<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>Chapter 26. High Availability, Load Balancing, and Replication</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="continuous-archiving.html" title="25.3. Continuous Archiving and Point-in-Time Recovery (PITR)" /><link rel="next" href="different-replication-solutions.html" title="26.1. Comparison of Different Solutions" /></head><body><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">Chapter 26. High Availability, Load Balancing, and Replication</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="continuous-archiving.html" title="25.3. Continuous Archiving and Point-in-Time Recovery (PITR)">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="admin.html" title="Part III. Server Administration">Up</a></td><th width="60%" align="center">Part III. Server Administration</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 11.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="different-replication-solutions.html" title="26.1. Comparison of Different Solutions">Next</a></td></tr></table><hr></hr></div><div class="chapter" id="HIGH-AVAILABILITY"><div class="titlepage"><div><div><h2 class="title">Chapter 26. High Availability, Load Balancing, and Replication</h2></div></div></div><div class="toc"><p><strong>Table of Contents</strong></p><dl class="toc"><dt><span class="sect1"><a href="different-replication-solutions.html">26.1. Comparison of Different Solutions</a></span></dt><dt><span class="sect1"><a href="warm-standby.html">26.2. Log-Shipping Standby Servers</a></span></dt><dd><dl><dt><span class="sect2"><a href="warm-standby.html#STANDBY-PLANNING">26.2.1. Planning</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#STANDBY-SERVER-OPERATION">26.2.2. Standby Server Operation</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#PREPARING-MASTER-FOR-STANDBY">26.2.3. Preparing the Master for Standby Servers</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#STANDBY-SERVER-SETUP">26.2.4. Setting Up a Standby Server</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#STREAMING-REPLICATION">26.2.5. Streaming Replication</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#STREAMING-REPLICATION-SLOTS">26.2.6. Replication Slots</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#CASCADING-REPLICATION">26.2.7. Cascading Replication</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#SYNCHRONOUS-REPLICATION">26.2.8. Synchronous Replication</a></span></dt><dt><span class="sect2"><a href="warm-standby.html#CONTINUOUS-ARCHIVING-IN-STANDBY">26.2.9. Continuous archiving in standby</a></span></dt></dl></dd><dt><span class="sect1"><a href="warm-standby-failover.html">26.3. Failover</a></span></dt><dt><span class="sect1"><a href="log-shipping-alternative.html">26.4. Alternative Method for Log Shipping</a></span></dt><dd><dl><dt><span class="sect2"><a href="log-shipping-alternative.html#WARM-STANDBY-CONFIG">26.4.1. Implementation</a></span></dt><dt><span class="sect2"><a href="log-shipping-alternative.html#WARM-STANDBY-RECORD">26.4.2. Record-based Log Shipping</a></span></dt></dl></dd><dt><span class="sect1"><a href="hot-standby.html">26.5. Hot Standby</a></span></dt><dd><dl><dt><span class="sect2"><a href="hot-standby.html#HOT-STANDBY-USERS">26.5.1. User's Overview</a></span></dt><dt><span class="sect2"><a href="hot-standby.html#HOT-STANDBY-CONFLICT">26.5.2. Handling Query Conflicts</a></span></dt><dt><span class="sect2"><a href="hot-standby.html#HOT-STANDBY-ADMIN">26.5.3. Administrator's Overview</a></span></dt><dt><span class="sect2"><a href="hot-standby.html#HOT-STANDBY-PARAMETERS">26.5.4. Hot Standby Parameter Reference</a></span></dt><dt><span class="sect2"><a href="hot-standby.html#HOT-STANDBY-CAVEATS">26.5.5. Caveats</a></span></dt></dl></dd></dl></div><a id="id-1.6.13.2" class="indexterm"></a><a id="id-1.6.13.3" class="indexterm"></a><a id="id-1.6.13.4" class="indexterm"></a><a id="id-1.6.13.5" class="indexterm"></a><a id="id-1.6.13.6" class="indexterm"></a><a id="id-1.6.13.7" class="indexterm"></a><p> Database servers can work together to allow a second server to take over quickly if the primary server fails (high availability), or to allow several computers to serve the same data (load balancing). Ideally, database servers could work together seamlessly. Web servers serving static web pages can be combined quite easily by merely load-balancing web requests to multiple machines. In fact, read-only database servers can be combined relatively easily too. Unfortunately, most database servers have a read/write mix of requests, and read/write servers are much harder to combine. This is because though read-only data needs to be placed on each server only once, a write to any server has to be propagated to all servers so that future read requests to those servers return consistent results. </p><p> This synchronization problem is the fundamental difficulty for servers working together. Because there is no single solution that eliminates the impact of the sync problem for all use cases, there are multiple solutions. Each solution addresses this problem in a different way, and minimizes its impact for a specific workload. </p><p> Some solutions deal with synchronization by allowing only one server to modify the data. Servers that can modify data are called read/write, <em class="firstterm">master</em> or <em class="firstterm">primary</em> servers. Servers that track changes in the master are called <em class="firstterm">standby</em> or <em class="firstterm">secondary</em> servers. A standby server that cannot be connected to until it is promoted to a master server is called a <em class="firstterm">warm standby</em> server, and one that can accept connections and serves read-only queries is called a <em class="firstterm">hot standby</em> server. </p><p> Some solutions are synchronous, meaning that a data-modifying transaction is not considered committed until all servers have committed the transaction. This guarantees that a failover will not lose any data and that all load-balanced servers will return consistent results no matter which server is queried. In contrast, asynchronous solutions allow some delay between the time of a commit and its propagation to the other servers, opening the possibility that some transactions might be lost in the switch to a backup server, and that load balanced servers might return slightly stale results. Asynchronous communication is used when synchronous would be too slow. </p><p> Solutions can also be categorized by their granularity. Some solutions can deal only with an entire database server, while others allow control at the per-table or per-database level. </p><p> Performance must be considered in any choice. There is usually a trade-off between functionality and performance. For example, a fully synchronous solution over a slow network might cut performance by more than half, while an asynchronous one might have a minimal performance impact. </p><p> The remainder of this section outlines various failover, replication, and load balancing solutions. </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="continuous-archiving.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="admin.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="different-replication-solutions.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">25.3. Continuous Archiving and Point-in-Time Recovery (PITR) </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> 26.1. Comparison of Different Solutions</td></tr></table></div></body></html>
