Difference between revisions of "Streaming Replication"

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Following steps are described how to setup the streaming replication
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Migrated to [https://intranet.softwareheritage.org/index.php?title=Streaming_Replication intranet].
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between a master and slave.
 
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= Master =
 
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== Master information ==
 
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Example db:
 
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* host: somerset.internal.softwareheritage.org
 
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* port: 5434
 
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* db: softwareheritage-indexer
 
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== Replication user needed ==
 
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Make sure a replication user exists on the master server
 
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    su - postgres
 
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    createuser -p 5434 replicator --replication
 
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    psql -p 5434 template1
 
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    alter user replicator with password '<precious-secret>';
 
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Note: To retrieve the password.
 
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    swhpass ls infra/somerset/postgres/5434/replicator
 
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== Postgres configuration adaptation ==
 
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=== pg_hba.conf ===
 
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File: /etc/postgresql/10/indexer/pg_hba.conf
 
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    # TYPE  DATABASE        USER            CIDR-ADDRESS            METHOD
 
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    host    replication    replicator      192.168.200.0/21        md5 # Azure
 
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=== postgresql.conf ===
 
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File: /etc/postgresql/10/indexer/postgresql.conf
 
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    wal_level = replica      # or higher, logical is fine
 
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    wal_keep_segments = 500  # in logfile segments, 16MB each; 0 disables
 
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    wal_compression = on    # enable compression of full-page writes
 
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Note:
 
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* wal_compression is an optimization which reduces I/O and network
 
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  traffic at the expense of CPU time. It is not required.
 
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* wal_keep_segments needs to be set to a reasonable value in order to
 
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  keep enough wal data between the initial database backup and its
 
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  subsequent regular startup on the slave. Slave startup can fail at
 
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  first, so make sure to have at least a couple hours of slack in case
 
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  things go wrong.
 
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== Restart db ==
 
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Make the db aware of those changes:
 
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    # to determine the main pid of the concerned db
 
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    sudo systemctl status postgres@10-indexer.service
 
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    # Actually make the db aware
 
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    # kill -HUP <pid-of-db>
 
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    # or
 
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    sudo systemctl restart postgres@10-indexer.service
 
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= Slave =
 
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== Package Dependencies ==
 
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Install client for connection part, and server for replication one.
 
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    # client part and server
 
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    sudo apt install postgresql-client-10 postgresl-10
 
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== Postgres setup ==
 
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Make sure the connection is ok.
 
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As postgres user:
 
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    su - postgres
 
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    touch .pgpass
 
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    cat > .pgpass <<EOF
 
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    somerset.internal.softwareheritage.org:5434:replication:replicator:<precious-secret>
 
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    EOF
 
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    chmod 0600 .pgpass
 
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    # should permit connection
 
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== Create replica cluster ==
 
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This is a necessary and temporary step to permit to initialize
 
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configuration files that would need editing down the line.
 
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    # drop installed main cluster as this won't be used
 
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    sudo pg_dropcluster --stop 10 main
 
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    # prepare directories in data volume
 
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    sudo mkdir /srv/softwareheritage/postgres
 
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    sudo chown postgres:postgres /srv/softwareheritage/postgres
 
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      # create the cluster configuration
 
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    sudo -u postgres env LC_ALL=C.UTF-8 \
 
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        pg_createcluster 10 replica \
 
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            -D /srv/softwareheritage/postgres/10/replica \
 
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            -p 5434
 
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== Make sure the slave can connect to the master ==
 
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Edit the local configuration according to the master.  The slave must
 
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have the at least the same boundaries for some configuration options:
 
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file: /etc/postgresql/10/replica/postgresql.conf
 
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    max_connections = 128      # same as master somerset.internal.softwareheritage.org:5434:softwareheritage-indexer
 
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    max_worker_processes = 10  # same as master somerset.internal.softwareheritage.org:5434:softwareheritage-indexer
 
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Note: You will be notified in logs when trying to start the db that
 
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some setup are not correctly set.  Some form of:
 
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    ardumont@dbreplica1 $ sudo journalctl -xef -u postgresql@10-replica.service
 
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    ...
 
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    Jun 15 08:13:24 dbreplica1 postgresql@10-replica[654]: 2018-06-15 08:13:24.425 UTC [664] FATAL:  hot standby is not possible because max_worker_processes = 8 is a lower setting than on the ma
 
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    ster server (its value was 10)
 
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    ...
 
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== Retrieve backup ==
 
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From the replica node, as postgres user:
 
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    postgres@dbreplica1 $ pg_basebackup --progress --write-recovery-conf \
 
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            -h somerset.internal.softwareheritage.org -p 5434 -U replicator \
 
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            -D /srv/softwareheritage/postgres/10/somerset-swh-indexer.backup
 
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Note that this step could fail if your .pgpass is not setup correctly.
 
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== Install backup as db ==
 
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Move the backup files to the regular pgdata location and start the
 
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slave server:
 
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    postgres@dbreplica1 $ cd /srv/softwareheritage/postgres/10
 
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    # Remove 'temporary' cluster
 
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    postgres@dbreplica1 $ rm -rf replica
 
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    # Rename the backup as replica
 
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    postgres@dbreplica1 $ mv somerset-swh-indexer.backup replica
 
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    ardumont@dbreplica1 $ sudo systemctl status postgresql@10-replica.service
 
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== Start cluser ==
 
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Now that everything is ready, start the cluster.  This will start by
 
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redoing the necessary steps to synchronize the dbs.
 
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    sudo systemctl start postgresql@10-replica.service
 

Latest revision as of 09:38, 15 June 2018

Migrated to intranet.

Retrieved from "https://wiki.softwareheritage.org/index.php?title=Streaming_Replication&oldid=850"