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Diffstat (limited to 'docs/database.md')
| -rw-r--r-- | docs/database.md | 8 |
1 files changed, 4 insertions, 4 deletions
diff --git a/docs/database.md b/docs/database.md index 43fd61cf3cba..cf19a86ddba5 100644 --- a/docs/database.md +++ b/docs/database.md @@ -52,13 +52,13 @@ It's often the case that the best algorithm to use for a given task depends on w ## Lag -Lag primarily occurs when large write queries are sent to the primary. Writes on the primary are executed in parallel, but they are executed in serial when they are replicated to the replicas. The primary writes the query to the binlog when the transaction is committed. The replicas poll the binlog and start executing the query as soon as it appears. They can service reads while they are performing a write query, but will not read anything more from the binlog and thus will perform no more writes. This means that if the write query runs for a long time, the replicas will lag behind the primary for the time it takes for the write query to complete. +Lag primarily occurs when large write queries are sent to the primary. Writes are executed in parallel on the primary, but they are executed serially when replicated to the replicas. The primary database may not write its query to the binlog for replication, until after the transaction is committed. The replicas poll this binlog, and apply the query locally as soon as it appears there. They can respond to reads while they are applying the replicated writes, but will not read anything more from the binlog and thus will perform no more writes. This means that if the write query runs for a long time, the replicas will lag behind the primary for as long as it takes for the write query to complete. -Lag can be exacerbated by high read load. MediaWiki's load balancer will stop sending reads to a replica when it is lagged by more than 30 seconds. If the load ratios are set incorrectly, or if there is too much load generally, this may lead to a replica permanently hovering around 30 seconds lag. +Lag can be exacerbated by high read load. MediaWiki's LoadBalancer will avoid sending reads to a replica lagged by more than a few seconds. -If all replicas are lagged by more than 30 seconds, MediaWiki will stop writing to the database. All edits and other write operations will be refused, with an error returned to the user. This gives the replicas a chance to catch up. Before we had this mechanism, the replicas would regularly lag by several minutes, making review of recent edits difficult. +MediaWiki does its best for multiple queries during a given web request to represent a single consistent snapshot of the database at a given point in time. In addition to this, MediaWiki tries to ensure that a user sees the wiki change in chronological order, such as subsequent web requests see the same or newer data. In particular, it tries to ensure that the user's own actions are immediately reflected in subsequent requests. This is done by saving the primary's binlog position after a database write, and during subsequent connections to a replica it will wait as-needed to catch up to that position before sending any read queries. -In addition to this, MediaWiki attempts to ensure that the user sees events occurring on the wiki in chronological order. A few seconds of lag can be tolerated, as long as the user sees a consistent picture from subsequent requests. This is done by saving the primary binlog position in the session, and then at the start of each request, waiting for the replica to catch up to that position before doing any reads from it. If this wait times out, reads are allowed anyway, but the request is considered to be in "lagged replica mode". Lagged replica mode can be checked by calling `LoadBalancer::getLaggedReplicaMode()`. The only practical consequence at present is a warning displayed in the page footer. +If the wait for chronology protection times out, or more generally if a queried replica is lagged by more than 6 seconds (`LoadBalancer::MAX_LAG_DEFAULT`, configurable via `max lag` in `$wgLBFactoryConf`), the MediaWiki request is considered to be in "lagged replica mode" (`ILBFactory::laggedReplicaUsed`). In this mode, MediaWiki automatically shortens the expiry of object caching (via WANObjectCache) and HTTP/CDN caching to ensure that any stale data will soon converge. ## Lag avoidance |