MySQL Replication: Managed Existing Server

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Setting up replication on a new server is easy. A few config file entries and a couple of restarts are all you need. On an existing server, however, there's a bit more work to do because you can't simply point a new slave at a master and ask it to clone all the existing data.

On an Existing Server

There are a couple of ways to do get the job done. We'll look at two specific solutions in a moment. First, let's outline the work that needs to be done; we'll deal then with common solutions.

What needs to happen

Normally, to enable replication, you have to add binary logging to your server, which means subsequently restarting the server. If you happen to have binary logging already enabled, you don't have to restart the server. As described earlier, you'll need to add at least two lines to the server's my.cnf file:

log-bin server-id = 1

Optionally, specify a full path and base filename for the binary logs:

log-bin = /var/db/repl/binary-log

The other task involves getting a copy of all the data from the master and putting it on the new slave. But there's a twist. The data given to the slave must correspond to the exact moment in time the binary log begins. Said another way, the binary log should contain all the queries that are executed on the master after the snapshot was taken and none of the queries from before the snapshot.

If the binary log contains queries that are already reflected in the data given to the slave, the slave has no way to know that. Consequently, it reexecutes the queries, possibly producing strange errors or otherwise making the data inconsistent with what is on the master.

If the binary log misses a few queries that weren't reflected in the slave's copy of the data, it won't see those queries. This can cause strange and hard-to-diagnose problems. Maybe records that were supposed to have expired are still there, or perhaps there's data on the master that doesn't appear on the slave.

Getting the initial data from the master to the slave may be complicated. If you're using only MyISAM tables and can afford to shut down the master for enough time to copy all the data, it will be easy. Otherwise, you'll need to perform an online copy or dump of the data.

Snapshot or backup, then copy

The easiest way to get the necessary data is to perform a snapshot (online backup) or a more traditional offline backup and then copy the data to the slave. Using archive tools such as tar or zip, or your traditional backup software, shut down MySQL and copy the contents of the data directory to your slave; then extract the data on the slave.

This method works well if you intend to replicate all the data and can shut down MySQL for the time required to make a copy of the data. If, however, you can't afford to have MySQL offline for more than a few seconds, there's an alternative approach: restart the server once after making the config file changes and then perform an online snapshot of the data.

A snapshot works well only for MyISAM tables. InnoDB and BDB tables are best backed up when MySQL isn't running at all. A snapshot also requires a read lock on the data for the duration of the snapshot. So you'll be able to service read requests during the snapshot process, but all writers will be blocked.

To perform the actual snapshot, you can write your own script to do the job, or you can use mysqlhotcopy or mysqlsnapshot. If you roll your own script, you need to ensure that the binary log is reset before the locks are released. The easiest way to do that is by executing FLUSH TABLES WITH READ LOCK and then RESET MASTER (or FLUSH MASTER in versions older than 3.23.26).

Chapter 9 covers backups as well as the mysqlhotcopy and mysqlsnapshot utilities.

Online table copies

Another approach is to use MySQL's command:

 LOAD TABLE mytable FROM MASTER

Doing so instructs a slave to load an entire table from the master. By writing a relatively simple script, you can instruct the slave to clone all the tables it needs using a series of those commands.

The usefulness of this technique is relatively limited, however. Like the previous option, it requires a master that isn't being updated. In an environment in which there are frequent updates to the master, this technique is simply not viable. Furthermore, the slave copies only the data from the master. It then reconstructs the indexes locally, for which large amounts of data can take hours or even days.

Online copy and synchronize (MySQL 4.x only)

MySQL 4.0 introduced the LOAD DATA FROM MASTER command. It combines the previous two approaches by first obtaining a read lock on all the master's tables, then loading each table one by one using the LOAD TABLE mechanism.[6] It respects any slave-side database or table filtering. Once it completes the loading process, it releases the locks on the master and begins replicating.

While this option is very appealing, it suffers from the same limitations as scripting the LOADTABLE command yourself. It is much slower than using a master snapshot. It also requires that you grant the repl user SUPER and RELOAD privileges on the master. Finally, it works only with MyISAM tables.

Replication in 3.23

MySQL's original replication code provides basic replication services. The master logs all write queries to the binary log. The slave reads and executes the queries from the master's binary log. If the two are ever disconnected, the slave attempts to reconnect to the master.

If you follow a query from start to finish, here's what's happening behind the scenes:

  1. The client issues a query on the master.

  2. The master parses and executes the query.

  3. The master records the query in the binary log.

  4. The slave reads the query from the master.

  5. The slave parses and executes the query.

  6. The slave performs a sanity check, comparing its result with the master's. If the query failed on the slave but succeeded on the master, replication stops. The reverse is also true. If the query partially completed on the master but succeeds on the slave, the slave stops and complains.

  7. The slave updates the master.info file to reflect the new offset at which it is reading the master's binary log.

  8. The slave waits for the next query to appear in the master's binary log. When one appears, it starts over at Step 4.

That's a relatively simple arrangement. The master simply logs any queries that change data. The slave reads those queries from the master, one by one, and executes each of them. If there are any discrepancies between the results on the master and the slave, the slave stops replicating, logs an error, and waits for human intervention.

The simplicity of this system has problems, however. If the master and slave are separated by a slow network, the speed at which replication can occur becomes bounded by the network latency. Why? Because the process is highly serialized. The slave runs in a simple "fetch query, execute query, fetch query, ..." loop. If the "fetch query" half of the loop takes more than a trivial amount of time, the slave may not be able to keep up with the master during very heavy workloads. The master may be able to execute and log 800 queries per second, but if the slave requires 25 msec to fetch each query over the network, it can replicate no more than 40 queries per second.

This can be problematic even with a fast network connection. Suppose the master executes a query that takes five minutes to complete. Maybe it's an UPDATE that affects 50 million records. During the five minutes the slave spends running the same query, it isn't pulling new queries from the master. By the time it completes the query, it's effectively five minutes behind the master, in terms of replication. It has a fair bit of catching up to do. If the master fails during that five-minute window, there's simply no way for the slave to catch up until the master reappears. Some of these problems are solved in 4.0.

Replication in 4.0

To solve the problem of slaves falling behind because of slow queries or slow networks, the replication code was reworked for Version 4.0. Instead of a single thread on the slave that runs in a "fetch, execute, fetch, ..." loop, there are two replication threads: the IO thread and the SQL thread.

These two threads divide the work in an effort to make sure the slave can always be as up to date as possible. The IO thread is concerned only with replicating queries from the master's binary log. Rather than execute them, it records them into the slave's relay log.[7] The SQL thread reads queries from the local relay log and executes them.

To put this in context, let's look at the step-by-step breakdown for replication in MySQL 4.0:

  1. The client issues a query on the master.

  2. The master parses and executes the query.

  3. The master records the query in the binary log.

  4. The slave's IO thread reads the query from the master and appends it to the relay log.

  5. The slave's IO thread updates the master.info file to reflect the new offset at which it is reading the master's binary log. It then returns to Step 4, waiting for the next query.

  6. The slave's SQL thread reads the query from its relay log, parses it, and then executes it.

  7. The slave's SQL thread performs a sanity check, comparing its result with the master's. If the query failed on the slave but succeeded on the master, replication stops.

  8. The slave's SQL thread updates the relay-log.info file to reflect the new offset at which it is reading the local relay log.

  9. The slave's SQL thread waits for the next query to appear in the relay log. When one appears, it starts over at Step 6.

While the steps are presented as a serial list, it's important to realize that Steps 4-5 and 6-9 are running as separate threads and are mostly independent of each other. The IO thread never waits for the SQL thread; it copies queries from the master's binary log as fast as possible, which helps ensure that the slave can bring itself up to date even if the master goes down. The SQL thread waits for the IO thread only after it has reached the end of the relay log. Otherwise it is working as fast as it can to execute the queries waiting for it.

This solution isn't foolproof. It's possible for the IO thread to miss one or more queries if the master crashes before the thread has had a chance to read them. The amount of data that could be missed is greatly reduced compared to the 3.23 implementation, however.

Files and Settings Related to Replication

There are several files and configuration options related to replication in this chapter. Without going into a lot of detail on any one of them (that's done elsewhere), the files fall into three categories: log files, log index files, and status files.

Log files

The log files are the binary log and the relay log. The binary log contains all write queries that are written when the log is enabled. The log-bin option in my.cnf enables the binary log. Binary log files must be removed when they're no longer needed because MySQL doesn't do so automatically.

The relay log stores replicated queries from a MySQL 4.0 slave (from the master's binary log) before it executes them. It's best thought of as a spool for queries. The relay log is enabled automatically in 4.0 slaves. The relay-log option in my.cnf can customize the name and location of the relay log's base filename:

relay-log = /home/mysql/relay.log

Like the binary log, MySQL always appends a sequence number to the base name, starting with 001. Unlike the binary log, MySQL takes care of removing old relay logs when they are no longer needed. MySQL 3.23 servers don't use relay logs.

Log index files

Each log file has a corresponding index file. The index files simply list the names of the log files on disk. When logs are added or removed, MySQL updates the appropriate index file.

You can add settings to my.cnf to specify the log index filenames and locations:

log-bin-index = /var/db/logs/log-bin.index relay-log-index = /var/db/logs/relay-log.index

Never change these settings once a slave is configured and replicating. If you do, MySQL uses the new values when it is restarted and ignores the older files.

Status files

MySQL 3.23 and 4.0 slaves use a file named master.info to store information about their master. The file contains the master's hostname, port number, username, password, log file name, position, and so on. MySQL updates the log position and log file name (as necessary) in this file as it reads queries from the master's binary log. While you should never need to edit the file, it's worth knowing what it is used for.

The master-info-file option in my.cnf can be used to change the name and location of the master.info file:

master-info-file = /home/mysql/master-stuff.info

However, there's rarely a need to do so.

MySQL 4.0 slaves use an additional status file for the SQL thread to track its processing of the relay log, in much the same way the master.info file is used. The relay-log-info-file setting can be used to change the filename and path of this file:

relay-log-info-file = /home/mysql/logs/relay-log.info

Again, you won't need to change the default.

Filtering

There may be times when you don't need to replicate everything from the master to the slave. In such situations you can use the various replication filtering options to control what is replicated. This is well covered in the MySQL documentation, so we'll just recap the important parts.

There are two sets of configuration options for filtering. The first set applies to the binary log on the master and provide per-database filtering:

binlog-do-db=dbname binlog-ignore-db=dbname

Any queries filtered on the master aren't written to its binary log, so the slave never sees them either.

The second set of options applies to the relay log on the slave. That means the slave still has to read each query from the master's binary log and make a decision about whether or not to keep the query. The CPU overhead involved in this work is minimal, but the network overhead may not be if the master records a high volume of queries.

Here is the second set of options:

replicate-do-table=dbname.tablename replicate-ignore-table=dbname.tablename replicate-wild-do-table=dbname.tablename replicate-wild-ignore-table=dbname.tablename replicate-do-db=dbname replicate-ignore-db=dbname replicate-rewrite-db=from_dbname->to_dbname

As you can see, the slave options are far more complete. They not only offer per-table filtering but also allow you to change the database or table names on the fly.

The Replication Rules

Before looking at the architectures, it helps to understand the basic rules that must be followed in any MySQL replication setup:

  • Every slave must have a unique server ID.

  • A slave may have only one master.

  • A master may have many slaves.

  • Slaves can also be masters for other slaves.

The first rule isn't entirely true, but let's assume that it is for right now, and soon enough you'll see why it isn't always necessary. In any case, the rules aren't terribly complex. Those four rules provide quite a bit of flexibility, as the next sections illustrate.

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