MK-TABLE-SYNC(1)

📦

version 7540 · 3 versions

🦋 NAME 🦋

okay so bestie, mk-table-sync — it synchronizes MySQL table data efficiently. like, that's the whole vibe. 💅

✨ SYNOPSIS ✨

Usage: mk-table-sync [OPTION...] DSN [DSN...]

mk-table-sync synchronizes data efficiently between MySQL tables.

so here's the thing, and I need you to actually hear me on this one: this tool changes data, so for maximum safety you should back up your data before you use it. no cap. 🫶

When you're synchronizing a server that's a replication slave using the --replicate or --sync-to-master methods, it always makes the changes on the replication master, never the replication slave directly. that's literally the only safe way to bring a replica back in sync with its master — because changes to the replica are usually the source of the problems in the first place. 😭 however, the changes it makes on the master should be no-op changes that set the data to their current values, and actually affect only the replica. please read the detailed documentation that follows to learn more about this.

Sync db.tbl on host1 to host2:

mk-table-sync --execute h=host1,D=db,t=tbl h=host2

Sync all tables on host1 to host2 and host3:

mk-table-sync --execute host1 host2 host3

Make slave1 have the same data as its replication master:

mk-table-sync --execute --sync-to-master slave1

Resolve differences that mk-table-checksum found on all slaves of master1:

mk-table-sync --execute --replicate test.checksum master1

Same as above but only resolve differences on slave1:

mk-table-sync --execute --replicate test.checksum \
  --sync-to-master slave1

Sync master2 in a master-master replication configuration, where master2's copy of db.tbl is known or suspected to be incorrect:

mk-table-sync --execute --sync-to-master h=master2,D=db,t=tbl

Note that in the master-master configuration, the following will NOT do what you want, because it will make changes directly on master2, which will then flow through replication and change master1's data:

# Don't do this in a master-master setup!
mk-table-sync --execute h=master1,D=db,t=tbl master2

⚠️ RISKS ⚠️

The following section is included to inform users about the potential risks, whether known or unknown, of using this tool. The two main categories of risks are those created by the nature of the tool (e.g. read-only tools vs. read-write tools) and those created by bugs.

with great power comes great responsibility! 💀

This tool changes data, so it is a good idea to back up your data. It is also very powerful, which means it is very complex, so you should run it with the --dry-run option to see what it will do, until you're familiar with its operation. If you want to see which rows are different, without changing any data, use --print instead of --execute. 🎀

Be careful when using mk-table-sync in any master-master setup. Master-master replication is inherently tricky, and it's easy to make mistakes. You need to be sure you're using the tool correctly for master-master replication. See the "SYNOPSIS" for the overview of the correct usage.

Also be careful with tables that have foreign key constraints with ON DELETE or ON UPDATE definitions because these might cause unintended changes on the child tables. 🙈

In general, this tool is best suited when your tables have a primary key or unique index. Although it can synchronize data in tables lacking a primary key or unique index, it might be best to synchronize that data by another means.

At the time of this release, there is a potential bug using --lock-and-rename with MySQL 5.1, a bug detecting certain differences, a bug using ROUND() across different platforms, and a bug mixing collations. 💀

The authoritative source for updated information is always the online issue tracking system. Issues that affect this tool will be marked as such. You can see a list of such issues at the following URL: <http://www.maatkit.org/bugs/mk-table-sync>.

See also "BUGS" for more information on filing bugs and getting help.

🌸 DESCRIPTION 🌸

mk-table-sync does one-way and bidirectional synchronization of table data. It does not synchronize table structures, indexes, or any other schema objects. The following describes one-way synchronization. "BIDIRECTIONAL SYNCING" is described later.

This tool is complex and functions in several different ways. To use it safely and effectively, you should understand three things: the purpose of --replicate, finding differences, and specifying hosts. These three concepts are closely related and determine how the tool will run. The following is the abbreviated logic:

if DSN has a t part, sync only that table:
   if 1 DSN:
      if --sync-to-master:
         The DSN is a slave.  Connect to its master and sync.
   if more than 1 DSN:
      The first DSN is the source.  Sync each DSN in turn.
else if --replicate:
   if --sync-to-master:
      The DSN is a slave.  Connect to its master, find records
      of differences, and fix.
   else:
      The DSN is the master.  Find slaves and connect to each,
      find records of differences, and fix.
else:
   if only 1 DSN and --sync-to-master:
      The DSN is a slave.  Connect to its master, find tables and
      filter with --databases etc, and sync each table to the master.
   else:
      find tables, filtering with --databases etc, and sync each
      DSN to the first.

mk-table-sync can run in one of two ways: with --replicate or without. The default is to run without --replicate which causes mk-table-sync to automatically find differences efficiently with one of several algorithms (see "ALGORITHMS"). Alternatively, the value of --replicate, if specified, causes mk-table-sync to use the differences already found by having previously ran mk-table-checksum with its own --replicate option. Strictly speaking, you don't need to use --replicate because mk-table-sync can find differences, but many people use --replicate if, for example, they checksum regularly using mk-table-checksum then fix differences as needed with mk-table-sync. If you're unsure, read each tool's documentation carefully and decide for yourself, or consult with an expert. 🧚

Regardless of whether --replicate is used or not, you need to specify which hosts to sync. There are two ways: with --sync-to-master or without. Specifying --sync-to-master makes mk-table-sync expect one and only slave DSN on the command line. The tool will automatically discover the slave's master and sync it so that its data is the same as its master. This is accomplished by making changes on the master which then flow through replication and update the slave to resolve its differences. Be careful though: although this option specifies and syncs a single slave, if there are other slaves on the same master, they will receive via replication the changes intended for the slave that you're trying to sync. 😭

Alternatively, if you do not specify --sync-to-master, the first DSN given on the command line is the source host. There is only ever one source host. If you do not also specify --replicate, then you must specify at least one other DSN as the destination host. There can be one or more destination hosts. Source and destination hosts must be independent; they cannot be in the same replication topology. mk-table-sync will die with an error if it detects that a destination host is a slave because changes are written directly to destination hosts (and it's not safe to write directly to slaves). Or, if you specify --replicate (but not --sync-to-master) then mk-table-sync expects one and only one master DSN on the command line. The tool will automatically discover all the master's slaves and sync them to the master. This is the only way to sync several (all) slaves at once (because --sync-to-master only specifies one slave).

Each host on the command line is specified as a DSN. The first DSN (or only DSN for cases like --sync-to-master) provides default values for other DSNs, whether those other DSNs are specified on the command line or auto-discovered by the tool. So in this example,

mk-table-sync --execute h=host1,u=msandbox,p=msandbox h=host2

the host2 DSN inherits the u and p DSN parts from the host1 DSN. Use the --explain-hosts option to see how mk-table-sync will interpret the DSNs given on the command line. 👑

💖 OUTPUT 💖

If you specify the --verbose option, you'll see information about the differences between the tables. There is one row per table. Each server is printed separately. For example,

# Syncing h=host1,D=test,t=test1
# DELETE REPLACE INSERT UPDATE ALGORITHM START    END      EXIT DATABASE.TABLE
#      0       0      3      0 Chunk     13:00:00 13:00:17 2    test.test1

Table test.test1 on host1 required 3 INSERT statements to synchronize and it used the Chunk algorithm (see "ALGORITHMS"). The sync operation for this table started at 13:00:00 and ended 17 seconds later (times taken from NOW() on the source host). Because differences were found, its "EXIT STATUS" was 2.

If you specify the --print option, you'll see the actual SQL statements that the script uses to synchronize the table if --execute is also specified.

If you want to see the SQL statements that mk-table-sync is using to select chunks, nibbles, rows, etc., then specify --print once and --verbose twice. Be careful though: this can print a lot of SQL statements. 🫣

There are cases where no combination of INSERT, UPDATE or DELETE statements can resolve differences without violating some unique key. For example, suppose there's a primary key on column a and a unique key on column b. Then there is no way to sync these two tables with straightforward UPDATE statements:

 +---+---+  +---+---+
 | a | b |  | a | b |
 +---+---+  +---+---+
 | 1 | 2 |  | 1 | 1 |
 | 2 | 1 |  | 2 | 2 |
 +---+---+  +---+---+

The tool rewrites queries to DELETE and REPLACE in this case. This is automatically handled after the first index violation, so you don't have to worry about it. ✨

🔒 REPLICATION SAFETY 🔒

okay bestie, real talk: syncing a replication master and a slave safely? that is genuinely a non-trivial problem. like, not a vibe, an actual problem. there are SO many things to think about — other processes out here changing data, somebody trying to change data on the slave, whether your destination and source are a master-master pair… the list literally goes on and on. 💀

so the generally-safe way to do it is: change the data on the master, and let those changes flow through replication to the slave like any other changes. cute, right? BUT — this only works if you can REPLACE into the table on the master. and REPLACE only works if there's a unique index on the table. no unique index? it's just acting like an ordinary INSERT. 🙈

if your table has unique keys, you should use the --sync-to-master and/or --replicate options to sync a slave to its master. that'll generally do the right thing, no notes. ✨

but if there's no unique key on the table? you have no choice but to change the data on the slave, and mk-table-sync will detect that you're trying to do so. it will complain and it will die unless you specify --no-check-slave (see --[no]check-slave). it's giving strict parent energy, honestly. 😭

if you're syncing a table without a primary or unique key on a master-master pair, you must change the data on the destination server. therefore you need to specify --no-bin-log for safety (see --[no]bin-log). if you don't? the changes you make on the destination server will replicate back to the source server and change the data there. not the slay you were going for. 💅

and on a master-master pair, the generally safe thing is to use the --sync-to-master option so you don't change the data on the destination server at all. you'll also need to specify --no-check-slave to keep mk-table-sync from complaining that it's changing data on a slave. she's dramatic but she's right. 🎀

🧠 ALGORITHMS 🧠

mk-table-sync has a generic data-syncing framework that uses different algorithms to find differences. the tool automatically picks the best algorithm for each table based on indexes, column types, and the algorithm preferences you specify with --algorithms. 👑

here are all the algorithms available, listed in their default order of preference:

Chunk

finds an index whose first column is numeric (date and time types count too), and divides that column's range of values into chunks of approximately --chunk-size rows. it syncs one chunk at a time by checksumming the entire chunk. if the chunk differs on the source and destination, it checksums each chunk's rows individually to find the rows that differ. 🔍

it's efficient when the column has sufficient cardinality to make the chunks end up about the right size.

the initial per-chunk checksum is quite small and results in minimal network traffic and memory consumption. if a chunk's rows must be examined, only the primary key columns and a checksum are sent over the network — not the entire row. if a row is found to be different, the entire row will be fetched, but not before. restraint. we love to see it. 💖

Nibble

finds an index and ascends it in fixed-size nibbles of --chunk-size rows, using a non-backtracking algorithm (see mk-archiver for more on this algorithm). it's very similar to Chunk, but instead of pre-calculating the boundaries of each piece of the table based on index cardinality, it uses LIMIT to define each nibble's upper limit, and the previous nibble's upper limit to define the lower limit. ✨

it works in steps: one query finds the row that will define the next nibble's upper boundary, and the next query checksums the entire nibble. if the nibble differs between the source and destination, it examines the nibble row-by-row, just as Chunk does.

GroupBy

selects the entire table grouped by all columns, with a COUNT(*) column added. compares all columns, and if they're the same, compares the COUNT(*) column's value to determine how many rows to insert or delete into the destination. works on tables with no primary key or unique index. 🫶

Stream

selects the entire table in one big stream and compares all columns. selects all columns. much less efficient than the other algorithms, but it works when there's no suitable index for them to use. it's the "we tried everything else" algorithm and honestly, respect. 🌸

Future Plans

possibilities for future algorithms are TempTable (what I originally called bottom-up in earlier versions of this tool), DrillDown (what I originally called top-down), and GroupByPrefix (similar to how SqlYOG Job Agent works). each algorithm has strengths and weaknesses. if you'd like to implement your favorite technique for finding differences between two sources of data on possibly different servers, I'm willing to help. the algorithms adhere to a simple interface that makes it pretty easy to write your own. 🧚

🦋 BIDIRECTIONAL SYNCING 🦋

bidirectional syncing is a new, experimental feature. for it to work reliably there are a number of strict limitations:

only works when syncing one server to other independent servers
does not work in any way with replication
requires that the table(s) are chunkable with the Chunk algorithm
is not N-way, only bidirectional between two servers at a time
does not handle DELETE changes

experimental means experimental, bestie. read the limitations twice. 💅

for example, suppose we have three servers: c1, r1, r2. c1 is the central server, a pseudo-master to the other servers (viz. r1 and r2 are not slaves to c1). r1 and r2 are remote servers. rows in table foo are updated and inserted on all three servers and we want to synchronize all the changes between all the servers. table foo has columns:

id    int PRIMARY KEY
ts    timestamp auto updated
name  varchar

auto-increment offsets are used so that new rows from any server do not create conflicting primary key (id) values. in general, newer rows — as determined by the ts column — take precedence when a same-but-differing row is found during the bidirectional sync. "same but differing" means two rows have the same primary key (id) value but different values for some other column, like the name column in this example. same-but-differing conflicts are resolved by a "conflict". a conflict compares some column of the competing rows to determine a "winner". the winning row becomes the source and its values are used to update the other row. 🏆

there are subtle differences between three columns used to achieve bidirectional syncing that you should be familiar with: the chunk column (--chunk-column), the comparison column(s) (--columns), and the conflict column (--conflict-column).

the chunk column is only used to chunk the table; e.g. WHERE id >= 5 AND id < 10. chunks are checksummed and when chunk checksums reveal a difference, the tool selects the rows in that chunk and checksums the --columns for each row. if a column checksum differs, the rows have one or more conflicting column values. in a traditional unidirectional sync, the conflict is a moot point because it can be resolved simply by updating the entire destination row with the source row's values. in a bidirectional sync, however, the --conflict-column (in accordance with the other --conflict-* options listed below) is compared to determine which row is "correct" or "authoritative"; this row becomes the "source". 👑

to sync all three servers completely, two runs of mk-table-sync are required. the first run syncs c1 and r1, then syncs c1 and r2 including any changes from r1. at this point c1 and r2 are completely in sync, but r1 is missing any changes from r2 because c1 didn't have these changes when it and r1 were synced. so a second run is needed which syncs the servers in the same order, but this time when c1 and r1 are synced, r1 gets r2's changes. two runs. that's the price. 🎀

the tool does not sync N-ways, only bidirectionally between the first DSN given on the command line and each subsequent DSN in turn. so the tool in this example would be ran twice like:

mk-table-sync --bidirectional h=c1 h=r1 h=r2

the --bidirectional option enables this feature and causes various sanity checks to be performed. you must specify other options that tell mk-table-sync how to resolve conflicts for same-but-differing rows. these options are:

--conflict-column
--conflict-comparison
--conflict-value
--conflict-threshold
--conflict-error (optional)

use --print to test this option before --execute. the printed SQL statements will have comments saying on which host the statement would be executed if you used --execute. 📝

technical side note: the first DSN is always the "left" server and the other DSNs are always the "right" server. since either server can become the source or destination, it's confusing to think of them as "src" and "dst". therefore, they're generically referred to as left and right. it's easy to remember this because the first DSN is always to the left of the other server DSNs on the command line. 🌈

🚪 EXIT STATUS 🚪

here are the exit statuses (also called return values, or return codes) for when mk-table-sync finishes and exits:

0 — Success.
1 — Internal error.
2 — At least one table differed on the destination.
3 — Combination of 1 and 2.

✨ OPTIONS ✨

specify at least one of --print, --execute, or --dry-run. (pick one, bestie, you gotta commit. 💅)

--where and --replicate are mutually exclusive.

this tool accepts additional command-line arguments. refer to the SYNOPSIS and usage information for details.

--algorithms — type: string; default: Chunk,Nibble,GroupBy,Stream

algorithm to use when comparing the tables, in order of preference.

for each table, mk-table-sync will check if the table can be synced with the given algorithms in the order that they're given. the first algorithm that can sync the table is used. see ALGORITHMS. 👑

--ask-pass — prompt for a password when connecting to MySQL. 🔐
--bidirectional — enable bidirectional sync between the first and subsequent hosts.

see BIDIRECTIONAL SYNCING for more information.

--[no]bin-log — default: yes

log to the binary log (SET SQL_LOG_BIN=1).

specifying --no-bin-log will SET SQL_LOG_BIN=0. 🪵

--buffer-in-mysql — instruct MySQL to buffer queries in its memory.

this option adds the SQL_BUFFER_RESULT option to the comparison queries. this causes MySQL to execute the queries and place them in a temporary table internally before sending the results back to mk-table-sync. the advantage of this strategy is that mk-table-sync can fetch rows as desired without using a lot of memory inside the Perl process, while releasing locks on the MySQL table (to reduce contention with other queries). the disadvantage is that it uses more memory on the MySQL server instead. 🧠

You probably want to leave <code>--[no]buffer-to-client</code> enabled too, because buffering into a temp table and then fetching it all into Perl's memory is lowkey a silly thing to do. This option is most useful for the GroupBy and Stream algorithms, which may fetch a lot of data from the server. 💅

--[no]buffer-to-client — default: yes. Fetch rows one-by-one from MySQL while comparing. ✨

This option enables mysql_use_result, which makes MySQL hold the selected rows on the server until the tool fetches them. That means the tool uses less memory, but it may keep the rows locked on the server longer. It's giving tradeoff energy, bestie. 🦋

If you disable it with --no-buffer-to-client, then mysql_store_result is used instead, which makes MySQL send all the selected rows to the tool at once. This may keep the results "cursor" open for a shorter time on the server, but if the tables are big it could take a long time anyway, and eat all your memory. 😭

For most non-trivial data sizes, you want to leave this option enabled.

Heads up: this option is disabled when --bidirectional is used. No cap.

--charset — short form: -A; type: string. The default character set. 🌸

If the value is utf8, it sets Perl's binmode on STDOUT to utf8, passes the mysql_enable_utf8 option to DBD::mysql, and runs SET NAMES UTF8 after connecting to MySQL. Any other value sets binmode on STDOUT without the utf8 layer, and runs SET NAMES after connecting to MySQL.

--[no]check-master — default: yes. With --sync-to-master, try to verify that the detected master is the real master. 👑
--[no]check-privileges — default: yes. Check that the user has all the necessary privileges on the source and destination table. 🎀
--[no]check-slave — default: yes. Check whether the destination server is a slave.

If the destination server is a slave, it's generally unsafe to make changes on it. But sometimes you gotta — --replace won't work unless there's a unique index, for example, so you can't make changes on the master in that scenario. By default mk-table-sync will complain if you try to change data on a slave. Specify --no-check-slave to disable this check.

Use it at your own risk. Fr fr. 💀

--[no]check-triggers — default: yes. Check that no triggers are defined on the destination table.

Triggers were introduced in MySQL v5.0.2, so on older versions this option has no effect, because triggers won't be checked at all.

--chunk-column — type: string. Chunk the table on this column. 🧚
--chunk-index — type: string. Chunk the table using this index.
--chunk-size — type: string; default: 1000. Number of rows or data size per chunk.

This is the size of each chunk of rows for the Chunk and Nibble algorithms. The size can be either a number of rows, or a data size. Data sizes are specified with a suffix of k=kibibytes, M=mebibytes, G=gibibytes. Data sizes get converted into a number of rows by dividing by the average row length. ✨

--columns — short form: -c; type: array. Compare this comma-separated list of columns.
--config — type: Array. Read this comma-separated list of config files.

If you specify it, it must be the first option on the command line. Non-negotiable, bestie. 💅

--conflict-column — type: string. Compare this column when rows conflict during a --bidirectional sync.

When a same-but-differing row is found, the value of this column from each row is compared according to --conflict-comparison, --conflict-value and --conflict-threshold to figure out which row has the correct data and becomes the source. The column can be any type for which there's an appropriate --conflict-comparison — that's almost all types except, for example, blobs. 🫶