Redis Data Structures & Use Cases Practice Exam

Question 1 Redis Data Structures & Use Cases

What is Redis Pub/Sub?

O(N) where N is the set size A messaging model where publishers send to channels and subscribers receive in real time Evicts the least frequently used keys (volatile-lfu or allkeys-lfu) List

Question 2 Redis Data Structures & Use Cases

What is a Redis transaction?

A group of commands executed atomically using MULTI and EXEC, isolated from other clients Milliseconds-time plus a sequence number for that millisecond Returns write errors when memory is full instead of evicting any data ZINCRBY

Question 3 Redis Data Structures & Use Cases

What backs a Redis geospatial index?

A listpack (previously ziplist) A sorted set with latitude/longitude encoded as a 52-bit geohash score Pub/Sub is fire-and-forget: messages are not stored and are lost without a subscriber SETBIT

Question 4 Redis Data Structures & Use Cases

What command returns the length of a string value?

An append-only log data structure with consumer groups, message IDs, and acknowledgments XADD STRLEN REmote DIctionary Server

Question 5 Redis Data Structures & Use Cases

What is Redis Sentinel?

A cursor-based iterator over the keyspace that does not block the server Compacts the AOF file from the current dataset to keep it from growing unbounded A built-in HA system that monitors masters/replicas and performs automatic failover AOF with appendfsync always is stronger; RDB may lose the last seconds or minutes of writes

Question 6 Redis Data Structures & Use Cases

What does SUNIONSTORE do?

512 MB Stores the union of multiple sets into a destination key Replica nodes asynchronously copy data from a primary to provide read scaling and HA The distance between two members of a geospatial index

Question 7 Redis Data Structures & Use Cases

What is the time complexity of LPUSH?

O(1) Tagging, unique visitor tracking, and set-intersection recommendations LPUSH inserts at the head (left); RPUSH inserts at the tail (right) Records commands exceeding a configurable time threshold for diagnostics

Question 8 Redis Data Structures & Use Cases

What is the difference between SETEX and SET with PX?

A cursor-based iterator over the keyspace that does not block the server HEXPIRE (Redis 7.4+) SETEX takes seconds; SET ... PX takes milliseconds for finer TTL control LPUSH new items and LTRIM to keep only the latest N items

Question 9 Redis Data Structures & Use Cases

How is Redis primarily classified architecturally?

GEOSEARCH (replaces GEORADIUS from Redis 6.2) O(1) per key A small string up to 44 bytes stored inline in the Redis object to save memory An in-memory key-value data store used as a database, cache, and message broker

Question 10 Redis Data Structures & Use Cases

Which is the most basic Redis data type, storing a single binary-safe value per key?

Appends every write command to appendonly.aof, which is replayed at restart REmote DIctionary Server A group of commands executed atomically using MULTI and EXEC, isolated from other clients String

Question 11 Redis Data Structures & Use Cases

What does OBJECT IDLETIME return?

The seconds since the last access of a stored object Hashes use less memory and allow partial-field reads and writes A sorted set with latitude/longitude encoded as a 52-bit geohash score Atomically pops an element from one list and pushes it to another, with a configurable direction

Question 12 Redis Data Structures & Use Cases

Which command returns places within a given radius?

Durable, replayable messaging with per-consumer delivery and acknowledgment 3 masters with 1 replica each (6 nodes) for availability A string value treated as an array of bits addressable by index via SETBIT/GETBIT GEOSEARCH (replaces GEORADIUS from Redis 6.2)

Question 13 Redis Data Structures & Use Cases

Which Redis module provides Bloom and Cuckoo filters?

A sorted set with latitude/longitude encoded as a 52-bit geohash score Returns 1 if the member is in the set, 0 otherwise A probabilistic membership test with possible false positives but no false negatives RedisBloom

Question 14 Redis Data Structures & Use Cases

What is the time complexity of ZADD?

REmote DIctionary Server O(log N) LPUSH inserts at the head (left); RPUSH inserts at the tail (right) String

Question 15 Redis Data Structures & Use Cases

What is the maximum size of a single string value in Redis?

A substring within { } braces used by Cluster to force keys into the same slot String 512 MB XREAD

Question 16 Redis Data Structures & Use Cases

What does the volatile-lru policy do?

Evicts the least recently used keys among those with a TTL Consuming a stream in a consumer group so each entry is delivered to only one consumer Filters returned keys to a glob pattern A native sharding solution that distributes keys across nodes using 16384 hash slots

Question 17 Redis Data Structures & Use Cases

Which command reads entries from one or more streams?

XREAD About 0.81% standard error SETBIT A substring within { } braces used by Cluster to force keys into the same slot

Question 18 Redis Data Structures & Use Cases

How would you implement a recent-activity feed with a Redis List?

Returned when EVALSHA is called but the script is not in the cache; clients should fall back to EVAL A skip list plus a hash table LPUSH new items and LTRIM to keep only the latest N items GEOADD

Question 19 Redis Data Structures & Use Cases

What does MGET do?

ZREVRANK Returns the values of multiple keys in a single round trip Snapshot after 1 hour if ≥1 change, after 5 min if ≥100, after 1 min if ≥10000 Acknowledges processing of a stream entry so it can be trimmed or deleted

Question 20 Redis Data Structures & Use Cases

What is the difference between HGETALL and HVALS?

Records commands exceeding a configurable time threshold for diagnostics A listpack (previously ziplist) MOVED is permanent and points to the slot's owner; ASK is a transient hint during slot migration HGETALL returns fields and values; HVALS returns only values

Question 21 Redis Data Structures & Use Cases

Which command atomically sets a value only if the key already exists?

O(1) per element added SET key value XX Pub/Sub is fire-and-forget: messages are not stored and are lost without a subscriber Both are atomic, but Lua supports conditional logic and complex cross-key operations

Question 22 Redis Data Structures & Use Cases

What is a Redis HyperLogLog used for?

Counting unique items with a fixed 12 KB footprint regardless of cardinality Point-in-time snapshots are written to dump.rdb at configured intervals LPUSH new items and LTRIM to keep only the latest N items Evicts the least frequently used keys (volatile-lfu or allkeys-lfu)

Question 23 Redis Data Structures & Use Cases

What does used_memory represent?

AOF with appendfsync always is stronger; RDB may lose the last seconds or minutes of writes The total bytes Redis has allocated to store data, including overhead Storing object attributes (e.g., user, product, session) under one key SRANDMEMBER

Question 24 Redis Data Structures & Use Cases

What is a typical use case for Redis Bitmaps?

A substring within { } braces used by Cluster to force keys into the same slot String 6379 Daily active user tracking, feature flags, or boolean state per user

Question 25 Redis Data Structures & Use Cases

What is a NOSCRIPT error?

A small string up to 44 bytes stored inline in the Redis object to save memory LLEN Returned when EVALSHA is called but the script is not in the cache; clients should fall back to EVAL BRPOP or BLPOP

Question 26 Redis Data Structures & Use Cases

What command returns random members from a set without removing them?

Consuming a stream in a consumer group so each entry is delivered to only one consumer LPUSH inserts at the head (left); RPUSH inserts at the tail (right) A messaging model where publishers send to channels and subscribers receive in real time SRANDMEMBER

Question 27 Redis Data Structures & Use Cases

Which command subscribes to messages matching a glob pattern?

Evicts the key with the shortest remaining TTL among keys that have one Returns 1 if the member is in the set, 0 otherwise PSUBSCRIBE HDEL

Question 28 Redis Data Structures & Use Cases

What is the time complexity of HGET?

Returns the data type stored at a key (string, list, set, zset, hash, stream) O(1) Removes elements outside the specified index range, modifying the list in place SETNX is the legacy form; SET ... NX is the modern atomic form that also supports expiration

Question 29 Redis Data Structures & Use Cases

Which command appends an entry to a stream?

The total bytes Redis has allocated to store data, including overhead Avoiding expensive database lookups for likely-nonexistent keys (e.g., username or cache-miss checks) Removes elements outside the specified index range, modifying the list in place XADD

Question 30 Redis Data Structures & Use Cases

What does the allkeys-lru policy do?

Evicts the least recently used keys regardless of TTL O(N + log M) where N is items in the radius and M is the total Pub/Sub is fire-and-forget: messages are not stored and are lost without a subscriber Removes elements outside the specified index range, modifying the list in place

Question 31 Redis Data Structures & Use Cases

What is the structure of a Redis Stream entry ID?

Milliseconds-time plus a sequence number for that millisecond An in-memory key-value data store used as a database, cache, and message broker Atomically sets a key to a new value and returns the previous one Store timestamps as scores; use ZREMRANGEBYSCORE to drop old entries and ZCARD to count recent ones

Question 32 Redis Data Structures & Use Cases

What is a Redis Bitmap?

A string value treated as an array of bits addressable by index via SETBIT/GETBIT SRANDMEMBER does not remove the element; SPOP removes and returns it GEOSEARCH (replaces GEORADIUS from Redis 6.2) Appends every write command to appendonly.aof, which is replayed at restart

Question 33 Redis Data Structures & Use Cases

What command removes one or more fields from a hash?

HDEL Replica nodes asynchronously copy data from a primary to provide read scaling and HA Sorted Set (ZSET) SETNX is the legacy form; SET ... NX is the modern atomic form that also supports expiration

Question 34 Redis Data Structures & Use Cases

Why prefer a Hash over many String keys for an object?

GEOADD Tagging, unique visitor tracking, and set-intersection recommendations Hashes use less memory and allow partial-field reads and writes O(N) where N is the set size

Question 35 Redis Data Structures & Use Cases

Which Redis type stores an unordered collection of unique strings?

About 0.81% standard error Set The total bytes Redis has allocated to store data, including overhead String

Question 36 Redis Data Structures & Use Cases

What is the difference between SETNX and SET with the NX option?

Durable, replayable messaging with per-consumer delivery and acknowledgment SETNX is the legacy form; SET ... NX is the modern atomic form that also supports expiration A listpack (previously ziplist) Sorted Set (ZSET)

Question 37 Redis Data Structures & Use Cases

What is the purpose of XACK?

LPUSH to enqueue and BRPOP to dequeue gives a simple message queue HGETALL returns fields and values; HVALS returns only values All fields and values in the hash as a flat list Acknowledges processing of a stream entry so it can be trimmed or deleted

Question 38 Redis Data Structures & Use Cases

What is the difference between KEYS and SCAN?

KEYS returns all matches in one blocking call; SCAN iterates in small batches and is safe on large datasets Acknowledges processing of a stream entry so it can be trimmed or deleted An append-only log data structure with consumer groups, message IDs, and acknowledgments Subscribe to __keyevent@0__:expired (or the relevant db index)

Question 39 Redis Data Structures & Use Cases

Which Redis type maps field-value pairs within a single key?

C All fields and values in the hash as a flat list ZINCRBY Hash

Question 40 Redis Data Structures & Use Cases

What is the purpose of the TYPE command?

Runs a previously cached Lua script by its SHA1 hash to avoid resending the script body Returns write errors when memory is full instead of evicting any data Acknowledges processing of a stream entry so it can be trimmed or deleted Returns the data type stored at a key (string, list, set, zset, hash, stream)

Question 41 Redis Data Structures & Use Cases

What is Redis replication?

About 0.81% standard error SETBIT Replica nodes asynchronously copy data from a primary to provide read scaling and HA SRANDMEMBER does not remove the element; SPOP removes and returns it

Question 42 Redis Data Structures & Use Cases

What does SDIFF return?

The set of members in the first set that are not in the others SRANDMEMBER SETEX takes seconds; SET ... PX takes milliseconds for finer TTL control LPUSH new items and LTRIM to keep only the latest N items

Question 43 Redis Data Structures & Use Cases

Which command returns the rank of a member with 0 meaning the highest score?

A probabilistic membership test with possible false positives but no false negatives ZREVRANK O(1) per key SETNX is the legacy form; SET ... NX is the modern atomic form that also supports expiration

Question 44 Redis Data Structures & Use Cases

What underlying structure backs a small Redis list?

Snapshot after 1 hour if ≥1 change, after 5 min if ≥100, after 1 min if ≥10000 AOF with appendfsync always is stronger; RDB may lose the last seconds or minutes of writes noeviction A listpack (previously ziplist)

Question 45 Redis Data Structures & Use Cases

What is a Redis keyspace notification?

Both are atomic, but Lua supports conditional logic and complex cross-key operations HGETALL returns fields and values; HVALS returns only values An event published on a channel when a key is created, modified, expired, or evicted SETNX is the legacy form; SET ... NX is the modern atomic form that also supports expiration

Question 46 Redis Data Structures & Use Cases

Which command returns server statistics and configuration?

INFO A string value treated as an array of bits addressable by index via SETBIT/GETBIT APPEND The seconds since the last access of a stored object

Question 47 Redis Data Structures & Use Cases

What does the GETSET command do?

A skip list plus a hash table Atomically sets a key to a new value and returns the previous one Redis returns an error Returns the values of multiple keys in a single round trip

Question 48 Redis Data Structures & Use Cases

What is the main limitation of Pub/Sub compared to Streams?

The internal encoding of a key, e.g., embstr, int, listpack, skiplist, or hashtable Returns the values of multiple keys in a single round trip Pub/Sub is fire-and-forget: messages are not stored and are lost without a subscriber The set of members in the first set that are not in the others

Question 49 Redis Data Structures & Use Cases

What is a typical use case for Redis Hashes?

SMEMBERS returns the full set in one call; SSCAN iterates incrementally with a cursor, avoiding blocking on large sets The seconds since the last access of a stored object Storing object attributes (e.g., user, product, session) under one key XADD

Question 50 Redis Data Structures & Use Cases

What is the difference between ZRANGE and ZRANGEBYSCORE?

ZRANGE selects by index range; ZRANGEBYSCORE selects by score range The seconds since the last access of a stored object Returned when EVALSHA is called but the script is not in the cache; clients should fall back to EVAL The set of members in the first set that are not in the others