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Card class: Hero • Category: Replication

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At a glance

How far behind the primary an asynchronous replica is, measured in seconds. This is the replica’s Seconds_Behind_Master (now Seconds_Behind_Source): the gap between the timestamp of the last event the replica has applied and the current time on the primary. Zero means the replica is caught up; a rising number means the replica’s apply thread cannot keep pace with the write rate. For a DBA, lag is the single most important replication signal: lagging replicas serve stale reads, fail their freshness SLOs, and (most dangerously) cannot be promoted cleanly in a failover without losing the un-applied transactions. When lag crosses 10 seconds the card turns amber.
What it tracksAsync Replication Lag (seconds): the maximum Seconds_Behind_Source across active asynchronous replicas. The detail line is Async Replication Lag (seconds) for the selected period.
Data sourceMariaDB SHOW REPLICA STATUS on each replica, reading Seconds_Behind_Source (Seconds_Behind_Master on older versions). Where GTID is in use, the engine cross-checks Gtid_IO_Pos against the primary’s gtid_binlog_pos for a position-based view.
Time windowRT: real-time, refreshed on each poll. The headline is the current worst-case lag across replicas.
Alert trigger> 10s. Above this the card turns amber and surfaces in the Sensitivity feed.
Distinct fromGalera flow-control pause (synchronous cluster), which is a different replication model. This card is for classic async (binlog-based) replication.
RolesDBA, platform, SRE

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Calculation

The headline is the maximum lag across all active async replicas, so a single straggler sets the number. On each replica, MariaDB reports Seconds_Behind_Source from SHOW REPLICA STATUS. That value is computed as the difference between the current replica clock and the timestamp recorded in the binary-log event currently being applied, adjusted for the known clock offset between primary and replica. 
lag_seconds(replica) = now_on_replica - timestamp_of_event_being_applied
card_value           = max(lag_seconds across active replicas)
Two well-known quirks matter. First, Seconds_Behind_Source reports NULL when the replica is not actually replicating (either thread stopped); the engine treats NULL as “replication broken” and surfaces it distinctly from “lag is high”, because the two need different responses. Second, the value can read 0 deceptively if the I/O thread itself has stalled: the replica thinks it is caught up to the last event it received, even though it has stopped receiving new ones. To guard against that, where GTID is enabled the engine also compares the replica’s applied GTID position against the primary’s, which exposes a stalled I/O thread that the seconds-based metric would miss.

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Worked example

A platform team runs a MariaDB 10.11 primary with two read replicas behind an application that routes reporting and search reads to the replicas. Snapshot taken on 18 Mar 26 at 16:40 GMT.
ReplicaSeconds_Behind_SourceNote
replica-a2 shealthy, tracking write rate
replica-b34 slagging, single apply thread saturated
Card value (max)34 samber (threshold > 10s)
replica-a is fine, but replica-b has fallen 34 seconds behind, so the card reads 34 and turns amber. Because the card reports the worst case, the DBA knows at least one replica is serving reads that are over half a minute stale. The investigation: 
-- On replica-b
SHOW REPLICA STATUS\G
-- Seconds_Behind_Source: 34
-- Slave_SQL_Running_State: 'Reading event from the relay log'
-- Slave_IO_Running: Yes, Slave_SQL_Running: Yes

SHOW PROCESSLIST;   -- look at the replica SQL apply thread
Both threads are running, so replication is not broken; the apply thread simply cannot keep up. The cause: a large DELETE on the primary touched two million rows, and the replica is applying it single-threaded while new writes keep arriving. The fixes are well established:
  1. Enable parallel replication. Set slave_parallel_threads (and an appropriate slave_parallel_mode) so independent transactions apply concurrently instead of serially. This is the standard remedy for an apply-bound replica.
  2. Chunk large DML on the primary. A two-million-row DELETE replicates as one serial unit of work; break it into batches so the replica drains them between normal traffic.
  3. Check replica hardware. If the replica has slower disks than the primary, its apply thread is I/O-bound and will lag under any sustained write burst. Match replica I/O to the primary for read-replica topologies.
After enabling four parallel apply threads, replica-b drains the backlog and lag returns to 1 to 2 seconds. Three takeaways:
  1. The card reports the worst replica, not the average. One lagging straggler is enough to turn it amber, which is correct: a failover to that replica would lose the un-applied transactions.
  2. Lag is usually an apply-side problem. The I/O thread (pulling binlog) is rarely the bottleneck; the SQL apply thread is. Parallel replication and chunked DML are the two highest-leverage fixes.
  3. NULL is worse than a big number. A high lag means the replica is working through a backlog. NULL means a thread has stopped and the replica is not replicating at all. Treat the two differently: high lag needs tuning, NULL needs SHOW REPLICA STATUS to read the Last_Error and restart the thread.

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Sibling cards

CardWhy pair it with Async Replication LagWhat the combination tells you
Active Async ReplicasThe count of replicas that should be reporting lag.If a replica disappears from the count, its lag stops contributing; a dropped replica can mask a lag problem.
Failover ReadinessWhether a clean promotion is currently possible.High lag directly degrades failover readiness: you cannot promote a replica that is 30s behind without data loss.
Queries per Second (live)The write-rate context driving the lag.Lag that rises with a write-rate spike is load-driven; lag that rises at steady QPS is an apply-thread or hardware problem.
Query Latency p99 (ms)Long transactions that replicate as serial work.A p99 spike from a long transaction on the primary often precedes a lag spike on the replica that must apply it.
Database Disk Usage %Relay-log growth while the replica catches up.A lagging replica accumulates relay logs; sustained lag can fill replica disk.
Memory Usage %Buffer-pool pressure on the apply side.An apply thread paying for disk reads (cold cache) lags more under the same load.
MariaDB Health ScoreThe composite that weights replication health.Sustained lag pulls the composite down even when the primary is serving writes cleanly.
MariaDB Inventory Rows vs Ecom Inventory CountThe downstream effect of stale replica reads.If the storefront reads inventory from a lagging replica, counts diverge from the source of truth.

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Reconciling against the source

Where to look in MariaDB’s own tooling:
SHOW REPLICA STATUS\G on each replica: read Seconds_Behind_Source, Slave_IO_Running, Slave_SQL_Running, and Last_Error. For GTID topologies, compare SELECT @@gtid_slave_pos; on the replica against SELECT @@gtid_binlog_pos; on the primary for a position-based lag view that survives a stalled I/O thread. SHOW REPLICA HOSTS; on the primary to confirm which replicas should be reporting. pt-heartbeat (Percona Toolkit) writes a heartbeat row on the primary and measures true lag on the replica, immune to the Seconds_Behind_Source stalled-I/O quirk.
Why our number may legitimately differ from a manual SHOW REPLICA STATUS:
ReasonDirectionWhy
Max vs single replicaOurs higherThe card reports the worst replica; reading one healthy replica manually will show less lag than our maximum.
Stalled I/O threadOurs higher (GTID path)Seconds_Behind_Source can read 0 when the I/O thread has stalled; our GTID cross-check exposes the real gap.
Clock skewMarginalSeconds_Behind_Source assumes synchronised clocks; drift between primary and replica adds or removes a second or two.
NULL handlingSurfaced separatelyA stopped thread reports NULL; we surface that as broken replication rather than as numeric lag.
On managed services: Amazon RDS / Aurora for MariaDB exposes lag as the ReplicaLag CloudWatch metric (in seconds) and in the console replication view; SkySQL and Azure Database for MariaDB report lag in their own metrics consoles. Aurora’s storage-level replication behaves differently from classic binlog replication, so its lag metric is typically far lower; align the replication model before comparing.

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Known limitations / FAQs

Q: The card reads zero but I suspect the replica is stale. Can that happen? Yes, and it is the classic Seconds_Behind_Source trap. If the replica’s I/O thread has stalled, the replica believes it is caught up to the last event it received and reports 0, even though new events on the primary are not arriving. Where GTID is enabled the engine cross-checks the applied GTID position against the primary and surfaces the real gap. For a definitive check run pt-heartbeat, which measures true lag independent of the I/O thread state. Q: What is the difference between high lag and NULL? High lag means replication is working but the apply thread is behind: the replica is draining a backlog. NULL means a replication thread has stopped (Slave_IO_Running or Slave_SQL_Running is No), so the replica is not replicating at all. Run SHOW REPLICA STATUS\G, read Last_Error, fix the cause (often a duplicate-key or missing-row error), then START REPLICA. The two states need different responses, which is why the card distinguishes them. Q: Lag keeps spiking under load. How do I reduce it? The apply thread is almost always the bottleneck. Enable parallel replication (slave_parallel_threads with an appropriate slave_parallel_mode) so independent transactions apply concurrently. Chunk large DML on the primary so a single big UPDATE/DELETE does not replicate as one serial unit. Ensure the replica’s disk and memory match the primary; an under-provisioned replica is I/O-bound on apply. Check Queries per Second (live) to confirm the lag tracks write-rate spikes. Q: Does this card cover Galera (synchronous) clusters? No. Galera is synchronous multi-primary replication with a different lag model (flow control rather than seconds-behind). For Galera use Galera Flow Control Paused % and Galera Cluster Status. This card is for classic async binlog replication. Q: Why does the card sometimes show a higher number than my managed-service console? The card reports the maximum lag across replicas, so it tracks your worst follower. A console that shows the lag for one specific replica (or an average) will read lower. On Aurora the storage-level replication metric is naturally much smaller than classic binlog lag, so align the replication model before treating the gap as an error. Q: Is a few seconds of lag a problem? It depends on what reads from the replica. For analytics and reporting, several seconds is harmless. For read-after-write paths (a user updates a record then immediately reads it from a replica) even one second causes visible inconsistency. The > 10s default is a generic starting point; if your application routes freshness-sensitive reads to replicas, tighten the threshold in the Sensitivity tab, and consider routing those reads to the primary.

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Tracked live in Vortex IQ Nerve Centre

Async Replication Lag (seconds) is one of hundreds of KPI pulses Vortex IQ tracks across MariaDB and 70+ other ecommerce connectors. Nerve Centre runs the detection layer; Vortex Mind investigates the cause when something moves; Ask Viq lets you interrogate any number in plain English. Start for free or book a demo to see this metric running on your own data.