in the DNS interface or by defining the service with a health check.

Answer

Enable 'only_passing' in the DNS configuration.

Answer

Disable the agent's 'allow_stale' read setting.

Answer

Set 'deregister_critical_service_after' to 0.

Question 9

What is the purpose of the 'service-tag' in a Consul service definition?

Accepted Answer

To provide additional categorization or versioning for services.. Tags are used for filtering and identifying specific instances or versions of a service.

Answer

To define the physical location of the server.

Answer

To set the priority of the service in DNS.

Answer

To encrypt communication between services.

Question 10

When using Consul DNS for a service, what happens if no instances are healthy?

Accepted Answer

It returns an empty response (NXDOMAIN or empty set).. By default, if no healthy instances are found, the DNS query will return no results.

Answer

It returns a 500 error.

Answer

It redirects to a standby service.

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It returns all instances regardless of health.

Question 11

A service definition includes a 'meta' block. What is the intended use of this block?

Accepted Answer

To define arbitrary key/value metadata for the service.. The meta block is for arbitrary key/value pairs related to the service instance.

Answer

To configure service discovery timeouts.

Answer

To override the default health check interval.

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To store authentication credentials.

Question 12

Which command is used to check the status of a registered service?

Accepted Answer

consul health service. 'consul catalog' or 'consul health' commands are used for status inspection.

Answer

consul check

Answer

consul status

Answer

consul list

Question 13

You are configuring a service check that needs to be ignored if the node is in maintenance mode. What is the correct approach?

Accepted Answer

Use the 'consul maintenance' command on the node.. Maintenance mode stops health checks from affecting the service's passing/failing status.

Answer

Use a tag named 'maintenance'.

Answer

Set 'maintenance_mode' to true in the service definition.

Answer

Set the 'status' to 'maintenance' in the API.

Question 14

You need to register a health check that triggers an alert when memory usage is high. Which type of check is most appropriate?

Accepted Answer

Script Check. A script check can execute a local binary or script to monitor memory and return an exit code.

Answer

gRPC Check

Answer

HTTP Check

Answer

TCP Check

Question 15

How does Consul handle service discovery in multi-datacenter environments when a service is requested by name?

Accepted Answer

It requires the user to append the datacenter name to the DNS query.. Consul can perform cross-datacenter lookups if the query includes the datacenter or if configured to do so.

Answer

It only queries the leader of the entire WAN.

Answer

It automatically replicates all services to all datacenters.

Answer

It always returns instances from all datacenters.

Question 16

When multiple agents are in a cluster, how does Consul ensure that service discovery information is consistent across all nodes?

Accepted Answer

Via the Raft consensus protocol for catalog changes.. The Gossip Protocol (serf) and Raft consensus maintain state across nodes.

Answer

Using a centralized database like PostgreSQL.

Answer

By polling the leader every second.

Answer

By broadcasting all service updates to every node.

Question 17

Which of the following best describes the 'consul agent -dev' mode in relation to service discovery?

Accepted Answer

It provides a single-node Consul environment for testing.. Dev mode is for testing and has ephemeral storage.

Answer

It disables DNS interfaces.

Answer

It forces the service catalog to be stored in Redis.

Answer

It is suitable for production clusters.

Question 18

What is the impact of setting 'enable_tag_override' to true in a service definition?

Accepted Answer

It allows tags to be modified via the API without updating the config file.. It allows external systems to modify service tags without updating the service definition file.

Answer

It ignores all tags in the catalog.

Answer

It forces all nodes to update their tags.

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It prevents tags from being used in DNS lookups.

Question 19

Which interface would you use to register a service dynamically using standard tools like 'curl'?

Accepted Answer

HTTP API. The Consul HTTP API is the standard interface for dynamic registration.

Answer

DNS interface

Answer

Gossip protocol

Answer

CLI agent command

Question 20

What is the effect of changing the 'interval' parameter in a service health check?

Accepted Answer

It changes the frequency at which the check runs.. The interval defines how often the health check executes.

Answer

It changes the DNS TTL.

Answer

It changes the weight of the service.

Answer

It changes the timeout of the check.