Database Reference
In-Depth Information
Master
The master server is the database server sourcing the data being replicated and
where all updates happen. You're allowed only one master when using the built-in
replication features of PostgreSQL. Plans are in place to support multimaster rep‐
lication scenarios. Watch for it in future releases.
Slave
A slave server consumes the replicated data and provides a replica of the master.
More aesthetically pleasing terms such as
subscriber
and
agent
have been bandied
about, but
slave
is still the most apropos. PostgreSQL built-in replication supports
only read-only slaves at this time.
Write-ahead log (WAL)
WAL is the log that keeps track of all transactions, often referred to as the
transaction
log
in other database products. To stage replication, PostgreSQL simply makes the
logs available to the slaves. Once slaves have pulled the logs, they just need to execute
the transactions therein.
Synchronous
A transaction on the master will not be considered complete until at least one slave
is updated. If you have multiple synchronous slaves, they do not all need to respond
for success.
Asynchronous
A transaction on the master will commit even if slaves haven't been updated. This
is useful in the case of distant servers where you don't want transactions to wait
because of network latency, but the downside is that your dataset on the slave might
lag behind, and the slave might miss some transactions in the event of transmission
failure.
Streaming
The streaming replication model was introduced in PostgreSQL 9.0. Unlike prior
versions, it does not require direct file access between master and slaves. Instead,
it relies on the PostgreSQL connection protocol to transmit the WALs.
Cascading replication
Starting with version 9.2, slaves can receive logs from nearby slaves instead of di‐
rectly from the master. This allows a slave to also behave like a master for replication
purposes. The slave remains read-only. When a slave acts as both a receiver and a
sender, it is called a
cascading standby
.
Remastering
Remastering is the process whereby you promote a slave to be the master. Up to
and including version 9.2, this was a process that required using WAL file archiving
instead of streaming replication. It also required all slaves to be recloned. Version
9.3 introduced streaming-only remastering, which means remastering no longer
