Clustering Redis Databases

Redis Cloud uses clustering to manage very large databases (25 GB and larger). Here, you'll learn how to manage clustering and how to use hashing policies to control how data is managed.

For very large databases, Redis Cloud distributes database data to different cloud instances. For example:

• When data grows beyond the RAM resources of a single server.

  Multiple shards should be used when data grows to 25 GB (50 GB for Auto Tiering) to create multiple shards.

• The operations performed against the database are CPU intensive enough to degrade performance.

  Clustering distributes operational load, whether to instances on the same server or across multiple servers.

This distribution is called clustering because it manages the way data is distributed throughout the cluster of nodes that support the database.

How data is distributed

A Redis Cloud cluster is a set of managed Redis processes and cloud instances, with each process managing a subset of the database keyspace. Clustering uses multiple cores and resources of multiple instances to overcome scaling challenges.

In a Redis Cloud cluster, the keyspace is partitioned into hash slots. At any given time a hash slot resides on and is managed by a single Redis server.

An instance that belongs to a cluster can manage multiple hash slots. This division of the key space, known as sharding, is achieved by hashing the key names, or parts of these (key hash tags), in order to obtain the hash slot in which a key should reside. Redis Cloud supports several hashing policies.

Even when using multiple Redis processes, the use of a Redis Enterprise Cloud cluster is nearly transparent to the application that uses it. The cluster is accessible via a single endpoint that automatically routes all operations to the relevant shards, without the complexity of a cluster-aware Redis client. This allows applications to benefit from using the cluster without performing any code changes, even if they were not designed to use it beforehand.

When creating or editing a Redis database on Redis Cloud, the system automatically calculates the number of shards needed based on the database memory limit and required throughput.

Multi-key operations

Operations on multiple keys in a sharded Redis Cloud cluster are supported, with the following limitations:

1. Multi-key commands: Redis offers several commands that accept multiple keys as arguments. In a sharded setup, you can run multi-key commands only if all the affected keys reside in the same slot (thus on the same shard). This restriction applies to all mulit-key commands, including BITOP, BLPOP, BRPOP, BRPOPLPUSH, MSETNX, RPOPLPUSH, SDIFF, SDIFFSTORE, SINTER, SINTERSTORE, SMOVE, SORT, SUNION, XREAD, XREADGROUP, ZINTER, ZINTERSTORE, ZUNION, ZUNIONSTORE, ZDIFF, ZDIFFSTORE
2. Geo commands: In GEORADIUS/GEORADIUSBYMEMBER/GEOSEARCHSTORE commands, the STORE and STOREDIST options can only be used when all affected keys reside in the same slot.
3. Transactions: All operations within a WATCH/MULTI/EXEC block should be performed on keys that are in the same slot.
4. Lua scripts: All keys that are used by the script must reside in the same slot and need to be provided as arguments to the EVAL/EVALSHA commands (as per the Redis specification).
5. Renaming/Copy keys: The use of the RENAME/RENAMENX/COPY commands is allowed only when both the key's original name and its new name are mapped to the same hash slot.
6. Variadic commands: The use of (MGET, MSET, HMGET, HMSET, etc..) and pipelining are supported with Redis Cloud cluster like if it were a non-cluster DB.

Hashing policies and hash tags

The hashing policy determines how data is distributed across multiple Redis processes of a database. It uses a hashing function to map keys to hash slots in these processes, ensuring an even distribution of data for optimal performance and scalability.

The hashing function uses the entire key name to calculate the hash slot, unless the key name contains a hash tag, represented by a {...} pattern.

If the key contains a {...} pattern, only the substring between { and } is hashed in order to obtain the hash slot.

You can use the {...} pattern to direct related keys to the same hash slot, so that multi-key operations are supported on them. On the other hand, not using a hashtag in the key's name results in a (statistically) even distribution of keys across the keyspace's shards, which improves resource utilization. If your application does not perform multi-key operations, you don't need to construct key names with hashtags.

Redis Cloud offers 3 hashing policies, which differ in how hash tags are processed. These hashing policies are not always available.

For accounts created after March 31, 2025, Redis defaults to the Redis hashing policy when creating a new database. For all other accounts, Redis defaults to the standard hashing policy.

Redis hashing policy

The Redis hashing policy is identical to the hashing policy used by Redis Community Edition. This policy is recommended for most users and you should select it if any of the following conditions apply:

• This is your first Redis Cloud account, and you are starting fresh.
• You are migrating data from Redis Community Edition or other Redis-managed platforms.
• Your application does not use hashtags in database key names.
• Your application uses binary data as key names.

Standard hashing policy

The Standard hashing policy is mostly consistent with the Redis hashing policy, and will generate the same hash-slot calculation in the following cases:

1. Keys with a single hashtag: a key's hashtag is any substring between '{' and '}' in the key's name. That means that when a key's name includes the pattern '{...}', the hashtag is used as input for the hashing function. For example, the following key names have the same hashtag and are mapped to the same slot: foo{bar}, {bar}baz & foo{bar}baz.
2. Keys without a hashtag: when a key doesn't contain the '{...}' pattern, the entire key's name is used for hashing

However, this policy is less recommended and you should select it only if any of the following conditions apply:

• Your application uses empty hashtags to hash different keys to the same hashslot
• Your application uses multiple curly brackets within a key’s name

In some cases, the Standard hashing policy behaves differently from the Redis hashing policy:

1. Using empty hashtags (“{}”): the Standard hashing policy does not ignore empty hashtags, so two keys that start with empty hashtags will be hashed to the same hashslot (while the Redis hashing policy would ignore them). For example: given 2 keys {}foo and {}bar, hashing would be:
   • Standard hashing policy: to the same hash slot
   • Redis hashing policy: to different hash slots
2. Using multiple curly brackets: when a key’s name contains multiple curly brackets, the Standard hashing calculation might be different than the Redis hashing policy. For example: given 2 keys {foo}bar} and {foo}qux}:
   • Standard hashing policy: substrings “foo}bar” and “foo}qux” will be used for the 1st and 2nd key respectively, hashed each key to a different hash-slot.
   • Redis hashing policy: the substring “foo” will be used for both keys, hashing them to the same slot.

Custom hashing policy

A Redis Cloud cluster can be configured to use a custom hashing policy. A custom hashing policy is required when different keys need to be kept together on the same shard to allow multi-key operations. Redis Cloud's custom hashing policy is provided via a set of Perl Compatible Regular Expressions (PCRE) rules that describe the dataset's key name patterns.

To configure a custom hashing policy, enter regular expression (RegEx) rules that identify the substring in the key's name - hashtag - on which hashing will be done. The hashing tag is denoted in the RegEx by the use of the `tag` named subpattern. Different keys that have the same hashtag will be stored and managed in the same slot.

Once you enable the custom hashing policy, the Redis Cloud's default RegEx rules that implement the standard hashing policy are:

You can modify existing rules, add new ones, delete rules, or change their order to suit your application's requirements.

Custom hashing policy notes and limitations

1. You can define up to 32 RegEx rules, each up to 256 characters.
2. RegEx rules are evaluated by their order.
3. The first rule matched is used; strive to place common key name patterns at the beginning of the rule list.
4. Key names that do not match any of the RegEx rules trigger an error.
5. The '.*(?<tag>)' RegEx rule forces keys into a single slot as the hash key is always empty. When used, this should be the last catch-all rule.
6. The following flag is enabled in our regular expression parser:
   • PCRE_ANCHORED: the pattern is constrained to match only at the start of the string which is being searched.

Cluster API

OSS Cluster API reduces access times and latency with near-linear scalability. The OSS Cluster API provides a simple mechanism for Redis clients to know the cluster topology.

Clients must first connect to the master node to get the cluster topology, and then they connect directly to the Redis proxy on each node that hosts a master shard.

The Cluster API is only supported on Redis Cloud Pro databases. You can enable it in the Performance section of the configuration screen.

After you select OSS Cluster API, you can select Use external endpoint if you want to use the external endpoint for the database. Selecting Use external endpoint will block the private endpoint for this database.

The Redis Cluster API is supported only when a database uses the standard hashing policy and does not use Search and Query or Time Series advanced capabilities.

Review Redis Cluster API architecture to determine if you should enable this feature for your database.