Client discovery through Sentinel in Redis - Step-by-Step Execution

Concept Flow - Client discovery through Sentinel

Client starts

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Query Sentinel for master info

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Sentinel responds with master address

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Client connects to master

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If master fails

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Sentinel elects new master

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Client queries Sentinel again

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Client connects to new master

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Repeat as needed

Client asks Sentinel for the current master address, connects to it, and if the master changes, repeats the discovery to stay connected to the right server.

Execution Sample

Redis

SENTINEL get-master-addr-by-name mymaster
// Client receives master IP and port
// Client connects to master
// On failover, client queries Sentinel again

Client asks Sentinel for the master address, then connects to that master, updating if Sentinel reports a new master.

Execution Table

Step	Action	Sentinel Response	Client Action	Result
1	Client sends 'SENTINEL get-master-addr-by-name mymaster'	Returns ['192.168.1.10', '6379']	Connect to 192.168.1.10:6379	Connected to master
2	Master fails	Sentinel elects new master '192.168.1.11:6379'	Client sends 'SENTINEL get-master-addr-by-name mymaster' again	Gets new master address
3	Client receives new master address	Returns ['192.168.1.11', '6379']	Connect to 192.168.1.11:6379	Connected to new master
4	No further failover	Sentinel continues monitoring	Client continues normal operations	Stable connection
5	Client stops querying Sentinel	No new info	Client stays connected to last known master	Potential stale connection if failover occurs
6	Exit	N/A	Client disconnects or stops	End of discovery process

💡 Client stops querying Sentinel or disconnects, ending the discovery process

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
master_address	null	192.168.1.10:6379	192.168.1.10:6379 (failed)	192.168.1.11:6379	192.168.1.11:6379

Key Moments - 2 Insights

Why does the client ask Sentinel for the master address again after a failover?

Because the original master failed and Sentinel elected a new master, so the client must get the updated address to connect to the correct server (see execution_table step 2 and 3).

What happens if the client stops querying Sentinel after a failover?

The client may stay connected to the old master address, which is no longer valid, causing errors or stale data (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution_table, what master address does the client connect to at Step 1?

A 192.168.1.10:6379

B 192.168.1.11:6379

C 127.0.0.1:6379

D No connection yet

💡 Hint

Check the 'Sentinel Response' and 'Client Action' columns in Step 1.

At which step does the client receive the new master address after failover?

A Step 2

B Step 3

C Step 1

D Step 5

💡 Hint

Look for when the client gets a new address from Sentinel in the table.

If the client never queries Sentinel again after Step 1, what is the likely outcome?

A Client connects to new master automatically

B Sentinel pushes new master info to client

C Client stays connected to old master even if it fails

D Client disconnects immediately

💡 Hint

Refer to Step 5 in the execution_table and variable_tracker for stale connection.

Concept Snapshot

Client discovery through Sentinel:
- Client asks Sentinel for master address using 'SENTINEL get-master-addr-by-name <name>'.
- Sentinel replies with current master IP and port.
- Client connects to that master.
- On failover, Sentinel elects new master.
- Client queries Sentinel again to get updated master address.
- Repeat to maintain connection to correct master.

Full Transcript

Client discovery through Sentinel means the client asks Sentinel for the current master server address. The client sends the command 'SENTINEL get-master-addr-by-name' with the master name. Sentinel responds with the IP and port of the master. The client then connects to that master. If the master fails, Sentinel chooses a new master. The client must ask Sentinel again to get the new master's address and connect to it. This process repeats to keep the client connected to the right master. If the client stops asking Sentinel, it might stay connected to an old master that no longer works.