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      How To Troubleshoot Issues in Redis


      Introduction

      Redis is an open-source, in-memory key-value data store. It comes with several commands that can help with troubleshooting and debugging issues. Because of Redis’s 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.

      How To Use This Guide
      This guide is written as a cheat sheet with self-contained examples. We encourage you to jump to any section that is relevant to the task you’re trying to complete.

      The commands and outputs shown in this guide were tested on an Ubuntu 18.04 server running Redis version 4.0.9. To obtain a similar setup, you can follow Step 1 of our guide on How To Install and Secure Redis on Ubuntu 18.04. We will demonstrate how these commands behave by running them with redis-cli, the Redis command line interface. Note that if you’re using a different Redis interface — Redli, for example — the exact outputs of certain commands may differ.

      Alternatively, you could provision a managed Redis database instance to test these commands, but note that depending on the level of control allowed by your database provider, some commands in this guide may not work as described. To provision a DigitalOcean Managed Database, follow our Managed Databases product documentation. Then, you must either install Redli or set up a TLS tunnel in order to connect to the Managed Database over TLS.

      memory usage tells you how much memory is currently being used by a single key. It takes the name of a key as an argument and outputs the number of bytes it uses:

      • memory usage key_meaningOfLife

      Output

      (integer) 42

      For a more general understanding of how your Redis server is using memory, you can run the memory stats command:

      This command outputs an array of memory-related metrics and their values. The following are the metrics reported by memory stats:

      • peak.allocated: The peak number of bytes consumed by Redis
      • total.allocated: The total number of bytes allocated by Redis
      • startup.allocated: The initial number of bytes consumed by Redis at startup
      • replication.backlog: The size of the replication backlog, in bytes
      • clients.slaves: The total size of all replica overheads (the output and query buffers and connection contexts)
      • clients.normal: The total size of all client overheads
      • aof.buffer: The total size of the current and rewrite append-only file buffers
      • db.0: The overheads of the main and expiry dictionaries for each database in use on the server, reported in bytes
      • overhead.total: The sum of all overheads used to manage Redis’s keyspace
      • keys.count: The total number of keys stored in all the databases on the server
      • keys.bytes-per-key: The ratio of the server’s net memory usage and keys.count
      • dataset.bytes: The size of the dataset, in bytes
      • dataset.percentage: The percentage of Redis’s net memory usage taken by dataset.bytes
      • peak.percentage: The percentage of peak.allocated taken out of total.allocated
      • fragmentation: The ratio of the amount of memory currently in use divided by the physical memory Redis is actually using

      memory malloc-stats provides an internal statistics report from jemalloc, the memory allocator used by Redis on Linux systems:

      If it seems like you’re running into memory-related issues, but parsing the output of the previous commands proves to be unhelpful, you can try running memory doctor:

      This feature will output any memory consumption issues that it can find and suggest potential solutions.

      Getting General Information about Your Redis Instance

      A debugging command that isn’t directly related to memory management is monitor. This command allows you to see a constant stream of every command processed by the Redis server:

      Output

      OK 1566157213.896437 [0 127.0.0.1:47740] "auth" "foobared" 1566157215.870306 [0 127.0.0.1:47740] "set" "key_1" "878"

      Another command useful for debugging is info, which returns several blocks of information and statistics about the server:

      Output

      # Server redis_version:4.0.9 redis_git_sha1:00000000 redis_git_dirty:0 redis_build_id:9435c3c2879311f3 redis_mode:standalone os:Linux 4.15.0-52-generic x86_64 . . .

      This command returns a lot of information. If you only want to see one info block, you can specify it as an argument to info:

      Output

      # CPU used_cpu_sys:173.16 used_cpu_user:70.89 used_cpu_sys_children:0.01 used_cpu_user_children:0.04

      Note that the information returned by the info command will depend on which version of Redis you’re using.

      Using the keys Command

      The keys command is helpful in cases where you’ve forgotten the name of a key, or perhaps you’ve created one but accidentally misspelled its name. keys looks for keys that match a pattern:

      The following glob-style variables are supported

      • ? is a wildcard standing for any single character, so s?mmy matches sammy, sommy, and sqmmy
      • * is a wildcard that stands for any number of characters, including no characters at all, so sa*y matches sammy, say, sammmmmmy, and salmony
      • You can specify two or more characters that the pattern can include by wrapping them in brackets, so s[ai]mmy will match sammy and simmy, but not summy
      • To set a wildcard that disregards one or more letters, wrap them in brackets and precede them with a carrot (^), so s[^oi]mmy will match sammy and sxmmy, but not sommy or simmy
      • To set a wildcard that includes a range of letters, separate the beginning and end of the range with a hyphen and wrap it in brackets, so s[a-o]mmy will match sammy, skmmy, and sommy, but not srmmy

      Warning: The Redis documentation warns that keys should almost never be used in a production environment, since it can have a major negative impact on performance.

      Conclusion

      This guide details a number of commands that are helpful for troubleshooting and resolving issues one might encounter as they work with Redis. If there are other related commands, arguments, or procedures you’d like to see outlined in this guide, please ask or make suggestions in the comments below.

      For more information on Redis commands, see our tutorial series on How to Manage a Redis Database.



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      How To Troubleshoot MySQL Queries



      Part of the Series:
      How To Troubleshoot Issues in MySQL

      This guide is intended to serve as a troubleshooting resource and starting point as you diagnose your MySQL setup. We’ll go over some of the issues that many MySQL users encounter and provide guidance for troubleshooting specific problems. We will also include links to DigitalOcean tutorials and the official MySQL documentation that may be useful in certain cases.

      Sometimes users run into problems once they begin issuing queries on their data. In some database systems, including MySQL, query statements in must end in a semicolon (;) for the query to complete, as in the following example:

      If you fail to include a semicolon at the end of your query, the prompt will continue on a new line until you complete the query by entering a semicolon and pressing ENTER.

      Some users may find that their queries are exceedingly slow. One way to find which query statement is the cause of a slowdown is to enable and view MySQL's slow query log. To do this, open your mysqld.cnf file, which is used to configure options for the MySQL server. This file is typically stored within the /etc/mysql/mysql.conf.d/ directory:

      • sudo nano /etc/mysql/mysql.conf.d/mysqld.cnf

      Scroll through the file until you see the following lines:

      /etc/mysql/mysql.conf.d/mysqld.cnf

      . . .
      #slow_query_log         = 1
      #slow_query_log_file    = /var/log/mysql/mysql-slow.log
      #long_query_time = 2
      #log-queries-not-using-indexes
      . . .
      

      These commented-out directives provide MySQL's default configuration options for the slow query log. Specifically, here's what each of them do:

      • slow-query-log: Setting this to 1 enables the slow query log.
      • slow-query-log-file: This defines the file where MySQL will log any slow queries. In this case, it points to the /var/log/mysql-slow.log file.
      • long_query_time: By setting this directive to 2, it configures MySQL to log any queries that take longer than 2 seconds to complete.
      • log_queries_not_using_indexes: This tells MySQL to also log any queries that run without indexes to the /var/log/mysql-slow.log file. This setting isn't required for the slow query log to function, but it can be helpful for spotting inefficient queries.

      Uncomment each of these lines by removing the leading pound signs (#). The section will now look like this:

      /etc/mysql/mysql.conf.d/mysqld.cnf

      . . .
      slow_query_log = 1
      slow_query_log_file = /var/log/mysql-slow.log
      long_query_time = 2
      log_queries_not_using_indexes
      . . .
      

      Note: If you're running MySQL 8+, these commented lines will not be in the mysqld.cnf file by default. In this case, add the following lines to the bottom of the file:

      /etc/mysql/mysql.conf.d/mysqld.cnf

      . . .
      slow_query_log = 1
      slow_query_log_file = /var/log/mysql-slow.log
      long_query_time = 2
      log_queries_not_using_indexes
      

      After enabling the slow query log, save and close the file. Then restart the MySQL service:

      • sudo systemctl restart mysql

      With these settings in place, you can find problematic query statements by viewing the slow query log. You can do so with less, like this:

      • sudo less /var/log/mysql_slow.log

      Once you've singled out the queries causing the slowdown, you may find our guide on How To Optimize Queries and Tables in MySQL and MariaDB on a VPS to be helpful with optimizing them.

      Additionally, MySQL includes the EXPLAIN statement, which provides information about how MySQL executes queries. This page from the official MySQL documentation provides insight on how to use EXPLAIN to highlight inefficient queries.

      For help with understanding basic query structures, see our Introduction to MySQL Queries.



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      How To Troubleshoot Socket Errors in MySQL


      MySQL manages connections to the database server through the use of a socket file, a special kind of file that facilitates communications between different processes. The MySQL server’s socket file is named mysqld.sock and on Ubuntu systems it’s usually stored in the /var/run/mysqld/ directory. This file is created by the MySQL service automatically.

      Sometimes, changes to your system or your MySQL configuration can result in MySQL being unable to read the socket file, preventing you from gaining access to your databases. The most common socket error looks like this:

      Output

      ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

      There are a few reasons why this error may occur, and a few potential ways to resolve it.

      One common cause of this error is that the MySQL service is stopped or did not start to begin with, meaning that it was unable to create the socket file in the first place. To find out if this is the reason you’re seeing this error, try starting the service with systemctl:

      • sudo systemctl start mysql

      Then try accessing the MySQL prompt again. If you still receive the socket error, double check the location where your MySQL installation is looking for the socket file. This information can be found in the mysqld.cnf file:

      • sudo nano /etc/mysql/mysql.conf.d/mysql.cnf

      Look for the socket parameter in the [mysqld] section of this file. It will look like this:

      /etc/mysql/mysql.conf.d/mysqld.cnf

      . . .
      [mysqld]
      user            = mysql
      pid-file        = /var/run/mysqld/mysqld.pid
      socket          = /var/run/mysqld/mysqld.sock
      port            = 3306
      . . .
      

      Close this file, then ensure that the mysqld.sock file exists by running an ls command on the directory where MySQL expects to find it:

      If the socket file exists, you will see it in this command's output:

      Output

      . .. mysqld.pid mysqld.sock mysqld.sock.lock

      If the file does not exist, the reason may be that MySQL is trying to create it, but does not have adequate permissions to do so. You can ensure that the correct permissions are in place by changing the directory's ownership to the mysql user and group:

      • sudo chown mysql:mysql /var/run/mysqld/

      Then ensure that the mysql user has the appropriate permissions over the directory. Setting these to 775 will work in most cases:

      • sudo chmod -R 755 /var/run/mysqld/

      Finally, restart the MySQL service so it can attempt to create the socket file again:

      • sudo systemctl restart mysql

      Then try accessing the MySQL prompt once again. If you still encounter the socket error, there's likely a deeper issue with your MySQL instance, in which case you should review the error log to see if it can provide any clues.



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