RS: Added tombstone explanation to Active-Active docs

content/operate/rs/databases/active-active/develop/develop-for-aa.md

@@ -17,20 +17,21 @@ conditions between updates to various sites, network, and cluster
 failures that could reorder the events and change the outcome of the
 updates performed across geo-distributed writes.
 
-Active-Active databases (formerly known as CRDB) are geo-distributed databases that span multiple Redis Software (RS) clusters.
-Active-Active databases depend on multi-master replication (MMR) and Conflict-free
-Replicated Data Types (CRDTs) to power a simple development experience
+Active-Active databases (formerly known as CRDB) are geo-distributed databases that span multiple Redis Software clusters.
+Active-Active databases depend on [multi-master replication](https://en.wikipedia.org/wiki/Multi-master_replication) and [conflict-free
+replicated data types (CRDTs)](https://en.wikipedia.org/wiki/Conflict-free_replicated_data_type) to power a simple development experience
 for geo-distributed applications. Active-Active databases allow developers to use existing
-Redis data types and commands, but understand the developers intent and
-automatically handle conflicting concurrent writes to the same key
+Redis data types and commands, but automatically handle conflicting concurrent writes to the same key
 across multiple geographies. For example, developers can simply use the
-INCR or INCRBY method in Redis in all instances of the geo-distributed
-application, and Active-Active databases handle the additive nature of INCR to reflect the
-correct final value. The following example displays a sequence of events
-over time : t1 to t9. This Active-Active database has two member Active-Active databases : member CRDB1 and
-member CRDB2. The local operations executing in each member Active-Active database is
-listed under the member Active-Active database name. The "Sync" even represent the moment
-where synchronization catches up to distribute all local member Active-Active database
+`INCR` or `INCRBY` method in Redis in all instances of the geo-distributed
+application, and Active-Active databases handle the additive nature of `INCR` to reflect the
+correct final value.
+
+The following example displays a sequence of events
+over time: t1 to t9. This Active-Active database has two member Active-Active databases: member CRDB1 and
+member CRDB2. The local operations running in each member Active-Active database are
+listed under the member Active-Active database name. The sync events represent the moment
+when synchronization catches up to distribute all local member Active-Active database
 updates to other participating clusters and other member Active-Active databases.
 
 |  **Time** | **Member CRDB1** | **Member CRDB2** |
@@ -47,44 +48,44 @@ updates to other participating clusters and other member Active-Active databases
 
 Databases provide various approaches to address some of these concerns:
 
-- Active-Passive Geo-distributed deployments: With active-passive
-    distributions, all writes go to an active cluster. Redis Software
-    provides a "Replica Of" capability that provides a similar approach.
-    This can be employed when the workload is heavily balanced towards
-    read and few writes. However, WAN performance and availability
-    is quite flaky and traveling large distances for writes take away
+- **Active-Passive geo-distributed deployments**: With active-passive
+    distributions, all writes go to an active cluster. Redis Sofware
+    provides a [Replica Of]({{<relref "/operate/rs/databases/import-export/replica-of/">}}) capability that provides a similar approach.
+    This can be employed when the workload is heavily balanced toward
+    reads and few writes. However, WAN performance and availability
+    can be unreliable, and traveling large distances for writes takes away
     from application performance and availability.
-- Two-phase Commit (2PC): This approach is designed around a protocol
+- **Two-phase commit (2PC)**: This approach is designed around a protocol
     that commits a transaction across multiple transaction managers.
     Two-phase commit provides a consistent transactional write across
     regions but fails transactions unless all participating transaction
-    managers are "available" at the time of the transaction. The number
+    managers are available at the time of the transaction. The number
     of messages exchanged and its cross-regional availability
     requirement make two-phase commit unsuitable for even moderate
     throughputs and cross-geo writes that go over WANs.
-- Sync update with Quorum-based writes: This approach synchronously
-    coordinates a write across majority number of replicas across
+- **Sync update with quorum-based writes**: This approach synchronously
+    coordinates a write across the majority of replicas across
     clusters spanning multiple regions. However, just like two-phase
-    commit, number of messages exchanged and its cross-regional
+    commit, the number of messages exchanged and its cross-regional
     availability requirement make geo-distributed quorum writes
-    unsuitable for moderate throughputs and cross geo writes that go
+    unsuitable for moderate throughputs and cross-geo writes that go
     over WANs.
-- Last-Writer-Wins (LWW) Conflict Resolution: Some systems provide
-    simplistic conflict resolution for all types of writes where the
+- **Last-Writer-Wins (LWW) conflict resolution**: Some systems provide
+    simplistic conflict resolution for all types of writes where
     system clocks are used to determine the winner across conflicting
     writes. LWW is lightweight and can be suitable for simpler data.
     However, LWW can be destructive to updates that are not necessarily
-    conflicting. For example adding a new element to a set across two
+    conflicting. For example, adding a new element to a set across two
     geographies concurrently would result in only one of these new
     elements appearing in the final result with LWW.
-- MVCC (multi-version concurrency control): MVCC systems maintain
+- **MVCC (multi-version concurrency control)**: MVCC systems maintain
     multiple versions of data and may expose ways for applications to
-    resolve conflicts. Even though MVCC system can provide a flexible
-    way to resolve conflicting writes, it comes at a cost of great
+    resolve conflicts. Even though an MVCC system can provide a flexible
+    way to resolve conflicting writes, it comes at the cost of great
     complexity in the development of a solution.
 
 Even though types and commands in Active-Active databases look identical to standard Redis
-types and commands, the underlying types in RS are enhanced to maintain
+types and commands, the underlying types in Redis Software are enhanced to maintain
 more metadata to create the conflict-free data type experience. This
 section explains what you need to know about developing with Active-Active databases on
 Redis Software.
@@ -97,7 +98,8 @@ execute them in script-replication mode.
 
 ## Eviction
 
-The default policy for Active-Active databases is _noeviction_ mode. Redis Software version 6.0.20 and later support all eviction policies for Active-Active databases, unless [Redis Flex or Auto Tiering]({{< relref "/operate/rs/databases/flash" >}})(previously known as Redis on Flash) is enabled.
+The default policy for Active-Active databases is _noeviction_ mode. Redis Software version 6.0.20 and later support all eviction policies for Active-Active databases, unless [Auto Tiering]({{< relref "/operate/rs/databases/auto-tiering" >}}) (previously known as Redis on Flash) is enabled.
+
 For details, see [eviction for Active-Active databases (Redis Software)]({{< relref "/operate/rs/databases/memory-performance/eviction-policy#active-active-database-eviction" >}}) or [eviction for Active-Active databases (Redis Cloud)]({{< relref "/operate/rc/databases/configuration/data-eviction-policies#active-active-replication-considerations" >}}).
 
 ## Expiration
@@ -106,21 +108,21 @@ Expiration is supported with special multi-master semantics.
 
 If a key's expiration time is changed at the same time on different
 members of the Active-Active database, the longer extended time set via TTL on a key is
-preserved. As an example:
+preserved.
 
-If this command was performed on key1 on cluster #1
+If this command was performed on key1 on cluster #1:
 
 ```sh
 127.0.0.1:6379> EXPIRE key1 10
 ```
 
-And if this command was performed on key1 on cluster #2
+If this command was performed on key1 on cluster #2:
 
 ```sh
 127.0.0.1:6379> EXPIRE key1 50
 ```
 
-The EXPIRE command setting the key to 50 would win.
+The `EXPIRE` command setting the key to 50 would win.
 
 And if this command was performed on key1 on cluster #3:
 
@@ -131,54 +133,61 @@ And if this command was performed on key1 on cluster #3:
 It would win out of the three clusters hosting the Active-Active database as it sets the
 TTL on key1 to an infinite time.
 
-The replica responsible for the "winning" expire value is also
-responsible to expire the key and propagate a DEL effect when this
-happens. A "losing" replica is from this point on not responsible
-for expiring the key, unless another EXPIRE command resets the TTL.
-Furthermore, a replica that is NOT the "owner" of the expired value:
+The replica responsible for the winning expire value is also
+responsible for expiring the key and propagating a DEL effect when this
+happens. From this point on, a losing replica is not responsible
+for expiring the key, unless another `EXPIRE` command resets the TTL.
+Furthermore, a replica that is not the owner of the expired value:
 
 - Silently ignores the key if a user attempts to access it in READ
-    mode, e.g. treating it as if it was expired but not propagating a
+    mode, for example, treating it as if it was expired but not propagating a
     DEL.
 - Expires it (sending a DEL) before making any modifications if a user
     attempts to access it in WRITE mode.
 
     {{< note >}}
-Expiration values are in the range of [0,&nbsp;2^49] for Active-Active databases and [0,&nbsp;2^64] for non Active-Active databases.
+Expiration values are in the range of [0,&nbsp;2^49] for Active-Active databases and [0,&nbsp;2^64] for regular databases.
     {{< /note >}}
 
+## Tombstones
+
+For conflict resolution purposes, Active-Active databases cannot immediately release a deleted key. Instead, the key is logically deleted but remains in memory as a tombstone until the garbage collector can safely remove it.
+
+When a deleted key becomes a tombstone, it frees some memory previously consumed by the key. The size of each tombstone varies depending on the data type and the key's history.
+
+The garbage collector automatically removes a tombstone when all instances in the Active-Active database have observed the deletion operation.
+
+To monitor tombstones, you can use shard-level metrics exposed by [`INFO crdt`](#info) and Grafana.
+
 ## Out-of-Memory (OOM) {#outofmemory-oom}
 
-If a member Active-Active database is in an out of memory situation, that member is marked
-"inconsistent" by RS, the member stops responding to user traffic, and
+If a member Active-Active database is out of memory, that member is marked as
+"inconsistent", the member stops responding to user traffic, and
 the syncer initiates full reconciliation with other peers in the Active-Active database.
 
-## Active-Active Database Key Counts
+## Active-Active database key counts
 
 Keys are counted differently for Active-Active databases:
 
 - DBSIZE (in `shard-cli dbsize`) reports key header instances
     that represent multiple potential values of a key before a replication conflict is resolved.
 - expired_keys (in `bdb-cli info`) can be more than the keys count in DBSIZE (in `shard-cli dbsize`) 
     because expires are not always removed when a key becomes a tombstone.
-    A tombstone is a key that is logically deleted but still takes memory
-    until it is collected by the garbage collector.
 - The Expires average TTL (in `bdb-cli info`) is computed for local expires only.
 
 ## INFO
 
-The INFO command has an additional crdt section which provides advanced
-troubleshooting information (applicable to support etc.):
+The `INFO` command has an additional CRDT section that provides advanced troubleshooting information:
 
 |  **Section** | **Field** | **Description** |
 |  ------ | ------ | ------ |
-|  **CRDT Context** | crdt_config_version | Currently active Active-Active database configuration version. |
+|  **CRDT Context** | crdt_config_version | Current Active-Active database configuration version. |
 |   | crdt_slots | Hash slots assigned and reported by this shard. |
-|   | crdt_replid | Unique Replica/Shard IDs. |
+|   | crdt_replid | Unique replica/shard IDs. |
 |   | crdt_clock | Clock value of local vector clock. |
 |   | crdt_ovc | Locally observed Active-Active database vector clock. |
-|  **Peers** | A list of currently connected Peer Replication peers. This is similar to the slaves list reported by Redis. |  |
-|  **Backlogs** | A list of Peer Replication backlogs currently maintained. Typically in a full mesh topology only a single backlog is used for all peers, as the requested Ids are identical. |  |
+|  **Peers** | | A list of currently connected peer replication peers. This is similar to the replicas list reported by Redis. |
+|  **Backlogs** | | A list of peer replication backlogs currently maintained. Typically in a full mesh topology, only a single backlog is used for all peers, as the requested IDs are identical. |
 |  **CRDT Stats** | crdt_sync_full | Number of inbound full synchronization processes performed. |
 |   | crdt_sync_partial_ok | Number of partial (backlog based) re-synchronization processes performed. |
 |   | crdt_sync_partial-err | Number of partial re-synchronization processes failed due to exhausted backlog. |
@@ -187,13 +196,13 @@ troubleshooting information (applicable to support etc.):
 |   | crdt_ovc_filtered_effect_reqs | Number of inbound effect requests filtered due to old vector clock. |
 |   | crdt_gc_pending | Number of elements pending garbage collection. |
 |   | crdt_gc_attempted | Number of attempts to garbage collect tombstones. |
-|   | crdt_gc_collected | Number of tombstones garbaged collected successfully. |
+|   | crdt_gc_collected | Number of tombstones garbage collected successfully. |
 |   | crdt_gc_gvc_min | The minimal globally observed vector clock, as computed locally from all received observed clocks. |
-|   | crdt_stale_released_with_merge | Indicates last stale flag transition was a result of a complete full sync. |
-|  **CRDT Replicas** | A list of crdt_replica \<uid> entries, each describes the known state of a remote instance with the following fields: |  |
+|   | crdt_stale_released_with_merge | Indicates the last stale flag transition was a result of a complete full sync. |
+|  **CRDT Replicas** | | A list of crdt_replica \<uid> entries, each describes the known state of a remote instance with the following fields: |
 |   | config_version | Last configuration version reported. |
 |   | shards | Number of shards. |
 |   | slots | Total number of hash slots. |
-|   | slot_coverage | A flag indicating remote shards provide full coverage (i.e. all shards are alive). |
-|   | max_ops_lag | Number of local operations not yet observed by the least updated remote shard |
-|   | min_ops_lag | Number of local operations not yet observed by the most updated remote shard |
+|   | slot_coverage | A flag indicating remote shards provide full coverage (all shards are alive). |
+|   | max_ops_lag | Number of local operations not yet observed by the least updated remote shard. |
+|   | min_ops_lag | Number of local operations not yet observed by the most updated remote shard. |