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Replication lag monitoring in Redis - Cheat Sheet & Quick Revision

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Recall & Review
beginner
What is replication lag in Redis?
Replication lag is the delay between when data is written on the master and when it is updated on the replica.
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beginner
Which Redis command helps check the replication lag?
The INFO REPLICATION command shows replication details including the lag time.
Click to reveal answer
intermediate
What does the master_repl_offset field represent in Redis replication info?
It shows the current replication offset of the master, indicating how much data has been sent to replicas.
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intermediate
How can you estimate replication lag using offsets in Redis?
By comparing the master's master_repl_offset with the replica's slave_repl_offset, the difference shows lag in bytes.
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beginner
Why is monitoring replication lag important in Redis?
Because high lag means replicas have outdated data, which can cause stale reads and affect application performance.
Click to reveal answer
Which Redis command provides replication status including lag information?
AINFO REPLICATION
BMONITOR
CCLIENT LIST
DCONFIG GET
What does a large difference between master_repl_offset and slave_repl_offset indicate?
AFast replication
BLow memory usage
CHigh replication lag
DDisconnected replica
If replication lag is high, what problem might occur?
AReplicas serve outdated data
BMaster crashes
CData loss on master
DFaster writes
Which metric is NOT directly related to replication lag in Redis INFO REPLICATION output?
Amaster_repl_offset
Bconnected_clients
Cslave_repl_offset
Dmaster_link_status
How often should you monitor replication lag in a production Redis setup?
AOnce a month
BNever
COnly after failures
DContinuously or frequently
Explain how to check replication lag in Redis using command line tools.
Look at replication offsets in INFO REPLICATION output.
You got /4 concepts.
    Why is it important to monitor replication lag in Redis and what issues can arise if lag is ignored?
    Think about how delayed data affects users.
    You got /3 concepts.

      Practice

      (1/5)
      1. What does the master_last_io_seconds_ago field in INFO replication indicate in Redis?
      easy
      A. The number of seconds since the master last communicated with the replica
      B. The total number of connected replicas
      C. The current memory usage of the master
      D. The replication backlog size in bytes

      Solution

      1. Step 1: Understand the INFO replication output

        This command shows replication details including communication status between master and replicas.

      2. Step 2: Interpret master_last_io_seconds_ago

        This field tells how many seconds ago the master last sent data to the replica, indicating replication lag.

      3. Final Answer:

        The number of seconds since the master last communicated with the replica -> Option A

      4. Quick Check:

        Replication lag = master_last_io_seconds_ago [OK]
      Hint: Lag time is how long since last master-replica communication [OK]
      Common Mistakes:
      • Confusing lag with number of replicas
      • Mixing memory usage with replication info
      • Thinking backlog size shows lag
      2. Which Redis command correctly shows replication lag details?
      easy
      A. SHOW replication
      B. INFO replication
      C. MONITOR replication
      D. REPLICA STATUS

      Solution

      1. Step 1: Recall Redis commands for replication info

        The INFO command with the replication section shows replication status and lag.

      2. Step 2: Verify command correctness

        MONITOR shows all commands, SHOW and REPLICA STATUS are invalid commands.

      3. Final Answer:

        INFO replication -> Option B

      4. Quick Check:

        Correct command = INFO replication [OK]
      Hint: Use INFO replication to check lag, not monitor or show [OK]
      Common Mistakes:
      • Using MONITOR instead of INFO
      • Trying non-existent commands
      • Confusing SHOW with INFO
      3. Given this INFO replication snippet:
      master_link_status:up
master_last_io_seconds_ago:5

      What does the master_last_io_seconds_ago value tell you?
      medium
      A. The replica is 5 seconds behind the master
      B. The master has been down for 5 seconds
      C. The master last sent data 5 seconds ago
      D. There are 5 replicas connected

      Solution

      1. Step 1: Analyze master_link_status and master_last_io_seconds_ago

        The status 'up' means the master and replica are connected. The number 5 means 5 seconds since last data sent.

      2. Step 2: Interpret the lag meaning

        The value shows how many seconds ago the master last communicated, not the exact lag or downtime.

      3. Final Answer:

        The master last sent data 5 seconds ago -> Option C

      4. Quick Check:

        master_last_io_seconds_ago = last data sent seconds [OK]
      Hint: Lag means last data sent time, not downtime or replica count [OK]
      Common Mistakes:
      • Confusing lag with downtime
      • Assuming number means replicas count
      • Thinking it shows replica delay exactly
      4. You run INFO replication and see master_link_status:down and master_last_io_seconds_ago:1000. What is the best first step to fix replication lag?
      medium
      A. Restart the Redis master server immediately
      B. Flush all data on the replica
      C. Increase the replication backlog size
      D. Check network connectivity between master and replica

      Solution

      1. Step 1: Understand the meaning of master_link_status:down

        This means the replica lost connection to the master, causing lag.

      2. Step 2: Identify the cause and fix

        Network issues are the common cause; checking connectivity is the first step before restarting or changing configs.

      3. Final Answer:

        Check network connectivity between master and replica -> Option D

      4. Quick Check:

        Link down means network check first [OK]
      Hint: Link down? Check network before restarting or flushing [OK]
      Common Mistakes:
      • Restarting master without checking network
      • Changing backlog size without cause
      • Flushing replica data unnecessarily
      5. You want to alert if replication lag exceeds 10 seconds. Which approach using INFO replication is best?
      hard
      A. Parse master_last_io_seconds_ago and trigger alert if > 10
      B. Check connected_slaves count and alert if less than 1
      C. Monitor master_link_status only for 'down' status
      D. Use INFO memory to check memory usage spikes

      Solution

      1. Step 1: Identify the lag metric

        master_last_io_seconds_ago directly shows how many seconds since last master-replica communication, ideal for lag monitoring.

      2. Step 2: Set alert condition

        Trigger alert when this value exceeds 10 seconds to catch lag early.

      3. Final Answer:

        Parse master_last_io_seconds_ago and trigger alert if > 10 -> Option A

      4. Quick Check:

        Lag alert = master_last_io_seconds_ago > threshold [OK]
      Hint: Alert on master_last_io_seconds_ago > threshold seconds [OK]
      Common Mistakes:
      • Using connected_slaves count for lag
      • Only watching link status without lag value
      • Checking memory instead of replication lag
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