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PostgreSQL comes with various ways of replicating data between servers, alongwith configuration options to tweak and tune the replication process to fityour needs. Regardless of what type of replication you set up, it is importantto monitor all endpoints of replication to ensure that your data is safe andsound.
Physical replication slots were introduced to PostgreSQL with version 9.4, while logical replication slots were added beginning with version 10.0. The default authentication assumes that you are either logging in as or sudo’ing to the postgres account on the host. This is a component for Apache Camel which allows for consuming from PostgreSQL replication slots. The component works with PostgreSQL 10 or later. Maven users will need to add the following dependency to their pom.xml for this component. Streaming replication slots are a pending feature in PostgreSQL 9.4, as part of the logical changeset extraction feature. What are they for, what do you need to know, what changes? What are replication slots? Streaming replication slots are a new facility introduced in PostgreSQL 9.4. They are a persistent record of the state of a. #define PGGETREPLICATIONSLOTSCOLS 13. Generated on Sat Dec 12 2020 00:13:42 for PostgreSQL Source Code by 1.8.13.
Read on to learn more about monitoring replication in Postgres.
Types of Replication
Broadly, there are 2 types of replication in PostgreSQL – physical andlogical. Here is a quick overview picture:
Let’s have a quick look at each before we see how to monitor them. In the textbelow, per convention, the term primary refers to the master or originatingserver, and standby refers to the slave or receiving server.
Physical Replication
Physical replication has been available in core Postgres for more than adecade. It allows you to replicate the data in a primary server byte for byteinto a standby server.
Essentially, physical replication relies on sending WAL (write ahead log)files generated by the primary over to the secondary. When changes happen onthe primary server, these are serialized and flushed to disk as a series ofWAL files. These files can be “replayed” to get the familiar SQL-structuredrepresentation of data. Even the primary server itself writes the recordfirst, then a bit later, replays the record and updates the SQL-structuredrepresentation.
Therefore, it stands to reason that you can send over these WAL files to anotherserver and have it replay them instead. And that’s exactly how physicalreplication works.
The standby server is setup not to accept changes via the normal DDL/DMLstatements but only via accepting and replaying WAL files. A serverconfigured this way is said to be in recovery mode.
A standby in recovery mode will not (cannot) perform changes to its data usingnormal SQL transactions. You can however allow read-only SQL transactionson it by making it a hot standby. Because the original changes at theprimary have to be brought to the standby and replayed, the queries at the hot standbywill potentially see an earlier version of the data. This is thereplication lag. The replication lag at each standby is one of the thingsyou should be monitoring.
Log Shipping
Since WAL files are actual, simple physical files typically 16MB in size andliving in the pg_wal (or pg_xlog prior to v10) subdirectory of $PGDATA, youcan simply copy them over to another server with scp or via NFS or the like.
The standby server is setup to accept these WAL files using therestore_commandor dropping them in the pg_wal directory.
Streaming Replication
Rather than copying WAL files through scripts, streaming replication allowsthe standby to establish a connection the primary and “stream” the contentsof the WAL file over this connection.
Streaming replication is not only much cleaner, but also has the advantage ofsmaller replication lags for low-TPS servers. This is because the WAL filesin turn contain smaller records arranged sequentially – in streaming replicationeach record can be streamed when created, whereas in log shipping you’ll have towait for the whole WAL file to be assembled, which may take a while.
Streaming replication was introduced in Postgres 9.0.
Physical Replication Slots
In Postgres 9.4, replication slots were introduced. A process receiving changesvia streaming replication can create a replication slot on the primary server.The primary will then use this slot to store the location up to where thechanges have been sent to the receiver.
If the receiver were to disconnect for a long while and then reconnect, theprimary would now retain those WAL files that are required for the receiver to catchup.
Streaming replication is possible with and without the use of replication slots.Using replication slots of course, makes it more reliable.
Logical Replication
Logical replication was introduced in core PostgreSQL in version 10. Unlikephysical replication, logical replication brings over only the logical, orSQL-like changes. It aims to be the foundation for currently unavailablefeatures like multi-master replication and zero-downtime major-version upgrades.
Logical replication works only via replication slots. A replication slot canbe created for either physical replication or logical replication.
The logical replication data received by a process has to be “decoded” usinga “plugin”. For more information see the docsand the wiki.
Subscriptions
The publication / subscription feature built into Postgres 10 is a means ofusing logical replication. In a way, this feature is a “UI” for logicalreplication.
Replication Topology
PostgreSQL allows sending one primary’s changes to multiple standbys and evencascading them further. This allows for complex replication topologies (checkout the article Replication Topologies in PostgreSQLto learn more).
There are however, a couple of simple rules to keep the story straight:
- a server is either in recovery mode (that is, it is a standby) or not
- a standby will receive incoming WALs and replay them
- a server can send it’s own WALs to 0 or more other servers
These rules are primarily for physical replication. For logical replication,changes can be sent from anywhere to anywhere and they are monitored only viatheir replication slots.
Monitoring Standby Servers
A standby server has incoming replication, and remains in recovery mode sothat it can replay the WALs/records as they come in.
Monitoring Incoming Replication
The stats related to the incoming replication can be monitored usingpg_stat_wal_receiverview (available in v9.6 and above). On a standby, it contains values similar to:
This shows that there is a single “WAL receiver” process with PID 2084, whichis currently active, and has received data from 0/3000000 up to 0/11233E20.The timeline IDwas 1 when it started, and is 1 even now. The difference between the send andreceipt times of the last message indicates the latency of the connection,which in this case is less than 1 millisecond. The difference between the twoLSNs (start and end) shows the amount of data received over the connection(0x11233E20 - 0x03000000 = about 226 MiB). It also says that this connectiondoes not use a replication slot – if it did, the “slot_name” column would notbe null.
Here is another standby which does use a replication slot:
Monitoring Recovery Status
The pg_stat_wal_receiver only gives stats about the receiving of WAL records,not of it’s replay. For that, we need to use:
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This shows that the last received WAL position is ahead of the last replayedWAL position, and there is a replay lag of 0/1A000000 - 0/19FFE928 = 5848 bytes.If there is a replay lag, the difference between the time the query was runand the last transaction replay timestamp is a measure of the replay lag inunits of time.
Monitoring Primary Servers
There are two type of things to monitor on primary servers – outgoing replicationconnections and replication slots.
Monitoring Outgoing Replication
The system view pg_stat_replicationreturns one row for each open outgoing replication connection.
We can see the WAL locations corresponding to send, write, flushand replay. In Postgres 10, the write, flush and replay lags are alsoavailable. The value of async for sync_state shows that the clients arenot synchronous standbys.
Monitoring Physical Replication Slots
The view pg_replication_slotsgives one row for each replication slot in the primary. Unlike streamingreplication connections, replication slots have to be created and deletedexplicitly. When there is an associated client connection, slots are consideredactive. Inactive slots are dangerous, because the server will retain theWAL files required for that slot forever, at the risk of filling up the disk.
Replicaton slots can also be temporary, which will go away when the replicationclient connection goes away. Newer versions of pg_basebackup use temporaryreplication slots.
The physical replication slot “myreplslot1” (value of slot_type is physical) is active,not temporary and it’s client has received up to 0/3B44A5D8.
Monitoring Logical Replication Slots
The logical replication slot “mylogslot” (value of slot_type is logical)is also active and permanent. It’s client has received up to 0/3B44A5A0 and hasconfirmed that there is no flush lag. The plugin used is “test_decoding” (because the client for this setup was pg_recvlogical).
Monitoring Using pgmetrics
The open-source tool pgmetrics can query Postgresservers and produce detailed reports, including for replication. You caninvoke pgmetrics pretty much like psql, with the same command-line optionsand environment variables.
Here is the pgmetrics output for a primary replicating to two physical standbysand one logical receiver. It also shows the physical and logical replication slots:
On the receiving end, it shows the recovery and the incoming replication status,including the lags in both bytes and time where relevant:
pgmetrics can also generate the results in JSON format, which can be used inscripts for monitoring or alerting.
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Monitoring Using pgDash
pgDash is an in-depth monitoring solution designedspecifically for PostgreSQL deployments. pgDash shows you information andmetrics about every aspect of your PostgreSQL database server, collected usingthe open-source tool pgmetrics.
In addition displaying the data interactively, it also stores metrics extractedfrom the pgmetrics report as timeseries data and displays them graphically. Hereis how it displays the replication information on the primary side:
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And for the standby server:
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pgDash provides core reporting and visualizationfunctionality, including collecting and displaying PostgreSQL information andproviding time-series graphs and detailed reports. We’re actively working toenhance and expand pgDash to include alerting, baselines, teams, and more.