- Getting Started With PostgreSQL Streaming Replication
- PostgreSQL: Documentation: High Availability, Load Balancing, and Replication
- What is WAL?
- EDB Postgres Replication Server
You will create an example database with a sample table, which you can then use to test logical replication between your servers. You will also create a dedicated user and assign them privileges over both the database and the table. First, open the psql prompt as the postgres user with the following command on both db-master and db-replica :. Note: The final ; in these commands is required. For more on meta-commands and the psql client, please refer to the PostgreSQL documentation. Create a new table called widgets with arbitrary fields on both hosts:.
The table on db-replica does not need to be identical to its db-master counterpart. However, it must contain every single column present on the table at db-master.
Getting Started With PostgreSQL Streaming Replication
If they do, replication will fail. We will call our user sammy , but you can replace this with your own username. Make a note of your password, as you will use it later on db-replica to set up replication. Still on db-master , grant full privileges on the example database to the user role you just created:.
The public schema is a default schema in each database into which tables are automatically placed. With these privileges set, you can now move on to making the tables in your example database available for replication. Publications are the mechanism that PostgreSQL uses to make tables available for replication. The database server will keep track internally of the connection and replication status of any replica servers associated with a given publication.
Subscriptions are used by PostgreSQL to connect to existing publications. A publication can have many subscriptions across different replica servers, and replica servers can also have their own publications with subscribers. Upon creating a subscription, PostgreSQL will automatically sync any pre-existing data from the master to the replica. In our case there is no data to sync since the widgets table is empty, but this is a useful feature when adding new subscriptions to an existing database. On db-master , insert the following data on the widgets table:.
The entries have been successfully replicated from db-master to db-replica. One thing to note about write queries on replica servers is that they are not replicated back to the master server. PostgreSQL currently has limited support for resolving conflicts when the data between servers diverges. If there is a conflict, the replication will stop and PostgreSQL will wait until the issue is manually fixed by the database administrator.
For that reason, most applications will direct all write operations to the master server, and distribute reads among available replica servers. After resolving the existing problem s , replication should take place automatically. You now have the required knowledge to experiment with horizontal read scaling, high availability, and the geographical distribution of your PostgreSQL database by adding additional replica servers.
Redis comes with several commands that can help with troubleshooting and debugging issues. Because of its nature as an in-memory key-value store, many of these commands focus on memory management, but there are others that are valuable for providing an overview of the state of your Redis server. This tutorial will provide details on how to use some of these commands to help diagnose and resolve issues you may run into as you use Redis. A Redis hash is a data type that represents a mapping between a string field and a string value.
A single hash can hold many field-value pairs and are designed to not take up much space, making them ideal for representing data objects. This tutorial will go over how to manage hashes in Redis, from creating them to retrieving and deleting the data held within a hash. In Redis, a list is a collection of strings sorted by the order in which they were inserted unless otherwise specified, similar to linked lists. This tutorial details how to create lists and retrieve and delete elements from lists. In Redis, strings are the most basic type of value you can create and manage.
This tutorial provides an overview of how to create and retrieve strings, as well as how to manipulate the values held by string keys.
PostgreSQL: Documentation: High Availability, Load Balancing, and Replication
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What is WAL?
You can also take a tar format backup from master and copy that to the slave server. The following step can be used to stream data directory from master to slave. This step can be performed on a slave. In the above command, you see an optional argument -R. When you pass -R, it automatically creates a recovery. It is mandatory to create the recovery. If you are not using the backup type mentioned above, and choose to take a tar format backup on master that can be copied to slave, you must create this recovery.
Here are the contents of the recovery. This recovery. If you have configured a backup remotely using the streaming method mentioned in Step 4 , it just copies all the files and directories to the data directory of the slave. Which means it is both a back up of the master data directory and also provides for restore in a single step. Once done, you may proceed to start your PostgreSQL instance on the slave using the following command. This parameter takes a shell command or a script that can be used to fetch the WAL needed by a slave if the WAL is not available on the master.
To achieve that, you have to add a line similar to the next example to your recovery. Check for these processes on both master and slave using the following commands. You must see those all three processes running on master and slave as you see in the following example log.
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- postgresql - check postgres replication status - Stack Overflow.
- Step 2 - Install PostgreSQL on Master and Slave Server.
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- Manual Installation.
Avinash Vallarapu. Avinash Vallarapu joined Percona in the month of May He has given several talks and trainings on PostgreSQL. WAL shipping is not strictly needed for streaming replication.
And once the slave is up and running, it will only ever use 1. Hi Douglas. Thats true. Thus, we made sure to ensure we suggest the best practices while building streaming replication, rather than just building it. In every Production environment, these parameters are a must. Also archiving of WALs ensure a better backup strategy. So, you consider all these factors while setting up a replica. At the same time, if there is a backup job running on Master due to whatever reason, you need another wal sender process.
EDB Postgres Replication Server
It should not in fact hurt setting additional wal sender processes. I usually recommend 2 wal sender processes per each slave. And 1 dedicated to Master for many obvious reasons.