Learn vocabulary for communicating database replication: lag, read replicas, consistency trade-offs, and failover.
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What is 'replication lag' in database vocabulary?
Replication lag is the delay between a write committed on the primary and the same write being visible on read replicas. In asynchronous replication, replicas may lag by milliseconds to seconds depending on write volume and network conditions. High lag means replicas serve stale data.
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What is 'read replica' in database architecture vocabulary?
A read replica is a continuously replicated copy of the primary that can serve SELECT queries. Applications route reads to replicas (reducing primary load) and writes to the primary. The trade-off: reads from replicas may be slightly stale due to replication lag.
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What is 'eventual consistency' in the context of database replication?
Eventual consistency: if no new writes occur, all replicas will eventually converge to the same state. In practice, this means reads from replicas may return older data. Applications must be designed to tolerate stale reads or use techniques like read-your-writes consistency routing.
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What is a 'failover' in database replication vocabulary?
Failover is the process of making a replica the new primary after primary failure. Automatic failover (e.g., PostgreSQL with Patroni, MySQL Group Replication) detects failure and promotes a replica within seconds. Manual failover requires operator intervention. RTO (Recovery Time Objective) measures how fast failover must complete.
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What is 'replication slot' in PostgreSQL vocabulary?
A PostgreSQL replication slot tracks a replica's replication progress and retains WAL segments until the replica has consumed them. Risk: if a replica falls far behind or disconnects permanently, the slot causes unbounded WAL accumulation on the primary — a common cause of disk exhaustion.
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PR Description
During code review of a new feature implementing read replicas for the user profile service, Sarah comments on David's PR description:
'David, this mentions 'replication lag' but doesn't explain how we'll monitor it or what actions we take if it exceeds 5 seconds. It also doesn't specify which replica is being used for reads.'
Which of the following best describes Sarah's concern regarding David's PR description?
Sarah's concern isn't about the code itself, but about the *description*'s lack of detail. Replication lag refers to the delay between updates on the primary and replicated databases. The PR description needs to articulate how Sarah will monitor this latency (e.g., using metrics) and what steps are in place if it becomes problematic – a key aspect of managing read replicas effectively. Option A is incorrect because replication lag *does* affect reads, leading to stale data. Option C accurately reflects the critical missing information about monitoring and mitigation.
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PR Description
During code review of a new feature implementing read replicas for the user profile service, Sarah comments on David's PR description:
'David, this mentions 'replication lag' but doesn't explain how we'll monitor it or what actions we take if it exceeds 5 seconds. It also doesn't specify which replica is being used for reads.'
Which of the following best describes Sarah's concern regarding David's PR description?
Sarah's concern isn't about the code itself, but about the *description*'s lack of detail. Replication lag refers to the delay between updates on the primary and replicated databases. The PR description needs to articulate how Sarah will monitor this latency (e.g., using metrics) and what steps are in place if it becomes problematic – a key aspect of managing read replicas effectively. Option A is incorrect because replication lag *does* affect reads, leading to stale data. Option C accurately reflects the critical missing information about monitoring and mitigation.
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PR Description
During code review of a new feature implementing read replicas for the user profile service, Sarah comments on David's PR description:
'David, this mentions 'replication lag' but doesn't explain how we'll monitor it or what actions we take if it exceeds 5 seconds. It also doesn't specify which replica is being used for reads.'
Which of the following best describes Sarah's concern regarding David's PR description?
Sarah's concern isn't about the code itself, but about the *description*'s lack of detail. Replication lag refers to the delay between updates on the primary and replicated databases. The PR description needs to articulate how Sarah will monitor this latency (e.g., using metrics) and what steps are in place if it becomes problematic – a key aspect of managing read replicas effectively. Option A is incorrect because replication lag *does* affect reads, leading to stale data. Option C accurately reflects the critical missing information about monitoring and mitigation.
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PR Description
During code review of a new feature implementing read replicas for the user profile service, Sarah comments on David's PR description:
'David, this mentions 'replication lag' but doesn't explain how we'll monitor it or what actions we take if it exceeds 5 seconds. It also doesn't specify which replica is being used for reads.'
Which of the following best describes Sarah's concern regarding David's PR description?
Sarah's concern isn't about the code itself, but about the *description*'s lack of detail. Replication lag refers to the delay between updates on the primary and replicated databases. The PR description needs to articulate how Sarah will monitor this latency (e.g., using metrics) and what steps are in place if it becomes problematic – a key aspect of managing read replicas effectively. Option A is incorrect because replication lag *does* affect reads, leading to stale data. Option C accurately reflects the critical missing information about monitoring and mitigation.
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During a standup meeting discussing the recent deployment of our new database replication setup, Mark says: 'We're seeing increased latency on read queries. It seems like there's a significant 'replication lag' between the primary and replica databases.' Which of the following BEST describes what Mark is referring to?
Replication lag specifically refers to the *delay* that occurs when changes made to the primary database haven't yet been applied to the replicas. This delay directly impacts read query performance as the application might be querying outdated data on the replica. Options B and C are incorrect because they describe bandwidth or schema inconsistency, respectively.
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You're reviewing an API response from our read replicas service. The response includes a timestamp indicating when the data was last synchronized with the primary database. This timestamp is labeled as 'replication lag'. What does this value primarily indicate?
'Replication lag' is a direct measure of the time difference between when a write operation occurs on the primary database and when that change becomes reflected in the replica. A higher value indicates a greater delay in read operations from the replica. Options A, B, and C are related to different metrics.
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During a Slack conversation with another developer, Alex mentions 'eventual consistency' in relation to our read replicas. What does Alex *most likely* mean?
'Eventual consistency' is a common characteristic of distributed database systems like our read replicas. It acknowledges that there might be a period where data on the replica isn't immediately consistent with the primary, but guarantees that over time, they will eventually become synchronized. Options A and B are incorrect; options C and D misrepresent the core concept.
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David is troubleshooting slow read queries on our user profile service that uses read replicas. He discovers that a 'replication slot' is consuming significant resources. What function does this slot primarily represent?
A replication slot is a critical component in PostgreSQL's replication setup. It represents the state of changes prepared for replication on the primary database. The slot itself doesn't hold the data; it holds the information necessary to ensure that the replica can continue replicating from the primary efficiently. Options A, B and C are incorrect descriptions.
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Sarah is explaining the concept of 'failover' in our database replication architecture to a new team member. She says: 'If the primary database goes down, we automatically switch to a read replica, minimizing downtime.' What aspect of replication does Sarah's explanation primarily highlight?
'Failover' in this context refers to the automated process of switching to a secondary replica when the primary database becomes unavailable. This ensures that read operations can continue without interruption, maintaining application availability and minimizing data loss, which is the core function of replication.
What does the "Replication Lag — Vocabulary and Communication" exercise practise?
Learn vocabulary for communicating database replication: lag, read replicas, consistency trade-offs, and failover.
How many questions are in this exercise?
This exercise has 14 questions, each multiple-choice with a full explanation shown after you answer.
What English level is this exercise for?
This exercise is tagged Intermediate. If the vocabulary feels difficult, browse the Database Optimization category page for an easier module to start with.
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Do I get feedback if I answer incorrectly?
Yes — whichever option you choose, right or wrong, you'll immediately see an explanation clarifying the correct term and why the other options don't fit.
Can I retry this exercise?
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Is "Replication Lag — Vocabulary and Communication" part of a larger series?
Yes — it's one exercise in the Database Optimization category on CoderSlingo. See the category page for the full list of related exercises on similar terminology.
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Where can I find more exercises like this one?
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