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      How To Run Transactions in Redis


      Introduction

      Redis is an open-source, in-memory key-value data store. Redis allows you to plan a sequence of commands and run them one after another, a procedure known as a transaction. Each transaction is treated as an uninterrupted and isolated operation, which ensures data integrity. Clients cannot run commands while a transaction block is being executed

      This tutorial goes over how to execute and cancel transactions, and also includes some information on pitfalls commonly associated with transactions.

      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 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 output 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.

      Running Transactions

      The multi command tells Redis to begin a transaction block. Any subsequent commands will be queued up until you run an exec command, which will execute them.

      The following commands form a single transaction block. The first command initiates the transaction, the second sets a key holding a string with the value of 1, the third increases the value by 1, the fourth increases its value by 40, the fifth returns the current value of the string, and the last one executes the transaction block:

      • multi
      • set key_MeaningOfLife 1
      • incr key_MeaningOfLife
      • incrby key_MeaningOfLife 40
      • get key_MeaningOfLife
      • exec

      After running multi, redis-cli will respond to each of the following commands with QUEUED. After you run the exec command, it will show the output of each of those commands individually:

      Output

      1) OK 2) (integer) 2 3) (integer) 42 4) "42"

      Commands included in a transaction block are run sequentially in the order they’re queued. Redis transactions are atomic, meaning that either every command in a transaction block is processed (meaning that it’s accepted as valid and queued to be executed) or none are. However, even if a command is successfully queued, it may still produce an error when executed. In such cases, the other commands in the transaction can still run, but Redis will skip the error-causing command. See the section on understanding transaction errors for more details.

      Canceling Transactions

      To cancel a transaction, run the discard command. This prevents any previously-queued commands from running:

      • multi
      • set key_A 146
      • incrby key_A 10
      • discard

      Output

      OK

      The discard command returns the connection to a normal state, which tells Redis to run single commands as usual. You’ll need to run multi again to tell the server you’re starting another transaction.

      Understanding Transaction Errors

      Some commands may be impossible to queue, such as commands with syntax errors. If you attempt to queue a syntactically incorrect command Redis will return an error.

      The following transaction createst a key named key_A and then attempts to increment it by 10. However, a spelling error in the incrby command causes and error and closes the transaction:

      • multi
      • set key_A 146
      • incrbuy key_A 10

      Output

      (error) ERR unknown command 'incrbuy'

      If you try to run an exec command after trying to queue a command with a syntax error like this one, you will receive another error message telling you that the transaction was discarded:

      Output

      (error) EXECABORT Transaction discarded because of previous errors.

      In cases like this, you’ll need to restart the transaction block and make sure you enter each command correctly.

      Some impossible commands are possible to queue, such as running incr on a key containing only a string. Because such command is syntactically correct, Redis won’t return an error if you try to include it in a transaction and won’t prevent you from running exec. In cases like this, all other commands in the queue will be executed, but the impossible command will return an error:

      • multi
      • set key_A 146
      • incrby key_A "ten"
      • exec

      Output

      1) OK 2) (error) ERR value is not an integer or out of range

      For more information on how Redis handles errors inside transactions, see the official documentation on the subject.

      Conclusion

      This guide details a number of commands used to create, run, and cancel transactions in 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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