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Diffstat (limited to 'iredis/data/commands/cluster-forget.md')
| -rw-r--r-- | iredis/data/commands/cluster-forget.md | 48 |
1 files changed, 23 insertions, 25 deletions
diff --git a/iredis/data/commands/cluster-forget.md b/iredis/data/commands/cluster-forget.md index 7926091..6bff506 100644 --- a/iredis/data/commands/cluster-forget.md +++ b/iredis/data/commands/cluster-forget.md @@ -1,45 +1,45 @@ -The command is used in order to remove a node, specified via its node ID, from -the set of _known nodes_ of the Redis Cluster node receiving the command. In -other words the specified node is removed from the _nodes table_ of the node -receiving the command. +The command is used in order to remove a node, specified via its node ID, +from the set of *known nodes* of the Redis Cluster node receiving the command. +In other words the specified node is removed from the *nodes table* of the +node receiving the command. Because when a given node is part of the cluster, all the other nodes participating in the cluster knows about it, in order for a node to be -completely removed from a cluster, the `CLUSTER FORGET` command must be sent to -all the remaining nodes, regardless of the fact they are masters or replicas. +completely removed from a cluster, the `CLUSTER FORGET` command must be +sent to all the remaining nodes, regardless of the fact they are masters +or replicas. -However the command cannot simply drop the node from the internal node table of -the node receiving the command, it also implements a ban-list, not allowing the -same node to be added again as a side effect of processing the _gossip section_ -of the heartbeat packets received from other nodes. +However the command cannot simply drop the node from the internal node +table of the node receiving the command, it also implements a ban-list, not +allowing the same node to be added again as a side effect of processing the +*gossip section* of the heartbeat packets received from other nodes. ## Details on why the ban-list is needed -In the following example we'll show why the command must not just remove a given -node from the nodes table, but also prevent it for being re-inserted again for -some time. +In the following example we'll show why the command must not just remove +a given node from the nodes table, but also prevent it for being re-inserted +again for some time. -Let's assume we have four nodes, A, B, C and D. In order to end with just a -three nodes cluster A, B, C we may follow these steps: +Let's assume we have four nodes, A, B, C and D. In order to +end with just a three nodes cluster A, B, C we may follow these steps: 1. Reshard all the hash slots from D to nodes A, B, C. 2. D is now empty, but still listed in the nodes table of A, B and C. 3. We contact A, and send `CLUSTER FORGET D`. 4. B sends node A a heartbeat packet, where node D is listed. -5. A does no longer known node D (see step 3), so it starts an handshake with D. +5. A does no longer known node D (see step 3), so it starts a handshake with D. 6. D ends re-added in the nodes table of A. As you can see in this way removing a node is fragile, we need to send -`CLUSTER FORGET` commands to all the nodes ASAP hoping there are no gossip -sections processing in the meantime. Because of this problem the command -implements a ban-list with an expire time for each entry. +`CLUSTER FORGET` commands to all the nodes ASAP hoping there are no +gossip sections processing in the meantime. Because of this problem the +command implements a ban-list with an expire time for each entry. So what the command really does is: 1. The specified node gets removed from the nodes table. 2. The node ID of the removed node gets added to the ban-list, for 1 minute. -3. The node will skip all the node IDs listed in the ban-list when processing - gossip sections received in heartbeat packets from other nodes. +3. The node will skip all the node IDs listed in the ban-list when processing gossip sections received in heartbeat packets from other nodes. This way we have a 60 second window to inform all the nodes in the cluster that we want to remove a node. @@ -49,11 +49,9 @@ we want to remove a node. The command does not succeed and returns an error in the following cases: 1. The specified node ID is not found in the nodes table. -2. The node receiving the command is a replica, and the specified node ID - identifies its current master. +2. The node receiving the command is a replica, and the specified node ID identifies its current master. 3. The node ID identifies the same node we are sending the command to. @return -@simple-string-reply: `OK` if the command was executed successfully, otherwise -an error is returned. +@simple-string-reply: `OK` if the command was executed successfully, otherwise an error is returned. |